the wording of the shear ks factor in AS3600 sect 8.2

[rscassar]

I liked the previous revision of the code shear it stated that slabs shear reo is required if V*>0.5*phi*Vuc and for beams V*<phi*Vuc. The new code has clarified this a bit further and started making the requirements of shear reinforcement based on the depth of the concrete under consideration. However I don't really like the wording that if a shear failure does not cause collapse, then a ks of 1.0 can be used. For me the 2018 code was very good particularly around detailing for robustness, integrity and prevention of progressive collapse, I just feel that this statement in the code may be misconstrued and used incorrectly.

 

[rapt]

The problem with the old rule was that it was different for beams and slabs, and there was a sudden step at the minimum depth limit. And we had idiots calling beams slabs and slabs beams for different reasons to get around different rules, because there is no definition of beam/slab in the code and some designers have very hopeful/wishful definitions of what they can call a beam.

It was a lot more sensible to adopt a depth based logic, with kinks rather than steps at the change points, otherwise a 299mm section would use 1.0 and a 301mm section would use .5. Canadian Bridge Code has similar logic.

I do not like the not cause collapse bit either as it will be misused by those same people. But if it was not there, others would complain as there is no way around the rule where it can be ignored.

A case for ignoring it might be a footing beam where a shear crack will not lead to collapse as it is still ground supported.

 

[IDS]

rscassar - Unless I'm missing something the 2009 version of the code required shear reinforcement where V*>0.5*phi*Vuc for both beams and slabs. The original release of the new version in 2018 removed the 0.5 factor, so was significantly less conservative. In the latest amendment the ks factor has been introduced varying from 1.0 for members 300 mm deep or less to 0.5 for members >= 650 mm deep. For members >= 750 mm deep shear reinforcement is required regardless of the design shear force. and that requirement was also in the 2009 code.

The latest amendment has therefore partially removed the relaxation of the requirements introduced in 2018. The clause allowing ks = 1.0 for all depths (up to 750 mm) is new in the latest amendment, and states in full:

That seems to me a reasonable relaxation of the requirements, and is similar in principle to other statements allowing use of less ductile detailing requirements.

Regardless of that clause, my approach would be to just provide the minimum shear reinforcement where required, and not worry about it.

Doug Jenkins
Interactive Design Services

http://newtonexcelbach.wordpress.com/