Title: Acceptable Elongations and Low-Cycle Fatigue Performance for High-Strength Reinforcing Bars
Author(s): Drit Sokoli; Graham Hogsett; Albert A. Limantono; Ariel Suselo; Dhiaa Al-Tarafany; Wassim M. Ghannoum
Appears on pages(s):
Keywords: high-strength, hsrb, reinforcement, bars, low-cycle fatigue
High-strength reinforcing bars (HSRB) with varying mechanical properties and performance under low-cycle fatigue are being introduced to the U.S. market driven by constructability and economic incentives. The project described in this report is part of a larger national effort aimed at quantifying changes in the seismic collapse risk of concrete structures associated with switching from conventional grade 60 reinforcing bars to HSRB. Correlations between seismic collapse risk and bar properties are crucial for code bodies to set acceptable properties for HSRB, especially fracture elongations and low-cycle fatigue performance. However, in order to objectively and reliably evaluate the seismic collapse risk of concrete buildings, the effects of the varying mechanical properties of HSRB on the deformation capacity of seismically detailed concrete members must be quantified. The objective of this project in particular is to provide the necessary experimental data and behavioral models to identify when longitudinal bars in seismically detailed frame members reach fracture during seismic events across all types of bars and grades in production or under development in the U.S. Three tasks were undertaken to achieve project objectives:
1) a low-cycle fatigue-capacity model was calibrated to cyclic tests conducted on bars of different grades and mechanical properties;
2) a mechanics-based model was developed and calibrated to experimental data from the literature to correlate member global deformations with strain demands that govern the fatigue behavior of longitudinal bars in concrete members; and
3) based outcomes from tasks 1) and 2), a methodology was proposed to estimate the point during a seismic loading history at which longitudinal bars fracture in seismically detailed concrete frame members. Cyclic tests were performed on reinforcing bars to bolster available fatigue data. Additional tests included bars of varying grades of steel, manufacturing techniques, clear lateral bracing spans, and strain amplitudes.