Pivot Piers

PIVOT PIERS. Pivot piers, i.e., the center piers —for swing bridges, differ from piers for fixed spans only in that they are circular, are larger on top, and usually have plumb sides. Pivot piers are protected from the pressure of ice and from shock by boats, etc., by an ice breaker which is entirely distinct from the pier. The ice breaker is usually constructed by driving a group of 60 or 70 piles in the form of a V (the sharp end up stream), at a short distance above the pier. On and above these piles a strong timber crib-work is framed so as to form an in clined ridge up which the cakes of ice slide and break in two of their own weight. Between the ice breaker and the pier two or three rows of piles are driven, on which a comparatively light crib is constructed for the greater security of the pier and also for the protection of the river craft.

Fig. 149 shows the masonry of the pivot pier for the North ern Pacific R. R. bridge over the Red River at Grand Forks, N. Dak. The specifications for the grillage were as follows: " Fasten the first course of tim bers together with Finch by 20-inch drift bolts, 18 inches apart; fasten second course to first course with drift bolts of same size at every other inter section. Timbers to be laid with broken joints. Put on top course of 4-inch by 12-inch plank, nailed every 2 feet with -Finch by 8-inch boat spikes. The last course is to be thoroughly calked with oakum." Fig. 150 shows the pier of the swing span of the Illinois Central Railway's bridge across the Tennessee River at Gilbertsville, Ky.* The pier is reinforced the same as the piers of the fixed spans— see § 1111.