Here, it is described how the structural behaviour of a member exposed to fire can be, in most cases, enhanced by consideration of the whole structural assem- bly. This information is based on Buchanan (2002). Note that these effects can be applicable for all types of building materials, however, here, special attention is paid to the behaviour of concrete structures.
4.2.1 Redundancy
Redundancy is the effect where the failure of a single element does not cause failure of the whole structure. This is achieved through load sharing, the loads that were carried by the failed member are redistributed to other, stiffer, and stronger members. This may remind of continuity, which is discussed later, how- ever, in continuous members the load is redistributed within that member. In the case of redundancy, the internal forces are carried on from member to member. The efficiency of this effect depends on the load factor. This term is explained later, in Section 6.7.2, however basically this means that, in the accidental situa- tion of a fire, the total load on the structure is much less than the full design load. Thus fewer members may be necessary to carry the structure, provided that alternative load paths are available so that the load can get to the undamaged members. Ductility is also needed, making this effect more known in steel struc-
tures, however it can occur in concrete structures as well, especially when the reinforcement provides ductility to the members.
4.2.2 Disproportionate collapse
Disproportional collapse is conceptually the opposite of redundancy, meaning that the failure of one member could cause the failure of a whole structure or part of that structure. The magnitude of the damage is disproportionate to the initial event. A well known example of disproportionate collapse, also called the lack of robustness, is the Ronan Point disaster in the U.K. in 1968. An explosion in one room of the multi-storey building caused a whole section of the building to col- lapse, with the loss of many lives. Disproportionate collapse can also occur when elements providing lateral restraint to slabs or beams or lost. Design against dis- proportional collapse requires structural toughness and alternative load paths. 4.2.3 Continuity
A member with flexural continuity possesses a much greater fire resistance than if simply supported. Flexural continuity essentially means that the member is hyper static, meaning that more than one cross-section may fail before its load carrying capability is lost. Examples are continuous beams over several supports or beams built in a rigid frame, i.e. clamped support conditions. A simply sup- ported beam, on the other hand, has no flexural continuity. The mechanism is based on the formation of plastic hinges. These are segments of the beam where large displacements can occur without significant increase in bending moment. The internal forces that were previously carried by these segments are then re- distributed to the rest of the beam which is still intact. The large displacements in the plastic hinges require ductile material, as steel members and reinforced concrete. The latter material can even show additional benefits due to different positive and negative bending moment capacities. An example of moment redis- tribution is given in Section 7.3, where the load-bearing capacity of a reinforced T-beam, continuous over 2 supports, exposed to a standard fire is checked.
4.2.4 Axial restraint
Restraint to axial expansion of concrete members can have a significant influence on its behaviour in fire, positive or negative. Its effects may even be so strong, that it can overshadow other effects as steel cover, size and shape of the member, aggregate type, reinforcement type and load intensity (Fellinger and Breunese, 2004). In order for this phenomenon to occur, the surrounding structure must be rigid enough to provide the restraining forces. Therefore, the effect is the greatest
in for example a localised fire where only a part of a floor or a building is heated, leaving a considerable amount of the remaining structure at normal tempera- tures.
The effect of axial restraint is explained by an example. In Figure 4.1, a beam is shown located between two rigid supports that allow rotation but no elongation at the ends. The bottom of the beam is exposed to fire. As it heats up, it will try to expand, however this is not allowed by the supports. Thus, an axial thrust force T is developed. Its effect can be best compared to that of an external prestressing. Due to the eccentricity e between the line of action of the thermal thrust and the centroid of the compression block near the top of the beam, an additional moment T.e arises which may help carrying the external load. In fact, the flexural capac- ity of the beam becomes MR,total MR,fire T e where MR,fire is the moment capac-
ity at elevated temperature. It is even possible, provided that the surrounding structure is stiff enough, that the elevated temperature moment MR,fire, can drop
to zero without structural failure. However, the effect of axial restraint can also be negative. If the line of action of the axial thrust forces develops near the top of the beam, the eccentricity e will be negative, thus diminishing the total flexural capacity. Consequently, supports should be adequately designed and executed. Note that, due to this high dependency on the support detail, axial restraint may only be relied upon when explicitly designed for. However, this design can prove to be challenging in some cases, since the position of the axial thrust can vary as the deflection or rotation of the beam varies during the fire exposure. Further- more, note that axial restraint may also lead to additional failure modes that should be taken in account in the design, for example shear failure of columns and walls or buckling of the beam.
Figure 4.1: Beam between two rigid supports and the axial restraint thrust forces. Re- drawn from Buchanan (2002).