the riser pan 10 down toward other stackable risers not shown in fig2 that are interconnected in the passageway beneath the riser pan 10 as discussed in u s pat no 5 852 901 which is incorporated herein by reference the stackable risers 12 14 are provided with horizontal outwardly extending annular ledges 29 which provide a similar function as seen in fig2 and 10 the stackable risers 12 14 may each include a plurality of such outwardly extending annular ledges 29 as the ledges 29 advantageously improve rigidity of the risers 12 14 and thereby increase the rigidity of the entire passageway 31 this first embodiment of the riser pan 10 further includes a plurality of exterior vertically oriented ribs 32 extending above the annular ledge 30 along the outside of the upper cylindrical wall 16 which are provided to help distribute load transmitted to the riser pan 10 from a next higher riser 12 stacked thereon in addition to the ribs 32 several screw bosses 33 each having sidewalls 34 36 an inner wall defined by an outer portion of the upper cylindrical wall 16 and an outer wall 37 are provided at periodic locations about the upper cylindrical wall 16 which also extend above the annular ledge 30 these screw bosses 33 may take the form of a pair of closely spaced ribs which are adapted to securely receive a threaded screw therebetween but it.