A trailer frame assembly for a fifth wheel trailer frame in which at least a portion of the frame is torsionally stiffened with a rectangular closed section in a transverse cross section. The rectangular closed section is formed by longitudinal, parallel spaced vertical side plates, a top pan extending between top portions of the side plates, and a bottom pan between bottom portions of the side plates. Overlap joints between the side plates and the top and bottom pans allow for connection with threaded or rivet fasteners. An end dump semi trailer is also provided with a double acting lift cylinder on each side of the trailer. Also provided is a latch lift assembly for a dump gate using a linkage assembly to impart vertical upward movement to dump gate to unlatch the dump gate from a cradle catch.