Chapter XVII (2)

_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.

_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.

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.

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.

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.

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.)

_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
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Total $2.29
Concrete:
Materials $2.92
Labor 2.38
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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
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Total $6.73
Concrete:
Materials $3.22
Labor 3.57

Total $6.79
Grand total $13.52