Abstract: A floor member (10) of channel form is pushed downwardly to install it onto bearings (72) and hold down members (16) which were previously installed on longitudinal guide tubes (12) which extend in the same direction as the floor members (10). The bearings (72) have sidewall portions which slope downwardly and outwardly to lower edge (96, 98). These sidewalls (76, 78) are connected at their upper portions to the bearing (72) in such a way that they will flex inwardly, into spaces (112, 114) provided for them, in response to a downward "snap on" movement of the floor members (10). The hold down members (16) have hold down wings (50, 52) which slope downwardly and outwardly to lower edges (54, 56). The hold down wings (50, 52) also flex inwardly, into spaces provided for them, in response to a downward "snap on" movement of the floor member (10).