Mechanical Characterization of Normal and High-Strength Steel Bars in Reinforced Concrete Members under Fire
Publication: Journal of Structural Engineering
Volume 146, Issue 7
Abstract
This study investigates the high-temperature mechanical response of deformed steel bars used in the United States (ASTM A615 and A706, all grades) for the construction of reinforced concrete structural members that are at risk of fire exposure. Bars meeting both ASTM standards with nominal yield ranging from normal (420 MPa) to high strength (up to 690 MPa) were tested to fracture using a universal testing machine in combination with an electric split-tube furnace. A full stress–strain characterization at temperatures from ambient to 800°C was obtained, and all grades exhibited similar reductions in strength and stiffness as well as strain ductility at ultimate and fracture as a function of increasing temperature. Based on the experimental results, a modified version of the Eurocode 2 stress–strain model for hot-rolled steel rebar at elevated temperature is proposed. The reductions in steel ductility that are introduced by the proposed model are examined in a numerical study. A simple prototype floor beam, designed to have the same nominal strength using each grade of rebar, is analyzed for fire resistance according to ASTM E119 thermal and deflection criteria. The numerical results indicate that the reductions in strain ductility in the proposed model can reduce flexural performance for fire-exposed sections that use higher strength rebar grades. Also, reduced minimum cover requirements that are enabled by the use of higher strength bars with smaller diameter will allow faster temperature increases in the steel reinforcement. As a result, the fire resistance of the floor beam may be reduced in some cases below standard predictions based on nominal strength.
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Data Availability Statement
Some or all data, models, or code generated or used during this study are available from the corresponding author by request. These items include the following:
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Input and output files from thermal and structural analyses in SAFIR.
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Tabulated results of numerical modeling in SAFIR.
Tabulations of all experimental data and recommended material property reduction factors have been included within this paper.
Acknowledgments
This project was funded in part by a grant from the Commonwealth of Pennsylvania, Department of Community and Economic Development, through the Pennsylvania Infrastructure Technology Alliance (www.pitpa.org) under award number PIT-16-19. Additional funding was provided by Lehigh University via a Collaborative Research Opportunity (CORE) Grant. Prof. Vermaak was also supported by the Air Force Office of Scientific Research (AFOSR) under award number FA9550-16-1-0438. The authors would like to thank Joe Homic and Gerdau Long Steel North America in Sayreville, New Jersey, for their generous donation of all rebar specimens for this research effort. Many thanks also to Mike Mota at the Concrete Reinforcing Steel Institute (CRSI) for his guidance and assistance throughout this project. The authors would like to acknowledge the collaborative efforts of Lehigh graduate students Ahmed Abdulridha and Ismail Soner Cinoglu as well as postdoctoral researcher Ali Charbal who worked on other aspects of this project.
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Published In
Journal of Structural Engineering
Volume 146 • Issue 7 • July 2020
Copyright
©2020 American Society of Civil Engineers.
History
Received: May 31, 2019
Accepted: Nov 14, 2019
Published online: Apr 18, 2020
Published in print: Jul 1, 2020
Discussion open until: Sep 18, 2020
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