longitudinal members 20 the boards 14 insert into and are retained by the slots 34 on the mid rail 32 in one embodiment a plurality of cross beam straps 36 are arranged in an orientation that is substantially parallel to the opposing end members 18 in one embodiment each cross beam strap 36 comprises a flat metal bar and is connected at one end to the mid rail 32 and at the other end to a longitudinal member 20 the cross beam strap 36 may be butt welded to the longitudinal members 20 and the mid rail 32 such that they are disposed on top of the boards 14 and flush with the longitudinal members 20 and mid rail 32 in an alternative embodiment each cross beam strap 36 is lengthened slightly such that the ends overlap and slide under the longitudinal member 20 channel on one side and either within the mid rail 32 i beam channel on the other side or where there is no mid rail rail under the longitudinal member 20 channel 20 on the other side the strap 36 is then lap welded to the longitudinal member 20 and the mid rail 32 or the other longitudinal rail 20 a transverse groove having a depth approximately equal to the thickness of the cross beam strap 36 is cut into the boards 14 so that the strap 36 is interface of the strap 36 is flush with the major surfaces 28 formed by the boards 14 the cross beam straps 36 may be aligned on either side of the mat and held together by a pin 38 which is welded in place as shown in fig2 each end member 18 is an i beam 38 comprising a vertical.