Fire resistance of axially restrained Q690 H-shaped welded steel columns: Test, simulation and design

Highlights

• Fire tests on axially restrained Q690 H-shaped welded HSS columns were carried out.

• Numerical models based on ABAQUS were developed and validated by the tests.

• Parametric analyses on load ratios, axial restraint, and slenderness were conducted.

• A simplified design method for the critical temperature of axially restrained Q690 HSS columns was proposed.

Abstract

Six fire resistance tests of axially restrained Q690 high strength steel (HSS) columns were conducted in this paper to obtain its fire response and failure pattern. Slenderness, axially restraining stiffness, and load ratios were considered in the tests. The furnace temperature, specimen temperature, specimen deformation, and internal force were recorded during the tests. Furthermore, the critical temperature, fire resistance, failure pattern, and post-buckling behavior were determined based on the test results. Test results showed that increasing the load ratio, axial restraining stiffness ratio, and slenderness would reduce the critical temperature of the specimens. After that, numerical models incorporating residual stress and non-uniform temperature field were developed and calibrated with the test results. Finally, parametric analyses were performed, and a simplified design method was proposed to evaluate the critical temperature of restrained Q690 HSS columns.

Introduction

High strength steel (HSS) usually refers to the steel with yield strength higher than 460 MPa, thus realizing high strength by adopting special manufacturing technology and adding alloy elements. The application of HSS can reduce the structural weight and improve the structural seismic performance due to the nature of high yield strength. Fire resistance research is essential for the steel structure because steel was demonstrated to have sharp reductions in strength and stiffness with increasing temperatures. Concerning a local fire condition, columns in a frame are typically axially restrained by adjacent members that impose a thermal restraint on the columns and introduce additional force. Thus, the fire behavior (structural performance under fire condition) of the restrained column is entirely different from that of the unrestrained column.

Numerous investigations have been conducted to study the fire performance of axially restrained steel columns. Li et al. [[1], [2], [3]] employed experimental and numerical analyses to investigate the fire behavior of restrained mild steel columns and revealed the significant influence of the axial restraining stiffness and load ratio on its fire performance. Moreover, Li et al. [[1], [2], [3]] identified the presence of the post-buckling behavior in the restrained column, which should be considered in the fire-safety design. Correia and Rodrigues [4] indicated that increasing the axial restraining stiffness might not reduce the critical temperature of the column. Afterward, Correia and Rodrigues [5] conducted a series of parametric analyses and proposed a simplified design approach for the fire safety of restrained columns. Moreover, Ali et al. [6] and Neves et al. [7] also developed simplified methods to evaluate the critical temperature of the axially restrained column.

However, the aforementioned works mainly focused on mild steel columns, whereas research on the HSS columns was limited. The mechanical properties of HSS at elevated temperatures are generally quite different from mild steel due to the manufacturing and alloy composition. Thus, the current design standards and research, which adopted the mechanical properties of mild steel for the fire safety design, cannot be directly used for HSS members [8]. The lack of usable design information, which allows designers to harness the advantages of HSS, introduced barriers to the widespread use of HSS in construction. Wang et al. [9] conducted experimental research on the fire behavior of restrained Q460 HSS columns and indicated that it had better fire resistance than mild steel columns. Winful et al. [10] used numerical simulation to investigate the fire behavior of S690QL and S700MC HSS columns and found that the EN 1993-1-2 [11] approach can be unconservative (i.e., unsafe) and conservative (i.e., safe), respectively. Chen and Young [12] developed the numerical models to investigate the structural behavior and design of HSS columns with increasing temperatures. They found that the EN 1993-1-2 [11] and the direct strength method [13] conservatively predicted the HSS column strengths at elevated temperatures.

Q690 HSS, which has a nominal yield strength of 690 MPa, is one of the widely used HSS in China. Thus, numerous studies were conducted to investigate the mechanical behavior of Q690 HSS members at ambient temperature to promote its application. In recent years, researchers began to provide additional attention to the fire behavior of Q690 HSS but mainly focused on its material properties, including mechanical properties at elevated temperatures [14,15], post-fire mechanical properties [[16], [17], [18], [19], [20]], and high-temperature creep [14,21]. Research conducted for the fire behavior of Q690 HSS members remained limited. J. H. Luo et al. [22] investigated the mechanical behavior of Q690 HSS beams at room and high temperatures using experimental and numerical analyses. Their results indicated a sharp reduction in the load-bearing capacity of the beams in a fire with an increase in temperature. Moreover, elevated temperatures could influence the failure mode of Q690 HSS beams. Wang et al. [23] conducted fire resistance tests and numerical analyses on Q690 HSS unrestrained columns and revealed that the critical temperature of these columns as predicted by Chinese specification GB 51249–2017 [24] and EN 1993-1-2 [11] was unconservative. Wang et al. [23] also proposed a simplified design method to evaluate the critical temperature of unrestrained Q690 HSS columns. Therefore, additional research was required on the fire behavior of the restrained Q690 HSS columns.

The fire behavior of axially restrained Q690 HSS columns was experimentally studied in this paper. An elaborate numerical model was developed to reproduce the tests. Finally, a simplified design method was proposed based on parametric analyses to evaluate the critical temperature of the axially restrained Q690 HSS columns.