Assessment of material strain limits for defining different forms of plastic hinge region in concrete structures

Abstract

The New Zealand Concrete Structures Standards prior to the 2006 edition used the structural ductility factor as the primary parameter used to determine the form of detailing required in potential plastic regions. However, it has been found that this parameter gives a poor indication of the magnitude of inelastic deformation that individual plastic regions are required to sustain in a design level earthquake. The New Zealand Earthquake Actions Standard, NZS 1170.5-2004, recognised this deficiency and required the level of detailing in plastic regions to be based on the predicted “material strain” level sustained in individual plastic regions for the ultimate limit state involving seismic actions. The term “material strain” is a general term covering axial tensile or compressive strains, shear strains or curvatures.

The 2006 edition of the New Zealand Concrete Structures Standard, NZS 3101-2006, adopted the approach outlined in the Earthquake Actions Standard. Three levels of detailing were defined, namely:

1. Nominally ductile detailing, which is intended to give members sufficient ductility to satisfy the requirements for non-seismic design such as moment redistribution and protection against brittle failure due to accidental overloads;
2. Limited ductile detailing, for plastic regions which are only required to sustain low to moderate levels of inelastic deformation;
3. Ductile plastic regions that are intended for plastic regions that are required to sustain moderate to high levels of inelastic deformation.

While an appreciable data base of test results was found for establishing material strain limits for walls, columns and ductile beams there was a lack of suitable test results for nominally ductile beams. The principal objective of this project was to examine the performance of beams detailed as nominally ductile members, either to help in establishing material strain limits, or to reinforce proposed material strain limits for this form of detailing. To this end eight beams were built and tested. Extensive instrumentation was used on each beam so that flexural and shear deformation could be measured in each beam together with elongation of the plastic region.

A limited number of test results for members with ductile or limited ductile detailing were added to an existing data base. An analysis of this data base supported the current material strain limits for ductile and limited ductile detailing given in Amendment 2 to the Concrete Structures Standard.