said ringlike covering part 31 in addition the lower end of said pushing plate 451 is formed with a pair of separate feet 453 the space between the separate feet 453 can be used for mounting with the section axis 421 of said locking bar 42 when the locking bar 42 is horizontally removed the passive key 45 then can swing to 90 degrees such that the pushing plate 451 becomes perpendicular to the side plates 321 322 when the upper locking construction 4 b is installed in the pivotal joint 32 the transverse step beam 5 is then provided onto the outside of said pivotal joint 32 on lower face of said transverse step beam 5 there is a hole 51 formed at a position opposite to the second panel 442 of said turning key 44 while a second hole 52 is formed at a position opposite to the said passive key 45 so as to let said pushing plate 451 to extrude upward please refer to fig4 the base locking construction 4 a has a spring 43 a turning key 44 and a passive key 45 which are identical in structure of the same elements of said upper locking construction.