Chapter XIX (2)

The work done on the metal consists chiefly of bending. The metal can usually be bent cold, but for sizes 1½-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, 1½ 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 500×75 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 1½ 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, 30×30×1½ ins., weighed about 150 lbs. each.

Some 2,000,000 lbs. of plain round steel rods from ¼ in. to 1½ 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 ¾ ct. per lb. The cost of placing the steel in the building was ¼ 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 13½ 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 2×10-in. stuff from car to the second floor and distributed it in 43 minutes. A ½-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 ½-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, 17½c. .35 4 men wheeling and shoveling stone, 17½c. .70 1 man helping up runway, 17½c. .17½ 2 men carrying cement, 17½c. .35

Gang Placing Cement:

1 foreman, 25c. .25 9 men wheeling concrete, 17½c. 1.57½ 3 men tamping concrete, 17½c. .52½ 1 man filling carts, 17½c. .17½ ------

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