American Concrete Institute Structural Journal, May, 2019

Effect of Sustained Service Loading on Post-Fire Flexural Response of Reinforced Concrete (RC) T-Beams

 by Chanachai Thongchom, Akhrawat Lenwari, and Riyad S. Aboutaha


Newswise — This research highlights the effect of sustained service load at elevated temperatures on the residual flexural response of reinforced concrete (RC) T-beams after being exposed to 1292°F (700°C) or 1652°F (900°C) for 3 hours, and then air cooled. After fire, the structural performance of RC members can significantly deteriorate due to the unrecoverable loss in mechanical properties of concrete, steel reinforcements, and bonding between steel bars and surrounding concrete. Typically, RC members are subjected to some level of service load beyond the cracking load during fire incidents.

The objectives of this research include:

1) Investigating the effects of sustained service load on thermal and creep deflection responses of simply-supported RC T-beams during fire exposure

2) Investigating the effects of sustained service load on the flexural response of fire-damaged RC beams

3) Proposing a simplified analytical model for predicting the flexural response of fire-damaged RC beams

In the experiments, two beams were simultaneously exposed to fire in an unrestrained condition. One beam was subjected to a constant simulated service load, while the counterpart beam was exposed to fire without any applied sustained load (only self-weight). The level of simulated service load 7.1 kips (31.6 kN) was 22.6 percent of the flexural strength 31.5 kips (140.0 kN) and higher than the unheated cracking load 6.1 kips (27.0 kN) of a control (unheated) beam. At this load level, the measured tensile stresses at bottom steel reinforcements in the control beam were 13 percent of yield strength of steel. Thermocouples were installed inside the RC beams to record the temperature histories at concrete and steel reinforcements during fire exposure. Linear variable differential transducers (LVDTs) were also installed to measure the creep deflections. After being heated for 3 hours to 1292°F (700°C)  or (1652°F) 900°C, and air cooled in the furnace, the fire-exposed beams were subsequently tested under static four-point bending to failure.

Based on the test results, the sustained loading promotes a creep deflection at the elevated temperatures. The rate of creep deflection of RC beams subjected to the sustained loading at elevated temperatures increases once the steel temperature exceeds 932° (500°C).

The post-fire static test results show that the sustained service loading causes a detrimental effect on the post-fire flexural response of RC beams (Fig. 1). It decreases the stiffness, strength, and ductility of RC beams after fire exposure. The effect on the stiffness and ductility is more pronounced than strength (Fig. 2).

This research also describes the simplified finite element models that can be used for predicting the temperature histories in fire-exposed RC beams and load-deflection relationships of fire-damaged RC beams.

The research can be found in a paper titled “Effect of Sustained Service Loading on Post-Fire Flexural Response of RC T-Beams,” published by ACI Structural Journal.



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