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.
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.