Comments on: Stability – AISC’s Direct Analysis Method

By: Rhett

Rhett — Tue, 15 Jul 2014 08:51:27 +0000

Thanks for your kind explanation.

In addition, regarding P-d (small d) effect, you mentioned that the moment may be “semi” captured if the column element is broken into several nodes.

I agree that the P-d moment may be added, but in that case there is a problem. Many software automatically calculate whether the AISC’s requirements are satisfied or not, and it is broadly called as ‘Code Check’.

If, like you said, the element is divided with some parts using additional nodes, the software performs code check along the divided length. So the unbraced length will be shorter than the original one and it causes wrong results. In this case, I think, the only way to solve the problem is to input the unbraced length manually.

Could it also be controlled easily in software?

By: Ryan Freund (@ryanfreund)

Ryan Freund (@ryanfreund) — Fri, 27 Jun 2014 14:34:07 +0000

That is a good point. Some software packages have the direct analysis method built in, however if it is not I suppose you could use the reduced E in your member design. Or manually determine the member capacity. Or even have two models. Actually you may need two models anyway or atleast run the model twice as you would not check the drift requirements using the reduced stiffness.
Here http://communities.bentley.com/products/structural/structural_analysis___design/w/structural_analysis_and_design__wiki/6011.asce-7-aisc-360-and-the-direct-analysis-method-in-the-ram-structural-system.aspx

Is one software that walks through the DAM. RISA is another one that has this option and I’m sure there are many more.

By: Rhett

Rhett — Fri, 27 Jun 2014 02:30:36 +0000

Actually, how do we reduce the stiffness about 20% in structural analysis software? If we adjust E or A (E or I), then the required strength will be correctly adjusted following the modified stiffness. But how about the available strength? If we adjust E or A (E or I), then the available strength will be wrong.

By: Ryan Freund

Ryan Freund — Wed, 24 Jul 2013 02:20:50 +0000

The short answer – yes, notional loads should be applied for all load combinations. However –
If the ratio of the second order drift to first order drift (referred to as B2 in the approximate second order method) is less than 1.5 (or less than 1.7 if you are running the analysis using the reduced stiffness) than the notional loads may be applied as minimums. Meaning – for load combinations that have lateral forces (i.e. wind and seismic) if the lateral force due to wind/seismic is greater than the notional load then the notional load does not need to be applied. However if the load combination does not include any lateral forces (no wind/seismic) than the notional load should be applied.

If the drift ratio is greater than 1.5 than you must apply the notional load as additive to all load cases in the same direction as the lateral force (i.e. worst case).

By: Girish

Girish — Tue, 23 Jul 2013 21:39:25 +0000

I am designing a three tier pipe rack by direct analysis method in Risa 3D using AISC 13th edition. Is it compulsory to apply notional loads when we are using direct analysis.