The upper sides of the projections (2) and wings (3) of the bridging element have inclined faces (4,5) with the slope in the same direction. Each inclined face (4) of the projections has a height displacement (H) in relation to the inclined face (5) of the wings. Each projection is provided with a recess (6) on the inclined face and a slotted recess (8) on its underside which mates positively with hooks (9) on the lower end of the wings.
