Concrete Construction Part 45

This derrick plant possessed several advantages of importance. In the first place the derricks would handle all cla.s.ses of material--concrete, forms, steel frames--equally well and could be transferred from one cla.s.s 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 bas.e.m.e.nt 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 a.s.sembled 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 a.s.sembled and handled. The derricks occupied only the area of four floor panels, the remainder of the area of each floor was left un.o.bstructed 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 bas.e.m.e.nt to a mixer located close enough to one of the distribution tracks to permit it to discharge directly into the cars.

[Ill.u.s.tration: Fig. 222.--Special Concrete Bucket for Large Warehouse Building.]

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 sh.o.r.ed, a portion of the wall was cut out and forms erected and the concrete column subst.i.tuted for the section of wall which was removed. The staging was then moved into position for another column.

[Ill.u.s.tration: Fig. 223.--Section of Rectangular Wall 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.

[Ill.u.s.tration: Fig. 224.--Staging and Forms Used in Building Column Shown by Fig. 223.]

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 1 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 7/8-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. 22 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:

Cost per Cost per Concrete. column cu. yd.

Lumber for forms $ 4.81 $0.40 Setting up and removing forms 11.32 0.95 Cement, 10.17 bbls. at $2.40 24.40 2.03 Sand, 5.87 yds. at $0.90 5.28 0.44 Stone, 8.75 yds. at $1.35 10.94 0.91 Mixing and wheeling 15.73 1.31 Hoisting by mule with driver 4.80 0.40 Handling bucket on scaffold 2.93 0.25 Tamping inside column 2.93 0.25 Painting with grout 3.89 0.32 Clearing away rubbish 1.97 0.16 Rigging, etc. 2.64 0.21 Tools 0.59 0.05 Moving scaffold 2.94 0.25 Moving mix board and rigging hoist 1.62 0.14 ------ ----- Total cost of concrete $96.79 $8.07

Cost per Cost cts. per Reinforcement. column. lb. of bars.

Iron bars, 1,034 lbs. $20.68 $2.00 Drilling iron bars 1.44 0.14 Setting iron bars in place 1.23 0.12 Bolts for splicing and s.p.a.cing 3.98 0.40 Wire cross ties at 2, cts. lb. 1.39 0.14 Labor forming 130 cross ties 1.13 0.11 ------ ----- Total cost of iron and steel $29.85 $2.91

Summary of Cost.

Per column. Per cu. yd.

Concrete in place $96.79 $8.07 Steel in place 29.85 2.49 Cutting out and removing brick 8.36 0.70 Shoring floors and roof, labor 5.87 0.49 Ditto for lumber used 3 times 3.44 0.29 ------- ------ Total $144.31 $12.04

[Ill.u.s.tration: Fig. 225.--Girder Plan for 6-Story Building.]

~FLOOR AND COLUMN CONSTRUCTION FOR SIX-STORY BUILDING.~--The building was 91112 ft.; 56 columns s.p.a.ced 16 ft. apart carried the girder system shown by Fig. 225, which in turn supported a 3-in. floor slab. The walls and part.i.tions 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 1-in. stuff, surfaced on four sides; the yokes were s.p.a.ced 2 ft.

apart. The 16-in. pieces were nailed to the 24'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 34's with 4412-in. head blocks nailed to their tops and wedges under their bottoms. The girder molds were 1-in. stuff, and to the side pieces were nailed 14-in. cleats; the bottom and side pieces were connected by 3/84-in. lag screws s.p.a.ced 28 ins. apart. The floor slab stringers were carried on the 14-in.

cleats; they were s.p.a.ced 28 ins. apart and were not nailed; neither were the 16-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 91112 ft., or, say, 10,200 sq. ft., was as follows:

Lumber for columns, ft. B. M. 9,000 Lumber for 1010-in. beams, ft. B. M. 7,600 Lumber for 510-in. beams, ft. B. M. 2,700 Intermediate 34-in. posts, ft. B. M. 1,000 Lagging, 16-in. boards, ft. B. M. 9,000 Stringers, 34 ins., ft. B. M. 4,500 ------ Total ft. B. M. 33,800

[Ill.u.s.tration: Fig. 226.--Column Form for 6-Story Building.]

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:

Lumber, cost per M. ft. B. M. $26.00 Labor, framing per M. ft. B. M. 4.00 ------ Total per M. ft. B. M. $30.00

[Ill.u.s.tration: Fig. 227.--Girder and Slab Forms for 6-Story Building.]

Since the lumber was used three times, $30 3 = $10 is the charge against each 1,000 ft. B. M. needed to encase the concrete on a floor.

There were nearly 34,000 ft. B. M. per floor, hence the cost of lumber ready for erection was $340 per floor. There were as shown below, 200 cu. yds. of concrete per floor, so that the cost was $340 200 = $1.70 per cu. yd. of concrete for forms ready for erection. It took a gang of 5 men 7 days to tear down and carry up the forms for one floor; hence 5 $2 7 = $70 per floor, or practically $2 per M. ft. B. M., or $0.35 per cu. yd. of concrete for taking down and carrying forms two stories.

It took a gang of 10 carpenters 7 days to erect these forms, which at $3.50 per day was $245 per floor, or $7 per M. ft. B. M., or $1.20 per cu. yd. of concrete.

_Concrete._--The amount of concrete per floor was as follows:

Floor slab 3 ins. thick, 10,200 sq. ft. 110 cu. yds.

Beams, 1010 ins. 40 cu. yds.

Beams, 510 ins. 20 cu. yds.

Columns, 1515 ins. (average) 30 cu. yds.

----- Total concrete per floor 200 cu. yds.

A concrete mixer, a hoist and a gang of 14 men mixed and placed the concrete for a floor in 7 days. At $2 per day for labor this gives 14 7 $2 = $196, or say $1 per cu. yd. for mixing and placing the concrete.

_Reinforcement._--In each of the 1010-in. beams there were 4, 1-in.

round rods, 2 straight and 2 bent, and stirrups of 1/81-in. straps s.p.a.ced 5 ins. apart at columns and 15 ins. at the center. In each 510-in. beam there was half as much steel as in a 1010-in. beam. The floor slab reinforcement consisted of -in. rods s.p.a.ced 5 ins. apart and 2 cross-rods in 7-ft. panel. The column reinforcement consisted of 4 rods averaging 1 in. in diameter. In round numbers the amount of steel required for each floor was, therefore, as follows:

Lbs. steel rods in 1010-in. beams 16,200 Lbs. steel rods in 510-in. beams 4,000 Lbs. stirrups in beams 3,000 Lbs. steel rods in floor slabs 3,800 Lbs. steel rods in columns 1,400 ------ Total pounds steel per floor 28,400

This is equivalent to 142 lbs. of steel per cubic yard of concrete, or about 1 per cent of the total volume of reinforced concrete was steel.

The steel in the beams was about 3 per cent. It required a gang of 5 laborers 7 days at $2.25 per day, to bend and place the steel for each floor or $86 for labor on 28,400 lbs. of steel. This is equivalent to 0.3 ct. per lb., or 45 cts. per cu. yd. of concrete.

_Summary of Costs._--Summarizing the figures given we have the following cost per cubic yard of concrete in floors and columns:

Per cu. yd.

142 lbs. steel at 2 cts. $ 3.55 1 bbl. cement 2.50 1 cu. yd. gravel 1.10 cu. yd. sand 0.55 170 ft. B. M. lumber ready to erect at $10 (1/3 of $30) 1.70 170 ft. B. M. torn down at $2 0.35 170 ft. B. M. erected by carpenters at $7 1.20 Mixing and placing concrete 1.00 Shaping and placing steel 0.45 Superintendence 0.25 ------ Total $12.65

~WALL AND ROOF CONSTRUCTION FOR ONE-STORY CAR BARN.~--The barn was 50 ft.

wide and 190 ft. long, divided into three rooms by two transverse part.i.tions and covered with a 4-in. roof having a pitch of in. per foot. The main walls were 12 ins. thick and the part.i.tion walls 10 ins.

thick. The main room 110 ft. long had four car tracks its whole length with pits under each and a 6-in. reinforced concrete floor slab between.

The floor girders, one under each rail, were 12 ins. square, each reinforced by three 1-in. rods, and were carried on 1212-in. pillars.

The total yardage of concrete was 874 cu. yds. divided as follows:

Walls and foundations, cu. yds. 614 Pillars and girders in track pits, cu. yds. 44 Reinforced floors, cu. yds. 55 Roof 160 --- Total, cu. yds. 873

A 1-2-5 concrete was used for floors, roofs and girders and a 1-3-6 concrete for foundations and walls. There were 26 tons of reinforcing steel, or 61 lbs. per cu. yd., or 0.45 per cent. of the volume of the concrete was steel. The wages paid were: Foreman, $2.50; blacksmith, $2; engineer, $1.75; laborers, $1.50; two-horse team and driver, $3.67; one-horse team and driver, $2.92; carpenter, $2.25; carpenters worked 9 hours; all others 10 hours.

_Forms._--Carpenters framed and erected forms and common laborers under foreman carpenter took them down. Lagging was all 2-in. stuff and uprights 34-in. stuff. Props for roof forms were 18-ft. round timber procured on the job. They were 6 ins. in diameter at the top and cost 50 cts. each, 91 being used. These props are not included in the lumber listed below, but their cost is included in the costs given. No record was kept of the number of times the lumber was used, but as 54,643 ft.