Beam Behaviour

All beams in steel structures, including beams designed as simply supported, experience a certain degree of axial and rotational restraints at their ends. Beam internal forces are strongly related to the degree of end restraint and beam temperature. End restraints generate axial force and moment within the supported member at elevated temperature. At the initial stage of a fire (100°C – 400°C), a steel beam starts to experience compressive internal forces due to the restraint to thermal expansion and it starts to bow towards the fire. When the beam temperature reaches 400°C, the steel begins to lose its strength. the beam internal force begins to Progressive collapse is the collapse of a building due to a
failure of a single vertical load-carrying element, or a small number, which are disproportionate to the  complete building failure. Element failure could occur as a result of any of several extreme loading events on buildings, including strong earthquakes, blast, vehicle impact, fire, or similar incidents.turn from compression into tensile force, and then the connections begin to support the steel beam by resisting pull-in forces as well as vertical shears. At the advanced level, a steel beam hangs as a suspension cable from the cooler end connections. Such a phenomenon is called “Catenary Action”
Consequently, providing the end connections’ resistance still exists, the tensile horizontal axial force grows progressively, as the beam mid-span deflection grows. Furthermore, beam end connections are required to resist an additional moment, resulting from rotation due to excessive mid-span deflections. This leads to reductions in the beam mid-span moment. In essence, the behaviour of the beam is affected by the connections’ ductility and stiffness. In this manner, catenary action helps the beam by reducing the mid-span moment and it hangs from the cooler connections which need to sustain the tensile axial force of the beam and prevent the collapse. Therefore, Catenary Action works to improve the survival time of steel beams in a fire provided that the beam end connections still function. Beam-to- column joints in structural steel buildings transfer the floor and beam loads to the columns and provide the link between the principal structural elements for the overall structural stability. By this means, the forces transmitted through the joints can be axial and shear forces, bending and torsional moments. The effect of torsion on individual members can be neglected in plane frames, as a result of the lateral restraint provided by composite action with floor slabs. Nevertheless, the magnitudes of the other three actions transferred by the connections to the supporting columns depend on the connection type and the surrounding conditions. In the case of steel frame structures with moment-resistant connections, such as bolted end plates, the bending moments are predominant compared to axial and shear force.