_Shaping and Placing Reinforcement._--The 60 and 87-ft. spans were reinforced with 32 1-in. round longitudinal rods held in place by -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 s.n.a.t.c.h team being used to a.s.sist 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 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 2 ins. Larger spikes would have split the cross-ties, which were 34 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.
[Ill.u.s.tration: 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 s.p.a.ced 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.
[Ill.u.s.tration: Fig. 163.--Arrangement of Service Tracks at Mixer.]
[Ill.u.s.tration: 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 s.n.a.t.c.h 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 s.n.a.t.c.h block through which pa.s.sed 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 s.n.a.t.c.h 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 pa.s.sing 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.
[Ill.u.s.tration: Fig. 165.--General Plan of Rope Haulage System.]
[Ill.u.s.tration: 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 min. of its time being occupied in loading and another 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 sh.e.l.ls tied in by b.u.t.tresses 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.)
[Ill.u.s.tration: Fig. 168.--Details of Ribbed Arch Bridge.]
_Centers._--The center for the arch consisted of 4-pile bents s.p.a.ced about 12 ft. apart in the line of the bridge. The piles were 1212 in.24 ft. yellow pine and they were braced together in both directions by 210-in. planks. Each bent carried a 312-in. plank cap. Maple folding wedges were set in these caps over each pile and on them rested 1212-in. transverse timbers, one directly over each bent. These 1212-in. transverse timbers carried the back pieces cut to the curve of the arch. The back pieces were 212-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 s.p.a.ces 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 cla.s.s 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 1212-in.16-ft. yellow pine per M. ft. 29 1212-in.24-ft. yellow pine, piling per M. ft. 27 Maple wedges per pair 50 cts.
-in. corrugated bars per lb. 2.615 cts.
-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