of Petit trusses, four (4) panel lengths; and for main-truss members, the length of span loaded when the member under consideration receives its maximum stress.

In the case of bridges with exterior sidewalks, one sidewalk only and the roadway are to be considered loaded when proportioning the beam-hangers and primary truss members of all bridges, and when proportioning the main-truss members of all spans less than one hundred (100) feet for bridges of Class A, and of all spans less than eighty (80) feet for bridges of Classes B and C. In all other cases both of the sidewalks and the roadway are to be considered loaded. The eccentric loading increases the live load per truss. But, when a bridge has only one exterior sidewalk, the effect-of the eccentric loading is to be considered to act upon the whole of the nearer truss, and the sidewalk is to be considered empty when calculating the stresses in the farther truss. Floor-beams of bridges with one or two exterior Sidewalks are to be proportioned on the assumption that, first, the main roadway is loaded, and the sidewalk or sidewalks are empty and, second, that the main road way is empty, and the sidewalk or sidewalks are loaded, due account being taken of the effect of reversing stresses as hereafter, specified.
      In addition to the preceding loads, the floor, joists, floor-beams, beam-hangers, and primary-truss member are to be proportioned for the following concentrated loads, which are, however, supposed to occupy a whole panel length of the main roadway to the exclusion of the other live loads there (excepting only the electric-railway live load).

CLASS A.

A road-roller weighing thirty thousand (30,000) pounds, of which twelve thousand (12,000) pounds are concentrated upon the roller in front of the machine, and nine thousand (9000) pounds on each of the wheels at the rear, the distance between the central planes of these wheels being five (5) feet, and that between their axis and the axis of the front roller eleven (11) feet. The width of the front roller is to be four (4) feet, and that of each rear wheel one foot eight inches (1' 8").