U U

+U,U, + U,U, — hor. comp. m,U, + hor. comp. = 0 +252.0+ .•. U,U. = —300.0.

In a similar manner, by resolving the horizontal forces at it will be seen that the action of will neutralize that of as they are equal and pull in opposite directions, and is equal to = —300.0.

The live-load stresses in the chords, the end-post, and the sub diagonals are all proportional to the dead-load stresses in the same ratio as the live panel load is to the dead panel load. This ratio is 40 = 1.667. By reference to Fig. 70, it will be seen that the live 24 load stress in the sub-verticals is r40.0 for each one. The following stresses can now be determined: m,L, = - 16.96 X 1.667 = - 28.28 = m,U, = +16.96 X 1.667 = +28.28 The vertical will have its maximum live-load stress when points and are loaded, for these are the only loads which cause a stress in that member (see Fig. 76a). The equation is: from which, U,L, _ +60.0.

The maximum live-load stresses in and are obtained in a manner exact ly like that used in obtaining dead load stress, only the live-load posi tive shear is used. The stresses are: In the determination of the maximum live-load stress in the lower halves of the main diagonals, m,L,, and m,L;, one of the peculiarities of this truss becomes apparent. A section being passed

as in Fig. 82, the panel point ahead of the section, and all between the section and the right support, must be loaded. This of course produces a stress in and the vertical component of this enters the stress equation.' The shear in the section a — a under this load ing is: = +188.5 - 40 = +148.5; and the stress equation is: -m,L, X'0.707 + 0 + 148.5 = 0 m,L, = +238.0.

If the truss had been loaded from the section to the right, there being no load on no stress would result in and the stress in m2L2 The maximum live-load stresses in the main verticals occur when the panel points to the right of the section which cuts the member under considera tion are loaded. There being no load at the end of the sub-vertical just to the left of the section, there will be no stress in the sub-diag onalwhich the sec tion cuts. The chords, of course, do not exert a vertical com ponent; and so the only unknown term of the stress equation is the stress in the member itself. Fig. shows how the section should be passed when is considered. The stress equation is: