marking three rectilinear portions 4 a 4 b 4 c the edges 5 a 5 b are preferably equidistant from their respective contact area 6 a 6 b thus the three portions 4 a 4 b 4 c are of a length substantially equal in transverse section as the first mode of execution the connecting element 4 may also include at its ends an upper contact area 6 a intended for the connection of the element 4 with the upper wall 1 and eventually a lower contact area 6 b intended for the connection of the element with the lower wall 2 fig6 illustrates in perspective the connecting element 4 of fig5 the structure of panels 10 of the invention may be a generalization of the form of the connecting element 4 as bearing n longitudinal edges and n 1 rectilinear portions the distance between two adjacent edges is preferably the same on the entire connecting element 4 after folding the angles u03b11 u03b12 are less than 90 u00b0 and advantageously provide the following ratios these angles may be different but the ratios cited above ensure greater manufacturing simplicity fig7 illustrates an execution mode in which the angles of the panels 10 are different the oriented angles in fig7 are defined in the following manner the benefits of the invention include simple implementation because the folding of the connecting element may be done with standard tools furthermore the absence of a damper facilitates the assembly and removal while reducing the number of parts necessary for each operation furthermore the fastening of the connecting element may be carried out by standard methods or by welding.