Itemized Cost per Lineal Foot.
Sand and gravel $0.42 Cement 2.44 Mixing and wheeling concrete 0.98 Labor placing concrete 0.47 Forms and templates 0.30 Metal fabric 0.39 Setting up forms 0.43 Finishing 0.09 Tools, general and superintendence 0.43 ——- Total per lineal foot $5.95
The cost per cubic yard was thus $6.26. Wages were $1.75 per day.
METHOD AND COST OF MOLDING CULVERT PIPE, CHICAGO & ILLINOIS WESTERN R. R.—During 1906, the Chicago & Illinois Western R. R., Mr. O. P. Chamberlain, Chief Engineer, built a number of culverts of concrete pipe with an interior diameter of 4 ft., and 6-in. shells. Fig. 176 shows the forms in which the pipe was molded. Both forms are of ordinary wooden tank construction. The inner form has one wedge-shaped loose stave which is withdrawn after the concrete has set for about 20 hours, thus collapsing the inner form and allowing it to be removed. The outer form is built in two pieces with 25/8-in. semi-circular iron hoops on the outside, the hoops having loops at the ends. The staves are fastened to the hoops by wood screws 1 ins. long driven from the outside of the hoop. When the two sides of the outer form are in position, the loops on one side come into position just above the loops on the other side, and four -in. steel pins are inserted in the loops to hold the two sides together while the form is being filled with concrete and while the concrete is setting. After the inner form has been removed, the two pins in the same vertical line are removed and the form opened horizontally on the hinges formed by the loops and pins on the opposite side. The inner and outer forms are then ready to be set up for building another pipe.
The concrete used in manufacturing these pipes was composed of American Portland cement, limestone screenings and crushed limestone that has passed through a -in. diameter screen after everything that would pass through a -in. diameter screen had been removed. The concrete was mixed in the proportions of one part cement to three and one-half parts each of screenings and crushed stone. All work except the building of the forms was performed by common laborers. In his experimental work Mr. Chamberlain used two laborers, one of whom set the forms, and filled them and the other of whom mixed the concrete. The pipes were left in the forms till the morning of the day after molding. The two laborers removed the forms filled the day before, the first thing in the morning, and proceeded to refill them. The average time the concrete was allowed to set before the forms were removed was 16 hours. Mr. Chamberlain believes that with three men and six forms the whole six forms could be removed and refilled daily. Based on the use of only two forms with two laborers removing and refilling them each day, and on the assumption that a single set of forms costing $40 can be used only 50 times before being replaced, Mr. Chamberlain estimates the cost of molding 4-ft. pipes as follows:
2 per cent, of $40 for forms $0.80 1.1 cu. yds. stone and screenings at $1.85 2.04 0.8 bbls. cement at $2.10 1.68 10 hours' labor at 28 cts. 2.80 ——- Total per pipe $7.32
This gives a cost of $1.83 per lineal foot of pipe or practically $7 per cu. yd. of concrete. The pipe actually molded cost $2.50 per lin ft., or $9.62 per cu. yd. of concrete, owing to the small scale on which the work was carried on—the laborers were not kept steadily at work.
The pipes were built under a derrick and loaded by means of the derrick upon flat cars for transportation. At the culvert site they were unloaded and put in by an ordinary section gang with no appliances other than skids to remove the pipes from the cars. As each four-foot section of this pipe weighs about two tons, it was not deemed expedient to build sections of a greater length than 4 ft., to be unloaded and placed by hand. On a trunk line, however, where a derrick car is available for unloading and placing the pipes, there is no reason why they should not be built in 6 or 8-ft. sections.
METHODS AND COST OF REINFORCED CONCRETE BUILDING CONSTRUCTION.
If we set aside concrete block construction, virtually all concrete used in building construction is reinforced; plain monolithic or mass concrete now, as in the past, is one of the secondary building materials. It is reinforced concrete building construction that is discussed in this chapter. In no class of concrete work is the contractor's responsibility for the successful outcome of the work greater than in reinforced concrete building construction. No degree of excellence in design can make up for incompetent, careless or dishonest work in construction. This is true not merely in the general way that it is true of all engineering construction—it is true in a special way peculiar to the material. Except for the reinforcing steel, the contractor for concrete building work has no guarantee of the quality of any element of his work except his own faithful care in performing every task that combines to produce that element. The quality of his concrete depends upon the care with which he has chosen his cement, sand and stone, and on the perfection with which he has incorporated them into a homogeneous mixture. The quality of his beam or column, then, depends upon the care with which the concrete is placed in position with the reinforcement and with which the supporting forms are maintained until the member is amply strong to do without support. There is no certainty of any detail except the certainty that is had by performing every part of the work as experience has taught that it should be performed if perfect results are to be attained. We have dwelt thus emphatically on the responsibility in concrete building work of the contractor for the reason that in the past it has been upon the contractor that the burden of failure has been generally shifted.
The construction work of buildings is divided into (1) construction, erection and removal of forms; (2) fabrication and placing of reinforcement; (3) mixing, transporting and placing concrete.
CONSTRUCTION, ERECTION AND REMOVAL OF FORMS.
The stereotyped text-book statement that forms must be true to dimensions and shape and rigid enough in construction to maintain this condition under all loads that they have to sustain mentions only one of the factors that the constructing engineer or the contractor has to keep in mind in designing such forms. His design must be made true and rigid at the least possible cost for first construction of lumber and carpenter work; it must be made with the plan in mind of using either the same forms as a whole or the same form material several times in one structure; it must be made with a view to convenience in taking down, carrying and re-erecting the forms the second or third time; and it must be made with the object in sight of securing the greatest salvage value either in forms fit for use again or in form lumber that can be sold or worked up for other purposes.
The general conditions governing the computation and design of economic form work are discussed in Chapter IX.
COLUMN FORMS.—Concrete columns are usually square or rectangular in section, with, commonly, chamfered or beveled corners. The popularity of these sections is due very largely to the simplicity of the forms required. When hooped reinforcement is used, the column section is always circular or polygonal. Hollow sections, T-section and channel sections are rarely employed and then only for wall columns.
Column forms should be made in units which can readily be assembled, taken apart and re-assembled. The number, arrangement and size of the units are determined by the shape and size of the column and the means adopted for handling the forms. For square or rectangular columns there will be usually four units of lagging, one for each side, plus the number of clamps or yokes used to bind the sides together. Yokes or clamps will seldom be spaced over 3 ft. apart unless very heavy lagging is used; 2 ft. spacing for yokes is common. For circular columns two units of lagging are necessary and this is the number commonly used; the yokes or hoops are spaced about as for rectangular columns. Metal forms can be used to good advantage for cylindrical columns. Forms for polygonal columns are difficult to construct in convenient units. Forms built complete a full story high and concreted from the top are essential where wet and sloppy concretes are used. In Europe, where comparatively dry concretes are employed and where the reinforcement is commonly placed a piece at a time as concreting progresses, three sides of a rectangular form are erected full height and the fourth side is built up as the concrete and metal are placed. This construction is now less common, even abroad, than it was, since wetter mixtures are coming to be approved by European engineers to a greater extent now than formerly. It is a time consuming method and with wet mixtures it has nothing to recommend it. For lagging 1 and 2-in. plank are commonly used; with yokes spaced 2 ft. apart the lighter plank is amply strong and reduces the weight of the units to be handled as well as the amount of form lumber required.
Column forms should always be constructed with an opening at the bottom by means of which the reinforcement can be adjusted and sawdust, shavings and other material cleaned out.
Rectangular Columns.—The form shown in section by Fig. 177 was used in constructing a factory building at Cincinnati, O. Two 24-in. studs at each corner carry the horizontal side lagging boards and are clamped together by yokes composed of four hardwood corner saddles connected around the form by a hooked rod with center turnbuckle on each side. No nails are used in assemblying the parts; the same studding and yokes serve for several sizes of column, the lagging alone being changed. The lumber required for studding is 5 ft. B. M. per foot of column length. The lumber required for lagging, using 1 in. boards, would be 2-2/3 ft. B. M. for a 12-in. column, and 2/3 ft. B. M. would be added for every 2-in. increase in size of the column. About 3 ft. B. M. is required for each set of four corner saddles. With the studs rabbeted at the mill, the carpenter work is reduced to the simple task of sawing the boards and struts to length. The form is taken down by simply unscrewing the turnbuckles; it can be erected by common labor in charge of one carpenter to attend to the plumbing and truing-up. The form can be used over and over and for columns of different sizes without change except in the length of the lagging boards.
The form shown by Fig. 178 was used in constructing a nine-story warehouse at St. Paul, Minn.; it is a design which has become almost standard with a number of large building contractors. In this construction lagging boards the full length of the column are used and are held without nails by yokes. The yokes consist of two heads of wood held together by threaded rods with nuts; between the rods and the lagging are struts or blocks serving both as spacers and to hold the lagging to plane and surface. The yoke proper is adjustable to the extent of the threaded portions of the tie rods. It is to be noticed that the lagging boards are not connected by battens or cleats, therefore, two or three widths of stock serve for all ordinary changes in size of columns and carpenter work is limited to sawing them to length. Furthermore as the boards are full column length, their salvage value when removed from the forms is high. Common laborers under a carpenter foreman can assemble and erect the form. For a 12-in. column and using 34-in. yokes spaced 2 ft. apart and 1-in. lagging, this form requires about 12 ft. B. M. of lumber per foot length of column. The column form shown by Fig. 226 for the six-story building described in a succeeding section differs from the one described only in the details of the yoke construction. In place of the struts between the wooden heads of the yoke a cleat is nailed across the projecting ends which has to be pried loose every time the yoke is removed and nailed into place again every time the yoke is put onto another form; these repeated nailings soon destroy the yoke heads. This form as constructed requires about 8 ft. B. M. of lumber per foot length of 12-in. column, which is 3 ft. B. M. less than is required for the form shown by Fig. 177. The saving comes entirely in the yoke construction.
The form shown by Fig. 238 is of the same general type as are the two just described, the chief difference in detail being in the yoke construction and in the forming of the lagging boards into a panel or unit for each side by means of battens. This panel construction makes a lagging unit which is more convenient to handle, but less convenient to adapt to changes in size of column. The salvage value of the lumber is also reduced by the nailing. Assuming 1-in. lagging and a yoke spacing of 2 ft., to permit direct comparison, this form requires 10 ft. B. M. of lumber per foot length of 12-in. column as compared with 12 ft. B. M. for the form shown by Fig. 177 and 8 ft B. M. for the form shown by Fig. 178. As actually constructed with 2-in. lagging the form shown by Fig. 238 requires about 14 ft. B. M. of lumber per foot length of 12-in. column.
The French constructor, Hennebique, uses the column form construction shown by Fig. 179. Three sides of the forms are built full length of vertical plank and the fourth is built up of horizontal lagging nailed on a board at a time as concreting progresses. In place of rectangular yokes, steel clamps of special form are used to hold the lagging in place. To tear down this form requires drawing the nails in the horizontal lagging and the knocking loose of the clamps. The vertical lagging is of necessity connected by battens into panels to make it possible to hold it in place by the form of clamp used. Assuming 2-in. vertical lagging with 7/83-in. battens every 3 ft., and 7/8-in. horizontal lagging this form requires about 12 ft. B. M. of lumber for every foot length of 12-in. column. This form seems to offer no particular merits to American eyes: there is practically no saving in lumber over forms with rectangular yokes and the clamp shown, while adjustable, is not nearly so rigid and secure a bond for the lagging as is a good yoke.
The form shown by Fig. 180 is an extreme example of nailed construction throughout, no yokes or clamps being used. It was used in constructing a factory building in New York City. Horizontal lagging nailed to vertical studs was used for all four sides; three sides were built up full height and the fourth side was placed a board at a time as concreting progressed. This form required 7-1/3 ft. B. M. of lumber per foot length of 12-in. column, which is probably about as low in lumber as column form construction can be got. The labor of tearing down and re-erecting the form would be high as also would the waste of lumber. Nailed forms of this type are rarely used.
The form shown by Fig. 181 was used for molding T-section wall columns for a power station. It is noteworthy for its section; because of the provision for molding grooves in the two sides to which the curtain walls join, and because of the manner in which three of the eight sides were built up as the concreting progressed. The sides a b c, d e and f g h were erected in full column units and the sides c d, e f and h a were erected in sections 2 ft. high as concreting progressed. The yokes were spaced 2 ft. apart. Using 1-in. stuff for yokes and lagging this form as built required about 16 ft. B. M. per foot length of column. Except for the beveling of the mold for the curtain wall recesses, the framing is all plain saw and hammer work.
A corner wall column form is shown by Fig. 182 and as this was an example of hollow column work the section of the concrete within the form is shown. Forms of this shape and of T-section are properly classed as special form work so that the examples given here are helpful merely as indicating general methods that may be followed. This particular form required 15 ft-B. M. of 7/8-in. lagging per foot of column length, and, neglecting the special top frame, about 16 ft. B. M. of "staging" per foot to support the lagging. The core forms for molding the hollow spaces in the columns of this particular building are shown in Fig. 183. The cross pieces or keys carried on the 5/8-in. bolts as pivots are revolved a quarter turn to slip clear of the slots and permit the sides to close together and free the core for withdrawal. In many cases the contractor will find it preferable to use thin sheet metal core molds or light wooden cores and leave them in place. In one case known to the authors where hollow wall columns were used as hot air ducts for a heating system the duct was laid up of one row of bricks, encircled by the column form and the annular space concreted around the brick duct as a core. The rare use of irregular columns makes form and core construction for them a special problem requiring special detailed estimates in each case. The channel section wall column form shown by Fig. 230 is a case in point; here the form became practically a portable mold for duplicating columns as many times as was desired.
As an example of form work for very large columns or pillars that shown by Fig. 184 is particularly good; it was used for constructing eight 3-ft. square pillars for a water tank tower. The lagging consists of four panels made by nailing horizontal boards to vertical studs. The panels are clamped together by rectangular yokes spaced 3 ft. apart. There are nearly 27 ft. B. M. of lumber per foot length of 3-ft. column in this form.
The form shown by Fig. 185 was used by Mr. R. W. Maxton in constructing a large factory building at St. Louis, Mo., and is notable for the means adopted for centering the forms and for reducing their lateral dimensions to fit them for molding the decreasingly smaller columns of the upper floors. To center the forms the short angles A A are molded into the concrete so as to project slightly above the tops of the floor slab. Also the pieces of wood C are molded into the floor slab. The form is set over the angles and lined up truly by nailing the blocks B to the blocks C. It will be noticed also that the column mold bears only at the four corners the lagging being cut away somewhat on each side so as to afford an opening for cleaning. The lagging for the sides of the column mold is battened together to form four units or panels which are held together by iron clamps of the form shown. Lag screws are used everywhere in place of nails. The notable feature, however, is the piecing out of the lagging panels with 1-in. strips, one or more of which can be ripped off on each side to reduce the size of the forms as the columns grow smaller toward the top of the building.
Polygonal Columns.—Forms for polygonal columns require more lumber and more carpenter work and are less susceptible of ready arrangement into units than forms for rectangular columns. There is no approach to a uniform practice in their construction and the few forms shown here are merely specific examples.
The form shown by Fig. 186 was used for interior columns of octagonal section with hooped reinforcement for a factory building. This form for a 12-ft. octagonal column 24 ins. across between sides requires approximately 325 ft. B. M. of lumber. The form shown by Fig. 187 was used by the same engineer in another building; it is, as will be noted, in four units coming apart in joints at diagonally opposite corners. This form for an octagonal column 18 in. across between sides required about 13 ft. B. M. of lumber per foot of column length, with yokes spaced 3 ft. apart.
The form shown by Fig. 188 was used in a large warehouse at Chicago, Ill. It will be noted from the dotted lines that one yoke clamps the sides a a, the next the sides b b and so on. This does away with triangular blocking to hold the corner boards that is used in the form shown by Fig. 187. Six pairs of yokes were used for each column so that the yoke spacing was about 2 ft. With 26-in. yokes and 1-in lagging a form for a column 18 ins. between sides would require some 17 ft. B. M. per foot of column length.
Circular Columns.—Circular columns have been most frequently molded in steel forms, and these are by all odds the best for general work. Made in two parts of sheet steel and in sections that are set end to end one on another a form is obtained which is easy to erect, remove and transport. Wood forms for circular columns are rather clumsy affairs and are expensive to construct. Such a form, Fig. 190, is described in the succeeding section; another is shown by Fig. 189. This form was used successfully for filling and encasing steel columns for a fireproof building in Chicago, Ill., and is a favorite circular form construction in Europe. It is apparent that the hooping needs to be very heavy and that the form is one that will be hard to handle and rather expensive to make.
In several instances, where hooped reinforcement has been used, the hooping has been wrapped with, or made of, expanded metal or other mesh-+work, and the concrete deposited inside the cylinder thus formed, without other form work. A six-story factory building in Brooklyn, N. Y., was built with circular interior columns from 28 ins. to 12 ins. in diameter, reinforced by a cylinder of No. 10 3-in. mesh expanded metal, stiffened lengthwise by four round rods 1 in. in diameter for larger columns to in. in diameter for smaller columns. This reinforcement was set in place and wrapped with No. 24 -in. mesh metal lath, and the cylinder was filled with concrete and plastered outside. A moderately dry concrete is essential for such construction.
The method of molding shells with the hooping embedded described for the Bush terminal factory work in another section is another way of avoiding form work of the usual type.
Light steel forms as well as the special construction noted must be supplemented by staging to hold them in line and to carry the ends of the girder forms that are ordinarily carried by the column forms. Four uprights arranged around the column so as to come under the connecting girders are commonly used; they are set close enough to the column to hold the form plumb by means of blocks or wedges.
Ornamental Columns.—Forms for ornamental columns call for special design and construction. For many purposes, such as porch and portico work, the best plan is to mold the columns separately and erect them as stone columns of like character are erected. Metal forms of various patterns are made by firms manufacturing concrete block molds and can be purchased from stock or made to order. Where the column is to be molded in place form construction becomes a matter of pattern making, the complexity and cost of which depends entirely upon the architectural form and ornament to be reproduced. The molding of ornament and architectural forms in concrete is discussed in Chapter XXIII, and the two examples of ornamental column form work given here from recent work indicate the task before the builder.
The form shown by Fig. 190 was used for molding in place fluted columns used in a court house constructed at Mineola, N. Y. The lagging in the form of staves forms a 24-sided polygon and is held in position by hoops and yokes. The molds for the flutes were formed by inserting screws from the outside so as to penetrate the staves and molding half-round ribs of plaster of Paris over them by means of the simple device shown. To dismantle the form the screws were removed and the lagging taken down leaving the plaster of Paris in place as a protection to the thin edges until the final finishing of the building.
The methods illustrated by Fig. 191 were employed in molding columns in place for a church at Oak Park, Ill. The bottom portions of these columns were plain square sections molded in place in square molds. The top portions were heavily paneled. The four corner segments were cast in glue molds backed by wood with wires embedded as shown. After becoming hard they were set on end on the plain column and tied and braced as shown. The side openings were then closed by wooden forms and the interior space was filled with concrete. The surface facing for these columns was bird's-eye gravel and cement, with very little sand, mixed very dry and placed and tamped with the coarse concrete backing.
SLAB AND GIRDER FORMS.—Slab and girder construction for roofs and floors is of three kinds: (1) Concrete slab and steel beam construction in place; (2) concrete slab and girder construction in place (3) separately molded slab and beam construction. The third method of construction is distinct from the others in respect to form work as well as other details and is considered separately in Chapter XX.
Slab and I-Beam Floors.—Centers for floor slabs between steel I-beams are made by suspending joists from the beam flanges and covering them with lagging. Frequently the joists and lagging are framed together into panels of convenient size for carrying and erecting. The construction is a simple one in either case where slabs without haunches or plain arches form the filling between beams. Figure 192 shows an arch slab center; plain hook bolts, with a nut on the lower end, passing through holes in the joists are more commonly employed. For 1-in. lagging the joist spacing is 2 ft., for 1-in lagging, 4 ft., and for 2-in. lagging, 5 ft.
A more complex centering is required where the slab has to be haunched around the I-beams. The center shown by Fig. 193 was designed by Mr. W. A. Etherton for the floor construction of the U. S. Postoffice Building erected at Huntington, W. Va., in 1905. The center consists essentially of the pieces A (24 ins. for spans not exceeding 6 ft.) and the 23-in. triggers B, which rest on the lower flanges of the floor beams and thus support the forms. The trigger is secured at one end to the piece A by a 13-in. cleat C and at the other end by 13-in. cleats D on either side of A, which serve also as supports for the batter boards E. The six-penny nail F is but partly driven and it is to be drawn before removing the forms. When the supports of the beams are not fireproofed the cleats D extend to the bottom of the trigger B, but otherwise one cleat extends lower to secure the cross-strip G. To remove the forms, draw the partly driven nail F; knock off the strip G or loosen it enough to draw the nails in B>; pull the triggers on one beam, and the forms will drop. If the soffit board H is used it is necessary first to remove the strip G. For larger beams use the spacing blocks H as shown; for smaller beams omit the trigger B and extend A to rest on the flange of the beam, then to remove the form A must be cut preferably near the beam.
No complete records of the cost of these forms were obtained, but the following partial information is furnished by Mr. Etherton: Considering a panel 6 ft. span by 19 ft. long on 15-in. I-beams, the lumber consisting of 1-in. boards supported by 24-in. cross-pieces on 23-in. triggers spread 3 ft. on centers, soffit of beams not fireproofed, it required one carpenter five hours at 30 cts. per hour to complete the panel. Figuring from this alone I should say that 10 cts. per sq. yd. is a fair estimate for carpenter work. In working over the forms for another floor the 1-in. boards require more time to handle and I should say that the saving in cost of work over the first floor would be not over 2 cts. per sq. yd. Two laborers moved their scaffolding and took down the forms from three completed panels of 13 sq. yds. each in one hour. Smaller panels require a longer time per yard. Counting for the proper piling of lumber I should allow one hour for one man to take down the forms for a 13-sq. yd. panel when conditions are the best. We contracted with two laborers to remove the forms from the third floor and roof and pile them in good shape on the ground just outside of the building for an amount averaging about 4 cts. per sq. yd., and the men made but small wages on the contract. The lumber was used on three floors and the roof, and the best of the 1-in. boards and all of the 24-in. and 23-in. stuff were used on a second job. For a safe estimate based on the data secured I should figure the cost of labor and materials for a three or four-story building about as follows:
Per sq. yd. Lumber at $20 per thousand 28 cts. Carpenter work at 30 cts. per hour 10 cts. Labor tearing down at 15 cts. per hour 4 cts. ———- Total per square yard 42 cts.
Figure 194 shows an arrangement of centering between steel beams which is novel in that it provides for molding a slab with girders. The form was used in building the roof of a locomotive roundhouse. This roundhouse was of the usual circular form and had a radial width of 80 ft. Each radial roof girder, which was an 18-in. I-beam was carried by an outside wall pier and three I-beam columns encased in concrete. The space between main roof girders was spanned by reinforced concrete girders and roof slab. The center illustrated was employed for molding the concrete girders and slab, and carries out the idea of making a stiff and light center for considerable spans of slab without support by staging. The truss construction of the frames supporting the girder box will be noted.
Concrete Slab and Girder Floors.—The construction of forms for this type of floor should be such that the slab centers and the sides of the girder molds can be removed without disturbing the bottoms of the girder molds. This permits the beams to be supported as long as desirable and at the same time releases the greater part of the form work for use again. It is of advantage also to lay bare the concrete as soon as possible to the hardening action of the free air. The slabs may be similarly supported by uprights wedged up against plank caps; no very great amount of lumber is required for this staging and it gives a large assurance of safety. It is well also to give the girder molds a camber or to crown them to allow for settling of the falsework.
The form shown by Fig. 195 was used in constructing girders from 14 to 23 ft. long in a factory building at Cincinnati, O. The sides are separate from the bottom, being supported at the ends by cleats on the column form and at intermediate points by struts under the yokes. The floor lagging is carried by 24-in. stringers supported by the yokes. Uprights set under the bottom plank keep the girder supported after the sides and slab centers are removed. It will be noted that the form is given a camber of 1-in. The structural details are evident from the drawing. The form shows a method of molding a bracket for wind bracing; a simple modification fits it for molding girders without brackets. A rough computation gives 10 ft. B. M. of lumber per lineal foot of girder form as shown.
The form construction shown in Fig. 196 was employed in building the slab and girder floors for the United Shoe Machinery Co.'s factory at Beverly, Mass. In these buildings the main girders cross the building at 20-ft. intervals and midway between the main girders is a bridging beam also reaching across the building. Floor beams span the 10-ft. spaces between bridging beams and main girders at intervals of 3 and 4 ft. Referring first to the main girder form, tall horses are set up at 3-ft. intervals and connected by stringers laid on the caps. These stringers carry a cross piece, with a cleat at each end, over each horse. The bottom boards of the mold rest on these cross pieces and the side pieces are set up between verticals wedged tight between the cleats. The beam molds are a modification of the girder molds. The slab centers consist of panels just large enough to span the openings between beams and girders and composed of 1-in. boards fastened together by four 15-in. cleats. Except in attaching the quarter round and triangular moldings for fillets no nailing is necessary in erecting and taking down the forms.
The form construction shown by Fig. 197 is one used by a large firm of reinformed concrete builders. The slab centers can be struck and the sides of the girder mold removed without disturbing the support for the bottom of the beam. This form runs quite low in lumber, requiring for a 912-in. beam box including posts some 9 ft. B. M. per lineal foot of box. The joists and lagging as shown require about 2 ft. B. M. per square foot of floor slab. The practice is to give these girder boxes a camber of -in. in 10 ft.
The construction shown by Fig. 198 is designed to provide adjustability, to enable quick erection and removal and to do away with all nailing. The construction is as follows: Wooden posts carry at their tops steel T-beam cross-arms knee braced to the posts by steel straps. The cross-arms carry the two jaws of a clamp, each consisting of a vertical plate, and two diagonal braces, slotted so as to slide on the T-beam. A cut nail or other piece of metal driven into the slots fastens the jaws on the T-beam. The cross-arms carry the bottom boards of the girder molds and the vertical plates of the jaws support the side pieces. A blocking piece slipped between the braces carries the end of the joist for the floor slab centers. This form is the invention of Mr. W. H. Dillon and was used in constructing the nine-story, 260150-ft. wholesale hardware store Of Farwell, Osman & Kirk Co., St. Paul, Minn.
The form shown by Fig. 199 was used in constructing a factory building in Long Island City, N. Y., and it is given here chiefly for the purpose of exhibiting the unnecessary complexity of form work. Comparing this form with that of nearly any of the preceding designs will bring out the point. The design, however, was one of the earlier ones to recognize the advantage of stripping the slab centers and the sides of the girder boxes without disturbing the bottom plank of the boxes or the staging. The drawing shows the independent support of the bottom board and side pieces of the girder mold on the transverse caps of the staging posts. These posts are 68 ins. in section and are spaced from 6 to 8 ft. apart. Briefly described the bottom board is a single plank from 1 to 3 ins. thick, to which the side pieces are lag-screwed at the bottom. The side pieces are panels composed of 47/8-in. vertical boards nailed to top and bottom 24-in. horizontal timbers. A third horizontal timber near the top serves as a seat for the ends of the joists carrying the slab lagging and is braced from the bottom horizontal by vertical stiffeners. The edge boards of the slab lagging are nailed to the top edges of the side pieces of the girder mold and the tops of these side pieces are connected across the trough by strips of board; all the slab lagging boards except those at the edges of the girder molds are laid loose. In the building referred to, after the floor concrete had set about seven days the joists carrying the slab lagging were turned a quarter over thus dropping the slab form about 2 ins. A few days later the joists and lagging were taken down and the side pieces of the girder mold were unscrewed and removed. The bottom board and staging posts were left in position about three weeks longer and then dropped about 1 in. by removing fillers from the staging post caps. In another week the bottom boards and staging posts were taken down. This construction of form and method of removing it permitted the concrete to be stripped so that the air could get at it as fast as it was safe to take the support from any part and at the same time kept the supports in such position that they form a safety platform in case of collapse. A more important advantage is that the form timber can be removed as fast as any part of it is free and used again. Thus the lagging boards and joists and the side pieces for the girder molds were free for use again about every two weeks and yet the main supports of the girders were undisturbed until they were fully a month old.
Other examples of girder and slab forms are shown in the succeeding sections describing the construction of a six-story building and of a garage constructed at Philadelphia, Pa.
Another type of slab and girder form construction that deserves brief mention because of its variation from usual practice and also because of its extensive use by one prominent builder is shown by Fig. 200. Cores, or inverted boxes, with four vertical sides and rounded corners, are set side by side, with ends on stringers carried by the column forms, at intervals wide enough to enable the beam to be molded between. A plank resting on cleats on the sides of the cores forms the bottom of the beam mold. The main girders are molded in similar spaces between the ends of the cores in one panel and of those in the next panel. To permit the core to be loosened readily it is hinged; when in place spacers inside the core keep the sides from closing. These are knocked out, the core sides close together and the core is removed for use in another place. Cores similar to these were used in molding the ribbed floor for the Bush terminal factory building described in a succeeding section. These cores are capable of repeated use so that while they are somewhat expensive to frame they give a very low cost of form work when the beam and girder spacing is arranged largely in duplicate from floor to floor. It will ordinarily be cheaper to have these cores made to pattern by regular woodworking shops, and shipped to the building ready to erect.
WALL FORMS.—Wall work in modern commercial and manufacturing buildings, when we come to eliminate windows and wall columns and girders, is confined very largely to isolated curtain wall panels between windows and framework. In such buildings, therefore, wall forms consist merely of wooden panels, one for each face of the wall, constructed to fit the spaces to be walled up. Where these spaces are duplicated from bay to bay or story to story the same form panels will serve repeatedly. For residences and other buildings having greater proportionate area of blank wall the builder has a choice between continuous forms carried by staging and movable panel forms.
For one and two-story buildings, with the usual variation in architectural detail, panel work and window work, the continuous form has many advantages, and the superior economy of movable panels in retaining and other plain wall work is by no means always true here. One good type of continuous wall form construction is shown by Fig. 201. The gallows frames are spaced about 6 ft. apart along the wall and connected by horizontal stringers nailed to the uprights or by diagonal bracing. Each frame may be made up of 66-in. posts connected by 24-in. cross-struts and diagonals with bolted connections so that the frame can be taken down and put together easily and so that the bracing can be removed as the wall is built upward. The other details of the form work are shown by the drawing. This construction leaves a clear space for placing the concrete and the cross pieces give support to runways; it has been successfully used in a large amount of low building work.
Movable panel forms are of great variety in detail but are generally of either one or the other types shown by Figs. 202 to 204.
The form shown by Fig. 202 was used in constructing a church at Oak Park, Ill. For the back of the wall it consists of continuous lagging held by 24 studs. For the face 16-in. lagging 12 ft. long was nailed to 24-in. studs to form panels. It will be noted that the ends of the studs are scarfed so as to interlock in succeeding panels. This construction also shows a method of supporting the reinforcing bars inside the form.
The form shown by Fig. 203 was used in constructing a large factory building, and consisted of two side pieces or panels 3 ft. high and 16 ft. long, the distance between wall columns. For the first course these were seated on the carefully leveled and rammed ground and securely braced by inclined or horizontal struts inside and outside of the building. After the concrete had set for three days the molds were loosened and lifted until the lower edges were 2 ins. below the top of the concrete and there they were held by horizontal bolts through their lower edges and across the top of the concrete by ties nailed across their tops every 3 ft. and by bracing to the falseworks supporting the column and floor forms. The cross bolts passed through pasteboard sleeves which were left permanently embedded in the wall. By starting the molds level and finishing each course level with their tops no difficulty was had in keeping the forms plumb and to level as they were moved upward. This type of form has to be exteriorly braced to staging or adjacent column forms, etc.
The type of movable panel form shown by Fig. 204 depends for all support on the wall alone. The sketch shows the form filled ready to be shifted upward; this operation consists in removing the bottom bolts and loosening the top bolts enough to permit the studs to be slid upward the full length of the slots. The lagging boards left free are then removed and placed on top and the bolts are tightened, completing the form for another section of wall.
A type of wall form construction intended to do away with studding and bracing is illustrated by Figs. 205 and 206. In both cases metal plank holders are used in place of studs, and practically the only difference between the two is in the shape and material of the holders. The mode of procedure is to work in horizontal courses one plank high around the wall, removing the bottom plank and placing it on top as each new course is begun after the first few courses have been laid. Using the arrangement shown by Fig. 205 in constructing a building 10054 ft. in plan and 36 ft. high with 12-in. walls, a height of two 122-in planks was all the form work that was ever necessary at any one time, so that the amount of form lumber required for the building was 2,464 ft. B. M. plus 205 ft. B. M. of 24-in. flooring strip, or altogether 2,669 ft. B. M., or 0.24 ft. B. M. per square foot of exterior wall surface, or 6 ft. B. M. per cubic yard of concrete. This same form lumber with 16 additional plank was then used to construct a building 100100 ft.16 ft. high, so that some 3,000 ft. of form lumber sufficed for 17,548 sq. ft. (exterior surface) of wall or for 617 cu. yds. of concrete in 12-in. wall, which gives 0.17 ft. B. M. per square foot or 4.8 ft. B. M. per cubic yard of concrete.
ERECTING FORMS.—The organization of the erecting gang will depend very largely on the manner in which the forms have been constructed. If they have been constructed in sections which go together with wedges and clamps common laborers with a foreman carpenter in charge to direct and to line and level the work will do the erecting, but if they have to be largely built in place carpenters are necessary for all the work except carrying and handing. There should be at least one foreman for every 15 to 20 men and a head foreman in charge of all form work. The mode of procedure will differ for every job, but the following general directions apply to all work in greater or less measure.
Clamps, bolts and wedges and not nails should be used wherever possible in assembling parts of forms in erection; these devices are not only quickly and easily applied in erection but they are just as quickly and easily loosened in taking down forms and they can be loosened without jarring the concrete member.
Lining girder forms and lining and plumbing column and wall forms is high-class carpenter work and should be directed by competent carpenters. A column or girder which is out of line or plumb not only looks bad but may be required to be removed and corrected by the engineer. The expense for one such correction will be many times that which would have been involved by proper care in the first place.
Supports or staging for the forms should be used freely and well braced in both directions. Uprights should be set on wedges and bear against a cap piece and on a sill piece to distribute the load.
Erect, line and plumb the column forms first; then erect, line and level the girder forms and set the girder staging, and finally erect and level the slab centers and their supports.
Leave the foot of each column form open on one side at the bottom so that the column reinforcement can be adjusted and connected up and so that a clear view can be had through the form to detect any object that may have fallen into the form and become wedged; this same opening makes it possible to clean the form.
Give the forms a final inspection before concreting to check line and level, to close open joints and to tighten up clamps and wedges. Finally clean each form and wet it down thoroughly before placing the concrete—do this just before placing the concrete.
REMOVING FORMS.—Good judgment and extreme care are essential in removing centers. It goes without saying that forms should never be removed until the concrete has set and hardened to such strength that it will sustain its own dead weight and such live load as may come upon it during construction. The determination of this condition is the matter that calls for knowledge and judgment. Some cements set and harden more rapidly than others, and concrete hardens more and more slowly as the temperature falls. These and other circumstances must all be taken into account in deciding upon the safe time for removal. Many large contractors mold a cube of concrete for each day's work and leave it standing on the finished floor exposed to the same conditions as the concrete in the forms; examination of this sample block gives a line on the condition of the concrete in the work and on the probable safety of removing the forms at any time. In all cases it should be the superintendent's duty to determine when to remove forms, and he should satisfy himself by personal inspection that the concrete is in condition to stand without support. It is also wise at least as a matter of precaution for the contractor to secure the engineer's or the architect's approval before removing any formwork.
Care in removing forms is essential for the reason that green concrete is particularly susceptible to injury from shock or sudden strain. It is well, therefore, to have a separate gang always doing the work. These men will in a few days become trained under an experienced foreman so that they will not only do the work with greater safety but also more rapidly. This gang should, furthermore, be required to follow a regular system in its work; a system which may not be departed from without direct orders from the superintendent. An example of such a system is outlined below.
The time of beginning this work of removal shall be given by the superintendent. In warm, dry weather, with other conditions favorable, removal may be begun after seven days. Then the following schedule may be followed: At the end of seven days remove the sides of the column forms. This gives an opportunity to determine the soundness of the column casting and also serves the further desirable purpose of baring the concrete to the curing and hardening action of the air. At the end of 14 days loosen the wedges of the posts supporting the slab centers and drop these centers a couple of inches: leave the centers in this position for another day, meanwhile examining the tops of the slabs to note their condition. Then remove the sides of the beam molds and the slab centers, replacing the latter with temporary uprights supporting a plank bearing against the underside of the slab. This precaution is often neglected and with very little reason considering the importance of the safeguard thus secured. Ordinarily the shores need not be left in place more than a week, so that the amount of lumber thus tied up is small. At the end of three weeks remove the uprights under the beam and girder molds and strip the bottom plank. In this schedule it is assumed that the floor is free from any great load and that no unusual loading is put upon it; if a load of any consequence is to come on the floor the shores and uprights should be left in place longer. No schedule of removal can be blindly followed, and that given above is certain only when the conditions are right and as stated.
FABRICATION AND PLACING OF REINFORCEMENT.
The amount of reinforcing steel used varies from 50 lbs. to 275 lbs. per cu. yd. of concrete; the highest figure will be had only in very heavy work and where very heavily reinforced raft foundations are employed, and the lowest only in one-story buildings consisting of walls and roof. A fair average is perhaps 150 lbs. per cu. yd. The cost of fabricating and placing reinforcement will run from 1/3 ct. to 1 cts. per pound, but the last figure is exceedingly high; ct. per pound for fabricating and placing is a reasonable labor charge.
Contractors frequently have their choice whether the steel shall be fabricated into frames and placed as units or whether it shall be placed in separate bars. For girders and columns the difference in cost of the two methods is not so very great for steel in place when the fabrication is done in the field. The unit frames cost considerably more than separate bars to fabricate, but the cost of handling and placing them in the forms is materially less; on an average the differences balance each other. Where the frames are made up in regular mills unit frames generally cost less to fabricate and place than do separate bars. The use of unit frames in wall and floor slab reinforcement is generally more expensive than the use of separate bars. The chief gain that comes from the use of unit frames is the gain due to the certainty that the reinforcing bars, stirrups, etc., are all there and are properly spaced and placed.
FABRICATION.—Fabrication includes all the work necessary to prepare the reinforcement ready to place in the forms. It amounts to very little where separate bar types of reinforcement are used. Plain bending and shearing operations comprise the whole task. Where the beam or column reinforcement has to be made up into complete frames which can be handled and placed as units this task is more complex and considerable apparatus is essential to rapid and economical work. For this reason it is wise usually to contract with some metal working shop to assemble and connect up the various units and to furnish them ready for installation. In many cases these unit frame types of reinforcement are patented and the proprietors contract to fabricate and furnish them complete according to the plans of the engineer or architect. Even where the frame construction is not so controlled it will be economy generally to have the fabrication done at regular shops where the necessary tools and skilled workmen are had. In any case the bars should be ordered cut to length at the mill so far as possible.
Assuming the fabrication to be done in the field, the mode of procedure will be as follows: Order the bars or rods to be shipped in bundles of corresponding sizes and lengths of pieces with each bundle tagged with its proper shop number or mark. The bundles should weigh about 200 lbs.; this is a load easily handled by two men and so long as possible all handling should be done in the original package, for when once broken it is very hard to get men to carry a full load. As received, the bars of each size and length should be stored by themselves. For ordinary bars not having long prongs a rack of the general form shown by Fig. 207 serves the purpose excellently. When a great deal of metal must be kept stored for some time it is wise to roof over the racks, not only to protect the metal from rain and snow, but to enable the men to work dry shod in stormy weather. Usually it will pay to have one man whose sole duty it is to receive and check all metal and to attend to its systematic arrangement on the racks; this same man will also direct the removal of the metal to the shop where it is bent and otherwise worked up, and can, if he is competent, earn his pay many times over in time saved all along the line in handling and working up the reinforcement. The authors have seen enough time wasted in hauling over and rehandling metal in piles to get at what was wanted to pay for shed, racks and the wages of a storekeeper several times during a moderate sized job. In large work provide the storekeeper with a schedule showing the order in which the metal is wanted for the work so that he can arrange it in that order and can check up his receipts from the mills and report missing items in time for the deficit to be made up before some part of the work has to be stopped because of material missing. System in receiving and storing the metal is absolutely essential to rapid and accurate work at the bending and erecting tables.
The work done on the metal consists chiefly of bending. The metal can usually be bent cold, but for sizes 1-in. and upward some makes of bars require heating; this can be done by laying the bars side by side on the ground and arranging sticks and shavings on top of them in a strip 18 ins. to 2 ft. wide across the portion where the bend is to be. Only moderate heating is usually required. Ordinary bending is a simple process and can be done with very simple apparatus. Figures 208, 209 and 210 show frequently used devices, any of which can be made by an ordinary carpenter. For heavy bars, 1 and 2 ins., the device shown by Fig. 210, with its heavy, swinging beam, is particularly efficient. An example of more elaborate methods is had in the following description of the processes employed in fabricating girder frames and hooped column reinforcement for a large factory building. The building was 50075 ft., with six stories and a basement, built for the Bush Terminal Co., Brooklyn, N. Y., in 1905. Three longitudinal rows of round columns and two rows of rectangular wall columns carry heavy longitudinal girders supporting floor slabs with corrugated undersides as shown by Fig. 211, which also shows the floor slab reinforcement. About 12,000 cu. yds. of concrete and 1,000 tons of reinforcing steel were required; hence 167 lbs. of steel were required for each cubic yard of concrete. The floors, however, were designed to carry a load of 800 lbs. per sq. ft. The particular feature of interest in this building was the fabrication of all the column and girder reinforcement into unit frames and cylinders in temporary workshops on the site.
The circular interior columns, varying from 30 ins. to 12 ins., in diameter were molded in permanent shells of cinder concrete. The shells were made in sections about 30 ins. long, with walls 1 ins. thick, which were set one on another with mortar joints to form the column mold. In fabricating the shells the first step was to wind a helix of steel wire on a collapsible mandrel about 4 ft. long; the mandrel was set with the axis horizontal and was revolved by hand, the wire being fed on also by hand and under a slight tension. After the wire helix was completed it was wrapped with a sheet of expanded metal, the longitudinal edges of which lapped a few inches and were tied by wire ties. The expanded metal covering was also wire tied to the helix. Each of these cylinders of expanded metal and wire was 30 ins. long and formed the inner mold for making the shell. The outer mold consisted of a sheet metal cylinder in two parts assembled and supported by wooden yokes and framework. The two molds were assembled on a plank platform, one inside the other, and about a common center. The annular space was then filled with a 1-5 cinder concrete mixed moderately dry so that while it would exude slightly through the expanded metal mesh it would not waste to any extent. After from 18 to 24 hours the outer mold was removed for reuse and the shell was left standing on the molding platform until safe to handle. The larger shells, 30301 ins., weighed about 150 lbs. each.
Some 2,000,000 lbs. of plain round steel rods from in. to 1 ins. in diameter were required for reinforcing the concrete. For the main girders these rods were cut, bent and assembled into frames or trusses which were placed as units. The main rods were ordered cut to length, but the stirrup rods were ordered in lengths of 20 ft. and cut to lengths as required. The rods were brought to the work in carload lots and were stored according to lengths and sizes in racks under sheds. Another shed was provided for the steelworkers, who cut and bent the rods and assembled the girder frames ready for the workmen on the building. There were about 50 different patterns of frames required. They were made entirely by hand. For bending large size rods, heavy compound levers were used; the lighter rods were bent by the device shown in Fig. 212. The assembling of the trusses was accomplished as shown by Fig. 213, using the steel framework of the erection shed as a staging. Across the horizontals of the framework were placed two false temporary top chord bars marked to the stirrup spacing of the truss being assembled. On these bars, at the spaces marked, were suspended stirrups with their lower ends hooked. The lower chord bars were then suspended in the stirrup hooks and the whole assemblage of bars and stirrups was then clamped rigid by the lever bars and intermediate clamps. The loop ends of the stirrups were then bent by special wrenches to the position shown at 2, then closed by hammering to the position shown at 3, and finally they were wire tied. The process was a simple one, and by adopting a regular routine the men became so expert that two of them could complete many trusses in a working day. The contract price for shaping the steel and assembling it into frames was 1 ct. per lb.; the cost of the work to the contractor has been stated by Mr. E. P. Goodrich, Engineer, Bush Terminal Co., to have been about ct. per lb. The cost of placing the steel in the building was ct. per lb.
PLACING.—With unit frame reinforcement the number, size and location of the bars have been made certain in the shops where the frames are fabricated so that the erector has nothing to do but to line and level up the frames in the forms, place such temporary braces as are needed to hold them true, and make the end connections with abutting frames. Such frames are usually provided with "chairs" to hold the bottom bars up from the form so that little bracing or none is required. With separate bar reinforcement the erector may either place the reinforcement complete in the form by wire-tying the bars to each other, to temporary braces or templates and to the forms, or he may insert the various pieces of reinforcement in the concrete as the pouring advances, depending on the surrounding concrete to retain them where inserted. Generally a combination of both methods is employed.
The processes in detail of placing reinforcement are particularized in several places in other sections; they will differ for nearly every job. Here, therefore, general rules only will be given.
(1) See that the correct number and size of reinforcing bars, splices and stirrups are used and that they are spaced and placed strictly according to the working plans.
(2) Bars must be properly braced, supported and otherwise held in position so that the pouring of the concrete will not displace them.
(3) Splices are the critical parts of column reinforcement. See that the bars butt squarely at the ends and are held by pipe sleeves or wired splice bars; see that the longitudinal rods are straight and vertical; see that the horizontal ties or hooping are tight and accurately spaced. When the reinforcement is built up inside the form one side is left open for the work; ordinarily the column reinforcement will be fabricated into unit frames, then an opening in the form at the bottom to permit splicing will suffice.
(4) Take extreme care that beam and girder reinforcement is placed so that the bottom bars lie well above the bottom board of the mold; use metal or concrete block chairs for this purpose.
(5) See that the end connections and bearings of beam and girder frames are connected up and have the bearings called for by the plans.
(6) See that line and level of all bars and of the reinforcement as a whole are accurate; make particularly certain that expanded metal or other mesh-work reinforcement lies smooth and straight.
(7) Give all reinforcement a final inspection just previous to pouring the concrete; this is particularly essential where the reinforcement is placed some time in advance of the concreting.
MIXING, TRANSPORTING AND PLACING CONCRETE.
A reinforced concrete building requires from 0.2 to 0.5 cu. yd. of concrete per 100 ft. of cubical volume of the building, assuming walls, floors and roof to be all of concrete. The amount of concrete to be mixed, transported and placed is, therefore, large enough, even for a building of moderate dimensions, to warrant close study of and careful planning for this portion of the work. A few general principles can be set down, but as a rule there is one best way for each building and that way must be determined by individual conditions.
MIXING.—Concrete for building work has to be of superior quality so that no chances may be taken either in the process of mixing or with the type of mixer employed. Machine mixing and batch mixers should always be employed. Machine mixing gives generally a more homogeneous and uniform concrete than does hand mixing and is cheaper. Batch mixers are generally superior and more reliable than continuous mixers where a uniformly well mixed concrete is required. The capacity of the mixing plant is determined by the amount of concrete to be placed and the time available for placing it. Its division and arrangement is determined by the area of the work and the type and arrangement of the plant for transporting the materials and the mixed concrete. The following general principles may be laid down: Make the most use possible of gravity; it is frequently economy to carry all materials to the top of bins from which point they can move by gravity down through the mixer to the hoist buckets, and where natural elevations or basement floors below street level permit gravity handling they should be taken advantage of. The mixing should be done as near the place of concreting as practicable; in building work this is the point on the ground which is directly under the forms being filled. It is, of course, impracticable to secure so direct a route as this from mixer to forms, but it can be more or less closely approached; using two mixers, for example, one at the front and one at the rear of a building cuts down the haul from hoist to forms one-half. Other ways will suggest themselves upon a little thought. In the matter of the mixing itself, it must never be forgotten that a batch of concrete without cement which goes into a girder or column will result in the failure of that member and possibly the failure of the building. In massive concrete work a batch without cement will not endanger the stability of the structure, but in column and floor work in buildings it is certain disaster. Formanship at the mixer is, therefore, highly important and a cement man who realizes the responsibility of his task is equally important.
TRANSPORTING.—Transporting the mixed concrete is divided into three operations—delivering concrete from mixer to hoist, hoisting, and delivering hoisted concrete to the forms. The delivery from mixer to hoist may be by direct discharge into hoist bucket, by carts or wheelbarrows, or by cars carrying concrete or concrete buckets. Hoisting may be done by platform hoists or elevators, by bucket hoists, or by derricks. Handling from hoist to form may be direct in buckets, by carts or wheelbarrows, or by cars. These several methods can be worked in various combinations, and the following examples of plants show such combinations as are most typical of current practice.
In any system of transportation it is getting the concrete to the hoist and from hoist to form that eats up the money. Hoisting makes but a small part of the total transportation cost, and, moreover, the difference in cost of operation for different hoists is very small. Mr. E. P. Goodrich states that on three buildings the actual costs for the hoists installed and removed after the completion of the work were as follows:
Platform hoist $330 Bucket hoist 465 Derrick 225
In figuring on the form of hoist to be adopted, the capability of the hoist for general service has to be kept in mind. Platform hoists and derricks can be used for hoisting form lumber and reinforcing steel as well as for hoisting concrete, while bucket hoists cannot be so used except where they may be fitted with special carriages for lumber or steel. On the other hand, the bucket hoist is usually the quickest method of hoisting concrete, and it can readily be extended upward as the work progresses. The last is true also of platform hoists. The use of derricks necessitates frequent shifting for high work or else the building of expensive staging to raise the derrick into a position to command the final height of the building. The probable costs of moving and extending must be allowed for in choosing the hoist to be used.
Direct discharge of the mixer into the hoisting bucket is, of course, the ideal manner of transporting the concrete from mixer to hoist, and this can generally be obtained by planning, particularly where bucket hoists or derricks are employed. For platform hoists direct discharge is impossible; it can be somewhat closely approached, however, where conditions permit car tracks to be laid on the floors being built, so that a car holding a batch of concrete can be run onto the platform, hoisted and then run to shoveling boards near the forms that are being filled. The successful use of such an arrangement of car tracks is described in Chapter XX, but it was for handling concrete blocks. A direct discharge from hoisting bucket to forms is frequently possible where derricks are used for hoisting, but with bucket and platform hoists, wheeling or carting is necessary.
Where wheeling or carting has to be done either at the bottom or at the top of the hoist, or at both points, a great factor in the economy of work is the arranging of the operations in cycles. For example, in wheeling concrete to forms from a hopper fed by a bucket hoist, arrange the runways so that each man makes a circuit, passing by the form at one end and by the hopper at the other end, and goes and comes by a different route. The speed gained by avoiding confusion and delay saves many times the additional cost of runways which is small. In fact it is economy to employ a few extra men to arrange runways and keep them clean, because of the additional speed thus gained. Good organization effects more economy than special methods of hoisting as far as the labor of handling the concrete is concerned.
Bucket Hoists.—A bucket hoist construction which has been extensively used in building work on the Pacific coast is shown by the drawings of Figs. 214 to 216. Two T-bar guides made in sections connected by fishplates furnish a track for an automatic dumping bucket hoisted and lowered by steel cable from engine on the ground to head sheaves as shown. The sectional construction of the T-bar guides permits the hoist to be any height desired, it being lengthened and shortened by adding and taking out sections. The bucket is dumped automatically at any point desired by means of a tripping device attached to a chute which receives the contents of the bucket and delivers them to carts, wheelbarrows, or other receptacle. The hoist is set outside of the building with the mixer arranged, if possible, to discharge directly into the bucket. By setting the guide frame in a pit or on blocking any height of edge of bucket can be secured. The buckets are ordinarily 13 or 20 cu. ft. capacity. It is recommended, when greater hoisting capacity is necessary, to use two hoists set side by side and operated by one cable in the same manner as double wheelbarrow cages; as the weight of one bucket counterbalances the weight of the other, the power required for hoisting is reduced. To adapt this hoist to handling form lumber the bucket is replaced by the lumber carriage shown by Fig. 216; this carriage discharges over the head of the mixer and the spring buffer shown by Fig. 214 is to take the shock of the rising carriage. This buffer is omitted when concrete only is to be hoisted. In one case this device has hoisted 520 batches of 12 cu. ft. each to the fourth floor in 8 hours, or nearly 19 cu. yds. per hour. In another case 65 trips per hour were averaged to the fifth floor with a 12-cu. ft. load each trip; this is nearly 30 cu. yds. per hour. With the lumber carriage 8 men have unloaded 14,000 ft. B. M. of 210-in. stuff from car to the second floor and distributed it in 43 minutes. A -cu. yd. combination outfit for concrete and lumber, with 40 ft. of guide track, weighs 1,750 lbs., without the lumber carriage the outfit weighs 1,600 lbs. This hoist is made by the Wallace-Lindesmith Co., Los Angeles, Cal.
A popular construction for automatic bucket hoists is that shown by Figs. 217 and 218 by Mr. E. L. Ransome. The bucket is held upright by guides at its front and rear edges; to dump it a section of the front guide is removed at the desired dumping point which allows the bucket to overturn as shown. A friction crab hoist operated from the mixer engine runs the bucket. The mixer is located as shown. Figure 218 shows the foot of the hoist set in a pit with the mixer at surface level, but the hoist can be set on the surface and the mixer mounted on a platform. In the latter case a charging bucket, traveling from stock pile up an inclined track to the mixer platform, is generally used. A hoist like that illustrated, equipped with a -cu. yd. Ransome mixer, will cost about $1,500 and will deliver 15 cu. yds. of concrete per hour. Mr. F. W. Daggett gives the following figures of the cost of operation:
Mixing Gang: Total 1 hr. 1 mixer foreman, also engineer, 25c. $.25 1 man charging mixer, 20c. .20 1 man running hoist, 20c. .20 2 men wheeling sand, 17c. .35 4 men wheeling and shoveling stone, 17c. .70 1 man helping up runway, 17c. .17 2 men carrying cement, 17c. .35
Gang Placing Cement:
1 foreman, 25c. .25 9 men wheeling concrete, 17c. 1.57 3 men tamping concrete, 17c. .52 1 man filling carts, 17c. .17 ———
Total labor cost per hour $4.75
Fuel, etc. .50 ——— 5.25
This gives a cost of 35 cts. per cu. yd. for mixing and placing concrete.
In this particular case the mixer was charged by wheelbarrows. Frequently the stone and sand bins can be arranged to chute the materials directly into the charging hopper as shown by Fig. 217. In place of barrows two-wheeled carts of the type shown by Fig. 12 can be used. Mention has already been made of operating the charging bucket on an incline from stock pile to mixer. Such arrangements are described in Chapter IV.
In constructing a 9-story store at St. Paul, Minn., the concrete was hoisted by continuous bucket elevators. A lay-out of the construction plant is shown by Fig. 219. In the alley near the center of the north side of the building the surface grade was about 6 ft. above the third story level. A hopper was constructed at grade and provided with two chutes running to the basement. These chutes discharged on opposite sides of a vertical partition separating the sand and stone bins, and by closing either chute at its top by a suitably arranged deflector plate either sand or stone could be dumped into the same hopper and chuted to its proper bin. Cement was brought to the work in cars over the tracks shown and was wheeled from the cars over runways leading to the charging platforms near each mixer. Other runways connecting with these platforms provided for wheeling the sand and stone to the mixers. The runways were placed at the proper height to permit the barrows to be emptied directly into the charging hoppers. Two Smith mixers were used, located as shown, and each discharged through a chute into one of the bucket elevator boots. There were two elevators which were "raised" two stories at a move as the work progressed. Each elevator discharged into a hopper holding 1 batches, and from these hoppers the concrete was fed into wheelbarrows and wheeled to the forms. The bucket elevators were carried no higher than the eighth floor. When this floor had been completed the hoppers were moved down to the fifth floor and the wheelbarrows were taken to platform elevators and carried to the remaining floors and roof. Special 4-cu. ft. wheelbarrows were used for handling the concrete. A maximum of 155 cu. yds. of concrete was mixed, transported and placed in a 10-hour day with a gang of 28 men.
Platform Hoists.—The common builders' hoist or elevator, operating single or double platforms or cages, needs no special description. The wheelbarrow, cart or car containing the concrete is run onto the platform, hoisted and then run to the forms. The chief advantage of this device in concrete work is that it will handle all classes of material without any change of carriage or arrangement, it can thus be used for handling form lumber and reinforcing steel as well as for handling concrete.
Derricks.—The use of derricks for hoisting in concrete building work is limited by the necessity of supporting them independently of the structure being built; the formwork or the completed concrete work cannot be utilized to carry derricks during construction. For low structures the derrick can be set on the ground, but for high buildings a supporting tower or staging is necessary. The arrangement of such falsework can be illustrated best by specific examples.
In constructing a 7-story factory at Cincinnati, O., concrete was mixed on the ground and hoisted by a derrick with an 80-ft. boom mounted on a tower 55 ft. high. The derrick was located to one side of the building. For the lower floors the boom swing covered so large an area that the bucket was dumped at various places, but for the upper floors it was found more economical to dump buckets into a hopper from which wheelbarrows were filled. By this plan less time was consumed in placing the bucket and no tag rope man was required, as the engineman could swing the boom to a certain point on the wall which would bring the bucket directly over the hopper. A Smith mixer discharged directly into derrick buckets, which rested on a track long enough to hold two buckets. The buckets were filled and emptied alternately by shuttling the truck and attaching first one and then the other to the derrick.
In constructing an 11-story and basement office building in New York City a four-legged tower starting from the bottom of the excavation was erected at about the center of the lot. It was built of timber and extended upward as the progress of the work demanded until it overtopped the roof 11 stories above the street. The tower was square in plan and was divided into stories corresponding approximately to the several stories of the building. A floor was constructed in the tower at each story to be used in storing materials. For hoisting a 75-ft. boom was swung from each leg of the tower, each boom being operated by a separate engine and having a nominal capacity of 5 tons. The four booms covered the whole building area and were kept about two stories above the work by being shifted upward as the work progressed. This arrangement of derricks was used to handle the steel, lumber and concrete, the building being built up around the tower, which was so located that its only interference with the building structure was in the shape of square holes left in the floor slabs to accommodate the tower legs.
In constructing an 8-story warehouse covering some three acres of ground in Chicago, Ill., the derrick plant shown by Figs. 220 to 222 was installed. Some 7,500 tons of reinforcing steel, 125,000 cu. yds. of concrete and 4,000,000 ft. of form lumber had to be handled. Incidentally it is worth noting that there were about 120 lbs. of reinforcing steel and 32 ft. B. M. of form lumber used per cubic yard of concrete.
The controlling conditions governing the arrangement and character of the construction plant were as follows: The building, to be built entirely of reinforced concrete, was 135 ft. high. Its west front abutted on the river and its south front on the street; at the north end there was some ground available for plant and along the east front there was a strip about 20 ft. wide between the building wall and the main line tracks of a railway. At best, therefore, the area outside of the building and available for plant and storage was limited, while inside the building area the contractor was confronted by the insistence of the architect that an unbroken monolithic construction be obtained as nearly as possible, by reducing the floor openings for construction work to a minimum. The sketch plan, Fig. 220, shows the plant designed to meet the conditions.
To get the large amount of construction material onto the work a side track was built along the 20-ft. area on the east side of the building and another was turned into the area at the north end of the building. These side tracks handled all construction materials coming onto the work. Over the first there were built two sets of storage bins for sand and gravel and all concrete materials brought in in carload lots are unloaded at these two points, as will be described further on. Lumber for forms and steel for reinforcement shipped in similar manner were taken by the second siding to the lumber yard and steel mill at the north end of the building.
The raw materials after being worked up in the mixer plants and the saw and steel mills were distributed over the work by an industrial railway. The track system of this railway is shown by the dotted lines; it was located on the basement floor, with rampes leading to the No. 1 mixer plant and to the saw and steel mill tracks. The two main lines of track passed close to or under the elevator and stairway shaft openings in the several floors. This permitted the derrick buckets, lowered and hoisted through the shafts, to be loaded directly from the car tracks. All mixed concrete, forms and reinforcing frames were distributed by this railway to the several shafts and thence hoisted and placed by the derrick plant.
The derrick plant consisted of four derricks arranged as shown by the circles in Fig. 220. The view, Fig. 221 shows the first derrick installed and illustrates the general construction quite clearly. Briefly the derrick consisted of a vertical steel-work tower 10 ft. square and 85 ft. high, within which operated a steel mast 135 ft. high and carrying an 80-ft. boom connected just above the tower. The mast was pivoted at the bottom and had rollers turning against a horizontal ring inside the tower at the top. It was operated by a bull wheel above the top of the tower, the turning ropes running down inside the mast to the foot block and thence horizontally to the operating motor. The topping and hoisting lines also followed this route. The top of the tower was guyed by four ropes to anchorages in the basement floor. The boom commanded a circle 170 ft. in diameter and could lift 150 ft. above the base of the mast. The derrick was operated by a 25-HP. double drum electric hoist with a derrick swinging spool; this hoist was set on the basement floor. It will be noted that the guys are below the bull wheel so that the boom has a clear swing through a complete circle.
As stated above, four of these derricks were employed. Together they did not cover the entire building area, but by the use of a derrick bucket so designed that it could be used as a storage bin for feeding wheelbarrows, it was found possible to keep the number of derricks down to four.
This derrick plant possessed several advantages of importance. In the first place the derricks would handle all classes of material—concrete, forms, steel frames—equally well and could be transferred from one class of work to the other with practically no delay. In the second place, for a large area of the building, they handled the material from the basement direct to the place it was to occupy in the work, and did it in one operation. Finally they permitted the handling and erection of the forms and reinforcement in large units. Thus a column form would be assembled complete at the mill, moved as a unit by car to the proper shaft and then hoisted and set in place as a unit by the derrick. Girder forms, floor slab forms, girder and column reinforcing, etc., could be similarly assembled and handled. The derricks occupied only the area of four floor panels, the remainder of the area of each floor was left unobstructed for the work to be done. No materials or supplies needed be stored on the floors until they were in perfect condition to accommodate them, and not then, even, so far as the prosecution of form erection and concreting were concerned.
The sand and gravel for concrete were brought in by bottom or side dump gondola cars from pits located about 30 miles out on the Chicago, Milwaukee & St. Paul Ry. The cars were switched onto the main side track and unloaded under the bins which straddle this track. A receiving hopper, with its top at rail level and long enough to permit two cars to be unloaded at once, received the sand or gravel and distributed it through twelve gate openings onto an 18-in. horizontal belt conveyor 65 ft. long. This conveyor discharged into a second conveyor, 133 ft. long, which ran up a 22 incline, extending away from the bins and discharged onto a third conveyor 117 ft. long, which doubled back on a 22 incline reaching to and over the top of the bins. This third conveyor had two fixed trippers and an end discharge to distribute its cargo. All three conveyors were operated by a 35-HP. motor located at the junction of the two inclined conveyors, both of which were driven from the same shaft. A chain belt from the idler shaft of the first incline conveyor to the driving shaft of the horizontal conveyor operated that unit of the plant. This belt was operated as a cross belt by reversing alternate links. No manual labor was required to handle the sand and gravel from the cars to the storage bins.
The mixer arrangement at the two bins differed. At the No. 1 bins the mixer was located as shown in Fig. 220, close to the bin. Chutes led directly from the sand and gravel bins to the charging hopper and the bags of cement were stacked alongside this hopper. The mixer discharged either directly into the bucket of the first derrick or into cars for transportation on the railways. At the No. 2 bins a belt conveyor took the concrete materials down into the basement to a mixer located close enough to one of the distribution tracks to permit it to discharge directly into the cars.
The derrick buckets by which the concrete was hoisted and handled to the work were of special construction. A bucket was desired which would serve several distinct purposes. It must first be able to hold a full mixer batch of material, since, with the derrick arrangement, economy in hoisting necessitated hoisting in large units and also because storage capacity was required of the bucket for wheelbarrow work. The four derricks did not command the entire area of a floor; there were corners and other irregular areas outside of the circles covered by the several booms over which the concrete must be distributed by barrows or carts. A bucket large enough to supply the barrows, while a second bucket was being lowered, charged from the mixer and hoisted, was required. In the second place, a bucket was required whose contents could be discharged all at once or in smaller portion at will. Finally a bucket was desired which could be made to distribute its load along a narrow girder form or in a thin sheet for a floor slab.
To meet these requirements the bucket shown in Fig. 222 was designed. It held 42 cu. ft., or about 1.55 cu. yds. of concrete. It had a hopper bottom terminating in a short rectangular discharge spout closed by a lever operated under cut gate, which could be opened as much or as little as desired. To the underside of the bucket there was attached a four-leg frame in which the bucket stood when not suspended. Ordinarily, that is within the circles commanded by the derricks, the buckets were discharged suspended and directly into the forms, the character of the discharge gate permitting a thin sheet to be spread for floor slabs or a narrow girder or wall form to be filled without spilling or shock. For wheelbarrow work outside the reach of the derricks the mode of procedure was as follows: A timber platform about 3 ft. high and having room for standing two buckets was set just on the edge of the circle commanded by the derrick boom. Two buckets were used. A full bucket was hoisted and set on the platform, with its spout overhanging. This bucket served as a storage bin for feeding the wheelbarrows while the second bucket was being lowered, charged and hoisted to take its place on the platform, and serve in turn as a storage hopper.
PLACING AND RAMMING.—A wet concrete is usually used in building work except on occasions, for exterior wall work and except for pitch roof work, where a wet mixture would run down the slope. Placing and tamping are therefore, essentially pouring and puddling operations. The pouring should be done directly from the barrows, carts, or buckets if possible; dumping onto shoveling boards and shoveling makes an extra operation and increases the cost by the wages of the shoveling gang. Where shoveling boards are necessary, take care that they are placed close to the forms being filled, as it is wasteful of time to carry concrete in shovels, even for a half dozen paces. Before pouring any concrete, the inside of the forms should be wet down thoroughly with a hose or sprinkler, if a hose stream is not available. The final inspection of forms and reinforcement just before concreting will have made certain that they are ready for the concrete, so far as line and level of forms and presence and proper arrangement of the reinforcement are concerned, but the concrete foreman must watch that no displacement occurs in pouring and puddling, and must make certain particularly that the forms are clean.
In pouring columns it is essential that the operation be continuous to the bottom of the beam or girder. It is also advisable to pour columns several hours ahead of the girders. Puddling should be thorough, as its purpose is to work the concrete closely around the reinforcement and into the angles of the mold and to work out air bubbles. A tool resembling a broad chisel is one of the best devices for puddling or slicing. In slab and girder construction, the pouring should be continuous from bottom of girder to top of slab. Work should never be stopped-off at horizontal planes. As in columns, careful puddling is essential in pouring beams. In slab work, the concrete is best compacted by tamping or rolling. A broad faced rammer should be used for tamping wet concrete, or a wooden roller covered with sheet steel, weighing about 250 lbs., and having a 30-in. face.
Theoretically, concreting should be a continuous operation, but practically it cannot be made so. Bonding fresh concrete to concrete that has hardened, though it has been done with great perfection by certain methods as described in Chapter XXIV, must still be held as uncertain. Ordinarily, at least, a plane of weakness exists where the junction is made and in stopping off work it should be done where these planes of weakness will cause the least harm. Experts are by no means agreed on the best location of these planes, but the following is recognized good practice. Work once started, pouring a column, should not be stopped until the column is completed to the bottom of the girder. For beams and girders; stop concrete at center of girder with a vertical face at right angles to the girder, or directly over the center of the columns; in beams connecting with girders, stop concrete at center of span, or directly over center of connecting girder; stop always with a vertical face and never with a sloping face, and never with a girder partly filled. For slabs; stop concrete at center of span, or directly over middle of supporting girder or beam; stop always with vertical joints. If for any cause work must be stopped at other points, than those stated, the fresh concrete and the hardened concrete must be bonded by one of the methods described in Chapter XXIV.
CONSTRUCTING WALL COLUMNS FOR A BRICK BUILDING.—The columns, 12 in number, were constructed to strengthen the brick walls of a power station and were built as shown by Figs. 223 and 224, one at a time. The staging, 50 ft. high and 46 ft. in plan, was erected against the wall which had been shored, a portion of the wall was cut out and forms erected and the concrete column substituted for the section of wall which was removed. The staging was then moved into position for another column.
Two men, with sledge and drill, cut out the brick work amounting to about 12 cu. yds. for each column in 15 hours, at a cost of about 70 cts. per cu. yd., including removal to the street. The cost of moving and re-erecting the scaffolding was $2.94 per each move. The character of the reinforcement is shown by Fig. 223; it was erected as the concreting progressed, the main bars being in sections 15 ft. long, spliced with and distanced by side bars and cross bolts at the splices.