Structural Analysis and Design

There are many factors affecting structural behavior in fire, such as material degradation at elevated temperatures, restrained thermal expansion, thermal bowing, and the degree of redundancy available when the structure acts as a whole. Each factor is addressed separately, but in an integrated structure exposed to fire, they all will interact to generate more complex structural behavior. Traditionally, steel fire design has been based on fire-resistance testing, although fire resistance by calculation also has been implemented for many years.

The Eurocodes are a collection of the most recent methodologies for structural design, such as Eurocode 3 (EC3): Design of Steel Structures, Part 1.2. Structural Fire Design, and Eurocode 4 (EC4): Design of Steel and Composite Structures, Part 1.2. Each Eurocode is supplemented by a national application document (NAD) appropriate to the country. All Eurocodes are presented in a limitstate format, where partial safety factors are used to modify loads and material strengths. EC3 and EC4 are very similar to BS 5950, Part 8, although some of the terminology differs. EC3 and EC4, Part 1.2, and BS 5950, Part 8, are concerned only with calculating the fire resistance of steel or composite sections. Three levels of calculation are described in EC3 and EC4. tabular methods, simple calculation models, and advanced calculation models.

Tabular methods are look-up tables for direct design based on parameters such as loading, geometry, and reinforcement. They relate to most common designs. Simple calculations are based on principles such as plastic analysis, taking into account reduction in material strength with temperature. These are more accurate than the tabular methods. Advanced calculation methods relate to computer analyses and are not used in general design.

The structural engineering community needs structural fire load (SFL) information that is understandable by a reasonably intelligent professional. The main objective of this chapter is to obtain the approximate analytical solution of a structural system subjected to SFL, but in such a simple form that it can be used in ordinary structural engineering practice. The methodology of finding the solution in this case is similar to that for any other environmental structural design load (e.g., wind load, seismic load, etc.).

The main idea here is based on substituting a very complex analysis of corresponding dynamic structural system with an equivalent system with one degree of freedom (ODOF) that has a simple form and is easy to use. In the case of wind load, for example, most international codes and standards use the so-called gustloading factor approach for assessing the dynamic along wind loads and their effects on the structure. Variations of these models have been adopted by major international codes and standards. Although a similar theoretical basis is used in the SFL case by introducing a dynamic coefficient, there is a considerable difference in application of such a method based on fire severity classification.

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Text by Leo Razdolsky, LR Structural Engineering Inc., Lincolnshire, Illinois, USA,
Professor at Northwestern University, Evanston, Illinois, USA