Abstract
The New Zealand Structural Loadings Standard, until its latest revision, used the structural ductility factor as a measure of the deformation demand of all potential plastic hinges in a structure. In the new version of New Zealand Standard for Earthquake Actions (NZS 1170.5:2004) the detailing of potential plastic regions is determined according to the local deformation demand in these regions. The change has been prompted by evidence that the structural ductility factor gives a poor indication of the demand on individual plastic regions. This new approach has also been adopted by the revised New Zealand Concrete Structures Standard (NZS 3101:2006) which classifies potential plastic regions into three categories (namely ductile, limited ductile and nominally ductile) based upon their inelastic deformation demand specified in terms of material strain limits. The material strain limits currently set in NZS 3101:2006 for the three categories of plastic regions are based on limited experimental evidence and need a closer revision. This paper tries to obtain more justifiable values of material strain limits based on experimental data. In this research, reversed cyclic loading tests of beams are conducted to compensate for a lack of data in the nominally ductile range of detailing. Based on the results of the tests conducted, curvature limits for nominally ductile plastic hinges are derived. Combining the experimental results collected from literature and the tests conducted in this project, updated material strain limits for the three categories of plastic regions are proposed. To unify the design process for all types of plastic regions, curvature limits for nominally ductile plastic hinges are also proposed as the multiple of first yield curvature (similar to the existing approach for the other two categories of plastic regions) rather than the existing approach of specifying allowable compressive (concrete) and tensile (rebar) strain limits for nominally ductile plastic regions. To further simplify the process, the representative value of first yield curvature is approximated as two times the yielding strain to the beam height ratio, thereby relieving the designers from having to conduct section analysis to estimate neutral axis depth.
Highlights
The previous version of New Zealand Loadings Standard (NZS 4203:1992) defined the loads and forces to be used in the design of structures for dead, live, wind, snow and seismic loading, and the requirements for detailing of potential plastic regions were left to the appropriate materials standard
Comparisons were made with the curvature limits currently in the concrete structures standard which showed that the existing limits are conservative
The material strain limits proposed by Fenwick and Dhakal for limited ductile and ductile members were reassessed by including some additional experimental results
Summary
The previous version of New Zealand Loadings Standard (NZS 4203:1992) defined the loads and forces to be used in the design of structures for dead, live, wind, snow and seismic loading, and the requirements for detailing of potential plastic regions were left to the appropriate materials standard. In the previous version of the New Zealand Concrete Structures Standard (NZS 3101:1995); the minimum detailing requirements for potential plastic regions were largely based on the structural displacement ductility factor, μ. This factor has been shown to be a poor indicator of the level of inelastic deformation on individual plastic regions (Fenwick and Dhakal, 2007a). The revised New Zealand Standard for Earthquake actions (NZS 1170.5:2004) has recognised this inadequacy and requires that the detailing of potential plastic regions be based on the expected inelastic deformation in individual plastic regions. Beams and walls in flexure the material strains are given in the form of curvature, while for shear links in eccentrically braced frames or reinforced concrete coupling beams the material strains are based on shear deformation
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