Effect of Temperature on Thermal and Mechanical Properties of Steel Bolts Venkatesh Kodur, F.ASCE1; Sonali Kand2; and Wasim Khaliq, A.M.ASCE3 Abstract: When steel-framed structures are exposed to fire, connections play a crucial role in transferring forces from highly stressed members to less stressed members. The performance of bolted connections, under ambient and fire conditions, depends on the strength characteristics of bolts. At present, the fire resistance of bolts is evaluated by assuming the high-temperature properties to be same as those of conventional mild steel. This is due to lack of data on high-temperature properties specific to high-strength steel bolts. To overcome this drawback, high-temperature thermal and mechanical properties of Grade A325 and A490 high-strength steel bolts are evaluated. Thermal conductivity and specific heat of A325 and A490 were measured in the temperature range 20–735°C, whereas thermal expansion was measured in the range 20–1000°C in both the heating and cooling phases of fire. Steady-state single shear and tension tests were carried out on A325 and A490 bolts at 20–800°C. Results from these tests were used to generate thermal conductivity, specific heat, thermal expansion, shear, and tensile strength relationships as a function of temperature for A325 and A490 bolts. Test data indicate that strength properties of A325 and A490 steel degrade faster than those of conventional steel and also that A490 steel exhibits slightly higher strength and stiffness properties than A325 steel at 20–800°C. DOI: 10.1061/(ASCE)MT.1943-5533.0000445. © 2012 American Society of Civil Engineers. CE Database subject headings: Temperature effects; Bolts; Fire resistance; Steel; Tensile strength; Shear strength; Thermal factors; Mechanical properties. Author keywords: High-temperature properties; Steel bolts; Cooling phase of fire; Fire resistance; Conventional steel; Tensile and shear strength; Thermal properties.
Introduction Connections in a structural system play a critical role in transferring loads from one member to the other. The integrity of the structural system may be compromised in the event of failure of connections, leading to its damage or even collapse. The role of the connections is much more crucial under fire conditions, as significant fireinduced forces have to be withstood. Thus, bolted connections should be designed to withstand fire-induced forces. Past realistic fire incidents and laboratory fire experiments indicate that two critical factors that influence the fire performance of bolted connections are temperature-induced strength degradation in bolts and fireinduced forces generated in connections resulting from thermal expansion of members. These two factors are temperature dependent; hence, reliable prediction of temperatures in members and bolted connections becomes necessary. Thermal properties that influence temperature rise in connections include thermal conductivity, 1 Professor, Dept. of Civil and Environmental Engineering, Michigan State Univ., East Lansing, MI 48824 (corresponding author). E-mail:
[email protected] 2 M.S. Graduate, Dept. of Civil and Environmental Engineering, Michigan State Univ., East Lansing, MI 48824. E-mail: kandsona@ msu.edu 3 Ph.D. Candidate, Dept. of Civil and Environmental Engineering, Michigan State Univ., East Lansing, MI 48824. E-mail: khaliqwa@ msu.edu Note. This manuscript was submitted on September 26, 2011; approved on December 8, 2011; published online on December 10, 2011. Discussion period open until November 1, 2012; separate discussions must be submitted for individual papers. This paper is part of the Journal of Materials in Civil Engineering, Vol. 24, No. 6, June 1, 2012. ©ASCE, ISSN 08991561/2012/6-765–774/$25.00.
specific heat, and thermal expansion of bolts. These properties can vary with the composition of steel and also with the heating and cooling phases of fire. Once the temperatures in fire-exposed connections are known, strength of bolts at a particular temperature can be evaluated by applying room temperature design procedures specified in codes and standards. For this evaluation, the variation in shear and tensile strength properties of bolts with temperature is required. At present, there is a lack of data on the high-temperature strength properties of steel specific to A325 and A490 bolts. To overcome this knowledge gap, an extensive experimental program was carried out to evaluate thermal and strength properties of A36, A325, and A490 steel. Thermal properties, namely, thermal conductivity, specific heat, and thermal expansion, were measured in the heating and cooling phases of fire. In addition, steady-state single shear and tensile strength tests were carried out to evaluate stress—strain response, yield and ultimate strength, and shear strength at elevated temperatures.
High-Temperature Properties of Steel General In the last five decades, significant research and development activities in the field of metallurgy have led to new types of steel with improved properties. Increasing carbon content and adding other alloys results in high-strength steel. Furthermore, type of heat treatment has significant influence on strength properties of steel; for example, annealing and normalizing produce normal-strength steel, whereas quenching and tempering produce high-strength steel. High-strength steel is commonly used in Grade A325 and A490
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