Design of vertical members in large-span steel structures: Can relevance of seismic actions be exceeded on account of indirect fire actions?

Abstract

Effect of lateral actions evolving from restrained thermal expansions of horizontal structural elements (beams, slabs) on stability of vertical elements (columns, walls) of fire-engulfed buildings remains unaddressed sometimes in engineering practice. Discussion with engineers has revealed that two reasons may contribute most to such design inconsistency. The first is premature assumptions that the effects of these actions can not be sufficient to exceed the effects of other lateral design actions, especially if for the same building considerable seismic influences are also expected. The second reason is the fact that, according to the present version of Eurocodes, these actions need not be addressed specifically if the design is carried out under standard fire curve. This curve, however, is still applied often in practice. For certain vertical members, such as the ones positioned somewhere within the middle of fire compartments, indirect lateral fire actions may indeed be negligible, e.g., due to partial or complete balancation of these actions in fire evolving simultaneously at two opposite sides. However, more pronounced effect might be observed for vertical members at the boundary between a fire-engulfed and a neighbouring cold-side compartment, especially in steel or RC buildings with larger horizontal spans prone to striking lateral thermal elongations. While such structures represent a fair share of existing building stock (they are common, e.g., in prefabricated industrial and commercial buildings), this problem deserves a special attention. In this paper two real one-storey large-span halls are analysed for confirmation of the above hypothesis. Each of them comprises two structurally connected units, i.e. one RC- and the other steel-framed, where fire is assumed to engulf the steel-framed unit. Stability of the ’cold-side’ RC columns at the boundary between both sectors is discussed and for the latter indirect fire actions from the connecting fire-engulfed steel members are found to be of a comparable or a greater size to effect of moderate-size seismic actions. The greatest effects of the observed fire actions are found at average steel temperatures below 400 °C. This is a rather low temperature expected to be achieved (sooner or later) in most fire-affected steel elements with or without fire protection. At the same time, as in this paper also proven by results of exact visco-plastic structural computations, this is also a temperature at which no pronounced creep strains yet evolve in steel within time frames relevant for common structural fire analyses. In addition, the paper also shows that these conclusions would not change for other variations of the discussed two buildings where other possible kinematic boundary conditions were applied at the ends of the steel members or other possible thermal boundary conditions were assumed along their longitudinal surfaces. At most, the observed indirect fire effects would only increase further in these cases. Thus, at least for structures of a similar type (i.e. one-storey frame structures of horizontal spans of similar size and materials), the authors propose that the discussed lateral indirect fire actions are not dismissed from any design procedure regardless of the expected fire scenario as long as steel is allowed to heat to moderate temperatures (around 400 °C). In the last part of the paper the authors also discuss possible change of these findings for multi-storey frame structures and/or frame structures of other spans. However, for these (and other possible) structural types, a more detailed further investigation is proposed.