elements respectively are positioned in relation to these bridging span elements so that the corresponding connecting arm be caused in its downward swung lower position to bear upon these two stops 35 while being inclined with respect to the plane of symmetry by a predetermined angle value for example of about 55 u00b0 as shown in fig8 the putting in abutment of the connecting arms 4c of the intermediate bridging span has the advantage of vertically locking or jamming this same bridging span underneath the launching beam and on the same occasion the lower bridging span held in the ascending vertical direction each connecting arm 3c 4c 5c comprises a central portion of the upper leg of the u03c9 shaped formation of this arm two pairs of riding rollers 39 permitting friction between the central portion of the connecting arm and the upper portion of the depositing and launching beam 7 to be avoided during a relative displacement of this beam and of the connecting arm both rollers 39 of one pair are fastened onto yokes 4e made fast to the corresponding connecting arm while extending in perpendicular relation to the central portion of this arm as shown in fig1 both connecting arms 4c of the intermediate bridging span 4 are held by the rollers 39 in bearing relationship upon the corresponding lower portions of the beam 7 by the rightening torque of each arm exerted by one of the rollers 28 upon each corresponding cam 27 when the beam 7 is disengaged from its storage position between both upper and intermediate bridging spans 3 and 4 during the procedure of launching and depositing of one bridging span or of assembled bridging spans.