Mechanical Properties of Cold-drawn Steel Strand at Elevated Temperatures

Fire is a one of the most serious threats that a structure may experience during its service life. Thermal expansion, extreme temperature gradients, and degrading material properties can lead to structural failure. Structural fire resistance is addressed in building codes; however fire resistance for bridge structures is not subject to the same standards. Past researchers have shown that while bridge fires are a low probability event, the outcome is often of high consequence. Recent fires on cable-stayed bridges have led to stay cable loss. Past research has shown that cold-drawn steel is more susceptible to mechanical property degradation compared to hot-rolled steel. To ensure that the current standards of practice accurately predict the behavior of cold-drawn steel at elevated temperature, an experimental study was conducted at Lehigh University. Samples of 7-wire ASTM A416-12a steel strand were tested in tension using an electrically heated ceramic furnace and a universal testing machine. The stress strain behavior along with the ultimate strength and elastic modulus were determined through constant temperature testing. The rupture temperature was determined through a series of transient temperature tests. A metallurgical microstructure analysis was conducted on samples from the constant temperature testing to observe changes in the steel microstructure. The test methods were modelled after past research and the rate of loading and sample sizes conformed to current ASTM standards. The test results showed good agreement with the current reduction values for ultimate strength found in ACI 216 (2014). Faster heating rates provide a more conservative rupture temperature due to incomplete microstructure reorganization in the absence of an extended thermal soak. And the current data also shows that the Eurocode material model for cold-drawn steel at elevated temperature does not accurate predict the stress-strain behavior of these strands. Modifications to the Eurocode material model are proposed. This model is more suitable for a performance based approach to structural fire resistant design of cable-stayed bridges.