Coupling Beams - Ductility Factor - AS3600

[Drapes]

AS3600-2018 and AS1170.4 specifies that partially or fully ductile coupled walls shall be designed with a ductility factor (mu) of 4 and structural performance factor (sp) of 0.67, and that we should refer to the NZS 3101 code for details. Is there any scope to adopt a lower mu and sp for these types of walls - for example instead of ductile, adopting limited or moderately ductile?

The AS1170.4 code mentions that we can assume ductility of less than those specified in the code (so in this case say by using mu= 2 or mu= 3), but then there is no information on how to detail the coupling beams to suit. Are there any detailing requirements for coupling beams designed as limited or moderately ductile for a coupled wall system?

 

[Agent666]

By all means you can design for a lower ductility and higher load, as the designer you can do anything. Just adopt the same (higher) level of detailing from NZS3101 for a lower ductility, ductility is ductility. In NZS3101 the level of detailing is based on the localised plastic curvature, i.e. the local ductility demand.

People tend to think about ductility the wrong way round. The design ductility is just that, under lower load you may stay elastic, under higher load than the code design basis earthquake loads you might see a higher ductility response. The earthquake doesn't care, you see the load that it takes to form your ductile mechanism. In the case of coupling beams in walls this is usually every level yielding, so design needs to recognise this, not sure how AS3600 treats the requirement for capacity design. I've always found it odd that codes say you must adopt a ductility of X for Y type of structure, it should be treated as a max ductility for a given structural form (i.e. within the bounds of current knowledge). This is certainly how it works in NZ and has done since the 1982 code.

Global ductility for working out the base shear is not related to the detailing required at a particular plastic hinge region. This is a very important concept, don't fall into the trap of thinking the global ductility relates to the detailing required at potential plastic hinge regions.

Think about 2 beams, different spans, in the same system, they undergo the same level of interstorey drift but the shorter beam has a larger rotation at any hinge locations and hence a larger plastic curvature. Therefore the shorter beam potential plastic hinge regions should have a higher detailing requirements. NZS3101 recognises this aspect of detailing since our 2006 version.