Rigid Vs. Moment

On this exterior frame, the horizontal I beams are connected with bolts to a flange in the metal column. The difference between a rigid and moment connection is a bolt to weld connection, respectively. A rigid connection is made so that movement is allowed. The bolt from the beam to column allows for rotation of the beam when the building moves. A moment connection welds part of the beam to the column so that there isn't any movement. The contractor will "call out" why type of connections are to be made on the plans for each connection.

This is another photo, although harder to tell, of a rigid connection. This I-beam is used as a header over a door opening. A hole is drilled through the flange on beam, and a bolt is inserted through into the CMU. A washer and nut are then used to lock the beam into place.

If you notice, the beam doesn't sit flush with the CMU wall. This is because there needs to be a gap so that when there is movement(in most cases it is dealing with the heating and cooling of materials which causes expanding and contracting). If the beam were to sit flush with the CMU, it would put pressure on the interrior wall when the beam expanded in hotter temperatures. This could cause damage, most likely cracking, with the interrior wall.