Concrete Construction: Methods and Costs

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:

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:

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 1½ 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 4×6 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.

The concrete was hand mixed in 6-cu. ft. batches at the foot of the column, by three men with a fourth turning over and filling the buckets. The buckets, 12 ins. in diameter and 16 ins. high, were hoisted by a pulley line arranged as shown and pulled by a mule driven by a man, at $1 per day for the mule and $1.50 for the man, the cost of hoisting being 25 to 40 cts. per cu. yd., depending on the rapidity of the man inside the form. This man tamped the concrete which was emptied from the buckets by a man on the scaffolding. Each batch raised the level in the form 15 ins., and between batches a set of ties for the column rods was placed by the man during the tamping. It took from 1½ to 2 days to concrete a column of 12 cu. yds. The concrete was a 1-3.8-5.7 limestone screenings mixture, mixed wet enough to be easily pushed into the forms and worked around the reinforcement. The form construction is shown by Fig. 224. The form for one column required 650 ft. B. M. of lumber, and on an average, each form was used twice. As a matter of fact, the side strips and outside braces were used three times, while much of the ⅞-in. sheathing was destroyed by being used once. The lumber for shoring cost $23 per M. ft. B. M., and the light lumber for forms cost $18 per M. ft. B. M. All lumber was yellow pine. All labor was negro, at 15 cts. per hour; foremen who worked. 22½ cts. per hour. The cost of the several parts of the work compiled from records furnished by Mr. Keith O. Guthrie, engineer in charge, was as follows:

FLOOR AND COLUMN CONSTRUCTION FOR SIX-STORY BUILDING.—The building was 91×112 ft.; 56 columns spaced 16 ft. apart carried the girder system shown by Fig. 225, which in turn supported a 3½-in. floor slab. The walls and partitions were not concrete. The following records were kept by the authors:

Forms.—The column forms were built as shown by Fig. 226. The boards were 1½-in. stuff, surfaced on four sides; the yokes were spaced 2 ft. apart. The 1×6-in. pieces were nailed to the 2×4's with 8-d. nails with heads left projecting for easy pulling. The girder forms, Fig. 227, rested on the column forms and on intermediate posts half-way between columns. These intermediate posts were 3×4's with 4×4×12-in. head blocks nailed to their tops and wedges under their bottoms. The girder molds were 1½-in. stuff, and to the side pieces were nailed 1×4-in. cleats; the bottom and side pieces were connected by ⅜×4-in. lag screws spaced 28 ins. apart. The floor slab stringers were carried on the 1×4-in. cleats; they were spaced 28 ins. apart and were not nailed; neither were the 1×6-in. lagging boards nailed to the stringers. The point to be noted is the design and construction of the forms so that they could be put together and taken apart easily. The lumber required for forms for one floor 91×112 ft., or, say, 10,200 sq. ft., was as follows:

In round numbers, we can say that 34,000 ft. B. M. of lumber were used for 10,000 sq. ft. of floor area, or 3.4 ft. B. M. per 1 sq. ft. Enough forms were provided to erect two complete floors; the forms for the lower floor being removed and erected again for the second floor above, thus using all the lumber three times. With carpenters at $3.50 for 8 hours, the forms were framed ready for erection for $4 per M. ft. B. M. The lumber framed ready to erect cost them: