(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 b.u.t.t 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 s.p.a.ced.
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, a.s.suming 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 h.o.m.ogeneous 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 bas.e.m.e.nt 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 ma.s.sive 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, pa.s.sing 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.
[Ill.u.s.tration: Fig 214.--Bucket Hoist for Building Work (Wallace-Lindesmith).]
~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.
[Ill.u.s.tration: Fig, 215.--Wallace-Lindesmith Hoist Bucket in Discharging Position.]
[Ill.u.s.tration: Fig. 216.--Lumber Carriage for Wallace-Lindesmith Hoist.]
[Ill.u.s.tration: Fig. 217.--Mixer Plant with Gravity Feed from Material Bins to Hoisting Bucket.]
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 ill.u.s.trated, 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.
[Ill.u.s.tration: Fig. 218--Bucket Hoist for Building Work (Ransome).]
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 bas.e.m.e.nt. These chutes discharged on opposite sides of a vertical part.i.tion 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 cla.s.ses 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.
[Ill.u.s.tration: Fig. 219.--Plan of Concrete Mixing and Handling Plant for 9-Story Building.]
~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 ill.u.s.trated 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 bas.e.m.e.nt 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 ab.u.t.ted 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.
[Ill.u.s.tration: Fig. 220.--Plan of Concrete Mixing and Handling Plant for Large Warehouse 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 bas.e.m.e.nt floor, with rampes leading to the No. 1 mixer plant and to the saw and steel mill tracks. The two main lines of track pa.s.sed 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.
[Ill.u.s.tration: Fig. 221.--Derrick for Handling Concrete for Large Warehouse Building.]
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 ill.u.s.trates 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 bas.e.m.e.nt 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 bas.e.m.e.nt 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.