Seismic Torsional Sensitivity - AS1170.4 & NZS1170.5

[Trenno]

It's common for modern commercial buildings in Australia to have an offset core to allow greater open plan flexibility. The mode shapes of such buildings usually demonstrate torsional behaviour, especially within the first two modes.

Example:

I know many international codes will penalise you for these torsionally dominant lateral stability systems, usually restricting the ductility factors such that an elastic design (Mu=1) is adopted. However, from what I understand AS1170.4 is silent on this and technically allows higher ductility levels (eg Mu=2).

However, NZS1170.5 has a clause that helps to quantify whether or not a building is torsionally sensitive. If your building is found to be irregular / torsionally sensitive you must scale the RSA base shears up to equal ESA base shears. But I can't see that it restricts the use of Mu>1.

I'd be keen to hear what other AUS/NZ engineers are adopting in these scenarios.

 

[rscassar]

I've noticed that as well, especially for low-rise buildings, often the torsional mode does appear in the top 2 modes. High-rise buildings not so common. My approach has always been to limit the drift in the XY plane (not X & Y direction independently). Always account for torsional offset in the wind case even for buildings lower than 70m. Use mu=1.0 for earthquake design and adopt a minimum 70% scaling for dynamic force scaling compared to the equivalent static method.

I am not an expert in response-spectrum analysis and couldn't really tell you how the forces are derived other than knowing that it is a combination of all modal responses encompassing 90% modal mass participation. I use U1 and U2 = 9.81 for the response spectrum analysis (have seen some engineers use U1 = 9.81 and U2=0.3*9.81 an vice versa).

But my thoughts are as long as all these items are checked, the forces are followed thru including the forces which run thru the diaphragm then the seismic design is adequate.

 

[Trenno]

Thanks rscassar - I would agree that your design philosophy is conservative and should certainly be sufficient.

Anyone else?

Hoping Agent666 drops by and provides some Kiwi input.

 

[KootK]

My understanding of this in Canada and the US:

1) Depending on how seismic the project location is and how important the building is:

a) Sometimes you can do it without restriction.

b) Sometimes you can do it within limitations.

c) Sometimes you can't do it at all.

d) Sometimes you're obligated to use a fancier analysis method such as RSA or Non-Linear Time History Analysis.

e) Never are you penalized with having to assume a lower ductility system.

2) My personal feeling is that, where torsional resistance is provided by a single core, ductility should absolutely be penalized. When I imagine a closed-ish, thin walled tube responding to an earthquake torsionally, that doesn't feel very ductile to me. The portion of the response that would be St.Venant torsion would have to yield all of your wall horizontal bars prior to forming a torsional "hinge" wouldn't it?