357. The greatest load likely to meet on a bridge at one time is that produced by a loaded railway train or a dense crowd of people. The former, including the weight of the bridge itself, may be taken, for the purpose of calculation, at 350 lbs., or the .16 of a ton per superficial foot; and the latter, when the roadway is paved or gravelled, at about 300 lbs., or .14 of a ton, or at 200 lbs. = .09 of a ton when the roadway is merely planked.

358. Riga fir, when the length unsupported is such that the piece will not yield by bending, suffers compression in the direction of the length of about 1 part in 1500 under a load of 1000 lbs. upon the square inch. Oak requires a greater load to produce the same compression.*

Under such a pressure the curved rib of a bridge, 200 feet in length, would be shortened rather more than 1.6 inch.