Part 43 (2/2)
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 sh.o.r.es 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 a.s.sumed 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 sh.o.r.es 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 s.p.a.ced 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 a.s.semble 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.
[Ill.u.s.tration: Fig. 207.--Rack for Storing Reinforcing Bars.]
a.s.suming the fabrication to be done in the field, the mode of procedure will be as follows: Order the bars or rods to be s.h.i.+pped 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 p.r.o.ngs 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 bas.e.m.e.nt, 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.
[Ill.u.s.tration: Fig. 208.--Table for Bending Reinforcing Bars.]
[Ill.u.s.tration: Fig. 209.--Table for Bending Reinforcing Bars.]
[Ill.u.s.tration: Fig. 210.--Table for Bending Reinforcing Bars.]
[Ill.u.s.tration: Fig. 211.--Column and Floor Slab Construction for Factory Building.]
The circular interior columns, varying from 30 ins. to 12 ins., in diameter were molded in permanent sh.e.l.ls of cinder concrete. The sh.e.l.ls 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 sh.e.l.ls 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 sh.e.l.l. The outer mold consisted of a sheet metal cylinder in two parts a.s.sembled and supported by wooden yokes and framework. The two molds were a.s.sembled on a plank platform, one inside the other, and about a common center. The annular s.p.a.ce 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 sh.e.l.l was left standing on the molding platform until safe to handle. The larger sh.e.l.ls, 30301 ins., weighed about 150 lbs. each.
[Ill.u.s.tration: Fig. 212.--Device for Bending Reinforcing Rods.]
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 a.s.sembled 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 a.s.sembled 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 a.s.sembling 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 s.p.a.cing of the truss being a.s.sembled. On these bars, at the s.p.a.ces marked, were suspended stirrups with their lower ends hooked. The lower chord bars were then suspended in the stirrup hooks and the whole a.s.semblage 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 a.s.sembling 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.
[Ill.u.s.tration: Fig. 213.--Sketches Showing Methods of Fabricating Girder Reinforcing Frames.]
~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 ab.u.t.ting 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 s.p.a.ced and placed strictly according to the working plans.
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