Calculation of relative thermal elongation of structural steels

Abstract

In the recent past, there have been a number of studies toward predicting the thermal and structural behavior of components subjected to elevated temperature due to an exposure to fire. Numerical modeling has been applied quite extensively toward this end. An accurate numerical model requires the use of appropriate thermophysical properties of the materials such as steels, concretes etc. Recent investigations into the collapse of World Trade Center building 7 showed that thermal expansion played an important role in the generation of stresses at elevated temperatures in components that are restrained structurally [Structural fire response and probable collapse sequence of World Trade Center Building 7, NIST NCSTR 1-9, November 2008]. In this article, relative thermal elongations of structural steels are computed using an approach similar to the one proposed by Andres et al. [Scr Mater 39:791, 1998] but with a novel method of computing volume phase fractions using a thermodynamic database in conjunction with Thermo-Calc [Thermo-Calc Software (2009) http://www.thermocalc.com, Stockholm Technology Park, Bjornnasvagen 21, SE-113 47 Stockholm, Sweden], a software that performs standard equilibrium calculations and calculation of thermodynamic quantities based on thermodynamic databases. Thermal expansion coefficient values were computed for a steel of eutectoid composition and an ASTM A572 Grade 50 steel. While the volume fractions of phases for A572 steel were computed using the thermodynamic approach, those for the eutectoid steel were computed using an approach similar to the one proposed in reference [Scr Mater 39:791, 1998]. The computed values of relative thermal elongation were compared with the values obtained from Eurocode. While the match was reasonable for the most part, there are significant differences in the values obtained during the phase transformation range (967–1,086 K for ASTM A572 Grade 50 steel). Also, the computed values were compared against those recommended during the NIST (National Institute of Standards and Technology) World Trade Center failure investigation. Reasonable agreement was achieved in the temperature range between room temperature and 600 °C (873 K).