Abstract: A channel member having its flanges extending at an acute angle to the web and having inturned margins on the free edges of the flanges is imbedded in plastic concrete in approximately the position of a threshold plate or other structure to be mounted, the set concrete retaining the mounting channel in fixed position by the dovetailed relation of the flanges in the concrete with the open top of the mounting channel substantially flush with the concrete to also serve as a screed guide for the plastic concrete; a plurality of spanner bars of channel shape and of a length greater than the spacing of the flanges of the mounting channel are provided with slots which extend the full distance between the free edges of the inturned margins with a square lock nut such as the type having a nylon strip in a keyway is slidably positioned in each spanner bar channel and a flat head or oval head mounting screw for the threshold extends through corresponding countersunk apertures in the threshold and threaded into the squar