Experimental investigation of post-fire residual stresses in Q690 welded I-sections

In this paper, the pre- and post-fire residual stresses of twenty-three welded I-shaped sections fabricated from Q690 high strength steel plates with a nominal yield strength of 690 MPa obtained experimentally are presented and discussed. The two main variables considered in the present study are the level of temperature exposure and the cooling methods used to cool the test specimens. The temperature used for the tests ranges from 300 °C to 900 °C, and two cooling methods – natural air and submersion in water – are used. In addition, the effects of different heating rates and different flange width-to-thickness ratios are also investigated. The test results show that while the level of exposed temperature (when it exceeds 300 °C) has a noticeable influence on residual stresses, the effect of flange width-to-thickness ratio is not important. Furthermore, it is found that heating rate has a very minor effect on residual stresses. However, for specimens heated to a temperature at or above 700 °C and suddenly cooled by water quenching, noticeable residual stresses are generated on the edges of the flanges and at the web-flange junctions. Using the measured data, two residual stress distribution models which take into consideration the levels of temperature exposure and types of cooling methods used are developed and proposed to represent the magnitudes and distributions of residual stresses for Q690 welded I-shaped sections before and after fire exposure. These residual stress distribution models are shown to give reasonably good results when compared with the experimentally measured data.