Experimental Investigation on the Post-Fire Performance of Multiple-Strength-Grade Steel Wires

This study assessed the critical fire resistance of bridge cables by investigating post-fire tensile degradation of high-strength steel wires (1860, 1960, 2100 MPa) heated under initial stress (10% and 40% of design strength) to 300 °C, 400 °C, 500 °C, and 600 °C followed by cooling to an ambient temperature. Tensile tests determined reduction coefficients (RCs) for proportional limit (σp), yield strength (σy), ultimate strength (σu), elastic modulus (E), and elongation (δ) relative to ambient values, with constitutive models for σp, σy, and σuRCs integrating temperature, stress, and grade. Visual observation showed intensified wire hue and reduced necking with increasing temperature. All RCs remained stable up to 300 °C; beyond this temperature, σp, σy, and σuRCs progressively decreased, averaging 77.5%, 65.7%, and 61.9% at 600 °C under lower-level initial stress (10%) and 74.1%, 63.8%, and 60.6% at 400 °C under higher-level initial stress (40%). Elastic modulus exhibited minimal variation, whereas elongation reached its minimum at 400 °C under lower-level initial stress but its maximum at 400 °C under higher-level initial stress. The impact of lower-level initial stress on mechanical properties was negligible, showing a less than 3.2% average RC decrease at 600 °C. Higher-level initial stress accelerated mechanical property degradation with increasing temperature, with comparable degradation patterns observed across different steel wire strength grades. The models confirm maximum temperature dominance in degradation, supporting a recommended critical fire-resistant temperature of approximately 400 °C for bridge cable wires.