Drift-based and dual-parameter fragility curves for concentrically braced frames in seismic regions

• We developed drift-based and dual-parameter fragility curves for CBFs.
• Aleatory and epistemic uncertainties are considered for the fragility curves.
• High yield ratio braces fracture at lower SDRs than low yield ratio braces.
• Seismically compact steel braces typically have a 10% chance to fracture at 2% SDR.
• Round HSS braces subjected to pulse-like protocols fracture at about 5% SDR.

This paper discusses the development of drift-based and dual-parameter fragility curves for steel braces as part of concentrically braced frames designed in seismic regions. The experimental results from 24 different research programs are compiled into a database for this effort. Drift-based fragility curves are developed for three damage states of steel braces subjected to cyclic loading associated with brace flexural buckling, local buckling and brace strength loss due to fracture. The effects of material variability, brace cross sectional shape and loading protocol on the drift-based fragility curves are investigated. The effect of global and local slenderness ratios on the fracture ductility of various shapes of steel braces is examined through dual-parameter fragility curves that relate these geometric ratios with the expected story drift ratios that each of the three pre-described damage states occur. The proposed fragility curves can be employed for rapid assessment of the seismic vulnerability of concentrically braced frames.