( 385 )

to them, like the king-post of a truss, fastenedto the two rafters.

The rafters being pressed in the direction oftheir length, by the weight hanging to theridge, the feet of the frame are pushed out-wards by a part of that strain. To ascertain theforce of that push, find, first, the strains uponthe rafters A B, A C, fig. 12, as before, and setit off 1 upon their prolongation, as B G. DrawB I, I G, parallel and perpendicular to the hori-zontal line, and B I will represent the force bywhich the foot B is pushed outwards by theweight A D. There is an equal thrust at thefoot C, in the contrary direction; and the strainof these opposite thrusts is sustained by the tiebeam B C, which completes the construction ofa simple truss-frame.

A very slender tie-beam is sufficient to with-stand these horizontal thrusts, as in the roofsof buildings; but the use of a truss in the pre-sent instance is, to support the weight thatmay be brought upon the beam B C of abridge. The point D being sustained by theking-post A D, in the direction of its length,the strain represented by A D acts, at eachfoot of the frame, in the proportions B I, IG;and the thrust at C is equal, but in a contrarydirection, to B l.

c c