Abstract: A clamping system for "temporarily" fixedly joining together two "I" beam or girder type structures on for example an offshore oil rig to set up for example a portable crane on the platform structure. The system uses four sets of opposed pairs of clamping elements, each set of which includes two related pairs of clamping elements which grip the beams and are bolted together, the clamping system being used in place of welding the beams or girders together. When it is time to move the structure, the clamping elements are merely unbolted, allowing the removal or replacement of the portable crane on the platform structure. The four clamping elements of each set include two pairs of side-by-side members which are positioned along side one another to grip the opposite edges of the top flange of the bottom, support beam and bear down on the upper surface of one side of the bottom flange of the upper, crane girder.