Experimental study on thermo-mechanical behaviour of beam string structures

Abstract

Beam string structures (BSS) have been widely employed in large-span spatial structures, while the experimental study on BSS in fire conditions is limited. This paper presents experimental investigation on thermo-mechanical behaviour of BSS with different load ratio. ISO834 standard fire and large space fire are adopted as fire conditions, respectively. The strain of BSS at high temperatures is measured by advanced strain gauges. The test results show that the prestressed string is the most vulnerable component of BSS in fire. The temperature distribution is non-uniform, and the temperature difference between the top flag and string can be up to 396.3 °C. The critical string temperature is in a range of approximate 350 °C–450 °C. Load ratio has little effect on the critical string temperature, and fire conditions with higher heating rate can increase the critical string temperature. Furthermore, the drop of residual prestress causes the rapid increase of mid-span vertical displacement and horizontal displacement. The direction of horizontal displacement of BSS without external load is outward, due to the thermal expansion. The strain distribution along the cross section indicates the additional bending moment induced by temperature. In addition, three behaviour modes of BSS at elevated temperatures are proposed according to the contribution of prestress. The behaviour modes illustrate the working mechanism of BSS as the temperature increases. Finally, a thermo-mechanical finite element model of BSS in fire conditions is established by assembly load method. The numerically predicted temperature distribution, displacement and residual prestress have a good agreement with the experimental results.