and around it a circular piece or collar 18 this piece 18 comprises a widened circular flange 19 which is mounted on central stem 20 the height of stem 20 corresponds approximately to the width of base 11 of each stave 10 thereby assuring the pinching of sides 13 the circular half notches 14 have a diameter corresponding approximately to the exterior diameter of stem 20 which is engaged in half notches 14 finally each piece 18 is equipped with in its center a bore 21 which threaded shaft 17 goes through in the example illustrated in fig7 and 9 each screw 15 cooperates with piece 22 as with piece 18 this has a widened flange 19 mounted on a stem 20 but the flange 19 is locally rendered more narrow width 23 smaller than diameter 24 by the presence of two flat sections 25 on flange 19 in this case one cuts some half openings 26 the length of the longitudinal side 13 of each base 11 each half opening 26 has a rectangular contour the length 27 of each half opening 26 is equal to or greater than diameter 24 on the contrary the width 28 of each half opening 26 is approximately equal to half of the width 23 in this way at the assembly two adjacent bases 11 fig8.