AS3600 - 2018 Ammendment 2 - CL 8.2.1.6

[Legoman92]

Hey guys,

I've been using the original 2018 version of AS3600 and the changes to the Vuc capacity of beams has been brought to my attention.

It seems embarrassingly reckless to remove the reduction of concrete shear capacity in the 2018 version only to significantly reduce the capacity in a further ammendment using the ks Value.

I did a quick comparison using the new Ks value and found:

450 thick slab, 25 MPa concrete 0.7 * Vuc = 352 kN

600 thick slab, 25 MPa concrete 0.7 * Vuc = 328 kN.

Can someone please explain to me how this new provision makes any sense? How is it possible a thicker slab provides less shear capacity?

 

[rscassar]

I had some questions about ks a few years back in this thread. But it was similar to the previous version of the code where shear reinforcement in a slab was required above phi*Vuc and shear reinforcement in a beam was required above phi*0.5*Vuc (I think where the requirements).

thread744-489155

 

[IDS]

I would be interested to see details of the section and the design shear and moment. I can't get exactly the same values.

But regardless of the exact numbers, the value calculated is not the shear strength, it is the maximum design shear force that can be applied without providing minimum shear reinforcement. The equation provides a transition in the value of ks, instead of a sudden jump from 1.0 down to 0.5 as in the 2009 code. Also note that ks may be taken as 1.0 if it can be shown that shear failure will not cause collapse of the structure.

Also worth noting:
1. Cl 8.2.1.6(c) says that if D >= 750 shear reinforcement is always required, so ks is effectively 0 for this case.
2. The current bridge code (AS 5100.5) has the same requirements as AS 3600-2009, i.e. ks = 0.5 for all sections except slabs <= 300 mm deep and walls.
3. The current draft for public comment of AS 5100.5 has adopted the current AS 3600 requirements.
4. ... except for Cl. 8.2.1.6(c), where the proposed section depth that will always require shear reinforcement is increased from 0.75 to 3.0 m

Doug Jenkins
Interactive Design Services

http://newtonexcelbach.wordpress.com/

 

[Legoman92]

Images attached. Just playing with the Ks value, there seems to be a sweet spot around 450mm where increasing the thickness gives diminishing returns and actually lowers the capacity at the 600 thickness mark. It's annoying as sometimes its more economical to design a thicker foundation and use concrete capacity in lieu of shear reinforcement.

 

[Legoman92]

Just to add, is the spirit of the Ks value to just be used for narrow beams? Or is it also to be used for suspended slabs and ground rafts/footings?

 

[IDS]

Just to add, is the spirit of the Ks value to just be used for narrow beams? Or is it also to be used for suspended slabs and ground rafts/footings?

AS 3600 says slabs should be designed for one-way shear according to the beam shear provisions, but walls have their own provisions for shear.
AS 5100.5 says the ks factor should be applied to all sections except slabs <= 300 thick and walls.

So that seems pretty clear to me, it applies to everything except walls, but for slabs <= 300 ks = 1.

Thanks for the calcs.

If I set the compression steel to zero I agree with the flexural capacity exactly, but I get 5-10% lower Vuc, resulting in the ks.phi.Vuc value for 600 thick being slightly higher than 450 thick.

I will have a closer look to see where the difference is coming from, but the exact numbers are not that important, we agree that increasing the depth from 450 to 600 mm only has a very small influence on the maximum shear allowable without shear reinforcement.

I don't see that as being a problem. In the 2009 code there was a 50% reduction in the value at 300 mm depth, and this has been changed to a transition from no reduction to 50% between 300 and 650 mm, so in effect an additional reduction factor is applied over a range of depths, rather than as a step reduction at 300 mm depth.



Doug Jenkins
Interactive Design Services

http://newtonexcelbach.wordpress.com/

 

[rapt]

Yes IDS, agreed it is not a "capacity" reduction. Because a shear failure is a brittle failure and relies on concrete tension and "interlock", it is a recognition that an extra safety factor is needed when the member is getting closer to the estimated Vuc. And the step was the problem we were trying to get rid of with ks. And the difference between slabs versus beams rather than a depth effect. Wide thin sections have an ability to distribute shear forces around a local shear failure or local reduction in capacity area with shear capacity lower than the Vuc estimate but saying it is for slabs is a problem as a slab could be 1m deep and 2m wide, so it is really a beam in this regard. I would have still liked an allowance for a width/depth ratio be more lenient for slabs with a high width/depth ratio, but you cannot win them all, and the current rule is much better than the old one with 50% for beams and 100% for slabs.

I would think that this limit applies to any member for which the one way shear rules are used to to calculate shear capacity, including walls.

 

[IDS]

I have found the difference between our calculations.

Cl. 8.2.4.2.2 says that M* shall not be taken less than V*dv (if no prestress), so for low applied moment and zero axial load and prestress, M*/dv = V* and
strain = V*/Es.Ast = 204000/(200000*2454) = 4.16E-4.

So for this case the strain is the same for both depths, and considerably greater than that found using the input M* value.

Doug Jenkins
Interactive Design Services

http://newtonexcelbach.wordpress.com/