Premature destruction of two-span RC beams exposed to high temperature caused by a redistribution of shear forces

Abstract

When multi-span RC elements are exposed to fire one usually observes a yielding of span cross-sections while a safety reserve of support cross-sections is still significant. Due to this phenomenon a redistribution of bending moments occurs and the values of sagging moment in span cross-sections decrease while the values of hogging moment in support cross-sections increase. This paper shows the results of tests conducted on two-span RC beams in a situation when only one span has been exposed to high temperature from the bottom. The beams were 12×16 cm in their cross-section. The length of the span was 165 cm. The load has been applied by two forces put on each span. The beams were made of C25/30 concrete with siliceous aggregate. As a result of significant stiffness decrease of the heated span, redistribution of shear forces and bending moment occurs. Due to this redistribution the tested beams were prematurely damaged due to exhaust of the shear load bearing capacity in the middle part of the beam span where there was no transverse reinforcement.

Post-Fire Characteristics of Concrete Beams Reinforced with Hybrid FRP Bars

“…Obtained results show that deflections obtained from current investigation for beams reinforced with FRP bars were much smaller than the results received for similar beams reinforced with steel bars in other studies [13,32]. In addition, the behavior of FRP-RC beams was quite uncharacteristic when subjected to elevated temperature, as well as during the cooling phase.…”

“…In addition, it can also be seen that the maximum temperatures measured at the bottom edge were approximately 20% lower (at the last point) than the applied temperatures. The sample beam B2Ø14 from Set 1 showed a time-vertical deflection relationship more similar to a typical one for steel reinforced concrete beams [13,32]; the deflections were only increasing. Despite the larger initial deflections for beam samples B2Ø14, H2Ø14 and N2Ø14, the increase of deflections during fire exposure started to be twice lower than in the case of analogous beams with steel reinforcement.…”

“…As known from experiments on beams reinforced with steel reinforcement, the concrete clear cover, and the fire scenario have the most significant influence on the response of the concrete element subjected to fire [32]. Nevertheless, in the design of FRP-RC structures, the reinforcement type and obtaining of a specific high-temperature resistance for FRP bars is one of the main concerns.…”

Thermal Bowing of Reinforced Concrete Elements Exposed to Non-Uniform Heating

“…multi-span slabs or beams and columns or walls, can be exposed to fire from the side of either the tension or compression zone [1,2]. The redistribution of internal forces caused by significant changes in the cross-sections' stiffness with a heated tension or compression zone plays a major role in the analysis of complex RC structures exposed to fire [3,4]. The effect of the redistribution of internal forces is usually beneficial for multi-span RC elements heated on the bottom side.…”

Experimental approach to strength reduction and elongation of self‐tempered reinforcing bars tensioned at a steady and an increasing temperature

“…However, in a case of advanced analysis of complex statically indeterminate RC structures exposed to fire, it is crucial to adequately predict the redistribution of internal forces occurring due to the influence of high temperature . Therefore, it is necessary to model the change in stiffness of various parts (sections) of the structure.…”