at the base of wings 69 bridge 153 equipped with a locking catch 240 has a cross bracing flange 300 fixed onto deck 76 of which the locking fork holds a supporting member 56 of which a bridge 660 blocks the two mini sections 561 one of which has two lateral wings forming at the ends hook 562 which can hook web 301 of cross bracing flange 300 shown at the back of fig1 and also in fig1 is a cross bracing bridge 153bis i e two bridges 153 connected by their deck 76 which are welded web to web which permits the construction of either a closed main beam two supporting members can also be joined face to face forming an enclosed space by means of flange 300 provided by two bridges 153 joined back to back fig1 a fig1 illustrates a low bridge 153 with deck 76 c and pillar 70 connected by an element 74 which has perforations 77bis to increase the flexibility of entry of pillars 70 into the supporting member sharp edged spurs 92 are provided on the pillars fig1 illustrates a bridge 57 with very small deck 76 k and suitable pillars 70 it constitutes an excellent means of connecting supporting members it differs from the preceding bridges in its small width and is accordingly insertable into a very narrow channel this bridge.