It is probable that the carpenter work includes merely shifting and erecting forms and not the first cost of framing centers. No materials, of course, are included. It should be kept in mind that while the output and labor force are exact the wages are assumed.
Traveling Derrick Plant for 4-Span Arch Bridge.—The bridge consisted of four 70-ft. arch spans and was built close alongside an old bridge which it was ultimately to replace. The approach from the west was across a wide flat; at the east the ground rose more abruptly from the stream. Conditions prevented the use of a long spur track and also made it necessary to install all plant at and to handle all material from the west bank. A diagram sketch of the arrangement adopted is shown by Fig. 153.
Fig. 153.—Sketch Showing Traveling Derrick Plant for Concreting an Arch Bridge.
The track from the west approached the existing bridge on an embankment 25 ft. high. A spur track 175 ft. long from clear post to end was built on trestle as shown. The cement house and mixer platform were placed at the foot of the embankment at opposite ends of the spur track. Between the two the slope of the embankment was sheeted with 1-in. boards and a timber bulkhead 4 ft. high was built along the toe of the sheeting. Stone, sand and coal were stored behind the bulkhead on the sheeting. A runway close to the bulkhead connected the cement house with the mixer platform, all materials to the mixer being wheeled in barrows on this runway. A ¾-cu. yd. Smith mixer was set on a platform 5 ft. above ground with its discharge end toward the stream. Beginning under this platform a service track was carried across the flat and stream to the extreme end of the east abutment. This track consisted of three rails, two rails 4 ft. apart next to the work and a third rail 25 ft. from the first. The 4-ft. gage provided for cars carrying concrete buckets from the mixer and the 25-ft. gage provided for a traveling derrick; 18-lb. rails were used and they proved to be too light, 40-lb. rails are suggested. The derrick consisted of a triangular platform carrying a stiff leg derrick with a 25-ft. mast and mounted on five wheels. The wheels were double flange 16 ins. diameter and cost $30 each, being the most expensive part of the derrick. The derrick was made on the ground and took four carpenters between 3 and 4 days to build. Derrick and 350 ft. of service track, including pole trestle across the stream, cost between $600 and $800. The derrick was moved by means of a cable wrapped around one spool of the Flory double-drum hoisting engine and leading forward and back to deadmen set at opposite ends of the service track. Cars carrying concrete buckets were run out on the 4-ft. gage track and the buckets were hoisted by the derrick and dumped into a ½-cu. yd. car running on a movable transverse track across the bridge. This transverse track was necessary to handle the concrete to the far side of the work, the derrick being set too low and the boom being too short to reach. The derrick was used to handle material excavated from the pier foundations and also to tear down the centers and spandrel forms. Some rather general figures on the cost of this bridge are given by Mr. H. C. Harrison, the contractor. They are:
| Materials: | Total. |
| 6,000 bbls. cement at $2.05 | $12,300 |
| 2,500 cu. yds. sand at $0.80 | 2,000 |
| 5,000 cu. yds. stone at $0.85 | 4,250 |
| 260 M. ft. B. M. lumber at $17 | 4,420 |
| ——— | |
| Total | $22,970 |
| Labor: | |
| Cofferdams, excavation and pumping | $ 3,000 |
| Forms, falseworks and centers | 2,000 |
| Mixing and placing concrete | 4,000 |
| Placing reinforcement | 400 |
| Removing falseworks, forms, etc. | 1,200 |
| One coat pitch and paper | 150 |
| Building plant, etc. | 2,250 |
| ——— | |
| Total | $13,000 |
Mr. Harrison states that including plant cost, delays, floods and incidentals the cost per cubic yard of concrete was $8 and that excluding these items the cost was $6 per cu. yd.
COST OF CONSTRUCTING CONCRETE HIGHWAY BRIDGE, GREENE COUNTY, IOWA.—The following is the itemized cost of constructing a reinforced concrete slab highway bridge, one of several built by the Highway Commissioners of Greene County, Iowa, in 1906. The figures are given by Messrs. Henry Haag and D. E. Donovan, the last being the foreman of the concrete gang doing the work. All bridges consist of 10 to 12-in. slabs reinforced with old steel rails and of abutments and wing walls reinforced with old rods, bars or angles selected from junk. This junk metal cost 0.6 cts. per pound and the rails cut to length cost 1.15 cts. per pound f. o. b. cars. The work was done by a special gang, the men receiving $1.50 per day and board. As a rule the footings were made 2 ft. wide and as high as need be to get above the water and dirt. Before the footing concrete set steel rods, bars or angles were placed; they were long enough to reach the height of the wall and 3 to 6 ins. into the slab. The forms for the abutment and wing walls and for the floor slab were then erected complete before any more concrete was placed. No carpenter was employed, every man on the job having been taught to take his certain place in the work, then, the forms being erected, every man had his particular place in the work of mixing and placing the concrete. The foreman saw that the reinforcement was properly placed and watched over the accuracy of the work generally. The concrete was allowed to set on the centers for from 30 to 40 days; the other form work was taken down after three days and travel over the bridge permitted after three or four days. The concrete was mixed wet. The bridge whose cost is given was 22 ft. wide and 16 ft. span with 2-ft. wing walls.