טכניון מכון טכנולוגי לישראל
הטכניון מכון טכנולוגי לישראל - בית הספר ללימודי מוסמכים  
M.Sc Thesis
M.Sc StudentShachar Yedidya Moshe
SubjectInfluence of Exposure to Fire on The Residual Structural
Properties of Reinforced Concrete Beams
DepartmentDepartment of Civil and Environmental Engineering
Supervisor Professor Avraham Dancygier
Full Thesis text - in Hebrew Full thesis text - Hebrew Version


Abstract

Fire events can cause extensive damage to reinforced concrete (RC) structural elements, even up to their collapse. Yet, RC beams and slabs can also withstand considerable elevated temperatures and depending on the structure, the fire characteristics and the actual loads that act during the fire, remain stable after the fire has been put out. There are studies of the residual load bearing capacity of RC structural elements that survived exposure to high temperatures. Yet, other structural properties, such as flexural ductility, were not investigated.

This thesis describes an analytical study of the residual, post-fire structural properties of one-way slabs and of beams with rectangular cross-sections after being exposed to fire. The work includes calculation of the cross-sectional moment-curvature relation, load-deflection response of a simply-supported beam (or slab) and based on these calculations - evaluation of their residual moment capacity and flexural ductility. These calculations were done for various fire scenarios, above, below and at both sides of the analyzed structural member. The effect of the fire was evaluated by known models of the temperature rise according to the fire duration and then - by thermal analysis of the temperature distribution across the section, for the given fire durations and scenarios. Published models of the residual, temperature-dependent stress-strain curves for concrete and steel were then employed to perform numerically the moment-curvature analysis.

The research examined the influence of different parameters on the residual moment bearing capacity and the flexural ductility. These parameters included the concrete and steel strengths, the steel type, the cross-section dimensions and concrete cover.

It was found that the ratio of the tension reinforcing steel has a significant influence on the normalized residual moment capacity and on the normalized residual ductility of the members, which cool off from a fire event, compared with their original properties. The results show that higher reinforcement ratios cause more reduction of the residual properties. Similarly, the effective depth has also substantial influence on the residual properties, where lower depths lead to reduced residual moment and deformation capacities.

The results that were obtained in this research show that there are cases of beams whose post-fire condition may be stable, but their pre-fire flexural ductility is totally lost due to being exposed to elevated temperatures.

Therefore, these results show that consideration of the moment capacity may not be sufficient for a true evaluation of the residual structural condition. It is shown that under moderate to extreme fire events, a structure that has survived the fire, may have not only reduced capacity but also reduced ductility. Thus, the importance of considering the residual ductility in addition to the load bearing capacity has been demonstrated herein.