Effect of fire on the reinforced concrete members under loaded conditions

Abstract

In this modern era of rapid development and technological advancement, frequent fire phenomenon are the most dangerous man made catastrophe in the metropolis. Dhaka, the center of development of Bangladesh, is marked by its rapid development of commercial buildings and high rise buildings, which are mostly unplanned in respect of urban planning and prone to fire accidents. Since reinforced concrete is the main building material used in most of the cases in this mega city, a detailed study on the reinforced concrete member is needed to mitigate the scale of destruction of fire in case of accident. With this point kept in mind, a parametric study was performed to understand the effect of fire on the reinforced concrete members, especially on beam as it is the primal load bearing component of any structure in general. In the unidirectional coupled field analyses, i.e. thermal to structural coupling, in ANSYS, concrete compressive strength (28MPa, 35MPa and 42MPa), steel yield strength (400MPa and 500MPa), beam cover (37.5mm, 50mm, 62.5mm and 75mm) and reinforcement ratio (minimum, intermediate and maximum value) were combined in analytical modelling with different temperature (300C, 600C and 900C) and duration of burning (30minutes, 60minutes, 90minutes and 120minutes) to observe the developed stress levels inside the beam. To imitate real life scenario, the temperature effect was observed in loaded condition during the simulation. The constitutive laws of concrete and reinforcing bars were for the loading stage only, no information on unloading stage was input. The results obtained show that the stresses in concrete and reinforcement increases with the increase in temperature and time, though the maximum load the member can withstand is gradually decreasing. The results of the simulations suggest the safe use of reinforcing steel for any temperature for initial 30 minutes of burning. On the contrary, the preparation and selection of concrete strength must be carefully chosen since after the first 60 minutes of burning the concrete was found stressed drastically close to its plastic limit. The result also indicates, if the member is loaded at 20 percent of its maximum capacity, it can withstand temperature as high as 900C for a duration as long as two hours.