An upright support assembly for a gantry includes a base with primary ground engaging means, such as wheels and a stabilizing member which is pivotally connected to the support assembly so as to be moveable between a stowed position folded against the support assembly and a deployed position in which it engages the ground to hold the support assembly upright. The stabilizing member includes a ground engaging fitting at its outer end which is adapted to engage the ground in the deployed position to stabilize the support assembly in the upright position, and to engage and roll over the ground in the stowed position when the support assembly is lowered to the ground so that the base and primary ground engaging means can be lifted off the ground and the support assembly moved on the ground engaging fitting.