Response Spectrum Scaling Per As1170.4

Having been used to UBC97 and ASCE 7-10 in scaling of dynamic seismic loads, using the Australian code is totally new to me. So after reading a number of references, probing the internet for some detailed how to’s, and consulting people who have had previous experience in using the Australian code for the dynamic analysis via ETABS, what I’ve gathered thus far is: you DON’T SCALE the calculated dynamic forces in relation to static base shear. Hence, the scale factor to be provided in ETABS is g x 1000 in m/s2 or 9810.

In ASCE and UBC, you need to first multiply the forces obtained via response spectrum analysis by the importance factor I divided by the response modification coefficient R.

I’ve had the same expectations for the Australian code.

But given the premise above that you directly use g as the initial and final scale factor, it met my strong initial resistance. It just didn’t “seem” to make any sense.

That is until I looked long enough at the response spectrum curve.

It dawned on me then that for AS 1170.4 it is already multiplied by Sp/mu where Sp is the structural performance factor and mu (greek character frequently used for friction coefficient) is the structural ductility factor. So the point is, the scale factor equal to the gravitational acceleration g is ALSO reduced.

So that’s settled.

What I find intriguing though (and I find it perplexing) is that while American codes ensure that dynamic forces are calibrated via the shear derived from the conservative static equation, the dynamic base shear based on the Australian code isn’t limited by such restrictions.

Big deal. What’s wrong with that anyway?

If you come to think of it, based on my experience and I’m sure many of you will agree, dynamic loads almost always spare the structure of the more conservative loads of the static. So what if the reverse happens, what else will be the saving grace if it isn’t for this boundary condition? Maybe I’m bound to just wait and see.

As I’ve said I’m still new at this. And that’s the reason why I’m opening this up for scrutiny while reviewing other sources of information. Don’t worry, I’ll let you know what I’ll stumble upon.