span at Pont-y-Prydd, over the Taff, in Wales, erected in 1750, is supposed to have a pressure of 72 tons per sq. ft. (1,000 lb. per sq. in.) on hard limestone rubble masonry laid in lime mortar.* Rennie subjected good hard limestone rubble in columns 4 feet square to 22 tons per sq. ft. (300 lb. per sq. in.).t The granite piers of the Saltash Bridge sustain a pressure of 9 tons per sq. ft. (125 lb. per sq. in.).
The maximum pressure on the granite masonry of the towers of the Brooklyn Bridge is about 28i tons per sq. ft. (about 400 lb. per sq. in.). The maximum pressure on the limestone masonry of this bridge is about 10 tons per sq. ft. (125 lb. per sq. in.). The face stones ranged in cubical contents from 1 to 5 cubic yards; the stones of the granite backing averaged about 11 cu. yd., and of the limestone about 11 cu. yd. per piece. The mortar was 1 volume of Rosendale natural cement and 2 of sand. The stones were rough axed or pointed to finch bed-joints and finch vertical face joints.$ These towers are very fine examples of the mason's art.
In the Rookery Building, Chicago, granite columns about 3 feet square sustain 30 tons per sq. ft. (415 lb. per sq. in.) without any signs of weakness.
In the Washington Monument, Washington, D. C., the normal pressure on the lower joint of the walls of the shaft is 20.2 tons per sq. ft. (280 lb. per sq. in.), and the maximum pressure brought upon any joint under the action of the wind is 25.4 tons per sq. ft. (350
lb. per sq. in.).¶ The pressure on the limestone piers of the St. Louis Bridge was, before completion, 38 tons per sq. ft. (527 lb. per sq. in.); and after completion the pressure was 19 tons per sq. ft. (273 lb. per sq. in.) on the piers and 15 tons per sq. ft. (198 lb. per sq. in.) on the abut ments.** The limestone masonry in the towers of the Niagara Suspension Bridge failed under 36 tons per sq. ft., and were taken down,—how ever, the masonry was not well executed and was subjected to At the South Street Bridge, Philadelphia, the pressure on the limestone rubble masonry in the pneumatic piles is 15.7 tons per sq. ft. (220 lb. per sq. in.) at the bottom and 12 tons per sq. ft. at the top. "This is unusually heavy, but there are no signs of weak ness."$$ The maximum pressure on the rubble masonry (laid in cement mortar) of some of the large masonry dams is from 11 to 14 tons per sq. ft. (154 to 195 lb. per sq. in.). The Quaker Bridge dam was designed for a maximum pressure of 161 tons per sq. ft. (230 lb. per sq. in.) on massive rubble masonry in best hydraulic cement mortar.* Safe Pressure. In the light of the preceding examples it may be assumed that the safe load for the different classes of masonry is about as follows, provided each is the best of its class: