First Mode is Torsion - Design as Non-Ductile?

[structuree]

Hi All,

I’m currently designing walls for an irregularly shaped building where it was determined, following 3D analysis, that its first mode is torsion.

I’ve heard varied opinions on whether I should be designing these walls as non-ductile (ie. use u=1) because torsional/shear failure is brittle in nature, and the structure will not behave in a ductile manner if it fails as such.

Alternatively, I’ve heard that in all cases, no matter the failure mechanism or primary mode, shear should be designed using u=1, and bending and axial using u=>2.

Finally, I’ve had someone propose to me that if the building can be designed to take the increase in shear forces resulting from using u=2, the building will not fail in a brittle manner, so can justify designing as moderately ductile.

Could anyone shed some light or their thoughts on this? Considering the 2018 requirements for moderately ductile structures in AS3600, I want to get this right.

 

[Retrograde]

I’ve heard varied opinions on whether I should be designing these walls as non-ductile (ie. use u=1) because torsional/shear failure is brittle in nature, and the structure will not behave in a ductile manner if it fails as such.

A torsion mode of a building is a completely different thing from a torsional/shear failure of a element like a beam or wall.

Alternatively, I’ve heard that in all cases, no matter the failure mechanism or primary mode, shear should be designed using u=1, and bending and axial using u=>2.

What u value you use is up to you. It is a trade off between reducing your forces (which can reduce the amount of reinforcement you need) and designing for additional ductility (which can increase the amount of reinforcement you need).

Finally, I’ve had someone propose to me that if the building can be designed to take the increase in shear forces resulting from using u=2, the building will not fail in a brittle manner, so can justify designing as moderately ductile.

The shear force will be lower if you use u=2 (limited ductile) or u=3 (moderately ductile) as compared to u=1, so I'm not sure what they are on about there. You need to check minimum shear requirements to Clause 14.6.6 (walls) and/or 14.5.2.2 (beams) if using u>1.

 

[structuree]

If your building is in torsion, wouldn’t a torsional failure (ie shear failure) be more likely to occur before flexural failure (for example)?

Surely you can’t justify using u=1 for any regular concrete building though, even if the shear force is larger. The new detailing requirements for u=2 in AS3600 are very stringent. u=1 appears to be for unreinforced masonry and the like. Not sure if you could treat a reinforced concrete lateral stability system as such.

Should I be considering u=1 when looking at shear and u=2 when looking at bending and axial forces?

 

[Retrograde]

If your building is in torsion, wouldn’t a torsional failure (ie shear failure) be more likely to occur before flexural failure (for example)?

No

Surely you can’t justify using u=1 for any regular concrete building though, even if the shear force is larger.

AS3600 allows it.

The new detailing requirements for u=2 in AS3600 are very stringent.

Agreed

u=1 appears to be for unreinforced masonry and the like.

Not correct. Non-ductile structural walls are u=1. Check Table 14.3.

Should I be considering u=1 when looking at shear and u=2 when looking at bending and axial forces?

That is probably conservative. Why not just design in accordance with the Code?

 

[structuree]

@Retrograde, sorry for my ignorance - why wouldn’t a torsional failure be more likely to occur before flexural failure in an earthquake event that is likely to result in torsion first?

@O_O_O, so without your overstrength factor, you're looking at u=0. But you're referring to tensile stress? Your post above refers to u=1 for shear and u=2 for bending and axial.

 

[Retrograde]

why wouldn’t a torsional failure be more likely to occur before flexural failure in an earthquake event that is likely to result in torsion first

It will depend a lot on the layout of the walls in your building (assuming the walls are the lateral load resisting elements), but just because the building has a torsional mode shape, this does not necessarily means your walls will experience torsion. The wall forces will come out of the analysis - and the torsional movement of the building will likely result in flexure in the walls (bending and shear). For u>1, AS3600 attempts to force a bending failure before a shear failure through equation 14.6.6.

For u=1 it does not matter if a shear failure happens first, as you have not assumed a ductile response.