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.)
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: