The effects of different strain contributions on the response of RC beams in fire

Abstract

A two-step formulation, consisting of separate thermal and mechanical analyses, is presented for the thermo-mechanical analysis of reinforced concrete planar frames subject to fire conditions. The heating and the cooling phases are considered. Standard planar, four-node quadrilater finite elements are employed in the non-linear time-dependent thermal analysis of cross-sections, while the recently proposed strainbased planar beam finite elements are used in the non-linear mechanical analysis of the frame [Bratina S, Saje M, Planinc I. On materially and geometrically non-linear analysis of reinforced concrete planar frames. International Journal of Solids and Structures 2004;41:7181-7207]. The formulation includes both exact geometric and material non-linearities, and considers the temperature dependence of thermal and material parameters, the plastic, creep and thermal strains in concrete and steel, the transient strain in concrete, and the strain localization as a consequence of softening of material at high temperatures. A so called 'constant strain element' is introduced to resolve numerically the loss of uniqueness of strain measures at the point of localization. The formulation is validated by comparing some of the present numerically predicted results with the data, measured in experiments. Although the present model is essentially 1D stress-strain model, and is thus simple in terms of the number of degrees of freedom used and ignores transfer of water in concrete during heating, the comparisons with the measured data are found to be satisfactory. In particular, the agreement of the fire resistance times and critical deflections between the predicted and the experimental values was found very satisfactory. In contrast, a disagreement was found in distributions of temperature over the beam cross-sections. The results make it possible to draw several conclusions concerning behaviour of structures in fire. In particular, it is established that the consideration of creep and transient strains in concrete has little effect on the fire resistance time of statically determinate beams under bending or unconstrained centrically loaded columns; their effect on displacements is, however, remarkable.

Effects of transient creep strain on post-tensioned concrete slabs in fire

“…As reported by Bratina et al (2005Bratina et al ( , 2007, the model was verified by excellent agreement with test data under various elevated temperatures.…”

Effect of Transient Creep Strain Model on the Behavior of Concrete Columns Subjected to Heating and Cooling

“…The mechanical law for the steel reinforcement was taken from Eurocode. It should be noted that no creep is explicitly considered in these relationships for steel reinforcement, which is an important assumption as the creep of the steel reinforcement is known to have an effect on stress and strain state in reinforced concrete frames when temperature in reinforcement bars exceeds 400°C [21,22], as is the case here. The axial restraint stiffness remains unchanged during the simulations, which can be justified by the fact that the maximum displacement at the center of the restraining beam is only about 13 mm [14].…”

Modelling the bond between concrete and reinforcing steel in a fire

“…In recent years a number of numerical models have been developed to represent the behaviour of reinforced concrete structures in fire [5][6][7][8][9][10], but none of these have taken the influence of the bond characteristic between concrete and reinforcing steel into account. In a number of previous research projects for modelling the bond characteristic between concrete and reinforcing steel at ambient temperature [11][12][13][14][15][16][17][18][19][20][21], two common types of models have been used.…”