A novel active fire protection approach for structural steel members using NiTi shape memory alloy

The aim of designing a steel structure against fire is to ensure that the integrity of the structure can be maintained for a sufficient time period without collapse in order to allow more time for evacuation processes. This time period is called the fire resistance of the structure. Conventionally, the fire resistance of a structure is achieved by keeping the temperature of the structure below a certain temperature above which the structure experiences structural instability. Various insulation materials are used to passively protect the structure from high temperatures to avoid structural failure that may result in a tragic catastrophe. A novel mechanical approach, based on integrating a shape memory alloy, NiTi, with a steel structure is proposed to increase the fire resistance of structural steel members. To demonstrate the principles of this approach, a simple structure in the form of a simply supported steel beam was used. The internal action of the beam due to a transverse applied load was reduced by utilising the shape memory effect in the NiTi alloy with rising temperature. As a result, the net internal action from the applied design load was kept below the deteriorated load capacity of the beam for a longer period during fire, even though the steel temperature exceeded the original critical temperature which may have caused the original beam structure to collapse. Prior to testing the composite NiTi-steel beams at high temperatures, the two received NiTi alloys with different initial conditions were characterised at room and elevated temperatures. The NiTi alloy with the appropriate initial conditions was selected based on the optimum transformation temperatures and maximum recovery stresses generated from it. Further thermo-mechanical characterisation at very high temperatures was carried out on the selected NiTi alloy. The convective coefficient and the surface emissivity of the carbon steel and NiTi alloy were investigated using experimental and analytical approaches. The results obtained from this approach showed that the steel emissivity was a function of its temperature, whereas it was not for the NiTi alloy. The temperature dependency of the steel emissivity was due to the formation of a rough oxide layer on the steel surface exposed to fire. The measured steel emissivity was adopted in a heat transfer model, which accounted for the variable steel emissivity and the view factor as well as the participating medium effects, to predict the temperatures of I-shape steel members. The predicted temperatures obtained from using the proposed heat transfer model showed excellent correlations with that obtained from the research literature. A new technique to consider the temperature dependency of the steel emissivity in the heat exchange by radiation in ABAQUS software was proposed to accurately predict the temperature of I-shape members. The results show that adopting variable steel emissivity results in accurate temperature prediction compared with that when constant steel emissivity is used. A finite element model simulating the fire behaviour of the composite steel-NiTi beams was developed using ABAQUS software. The model was validated against the measured fire endurance, mid-span deflection and steel critical temperatures and a good correlation was achieved. The validated model was further extended to numerically investigate the fire response of a selected full-scale case study. A parametric study was also conducted to demonstrate the effects of a number of parameters on the fire resistance of the full-scale composite steel-NiTi beams. Simple and fixed-end boundary conditions were taken into consideration in the parametric study. It was shown that the fire endurance and the critical temperature of the lower flange in the I-beam are significantly improved for beams with simple end conditions when integrating the SMA alloy to the lower flange from the bottom. In contrast, modest improvements in the fire endurance of the beam and the critical temperature of the lower flange are achieved in beams with fixed end conditions. Connecting the SMA alloy at a distance equivalent to span/6 from the end supports results in an optimum fire endurance and critical temperature of the lower flange in the beams with simple and fixed-end conditions.