Fire testing and design of stainless steel structures

Abstract

Despite significant progress in recent years in the development of room temperature design guidance for stainless steel structures, fire resistant design has received relatively little attention. This paper reports on studies carried out to investigate the performance of unprotected stainless steel beams and columns in fire. Material tests were carried out on five grades of stainless steel to determine strength and stiffness retention factors at elevated temperatures; both strength and stiffness retention were shown to be superior to that of carbon steel beyond 600 ^∘C. The temperature development characteristics of a range of stainless steel sections were investigated, and compared to those of carbon steel sections. Full scale fire tests were conducted on six stainless steel columns, and four stainless steel beams. Finite element modelling of the tests was carried out, and parametric studies were performed to supplement the test data. All tests were carried out as part of the European project ‘Development of the use of stainless steel in construction’. Design recommendations for stainless steel columns and stainless steel beams supporting a concrete slab, based on the ECCS model code for fire engineering, were validated against the test and finite element results. These recommendations have been incorporated into the Euro Inox/SCI Design Manual for Structural Stainless Steel, and implemented in Eurocode 3: Part 1.2, with minor adjustments for consistency with carbon steel.

Introduction

With improving design guidance, wider product availability and a greater emphasis now being placed on the whole life performance of structures, the use of stainless steel in structural applications is increasing. However, although significant progress has been made in recent years in developing room temperature design guidance for structural stainless steel, its fire resistance has received less attention. Material and member tests at elevated temperature were performed as part of the ECSC project ‘Development of the use of stainless steel in construction’, and are described herein, together with fire resistant design recommendations. A general review of the use of stainless steel in structures has been recently carried out [1], and a number of case studies collated [2]. Up to date design guidance for structural stainless steel may be found in [3].

Whilst vast quantities of elevated temperature material data do exist, these have been generated for the service use of stainless steel at elevated temperature in applications such as boilers and pressure vessels and are not suitable for fire engineering design. For such applications, generally only the strength at fairly low strain levels has been measured (commonly the strength at 0.2% plastic strain), whereas fire engineering methods permit the use of higher strain limits. Furthermore, isothermal test methods were used to generate the data; anisothermal (transient) test methods are now considered more appropriate for fire engineering because the specimen is in a similar situation as a structure in a real fire. Finally, elevated temperature data in material standards are usually only given up to 550 ^∘C, whereas in fire engineering, behaviour at higher temperatures is of interest.

Material behaviour at elevated temperatures was studied as part of the ECSC project, with data generated for a range of stainless steel grades, based on a programme of isothermal and anisothermal tests 4., 5.. Other material studies of stainless steel at temperatures concurrent with structural fire design have also been conducted 6., 7..

Member behaviour at elevated temperatures was also studied in conjunction with the generated material data. Although knowledge of the degradation of material strength and stiffness is fundamental to understanding the performance of members in fire, the member tests also enabled the effects of instability, temperature gradients and full cross-sectional behaviour (including enhanced strength corner regions) to be studied. The stress–strain properties of the corner regions of cold-formed stainless steel cross-sections differ from those of the flat regions due to the material’s response to deformation; enhancements in 0.2% proof strengths of between 20% and 100% have been observed in the corners of roll-formed sections [8]. A review of the corner properties of cold-formed stainless steel sections has been conducted [9]. Elevated temperature tests have indicated that this strength enhancement is retained to temperatures in the region of 600–800 ^∘C, but by about 900 ^∘C, corner and flat material display similar proof strengths [6].

Results from the member tests on columns and beams supporting concrete slabs, together with results from a parallel numerical study, were used to develop expressions for stainless steel member design in fire [10]. Six fire resistance tests on stainless steel columns have previously been performed 11., 12., but instead of following the temperature–time relationship specified by the standard fire curve [13], the furnace temperature followed a bespoke, bi-linear variation with time.