۱- Introduction

۲- Prototype (code-designed) RC building

۳- Benchmark for computational modeling of RC structures under fire with heating and cooling phases

۴- Behavior of the prototype building under fire

۵- Computational model for a column part of a structure

۶- Parametric study of the residual deformations of RC columns

۷- Conclusion

References

Abstract

Reinforced concrete (RC) structures often remain stable under fire, but exhibit damage and residual deformations which require repairs. While repair operations and building downtime are expensive, current fire design approaches do not consider post-event resilience. The first step to enable predicting the resilience of RC structures under fire is to develop capabilities to model the damage of these structures after various fire exposures. This paper focuses on the prediction of the residual (post-fire) deformations of RC columns within a code-designed five-story RC frame building. Computational modeling approaches to capture the fire behavior of the columns are investigated. The models range from isolated columns with linear springs at the boundaries to full building model coupling beam and shell elements, with intermediate approaches. The analyses highlight the critical nonlinear role of the thermal expansion-contraction of the surrounding beams and slabs on the column deformations. Large transversal residual deformations develop particularly in perimeter columns, combined with residual shortening. This invalidates models based on isolated column or 2D frame. A parametric study of the residual deformations of RC columns is then conducted, with due consideration of the 3D restraints and interactions, to investigate the effects of different design parameters and fire scenarios on the residual deformations after a fire event. The results of the parametric study indicate that fire load density and opening factor significantly influence the residual deformations of RC columns, compared to the thermal conductivity of concrete and live loads. This research improves the understanding and provides recommendations for numerical modeling of the effect of fire on the residual capacity and deformations in RC structures.

Introduction

Reinforced concrete (RC) buildings generally exhibit a good structural performance under accidental fire events, as seen in, for instance, the 2005 Windsor Tower fire or the 2017 Grenfell Tower fire. In the two latter events, no global structural collapse of the concrete structure occurred, despite fires raging for hours. Yet, while fire does often not result in global collapse of RC structures, the potential loss due to downtime and repairs may be significant. In many instances, the fire accident is not as severe as in the two aforementioned cases, and a rehabilitation is possible [1,2]. The question of post-fire damage and downtime cost has gained increasing attention due to the requirements for resilience of structures under hazards. To develop optimum provisions to design fire resilient structures, engineers need the ability to accurately estimate the potential economic loss for different design alternatives due to fire damage. This in turn requires the ability to predict the behavior of RC structures under fire, including the residual deformations and residual load-bearing capacity of a structure after fire. Structural members are often tested as independent elements under fire, disregarding global behavior. However, the proper inclusion of structural continuity of fire induced effects is crucial for an accurate evaluation of RC building’s response. The heating of structural members leads to thermal expansion, which may cause the surrounding structure to impose high restraint forces. The significant impact of boundary conditions on the fire behavior of RC structures under fire has been observed in many historical accidents and previous research. In the Katrantzos Department Store fire (an eight-story RC building) in Greece in 1980, the restraints from differential thermal expansion in the structure led to the collapse of a major part of the 5th to 8th floors and the failures of various other floors and columns throughout the building [3]. Extensive research works have demonstrated the considerable effect of restraints on the behavior of RC structural members under fire, based on numerical or experimental studies of RC columns, beams, slabs and walls [4–۱۱].