155. The exact construction of the forms for one of the larger slabs is

shown by Fig. 156. The side and end pieces were so arranged as to be easily taken down and erected for repeated use. About 100 floors were used and they had to be leveled up each time used as the lifting of the hardened slab disarranged them. The side and end pieces were removed in about a week or ten days, but the slabs stood on the floor 90 days, being wetted each day for two weeks after molding. The plant for mixing and handling the concrete was mounted on cars. A flat car had a rotary drum mixer mounted on a platform at its forward end. Beneath the mixer was a hopper provided with a deflector which directed the concrete to right or left as desired. Under the hopper were the ends of two inclined chutes extending out sidewise beyond the car--one to the right and one to the left--and over the slab molds on each side. Above the mixer was another platform containing a charging hopper, and from the rear of this platform an incline ran down to the rear end of the car and then down to the track rails. A car loaded with cement and gravel in the proper proportions was hauled up the incline by cable operated by the mixer engine, until it came over the topmost hopper into which it was dumped. This hopper directed the charge into the mixer below; the mixer discharged its batch into the hopper beneath from which it flowed right or left as desired into one of the chutes and thence into the mold. The chutes reached nearly the full length of the molds and discharged as desired over the ends into the far end of the mold or through a trap over the end of the mold nearest the car. To the rear of the mixer car came a cement car provided with a platform overhanging its forward end. Two hoppers were set in this platform each holding a charge for one batch. Coupled behind the cement cars came three or four gravel cars. These were gondola cars and plank runways were laid along their top outer edges making a continuous runway for wheelbarrows on each side from rear of train to front of cement car. The sand and gravel were wheeled to the two measuring hoppers and the cement was handed up from the car below and added, the charge was then discharged into the dump car below and the car was hauled up the incline to the mixer as already described. Two measuring hoppers were used so that one was being filled while the other was emptied, thus making the work continuous. The molding gang consisted of 33 laborers, two foremen and one engineman. This gang averaged 7 of the large slabs per 10-hour day and at times made as many as 9 slabs. When molding small slabs an average of 12 were made per day. This record includes all delays, moving train, switching gravel cars on and off, building runways, etc. The distribution of the men was about as follows: Handling Materials: No. Men. Shoveling gravel into wheelbarrows 9 Wheeling gravel to measuring hoppers 9 Emptying cement into measuring hoppers 2 Handling cement to men emptying 1 In charge of loading dump car 1 On top of cement car 1 Sub-foreman in charge 1 Mixing and Placing: Engineer 1 In charge of mixer 1 Hoeing and spreading in mold 2 Spading in mold 2 Finishing sides of block 2 General laborers 3 Foreman in charge 1 -- Total men 36 This gang mixed and placed concrete for 7 blocks or 117¼ cu. yds. of concrete per day. Assuming an average wage of $2 per day the cost of labor mixing and placing was 61.4 cts. per cu. yd. or $10.28 per slab. It is stated that the slabs cost $11.80 per cu. yd. on storage pile. This includes labor and materials (concrete and steel); molds; loading into cars with locomotive crane, hauling cars to storage yard and unloading with crane into storage piles, and inspection, incidentals, etc. To load the slabs into cars from storage piles, transport them to the work and place them in position is stated to have cost $2 per cu. yd. The slabs were placed by means of a locomotive crane being swung from the flat cars directly into place. [Illustration: Fig. 157.--Sections Showing Construction of Connecticut Ave. Bridge.] ~METHOD AND COST OF CONSTRUCTING CONNECTICUT AVE. BRIDGE, WASHINGTON, D. C.~--The Connecticut Ave. Bridge at Washington, D. C., consists of nine 150-ft. spans and two 82-ft. spans, one at each end, all full centered arches of mass concrete trimmed with tool-dressed concrete blocks. Figure 157 is a part sectional plan and elevation of the bridge, showing both the main and spandrel arch construction. This bridge is one of the largest concrete arch bridges in the world, being 1,341 ft. long and 52 ft. wide, and containing 80,000 cu. yds. of concrete. Its total cost was $850,000 or $638.85 per lin. ft., or $10.63 per cu. yd. of masonry. It was built by contract, with Mr. W. J. Douglas as engineer in charge of construction. The account of the methods and cost of construction given here has been prepared from information obtained from Mr. Douglas and by personal visits to the work during construction. _General Arrangement of the Plant._--The quarry from which the crushed stone for concrete was obtained was located in the side of the gorge at a point about 400 ft. from the bridge. Incidentally, it may be added, the fact that the contractor had an option on this quarry gave him an advantage of some $30,000 over the other bidders. The stone from the quarry was hoisted about 50 ft. by derricks and deposited in cars which traveled on an incline to a Gates gyratory crusher, into which they dumped automatically. The stone from the crusher dropped into a 600-cu. yd. bin under the bottom of which was a tunnel large enough for a dump car and provided with top gates by which the stone above could be dropped into the cars. The cars were hauled by cable to the mixer storage bin and there discharged. Sand was brought in by wagons and dumped onto a platform about 50 ft. higher than the bottom of the main stone bin. A tunnel exactly similar to that under the stone bin was carried under the sand storage platform. The sand car was hauled from this tunnel by cable to the mixer storage bin using the same cable as was used for the stone cars, the cable being shifted by hand as was desired. Cement was delivered to the mixer platform from the crest of the bluff by means of a bag chute. The mixer used was one of the Hains gravity type. It had four drops and was provided with four mixing hoppers at the top. The concrete was made quite wet. The proportions of sand and water were varied to suit the stone according to its wetness and the percentage of dust carried by it. The head mixer regulated the proportions and his work was checked by the government inspector. From the bottom hopper the mixed concrete dropped into a skip mounted on a car. [Illustration: Fig. 158.--Center for Connecticut Ave. Bridge (Elevation).] To distribute the skip cars along the work a trestle was built close alongside the bridge and at about springing line level. This trestle had a down grade of about 2 per cent. from the mixer. Derricks mounted along the centering and on the block molding platform lifted the skips from the cars and deposited them where the concrete was wanted. The skip cars were large enough for three skips but only two were carried so that the derricks could save time by depositing an empty skip in the vacant space and take a loaded skip away with one full swing of the boom. Altogether nine derricks were used in the bridge, four having 70-ft. booms and five having 90-ft. booms. These derricks were jacked up as the work progressed. [Illustration: Fig. 158.--Center for Connecticut Ave. Bridge (Details).] _Forms and Centers._--The forms for wall and pier work consisted of 1-in. lagging held in place by studs about 2 ft. on centers and they in turn supported by wales which were connected through the walls by bolts, the outer portions of which were removed when the forms were taken down. The centers for the five 150-ft. arches were all erected at one time; those for the 82-ft. arches were erected separately. The seven centers required 1,500,000 ft. B. M. of lumber or 1,404 ft. B. M. per lineal foot of bridge between abutments, or 1,640 ft. B. M. per lineal foot of arch span. The centers for the main arch spans are shown in detail by Fig. 158; this drawing shows the sizes of all members and the maximum stresses to which they were subjected from the loading indicated, that is the arch ring concrete. The centers as a rule rested on pile foundations. Four piles to each post were used for the intermediate posts and two piles for the posts in the two rows next the piers. Concrete foundations, however, were put in Rock Creek and on the line of Woodley Lane Bridge where it was impracticable to drive piles. As considerable difficulty was experienced in driving the piles, the ground consisting mostly of rotten rock, it is thought that it would have cost less if the contractor had used concrete footings throughout. Some of the costs of form work and centering are given. The cost of lumber delivered at the bridge site was about as follows: M. ft. B. M. Rough Virginia pine $25 Dressed Virginia pine lagging 23 Rough Georgia, sizes up to 12×12 ins. 33 Rough Georgia, sizes over 12×12 ins. 35 Rough oak lumber 35 The following wages were paid: Foreman carpenter, $3.50; carpenters, $2 to $3; laborers, $1.70, with a few at $1.50. An 8-hour day was worked. The cost, of formwork is given in summary as follows: Lagging per M. ft. (used twice): Lumber at $23 $11.50 Erection 15.00 ------ Total cost erected $26.50 Studding and rough boards used in place of lagging per M. ft. (used twice): Lumber at $25 $12.50 Erection 10.00 ------ Total cost erected $22.50 Wales per M. ft. (used six times): Lumber at $36 $ 6.00 Erection 10.00 ------ Total cost erected $16.00 The total cost of the main arch span centers to the District of Columbia was $54,000 or $59 per lineal foot of arch span, or $37.33 per M. ft. B. M. The cost of center erection and demolition was as follows: Erection below springing line per M. ft. $15 Erection above springing line per M. ft. 25 Demolition 5 The salvage on the centers amounted to $11 per M. ft. B. M. The spandrel arch centers were each used twice and cost per M. ft. B. M. for Lumber at $25 per M. ft. $12.50 Erecting at $25 per M. ft. 25.00 Moving at $5 per M. ft. 5.00 Total per M ft. 42.50 _Molding Concrete Blocks._--The bridge is trimmed throughout with molded concrete blocks, comprising belt courses, quoin stones, chain stones, ring stones, brackets and dentils. The blocks were made of a 1-2-4½ concrete faced with a 1-3 mixture of Dragon Portland cement and bluestone screenings from 3/8-in. size to dust. They were cast in wooden molds with collapsible sides held together by iron rods. Each mold was provided with six bottoms so that the molded block could be left standing on the bottom to harden while the side pieces were being used for molding another block. The molding was done on a perfectly level and tight floor on mud sills, the perfect level of the molding platform having been found to be an important factor in securing a uniform casting. The blocks were molded with the principal showing face down and the secondary showing faces vertical. The facing mortar was placed first and then the concrete backing. Care was taken to tamp the concrete so as to force the concrete stone into but not through the facing. Mr. Douglas remarks that the back of the block should always be at the top in molding since the laitance or slime always flushes to the surface making a weak skin which will develop hair cracks. In this work the backs of the blocks were mortised by embedding wooden cubes in the wet concrete and removing them when the concrete had set. These mortises bonded the blocks with the mass concrete backing. The blocks were left to harden for at least 30 days and preferably for 60 days and were then bush hammered on the showing faces, some of the work being done by hand and some with pneumatic tools. Some precautions necessary in the molding and handling of large concrete blocks were discovered in this work and merit mention. In designing blocks for molding it is necessary to avoid thin flanges or the flanges will crack and break off; blocks molded with a 2¼ in. flange projecting 1¾ ins. gave such trouble from cracking on this work that a flange 5 ins. thick was substituted. Provide for the method of handling the block so that dog or lewis holes will not come in the showing faces. Dog holes can be made with a pick when the concrete is three or four weeks old. When it is not practicable to use dogs, two-pin lewises can be used. The lewis holes should be cast in the block and should be of larger size than for granite; they should not be located too near the mortar faces. In turning blocks it is necessary to provide some sort of cushion for them to turn on or broken arrises will result. When the work will permit, it is desirable to round the arrises to about a 3/8-in. radius. The following general figures of the cost of block work are available. Foreman cutters were paid $5 per day; foreman concrete workers $3 per day; stonecutters $4 per day; concrete laborers $1.70 per day, and common laborers $1.50 to $1.70 per day. Plain and ornamental blocks cost about the same, the large size of the ornamental blocks bringing down the cost. The following is given as the average cost of block work per cubic yard: Cement $ 1.95 Sand 0.35 Stone 1.14 Forms, lumber and making 0.80 Mixing and placing concrete 1.50 Dressing 4.73 Handling and setting 2.00 Superintendence, plant, incidentals at 25 per cent. 3.12 Condemnation at 5 per cent. 0.78 ------ Total cost blocks in place $16.37 It will be seen that the largest single item in the above summary of costs is the item of dressing. This was done, as stated above, partly by hand and partly by pneumatic tools. Hand tooling cost about twice as much as machine tooling, but its appearance was generally better. The average cost of tooling the several forms of blocks is shown by Table XIX. For 42,190 sq. ft. the average cost was 26 cts. per sq. ft. or $2.34 per sq. yd., or $4.73 per cu. yd. of block work. This tooling was done by stone cutters, and was unusually high in cost. _Mass Concrete Work._--All parts of the bridge except the molded block trim were built of concrete deposited in place. Briefly, the molded blocks were set first and then backed up with the mass concrete deposited in forms and on centers. The only features of this work that call for particular description are those in connection with the main arch ring and the spandrel arch construction. The main arch rings were concreted in transverse sections; Fig. 158 shows the size and order of construction of these sections. Back forms were necessary up to an angle of 45° from the spring line after which the concrete was made somewhat drier and back forms were not used. After Sections 1, 2, 3 and 4 had been concreted they were allowed to set and then the struts and back forms were taken out and the intervening sections were concreted. The large Sections 6 and 7 were concreted in five sections each, in order to permit the taking out of the timber struts supporting the sections above. The concrete in all sections was placed in horizontal layers as a rule and it is the judgment of the engineers in charge of this work that this is the preferable method. TABLE XIX.--SHOWING COST OF TOOLING CONCRETE ORNAMENTAL BLOCKS FOR CONNECTICUT AVENUE BRIDGE. =============================================================================== | | Per Superficial Foot of | Per Cubic Foot. | Showing Face. +------+-----+------+-----+------+------+------+------ DESCRIPTION. | | Num-| | | | | |Number | | ber | | |Super-| |Cost |super. 1: 2: 4½ Concrete Backing| Total|cubic|Total |Cost |ficial|Total |per |ft. to 1: 3 (Mortar Face). |Number|feet |cubic |per | feet |super-|super-| one |Stones|in | feet |cubic| in |ficial|ficial| cubic | Cut. |each.| cut. |foot.| each.| feet.|foot. | foot. -------------------------+------+-----+------+-----+------+------+------+------ Brackets under Lamps and | | | | | | | | Rail Posts (Cap and Base)| 344| 16.0| 5,500|$0.27| 10.5 | 3,630|$0.41 | 0.66 Moulding under coping | 770| 5.9| 4,560| 0.30| 3.8 | 2,930| 0.47 | 0.64 Dentils between Moulding | 520| 5.5| 2,860| 0.20| 8.0 | 4,160| 0.14 | 1.45 Coping | 494| 61.2|30,220| 0.12| 35.4 |17,490| 0.21 | 0.58 Pedestal (3 courses) | 162| 27.2| 4,400| 0.15| 14.1 | 2,290| 0.29 | 0.52 Rail Posts (Top and Base)| 296| 7.1| 2,100| 0.50| 17.3 | 5,100| 0.21 | 2.43 Lamp Posts and Parapets | | | | | | | | over Piers (Top and Base)| 248| 22.9| 5,690| 0.17| 26.5 | 6,580| 0.15 | 1.16 -------------------------+------+-----+------+-----+------+------+------+------ Average of above--Totals | 2,834| 19.5|55,330|$0.17| 14.8 |43,190|$0.26 | 0.77 -------------------------+------+-----+------+-----+------+------+------+------ TABLE XX.--SHOWING COST OF MASS CONCRETE WORK PER CUBIC YARD. [Transcriber's note: Table split] =========================================================================== | | | Cost Delivered | | | | | on Mixer. | | Description. | | +--------+------+-------+ | | | Average | | | | | | | Yardage | | | | | | Propor-| for Days| | | | Total | | tions.| Run. | Cement.| Sand.| Stone.| Materials.| -------------------+--------+---------+--------+------+-------+-----------+ Class A, in Piers | 1:2:4½ | 150 | 1.65 | 0.39 | 1.08 | 3.12 | Class A, in Arches | 1:2:4½ | 200 | 1.65 | 0.39 | 1.08 | 3.11 | Class B, in Piers | | | | | | | --Solid Work | 1:3:6 | 160 | 1.40 | 0.42 | 1.23 | 3.05 | Class B, in Piers | | | | | | | --Hollow Work | 1:3:6 | 110 | 1.40 | 0.42 | 1.23 | 3.05 | Class B, in | | | | | | | Spandrel Walls | 1:3:6 | 110 | 1.40 | 0.42 | 1.23 | 3.05 | Class B, in | | | | | | | Spandrel Arches | 1:3:6 | 200 | 1.40 | 0.42 | 1.23 | 3.05 | Class B, | | | | | | | in Abutments | 1:3:6 | 150 | 1.40 | 0.42 | 1.23 | 3.05 | Class C, Filling | | | | | | | over Bridge | 1:3:10 | 145 | 0.90 | 0.31 | 1.30 | 2.51 | -------------------+--------+---------+--------+------+-------+-----------+ =============================================== | Cost of | | Mixing and Placing. | Description. +--------+--------+--------+ | | | Total | | | | Mixing | | | | and | | Mixing.| Placing| Placing| -------------------+--------+--------+--------+ Class A, in Piers | 0.09 | 0.21 | 0.30 | | | | | Class A, in Arches | 0.05 | 0.28 | 0.33 | Class B, in Piers | | | | --Solid Work | 0.09 | 0.18 | 0.27 | Class B, in Piers | | | | --Hollow Work | 0.11 | 0.36 | 0.47 | Class B, in | | | | Spandrel Walls | 0.11 | 0.40 | 0.51 | Class B, in | | | | Spandrel Arches | 0.07 | 0.26 | 0.33 | Class B, | | | | in Abutments | 0.11 | 0.24 | 0.35 | Class C, Filling | | | | over Bridge | 0.11 | 0.28 | 0.39 | -------------------+--------+--------+--------+ ========================================================================= | Cost of Form Work. | | Description. +----------+-------+--------+-----------+-------------+ | | Taking| | Total | Total Cost | | Erecting.| Down | Lumber.| Form Work | per Yard.[G]| -------------------+----------+-------+--------+-----------+-------------+ Class A, in Piers | 0.17 | 0.05 | 0.16 | 0.38 | $3.80 | | | | | | | Class A, in Arches | 0.08 | 0.03 | 0.10 | 0.21 | 3.66 | Class B, in Piers | | | | | | --Solid Work | 0.17 | 0.05 | 0.16 | 0.38 | 3.70 | Class B, in Piers | | | | | | --Hollow Work | 0.77 | 0.25 | 0.64 | 1.66 | 5.18 | Class B, in | | | | | | Spandrel Walls | 0.85 | 0.28 | 0.73 | 1.86 | 5.42 | Class B, in | | | | | | Spandrel Arches | 0.94 | 0.30 | 0.86 | 2.10 | 5.48 | Class B, | | | | | | in Abutments | 0.10 | 0.03 | 0.12 | 0.25 | 3.65 | Class C, Filling | | | | | | over Bridge | 0.00 | 0.00 | 0.00 | .... | 2.90 | -------------------+----------+-------+--------+-----------+-------------+ [Footnote G: Add 25% to the cost here tabulated for superintendence, plant and incidentals.] Considerable difficulty was experienced in building the large arches with a concrete block facing on account of the fact that the edges of the blocks are liable to chip off when any concentrated pressure is brought on them. In order to permit the ring of blocks to deform as the centering settled under its load, sheet lead was placed in the joints between blocks at the points corresponding with the construction joints between sections of the mass concrete backing. The deflection of the centers at the crown was a maximum of 3¼ ins. and a minimum of 2½ ins. TABLE XXI--Detail Cost of Engineering and Inspection for Different Classes of Work. Engineering. Inspection. Kind of Work. Total. Unit. Total. Unit. Class A, concrete, 23,500 cu. yds $3,055.00 $0.13 $1,762.50 $0.075 Class B, concrete, 36,580 cu. yds 3,658.00 0.10 1,646.10 0.045 Class C, concrete, 2,150 cu. yds 107.50 0.05 53.75 0.025 Class D, concrete, 6,250 cu. yds 1,875.00 0.30 4,687.50 0.75 1,000 M. ft. B. M. centering 1,000.00 1.00 440.00 0.44 Cement, 73,000 barrels 365.00 0.005 730.00 0.01 Earth filling, 50,000 cu. yds 1,000.00 0.02 500.00 0.01 The centering of the main arches was not struck until the spandrel arches and all the work above the main arches to the bottom of the coping had been completed. The first and third spandrel arch on each side of the piers was made with an expansion joint in the crown. To permit further of the adjustment of the portion of the masonry above the backs of the main arches, the crown of the middle arch of each set of spandrel arches was left unconcreted until the center of the main arches had been struck. It may be noted here that the expansion joints in the first and third arches were carried up through the dentils and coping, and observations show that these joints are about 1/8 in. larger in winter than in summer. The cost of the mass concrete work is shown in Table XX. These figures are based on the wages already quoted and the following: Foreman riggers, $4.50; riggers, $1.50 to $1.75 and $2; skilled laborers, $2; engineers, $3.50. The detail cost of engineering and inspection is shown in Table XXI. ~ARCH BRIDGES, ELKHART, IND.~--At the new Elkhart, Ind., yards of the Lake Shore & Michigan Southern Ry. the tracks are carried over a city street by concrete arches 40, 60 and 160 ft. long. These arches all have a span of 30 ft., a height of 13 ft. and a ring thickness at crown of 28 ins. The reinforcement consists of arch and transverse bars; the arch bars are spaced 6 ins. on centers 2½ ins. from both extrados and intrados, and the transverse bars are spaced 24 ins. on centers inside both lines of arch bars. The proportions of the concrete were generally 1 cement, 3 gravel and 6 stone. The gravel was a material dug from the foundations and was about 50 per cent. sand and 50 per cent. gravel, ranging up to the size of pigeons' eggs. The concrete was machine mixed and was mixed very wet. The work was done by the railway company's forces, and Mr. Samuel Rockwell, Assistant Chief Engineer, gives the following figures of cost: Total. Per cu. yd. Temporary buildings, trestles, etc. $ 752.33 $0.15 Machinery, pipe fittings, etc. 416.34 0.08 Sheet piling and boxing 1,006.12 0.21 Excavation and pumping 1,619.74 0.33 Arch centers and boxing 3,528.92 0.73 --------- ----- Total $7,323.45 $1.50 Concrete masonry: Cement 8,860.55 1.84 Stone 1,788.50 0.36 Sand 240.00 0.05 Drain tile 103.03 0.02 Labor 8,091.41 1.68 ---------- ----- Total concrete $19,083.49 $3.95 Steel reinforcing rods $ 3,028.39 $0.63 Engineering, watching, etc. 508.40 0.11 ---------- ----- Grand total (4,833 cu. yds. concrete) $29,943.73 $6.19 ~ARCH BRIDGE, PLAINWELL, MICH.~--The following figures of cost of a reinforced concrete arch bridge are given by Mr. P. A. Courtright. The bridge crosses the Kalamazoo River at Plainwell, Mich., and is 446 ft. long over all with seven arches of 54 ft. span and 8 ft. rise. The arch rings were reinforced with 4-in., 6-lb. channels bent to a radius of 70 ft. and spaced 1.9 ft. c. to c. The contract price of the bridge was $19,900. The concrete was made of Portland cement and a natural mixture of sand and gravel in the proportions of 1-8 for the foundations, 1-6 for arches and spandrel walls and 1-4 for the parapet wall. The proportions were determined by measure; the wagon boxes being built to hold a cubic yard of sand and gravel. A sack of cement was taken as 1 cu. ft. For foundations the pit mixture was used without screening; stones over 4 ins. in diameter being thrown out at the pit or on the mixing board. For the arches and spandrel walls the gravel was passed over a 2-in. mesh screen on the wagon box. The aggregate for the parapet walls was screened to 1 in. largest diameter. The concrete was mixed in a McKelvey continuous mixer which turned the material eight times. The mode of procedure was as follows: The gravel was loaded upon wagons in the pit and hauled to a platform at the intake of the mixer. Half of the cement required in the concrete was then spread over the top of the load in the wagon box and the whole was dumped through the bottom of the wagon box onto the platform and spread with shovels. The remainder of the cement was spread over the mixture and the whole was shoveled by one man to a second man who shoveled it into the mixer. Water was added after the mixture had passed about one-third of the way through the mixer. The mixer delivered the concrete directly into wheelbarrows, by which it was delivered to the work. The concrete was spread in layers from 2 to 4 ins. in thickness and thoroughly rammed with iron tampers; two men were employed tamping for each man shoveling. The arches were concreted in three longitudinal sections, each section constituting a day's work. The work was done in 1903 and the concrete cost for mixing and placing: Labor: Per day. Per cu. yd. 13 men at $1.80 $23.40 $0.78 Engine and mixer 5.00 0.17 1 team 3.00 0.10 1 foreman 3.00 0.10 ------ ----- Totals for labor $34.40 $1.15 Materials: 0.65 bbl. cement at $2 $1.30 0.9 cu. yd. gravel at $0.50 0.45 ----- Total for materials $1.75 Grand total $2.90 ~METHODS AND COST OF CONSTRUCTING A FIVE-SPAN ARCH BRIDGE.~--This bridge consisted of five elliptical arch spans of 40, 45, 60, 87 and 44 ft., carried on concrete piers. The arch rings were 12 ins. thick at the crowns and 18 ins. thick 5 ft. from the centers of piers and carried 4-in. spandrel walls; there were 1,000 cu. yds. of concrete in the arches and 600 cu. yds. in the piers. Each arch ring was reinforced by a grillage of longitudinal and transverse rods. [Illustration: Fig. 159.--End View of Center for Short Elliptical Arch Spans.] _Forms and Centers._--Figure 159 is an end view of the center arch. It consists of a series of bents, 6 ft. c. to c., the posts of each bent being 5 ft. c. to c. These posts are made of 2×6-in. Washington fir. Upon the heads of the posts rest 2×6-in. stringers, extending from bent to bent. Resting on these stringers are wooden blocks, or wedges, which support a series of cross-stringers, also of 2×6-in. stuff, spaced 2 ft. c. to c. On top of these cross-stringers rest the sheeting planks, which are 1×6-in. stuff, dressed on the upper side, and bent to the curve of the arch. This sheeting plank was not tongue and grooved, and a man standing under it, after it is nailed in place, could see daylight through the cracks. It looked as if it would leak like a sieve, and let much of the wet concrete mortar flow through the cracks, but, as a matter of fact, scarcely any escapes. Figure 160 shows a front view of a bent, and indicates the manner of sway bracing it with 1×4-in. stuff. Figure 161 shows the outer forms for the parapet wall, or concrete hand railing, and it will be noted that the cross-stringers are allowed to project about 3 ft. so as to furnish a place to fasten the braces which hold the upright studs. The inner forms for the parapet wall are shown in dotted lines. They are not put in place until all the concrete arch is built. Then they are erected and held to the outer forms by wire, and are sway braced to wooden cleats nailed to the top surface of the concrete arch. [Illustration: Fig. 160.--Front View of Center for Short Elliptical Arch Spans.] [Illustration: Fig. 161.--Form for Parapet Wall for Arch Bridge.] For the five spans the total amount of lumber in the centers was in round figures 28 M. ft., distributed about as follows: Item. Ft. B. M. 1×6-in. sheeting 5,600 2×6-in. longitudinal stringers 2,600 2×6-in. cross stringers 2,600 2×6-in. posts 4,000 3×8-in. sills 1,500 1×4-in. braces 3,000 Outer forms for spandrel walls 4,000 Inner forms for spandrel walls 4,000 ------ Total 27,300 The aggregate span length of the arches was 276 ft., so that a little less than 100 ft. B. M. of lumber was used for centering per lineal foot of span. The superintendent at $5 per day and five carpenters at $3.50 per day erected the five centers in 18 days at a cost of $400, or a trifle more than $14 per M. ft. B. M.; the cost of taking down the centers was $2 per M. ft. B. M., and the lumber for the centers cost $24 per M. ft. B. M. making a grand total of $40 per M. ft. B. M. for materials and labor. As there were 1,000 cu. yds. of concrete in the arches and spandrels, the cost of centers and forms was $1.12 per cu. yd. This form lumber was, however, after taking down, used again in erecting a reinforced concrete building. Assuming that the lumber was used only twice, the cost of centers and forms for these five arches was less than 80 cts. per cu. yd. of concrete. _Shaping and Placing Reinforcement._--The 60 and 87-ft. spans were reinforced with 32 1½-in. round longitudinal rods held in place by ½-in. square transverse rods wired at the intersections; the reinforcement of the smaller spans was exactly the same except that 1-in. diameter rods were used. To bend the longitudinal rods to curve, planks were laid on the ground roughly to the curve of the arch; the exact curve was marked on these planks and large spikes were driven part way into the planks along this mark. The end of a rod was then fastened by spiking it against the first projecting spike head and three men taking hold of the opposite end and walking it around until the rod rested against all the spikes on the curve. It took three men two 8-hour days to bend 46,000 lbs. of rods. Their wages were $2.50 each per day, making the cost of bending 0.03 ct. per pound, or 60 cts. per ton. It took a man 5 mins. to wire a cross rod to a longitudinal rod. With wages at $2.50 per day the cost of shaping and placing the reinforcement per ton was as follows: Item. Per ton. Bending rods $0.60 Shearing rods to lengths 0.40 Carrying rods onto bridge 0.40 Placing and wiring rods 2.35 ----- Total $3.75 Including superintendence the labor cost was practically $4 per ton, or 0.2 cts. per lb. Altogether 66,000 lbs. of steel was used for reinforcing 1,000 cu. yds. of concrete, or 66 lbs. per cu. yd. The cost of steel delivered was 2 cts. per lb., and the cost of shaping and placing it 0.2 ct. per lb., a total of 2.2 cts. per lb. or 2.2 × 66 = $1.45 per cu. yd. of concrete. _Mixing and Placing Concrete._--A Ransome mixer holding a half-yard batch was used. The mixer was driven by an electric motor. The concrete for the piers was a mixture of 1 part Portland cement to 7 parts gravel; for the arches, the concrete was mixed 1 to 5. The gravel was piled near the mixer, a snatch team being used to assist the wagons in delivering the gravel into a pile as high as possible. Run planks supported on "horses" were laid horizontally from the mixer to the gravel, so that big wheelbarrow loads could be handled. The barrows were loaded with long-handled shovels, and the men worked with great vigor, as is shown by the fact that four men, shoveling and wheeling, delivered enough gravel to the mixer in 8 hrs. to make 100 cu. yds. of concrete. We have, therefore, estimated on a basis of six men instead of four. The mixer crew was organized as follows: Per day. 6 men shoveling and wheeling $12 2 men handling cement 4 1 man handling water 2 1 man dumping concrete 2 2 men handling dump cars 4 2 men handling hoisting rope 4 4 men spreading and ramming concrete 8 1 engineman 4 1 foreman 5 Fuel, estimated 3 --- Total $48 The output of this crew was 100 cu. yds. per day. The concrete was hauled from the mixer in two small dump cars, each having a capacity of 10 cu. ft. The average load in each car was ¼ cu. yd. Ordinary mine cars were used, of the kind which can be dumped forward, or on either side. The cars were hauled over tracks having a gage of 18 ins. The rails weighed 16 lbs. per yard, and were held by spikes ¼×2½ ins. Larger spikes would have split the cross-ties, which were 3×4 ins. Only one spike was driven to hold each rail to each tie, the spikes being on alternate sides of the rail in successive ties. No fish plates or splice bars were used to join the rails, which considerably simplifies the track laying. [Illustration: Fig. 162.--Trestle for Service Track.] Two lines of track were laid over the bridge. The tracks were supported by light bents, the cross-tie forming the cap of each bent, as shown in Fig. 162. The bents were spaced 3 ft. apart. There were two posts to each bent, toe-nailed at the top of the tie, and at the bottom to the arch sheeting plank. Two men framed these crude bents and laid the two rails at the rate of 150 lin. ft. of track per day, at a cost of 4 cts. per lin. ft. of track. As stated, there were two tracks, one on each side of the bridge, but they converged as they neared the concrete mixer, so that a car coming from either track could run under the discharge chute of the mixer; Fig. 163 shows the arrangement of the tracks at the mixer. The part of each rail from A to B (6 ft. long) was free to move by bending at A, the rail being spiked rigidly to the tie at A, leaving its end at B free to move. To move the end B, so as to switch the cars, a home-made switch was improvised, as shown in Figs. 163 and 164. [Illustration: Fig. 163.--Arrangement of Service Tracks at Mixer.] [Illustration: Fig. 164.--Improvised Switch for Service Cars, General Plan.] It will be remembered that this bridge was a series of five arches. There was a steep grade from the two ends of the bridge to the crown of the center arch. Hence the two railway tracks ascended on a steep grade from the mixer for about 175 ft., then they descended rapidly to the other end of the bridge. Hence to haul the concrete cars up the grade by using a wire cable, it was necessary to anchor a snatch block at the center of the bridge. This was done by erecting a short post, the top of which was about a foot above the top of the rails. The post stood near the track, and was guyed by means of wires, and braced by short inclined struts. To the top of the post was lashed the snatch block through which passed the wire rope. Fig. 165 shows this post, P. About 10 ft. from the post P, on the side toward the mixer, another post, Q, was erected, and a snatch block fastened to it. When the hoisting engine, which was set near the concrete mixer, began hauling the car along the track, a laborer would follow the car. Just before the car reached the post Q, he would unhook the hoisting rope from the front end of the car, then push the car past the post Q, and hook the hoisting rope to the rear of the car. The car would then proceed to descend in the direction T, being always under the control of the wire rope, except during the brief period when the car was passing the post Q. Each of the two cars was provided with its own hoisting rope, and one engineer, operating a double drum hoist, handled the cars. The hoist was belted to an 8 HP. gasoline engine, no electric motor being available for the purpose. [Illustration: Fig. 165.--General Plan of Rope Haulage System.] [Illustration: Fig. 166. Fig. 167. Details of Haulage Rope Guides.] Where hauling is done in this manner with wire ropes, it is necessary to support the ropes by rollers wherever they would rub against obstructions. A cheap roller can be made by taking a piece of 2-in. gas pipe about a foot long, and driving a wooden plug in each end of the gas pipe. Then bore a hole through the center of the wooden plugs and drive a 1-in. round rod through the holes, as shown in Fig. 166. The ends of this rod are shoved into holes bored into plank posts, which thus support the roller. Where the rope must be carried around a more or less sharp corner, it is necessary to provide two rollers, one horizontal and the other vertical, as shown in Fig. 167. When conveying concrete to a point on the bridge about 300 ft. from the mixer, a dump car would make the round trip in 3 mins., about ¼ min. of its time being occupied in loading and another ¼ min. in dumping. One man always walked along with each car, and another man helped pull the wire rope back. Including the cost of laying the track and installing the plant, the cost of mixing and placing the 1,600 cu. yds. of concrete was only 55 cts. per cu. yd., in spite of the high wages paid. However, the men were working for a contractor under a very good superintendent. Summing up the cost of the concrete in the arches of this bridge, we have: Per cu. yd. 1.35 bbl. cement at $3 $4.05 1 cu. yd. gravel at $1 1.00 66 lbs. of steel in place at 2.2 cts. 1.45 Centers in place (lumber used once) 1.12 Labor, mix and place concrete 0.55 ----- Total $8.17 The cost of the nails, wire, excavation and plant rental is not available, but could not be sufficient to add more than 10 cts. per cu. yd. under the conditions that existed in this case. ~CONCRETE RIBBED ARCH BRIDGE AT GRAND RAPIDS, MICH.~--The bridge consisted of seven parabolic arch ribs of 75 ft. clear span and 14 ft. rise. The five ribs under the 21-ft roadway were each 24 ins. thick, 50 ins. deep at skewbacks and 25 ins. deep at crown; the two ribs under the sidewalks were 12 ins. thick and of the same depth as the main ribs. Each rib carried columns which supported the deck slab. Columns and ribs were braced together across-bridge by struts and webs. All structural parts of the bridge were of concrete reinforced by corrugated bars. The abutments were hollow boxes with reinforced concrete shells tied in by buttresses and filled with earth. There were in the bridge including abutments 884 cu. yds. of concrete and 62,000 lbs. of reinforcing metal, or about 70 lbs. of reinforcing metal per cu. yd. of concrete. Of the 884 cu. yds. of concrete 594 cu. yds. were contained in the abutments and wing walls and 290 cu. yds. in the remainder of the structure. (Fig. 168.) [Illustration: Fig. 168.--Details of Ribbed Arch Bridge.] _Centers._--The center for the arch consisted of 4-pile bents spaced about 12 ft. apart in the line of the bridge. The piles were 12×12 in.×24 ft. yellow pine and they were braced together in both directions by 2×10-in. planks. Each bent carried a 3×12-in. plank cap. Maple folding wedges were set in these caps over each pile and on them rested 12×12-in. transverse timbers, one directly over each bent. These 12×12-in. transverse timbers carried the back pieces cut to the curve of the arch. The back pieces were 2×12-in. plank, two under each sidewalk rib and four under each main rib of the arch. The back pieces under each rib were X-braced together. The lagging was made continuous under the ribs but only occasional strips were carried across the spaces between ribs. This reduced the amount of lagging required but made working on the centers more difficult and resulted in loss of tools from dropping through the openings. Work on the centers and forms was tiresome owing both to the difficulty of moving around on the lagging and to the cramped positions in which the men labored. Carpenters were hard to keep for these reasons. _Concrete._--A 1-7 bank gravel concrete was used for the abutments and a 1-5 bank gravel concrete for the other parts of the bridge. The concrete was mixed in a cubical mixer operated by electric motor and located at one end of the bridge. The mixed concrete was taken to the forms in wheelbarrows. The mixture was of mushy consistency. No mortar facing was used, but the exposed surfaces were given a grout wash. In freezing weather the gravel and water were heated to a temperature of about 100° F.; when work was stopped at night it was covered with tarred felt, and was usually found steaming the next morning. _Cost of Work._--The cost data given here are based on figures furnished to us by Geo. J. Davis, Jr., who designed the bridge and kept the cost records. Mr. Davis states that the unit costs are high, because of the adverse conditions under which the work was performed. The work was done by day labor by the city, the men were all new to this class of work, the weather was cold and there was high water to interfere, and work was begun before plans for the bridge had been completed, so that the superintendent could not intelligently plan the work ahead. Cost keeping was begun only after the work was well under way. Many of the items of cost are incomplete in detail. The following were the wages paid and the prices of the materials used: Materials and Supplies: No. 1 hemlock matched per M. ft. $20 No. 1 hemlock plank per M. ft. 17 No. 2 Norway pine flooring per M. ft. 19 No. 2 yellow pine flooring per M. ft. 20 12×12-in.×16-ft. yellow pine per M. ft. 29 12×12-in.×24-ft. yellow pine, piling per M. ft. 27 Maple wedges per pair 50 cts. ½-in. corrugated bars per lb. 2.615 cts. ¾-in. corrugated bars per lb. 2.515 cts. 7/8-in. corrugated bars per lb. 2.515 cts. Coal per ton $4 Electric power per kilowatt 6 cts. Medusa cement per bbl. $1.75 Aetna cement per bbl. 1.05 Bank gravel per cu. yd. 0.85 Sand per cu. yd. 0.66 Carpenters per day $3 to 3.50 Common labor per day 1.75 The summarized cost of the whole work, with such detailed costs as the figures given permit of computation, was as follows: General Service: Total. Per cu. yd. Engineering $451 $0.512 Miscellaneous 75 0.084 Pumping: Total 110 days. Coal at $4 per ton $210 Machinery, tools and cartage 283 Labor 497 ---- Total $990 This gives a cost of $9 per day for pumping. Excavation: Total cost. P. C. Total. Timber cartage, etc. $ 375 17.6 Tools 69 3.3 Labor at $1.75 1,687 79.1 ------ ----- Total $2,131 100.0 Filling 5,711 cu. yds.: Total. Per cu. yd. Earth $1,142 $0.20 Labor including riprapping 396 0.07 ------ ----- Total $1,538 $0.27 Removing Old Wing Walls: Total. Labor and dynamite $ 346 Tools and sharpening 64 ----- Total $ 410 Hand Rail, 150 ft.: Total. Per lin. ft. Material $ 278 $1.85 Labor 29 0.19 ----- ----- Total $ 307 $2.04 Wood Block Pavement, 296 sq. yds.: Total. Per sq. yd. Wood block, etc. $ 695 $2.35 Labor 57 0.19 ----- ----- Total $ 752 $2.54 Steel, 62,000 lbs.: Total. Per lb. Corrugated bars, freight, etc. $1,498 2.41 cts. Plain steel, wire, etc. 75 0.12 cts. Blacksmithing, tools and placing 438 0.71 cts. ------ ---- Total $2,011 3.24 cts. Concrete. Centering: Total. Per cu. yd. Lumber and piles $ 332 $1.14 Labor 272 0.95 ----- ----- Total $ 604 $2.09 Total. Per cu. yd. Forms $ 3,312 $ 3.75 Concrete 5,532 6.25 ------- ------ Grand total $18,113 $20.50 In more detail the cost of the various items of concrete work was as follows for the whole structure, including abutments, wing walls and arch containing 884 cu. yds.: Form Construction: Total. Per cu. yd. Lumber and cartage $1,547 $1.75 Nails and bolts 129 0.15 Tools 110 0.12 Labor, erecting and removing 1,526 1.72 ------ ----- Total $3,312 $3.74 Concrete Construction. Materials: Aetna cement at $1.05 $1,218 $1.37 Medusa cement at $1.75 499 0.56 Sand at 66 cts. per cu. yd. 37 0.04 Gravel at 85 cts. per cu. yd. 915 1.04 ------ ----- Total materials $2,669 $3.01 Mixing: Machinery and supplies $ 549 $0.62 Power at 6 cts. per kw. 52 0.06 Tools 22 0.02 Labor 737 0.83 ----- ----- Total mixing $1,360 $1.53 Placing concrete $ 609 $0.69 Tamping concrete $ 481 $0.54 Heating Concrete: Apparatus and cartage $ 47 $0.05 Fuel 96 0.11 Labor 270 0.31 ----- ----- Total heating $ 413 $0.47 Grand total $8,844 $9.98 Considering the abutment and wing wall work, comprising 594 cu. yds., separately, the cost was as follows: Forms: Per cu. yd. Materials $1.20 Labor 1.09 ----- Total $2.29 Concrete: Materials $2.92 Labor 2.38 ----- Total $5.30 Heating water and gravel $0.70 Grand total $8.29 Considering the arch span, comprising 290 cu. yds., separately, the cost was as follows: Forms: Per cu. yd. Materials $3.70 Labor 3.03 ----- Total $6.73 Concrete: Materials $3.22 Labor 3.57 Total $6.79 Grand total $13.52