Time-Dependent and Post-Elevated Temperature Performance of Welded Steel Connections Subjected to Fire

Abstract

The objective of this research work is to investigate the time-dependent and the post-elevated temperature behavior of welded lap joints under elevated temperatures. Weld is a metal joining material that is commonly used in steel connections and can affect their governing failure modes during fire and post-fire events. Despite their importance, very limited studies are conducted to examine the time-dependent or the creep behavior of welds and welded connections subjected to elevated temperatures. To address this issue, an experimental program is first conducted to examine the implicit and explicit time-dependent thermal creep behavior of weld material in transverse welded lap joints. In order to investigate implicitly the creep behavior of transverse welded lap joints in fire, two different loading rate scenarios (fast and slow) are used. Peak loads and retention factors for the weld material under both fast and slow loading rates are computed for all temperatures. However, for the explicit time-dependent behavior, the transverse welded lap joints are subjected to a constant load at a specific temperature for 120 min or until failure. Critical times, loads, and temperatures at which weld material fails due to creep are examined. Then, the creep curves resulted from the experimental tests are used to develop a Norton-Bailey creep power law equation for the welded lap joints. Then, two creep models for the welds and steel base material are proposed by introducing temperature-dependent scaling factors to the creep power law equation of the welded lap joints. Further, the two creep models are then used in the material properties of the welded lap joint in ABAQUS and then calibrated in order to predict with reasonable accuracy the experimental results. Also, another experimental program is conducted to investigate the effect of load angles on the thermal behavior of welds in welded lap joints while considering different loading rates. The lap joints used in this analysis are classified as longitudinal, inclined, and transverse where the angle between the axis of the fillet weld and the direction of the applied load is 0°, 45°, and 90°, respectively. Retention factors for the weld material under different loading rate scenarios and load angles are calculated and compared with those available in the literature. Finally, the post-fire behavior of welded specimens subjected to different load angles is also studied experimentally. More specifically, in the post-fire analysis, the test specimen is heated up to a target temperature and then cooled back to ambient and then loaded until failure. The results of this analysis are presented as residual strength capacities for the weld material after being exposed to elevated temperatures. Effect of load angles on the ductility of weld material under time-dependent thermal conditions and post-fire analysis are also studied and presented. Design equations that consider the rate-dependent and post-fire effects on fillet welds subjected to different loading angles are also provided. This research work will help providing deep understanding into the influence of different major parameters that affect the behavior of weld material in fire and post-fire conditions. Consequently, considering these parameters can provide essential data that will be intended to support the development of design guidelines of welded connections for structural-fire engineering application.