ABSTRACT
Joints, also sometimes referred to as connections, play an important role in a structural framing since they transfer forces from one member to another. The performance of connections is highly important under extreme loading conditions since the critical regions of a steel structure are in plastic state and also due to the greater need for redistribution of forces from one critical region to another critical region. In most fire situations, it is likely that some parts of the structure in the non-fire exposed regions have much greater capacity than the fire exposed regions and thus the fire resistance of the structure is highly dependent on the extent of redistribution (through connections) that occurs from highly stressed regions to less stressed regions. Thus the performance of connections is crucial for the stability of structural systems in buildings when they are exposed to fire. This aspect, of the importance of connections, has been observed and documented (FEMA 2002) in the damaged WTC buildings around Ground Zero as a result of fires initiated after the collapse of twin-towers. In modern steel-framed buildings, connections between various members may be
either of bolted or welded construction, or a combination of these types. Most codes and standards require steel connections to be provided with some level of fire protection. However, many codes do not explicitly state fire resistance requirements for connections. Further, in establishing fire ratings through prescriptive approaches, connections are generally not included as part of the assembly tested in traditional fire-resistance tests. Furthermore, most modelling efforts assume that the pre-fire characteristics of a connection are preserved during the fire exposure. US codes generally give little guidance on the fire design of connections. No explicit
provisions are specified in US codes and standards. A closer look at the overall fire resistance provisions clearly imply that the connections should be protected to the same level of fire resistance as that of the connecting members. In Europe, it was also commonly assumed since the 70’s that there is no need to take
special provisions for the connections as long as they are protected at least in the same manner as the adjacent members that they connect (ECCS, 1983). This implies that, if none of the connected members is protected, then there is no need to protect the joint. This concept was based on the idea that the thermal massivity of the joint should be higher than the massivity of the members because of the presence of additional mass in the connection zone, either from end plates, fin plates, web cleats or stiffeners. It was also based on the observation of numerous unprotected steel structures that completely
collapsed in severe fire, and where the steel beams were severely distorted, but rarely detached from the columns. The perception has evolved during the last decade because of the appearance of the conceptual design of multi-storey buildings where the columns are protected while the beams and the joints are not. The demand on the connections in such systems is of course much higher. This is especially the case when axial restraint induces axial forces in the beams and thus in the connections. Compression forces first develop in the beams due to restraint against thermal expansion and tension forces may develop in a later stage when significant vertical deflections in the beams transform the beams from elements in bending to elements in tension, more like cables. More tension can even develop in the beams, and thus in the joints, in the cooling phase of a natural fire. It has also been observed that not only the resistance to the varying forces, but also the ductility of connections must be very important in order to accommodate the large rotations linked to the large displacements that develop when the beams act in a catenary mode. A good review of recent research performed on the behaviour of joints in the fire situation may be found in Al-Jabri et al (2008). However, in Eurocode 3, design rules for both bolted and welded connections in the
fire situation are only specified through the introduction of strength reduction factors. Moreover, simplified temperature distribution of joints in the fire situation is also proposed, which may be used in strength analysis. This approach, though only primitive in nature, is rational and one step ahead of the “connection provisions’’ in other Codes of practice. This Chapter is devoted to highlighting the Eurocode methodology for the design of connections.
