rail 13 in fig1 since the joint between the rail 12 and the bar 14 is the same as that between the bottom rail 13 and the bar 14 only the joint between the rail base 15 and the vertical bar 14 will be described since the rail base 15 rides on the end face of the vertical bar 14 a hole 36 having the same size and shape as the cross sectional size and shape of the interior of the bar 14 is provided in the rail base 15 in the same position as that of the bar 14 so that the peripheral portion of the hole 36 rides on the periphery of the bar 14 because of this relationship it is possible to pressingly insert the stud 20 34 from the upper side of the rail base 15 through the hole 36 into the bar 14 the fit of body 25 34 of the stud 20 34 into the interior of the bar 14 is snug to the extent that the body can be inserted thereinto by light hammering the head 24 34 of the stud 20 34 has grooves 37 on the opposite surfaces thereof for insertion of the self tapping screws 21 and has shoulders 24 34.