Development and experimental verification of self-prestressing strengthening systems for cold-formed steel purlins using Fe-SMA strips

Cold-formed steel purlins have been widely applied in the roof system of industrial buildings. With changes in roof functionality, many purlins require strengthening to improve their load-carrying capacity and stiffness. Prestressing is an effective strategy for enhancing the mechanical behavior of steel structures. However, conventional prestressing techniques typically rely on mechanical anchors and external tensioning devices. To overcome these limitations, practical self-prestressing strengthening systems for cold-formed steel purlins were developed using iron-based shape memory alloy (Fe-SMA) strips. The recovery stress behavior and anchorage methods, including bolted anchorage and adhesively bonded anchorage, were first examined. Subsequently, full-scale purlins consisting of two C-shaped cold-formed steel sections were strengthened and subjected to four-point bending tests under static loading. The effects of activation temperature and anchorage system on the strengthening effectiveness were investigated. Test results demonstrate that the recovery stress was largely insensitive to the initial pretensioning stress, but strongly affected by the activation temperature and the applied pre-strain. A recovery stress of about 380 MPa was generated under an activation temperature of 250 °C and a pre-strain level of 4%. Both the bolted and adhesively bonded anchorage systems proved to be effective, as they enabled the Fe-SMA strips to achieve tensile stresses of 680 MPa and 900 MPa, respectively, significantly exceeding the tensile stress demand required for strengthening cold-formed steel purlins. The flexural strengthening tests further indicated that flame-gun heating was a feasible activation method for practical on-site applications. Compared with non-prestressed strengthening, which resulted in only a 0.6% increase in yield load, prestressed Fe-SMA strengthening achieved enhancements of 9.4%-13.6%. Specimens using bolted and bonded methods exhibited comparable flexural performance, but the bonded anchorage system provided a slight advantage in improving the yield load and elastic stiffness.