Calibration of Australian Standard AS3600 concrete structures part II: reliability indices and changes to capacity reduction factors

Abstract

Capacity reduction factors (ϕ) for flexure, shear and axial loadings are derived for reinforced concrete (RC) structures based on a reliability-based calibration of the Australian Concrete Structures Standard AS3600. The structural reliability analysis considers the bias and variability of material properties, dimensions, loads and model error. The target reliabilities (βT) are selected based on consideration of past practice and Australian and international standards. The capacity reduction factors (ϕ) for the new code AS3600-2009 are selected using the most recent statistical parameters for material strengths for 20–100 MPa strength concrete using Class N (ductile) 500N reinforcement. The reliability-based calibration found that the ϕ-factor can be increased from 0.80 to 0.85 for members in bending, and increased from 0.60 to 0.65 for axial loading of short (stocky) columns where the ratio of the live load to the dead load is at least 0.25. No changes are recommended for shear or torsion, at this time, or for slender columns; further research is needed to better refine the design models for these cases and reduce the variation in their model error. The proposed increases in capacity reduction factors will result in up to an 8.3% increase in design strength that, in turn, provides efficiency in the use of materials. The proposed changes provide for modest savings in greenhouse gas emissions.