Box girder

( Engineering.) A form of girder resembling a box, made out of boiler-plate, and fastened together by means of angle irons, which are rivetted respectively to the top and bottom plates. For spans of from 30 to 60 feet opening, these girders present great advantages, and they are now almost exclusively used by English engineers; for a superior elasticity, and the power of resisting violent impact, is far greater in this cla of girders than in the old form of cast-iron. Mr. Fair bairn, their inventor, has given the following formula for calculating their resistance: W = "de in which W the breaking weight applied in the centre; a the area of the bottom fange, d = the depth of the beam in inches, e a co-efficient = 75 for wrought iron; and the span in inches.