raised or lowered the treads of the step modules 41 42 will remain horizontal as they raised or lowered referring to fig4 6 the pivot assembly 43 includes a stepped diameter pivot rod or cylindrical stud 61 which has a large exterior diameter part 62 and a small exterior diameter part 63 separated by a shoulder presenting a flat annular axially facing abutment surface 64 the cylindrical stud 61 had a hollow interior defined by a bore 66 which has a coaxial relationship with the cylindrical exterior surfaces of the large and small diameter portions 62 63 the stud 61 is secured to the joist 16 by a tension member in the form of a round headed bolt 67 having square section portion 68 adjacent its head 69 which nonrotatably mates with a square hole 71 in the joist 16 and a nut 72 on the threaded end 73 of the bolt applying axial thrust against the reduced diameter end of the stud 61 through a washer 74 the side plate 46 of the step module 41 has an annular opening or bore 76 into which a radially outer sleeve 77 of a sleeve bearing module 78 is pressed a radially inner sleeve 81 has a radially inward facing cylindrical bearing surface 82 in radial thrust bearing engagement with a radially outward facing cylindrical bearing surface 83 of the reduced diameter portion 63 of the stud 61 the sleeves 77 81 have complimentary spherical load bearing surfaces 91 92.