Elevated-temperature mechanical properties of high strength structural steel S460N: Experimental study and recommendations for fire-resistance design

Abstract

Fire-resistance design is one of the most important considerations when structural engineers conduct design of steel structures. As a basis of analyzing fire performance of steel structures, elevated-temperature mechanical properties of structural steels are significant for practical design. The recommendations of current European, American, Australian and British standards were mainly obtained from mild steels, which are in question when used to conduct fire-resistance design of high strength steel structures. In order to reveal the elevated-temperature mechanical properties of high strength steel S460N, tensile tests were conducted at various temperatures ranged 20–700 °C. The elevated-temperature reduction factors of elastic modulus, yield and ultimate strengths of S460N were obtained and compared with current design standards and available literature. According to the comparison between this research result on S460N and the available research results in literature on S460N, S460M and various mild steels, it is found that the deterioration of mechanical properties of structural steels at elevated temperature is dependent on steel grades. Thus the recommendations in current design standards are not applicable to high strength structural steels. Further unique predictive equations for the deterioration of high strength structural steel S460 at elevated temperatures were proposed and validated with available literature.

Introduction

The structural steel grade S355, which used to be considered as high strength steel (HSS) 20 years ago, is now the main constructional steel for hot rolled plates and H-sections in Europe. For the time being, the steels having a nominal yield stress equal to or more than 460 N/mm² are called high strength steel, based on the implication of the current European Standard Eurocode 3 (EC3) [1]. In the last decade, high strength steel has been applied in many structures all over the world because of its economical benefits in comparison to mild steels, which are hot-rolled carbon steels with normal strengths. Hence HSS is gaining more and more attention in the market of steel structures.

In Europe the cost of building a structure usually depends more on fabrication, transportation and erection than on the price of material. In the construction of steel structures HSS allows less material to be used, which reduces the volume of weld metals and correspondingly the time for welding. Further, less material has to be transported and the lighter weight simplifies the erection of structures. In some structural applications the light weight plays an important role, because the payload can be increased or the running expense can be decreased, such as cranes and vehicles. What is more, an increasing international concern on environment protection leads to more attention on saving energy and raw materials. Thus using HSS is a great environmental benefit in comparison with mild steel. Collin and Johansson [2] made an evaluation and proved that if the strength of steel material could be fully utilized the cost of material would be decreased with the increase of material strength. With the development of process and manufacture technology, the cost of manufacturing high strength steel will be closer and closer to that of mild steels. So the material cost increase by using HSS is less than the benefits of its improved yield strength. Therefore, it is economical to use HSS especially in structures where the strength can be fully utilized.

Since the 911 World Trade Centre Tragedy, a lot of researches have been conducted on the structural behaviour of constructions under fire conditions with the combined effects of weakening of materials, thermal restraint and accidental removal of some structural elements. As a basis of evaluating the performance of steel constructions in fire and even after fire, mechanical properties of some constructional steels at elevated temperatures and after cooling down have been reported by now [3], [4], [5], [6], [7], [8]. However, the previous researches mainly focus on mild steels. For high strength steels, only very limited information has been reported so far [9], [10], [11], [12], [13], [14], [15], [16], [17]. As a result, the fire-resistance design of steel structures with HSS is either too conservative or safety-compromise in practice, which seriously retards the application of HSS in construction as well as leads to safety risk.

In European design standard EC3 part 1–2 [1], it is assumed that the material properties of various structural steel grades at elevated temperatures can be evaluated uniformly. And the predictions recommended by EC3 are based on the test results mainly obtained from mild steels. However, pervious researches indicate that the elevated-temperature material properties of HSS are different from that of mild steels [9], [10], [11], [12], [13]. Hence, using the recommendations from EC3 to conduct fire-resistance design of structural members made of HSS runs a risk. Not only European design standard but also American, Australian and British standards, no current design standard for steel structures has specified recommendations for HSS under fire conditions. Therefore, accurate material properties of various HSS grades at elevated temperatures are urgently needed in practical design, in order to keep pace with the development of modelling techniques for predicting the fire response of steel structures.

In construction, S460 is currently the most normally used high strength structural steel. However, the researches on material properties of HSS S460 at elevated temperatures reported in English are very limited and mainly focused on two types, S460N and S460M. The difference between them is delivery condition; S460N is normalized rolled while S460M is thermo-mechanical rolled. Lange and Wohlfeil [9] conducted transient state tests on both S460N and S460M; their stress–strain relationships at elevated temperatures up to 3% total strain were reported. They proved the elevated temperature performance of S460M was better than S460N. Schneider and Lange [10], [11], [12] extended the above experimental investigation to 7 types of commercial HSS S460 with different chemical compositions and delivery conditions, using both steady state and transient state test methods. They pointed out that EC3 overestimated the elevated-temperature yield strengths if used for S460. Based on various heating rates, they worked out an empirical equation of creep for S460 at elevated temperatures. Outinen [6], [8] conducted transient state test on S460M at elevated temperatures, and compared his experimental results of elastic modulus and yield strength with the recommendations of EC3. However, there is a considerable discrepancy between the different data available in the literature, because of variations in test methods, heating conditions and data collection techniques. This results in a challenge for structural engineers to choose accurate elevated-temperature material properties of HSS S460 for predicting response of steel structure under fire conditions. In order to supply convincing proof for safe fire-resistance design of steel structures with HSS S460 and validate the available research results, an experimental research was conducted on S460N, using both steady state test method and transient state test method.