RC beams with rectangular openings in case of fire

Vigas de concreto armado com aberturas retangulares em situação de incêndio

Abstract

Openings in reinforced concrete (RC) beams may be required due to building installations (electrical, water, etc.). They weaken its cross-sectional area and, in case of fire, can increase the thermal field of the structure. NBR 15200 does not consider this. The paper evaluated the influence of rectangular openings on the Fire Resistance Rating (FRR) of RC beams by the Simplified Method of NBR 15200. This method combines a non-linear thermal analysis (isotherms) with the evaluation of the flexural strength of the beams using a manual design calculation method. The thermal field were obtained by a thermal model solved by finite element analysis (FEA) with Abaqus software. Thermal parameters of NBR 15200 were used. This FRR was compared with equivalent beams without openings solved by the Tabular Method of NBR 15200. Twelve 60 cm high beams with different widths and dimensions of openings were studied. Beams with openings had a FRR up to 60 min lower than the equivalent beam without openings. The larger the size of the rectangular opening, the greater the mechanical degradation of the beam in fire. NBR 15200 Tabular Method proved to be unsafe in this case. Prescriptions for beams with openings must be shown in NBR 15200.

Keywords:
reinforced concrete; beams; fire; openings; NBR 15200