Experimental study on post-fire mechanical properties of high strength Q460 steel

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Highlights

  • Tensile tests for high strength Q460 steel were undertaken after cooling down from temperatures up to 900 °C.

  • Post-fire mechanical properties of high strength Q460 steel were reported.

  • Predictive equations were given for evaluating performance of high strength Q460 steel after fire exposure.

  • Reference for safe evaluating and reuse of members made of high strength Q460 steel after fire was provided.

Abstract

In order to evaluate the residual strength of high strength Q460 steel structures after fire, an experimental study was undertaken to investigate the post-fire mechanical properties of high strength Q460 steel. Tensile coupons taken from high strength Q460 steel sheets were exposed to different temperatures up to 900 °C, and were then allowed to cool down to ambient temperature before they were tested to failure. Two cooling methods were considered, namely; natural air cooling and cooling by water. Tensile coupon tests were conducted to obtain their post-fire stress–strain curves and associated mechanical properties (yield strength, elastic modulus, ultimate strength and elongation). It was found that the post-fire mechanical properties of high strength Q460 steel are significantly reduced below the original ambient temperature mechanical properties if they had been exposed to temperatures exceeding 600 °C. Hence a new set of equations is proposed to predict the post-fire mechanical properties of high strength Q460 steel. Such post-fire mechanical property assessments allows structural and fire engineers to make an accurate prediction of the safety of fire exposed high strength Q460 steel buildings.

Introduction

High strength Q460 steel members are commonly used as load bearing members in large span structures and high-rise buildings. For example, China National Stadium (called “bird's nest”), in which the opening ceremony of 2008 Olympic Games was held, was built with 400 t of high strength Q460 steel. Inevitably, the buildings made of high strength Q460 steel can be exposed to fire events. Recent researches [1], [2], [3] have provided a good understanding of the mechanical properties of high strength Q460 steel at elevated temperatures and fire resistance of high strength Q460 steel members. Upon cooling from elevated temperatures, the structural engineer then has to decide if the residual strength of the load carrying members were still adequate for future use. As to now, the behavior of high strength Q460 steel members after a fire event has not been investigated yet. There are also no design guidelines in design codes [4], [5], [6] for assessing fire exposed high strength Q460 steel members.

As a result of this limited knowledge on the post-fire behavior of high strength Q460 steel members, over conservative decisions are likely to be made in relation to the residual capacities of high strength Q460 steel members after fire events. Improved knowledge of these capacities would help engineers make the right decisions. After a fire event, the exposure to extreme temperature variations could have reduced the section and member load bearing capacities of steel members. The main reason for this is the reduction in post-fire mechanical properties (yield strength, elastic modulus, ultimate strength and elongation) of steels.

This paper investigates the residual mechanical properties of high strength Q460 steel after being exposed to elevated temperatures, and proposes new equations to predict them. Information gained from this research on post-fire mechanical properties will assist engineers in assessing the axial and bending capacities of fire exposed high strength Q460 steel members while also enabling further development of the high strength Q460 steel design standards with regard to post-fire high strength Q460 steel member assessments.

Section snippets

Previous studies on post-fire mechanical properties of steels

There have been limited studies on the post-fire mechanical properties of steels. These studies mainly focused on evaluating the post-fire mechanical properties of cold-formed steel, mild steel and high strength steel. A brief review of the previous studies is presented here.

Outinen and Makelainen [7] carried out an experimental study to determine the mechanical properties of S355 cold-formed steels (nominal yield strength of 355 MPa) at elevated temperatures and after cooling. The average

Experimental program

Tensile coupon tests were conducted to obtain their residual stress–strain curves and mechanical properties after experiencing pre-selected temperatures up to 900 °C.

Test results

The test results include visual observation of specimen before and after failure, stress–strain relationship, yield strength, ultimate strength, elastic modulus and ultimate elongation.

Discussion of results

In this section, residual mechanical properties of high strength Q460 steel experiencing elevated temperatures as obtained from this study are compared with the post-fire mechanical properties of the mild Q235 steel (both for natural air cooling and cooling by water) and high strength S460 steel (only for natural air cooling due to unavailable of cooling by water). For this purpose the test data on post fire mechanical property reduction factor carried out by Zhang et al. [10] Chen et al. [11]

Proposed equations

Comparison of reduction factors for high strength Q460 steel with mild Q235 steel and high strength S460 steel showed that the predictions for Q235 and S460 is not applicable to predict the reduction factors for high strength Q460 steel. Therefore, new predictive equations were proposed for the mechanical properties reduction factors obtained in this study. Test results from this study also showed that the cooling method has some influence on the mechanical properties reduction factors. Hence

Conclusion

This paper has presented a detailed experimental study of the post-fire mechanical properties of high strength Q460 steel. This study included tensile coupon tests conducted on high strength Q460 steel for experiencing temperature range of 20–900 °C. Test specimens were heated to various elevated temperatures before being allowed to cool down to ambient temperature with natural air cooling and cooling by water, respectively. The stress–strain curves, yield and ultimate strengths, elastic modulus

Acknowledgment

The authors wish to acknowledge the support from the Fundamental Research Funds for the Central Universities (Grant No.: CDJZR12200004 and 106112013CDJZR200004). Any opinions, findings, and conclusions or recommendations expressed in this paper are those of the authors and do not necessarily reflect the views of the sponsors.

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