JAE
Structural
- Jun 27, 2000
- 15,591
In Appendix 7 of AISC 13th Edition the Direct Design Method is described. I understand it OK - have gone to seminars, etc. and written an in-house procedure to use it along with our RISA 3D program.
The only thing that has come up that has us questioning the use of it is that it doesn't seem to take into consideration the possibility of a long, flexible diaphragm in the building being designed.
Notional Loads are applied laterally to the structure being analyzed which represent an initial out-of-plumbness of the frame of L/500 (0.002 x dead load).
If you take a large warehouse - say 200 ft x 200 ft - and have a flexible metal deck vs. a theoretically rigid diaphragm, it seems that the notional load doesn't get affected by it.
The AISC procedure doesn't seem to directly include any provision for the [blue]rigid vs. flexible[/blue] deck behavior.
With a flexible diaphragm, the [Δ] of the braces on either end of the diaphragm will induce leaning in the interior gravity-only columns. Also, the distortion of the metal deck under lateral loads will also add to that leaning of gravity columns. This combined leaning will create additional P[Δ] lateral thrust forces that will further distort the deck AND the braces. This further distortion will cause additional P[Δ] forces etc. until the building converges or collapses.
Again - the Direct Design Method doesn't explicitly address this other than in the commentary it mentions that Notional Loads are for the purpose of accounting for "...any other effects that could induce sway that is not explicitly considered in the analysis".
So my question is - If we first design the diaprhagm, and determine some lateral deflection - and use an INCREASED Notional load (i.e. higher than 0.002Yi) would we be OK? If so how much higher is appropriate?
Are there any papers or articles out there that address this issue? I've posted a question to AISC Technical staff and awaiting a reply but I thought I'd ask here.
The only thing that has come up that has us questioning the use of it is that it doesn't seem to take into consideration the possibility of a long, flexible diaphragm in the building being designed.
Notional Loads are applied laterally to the structure being analyzed which represent an initial out-of-plumbness of the frame of L/500 (0.002 x dead load).
If you take a large warehouse - say 200 ft x 200 ft - and have a flexible metal deck vs. a theoretically rigid diaphragm, it seems that the notional load doesn't get affected by it.
The AISC procedure doesn't seem to directly include any provision for the [blue]rigid vs. flexible[/blue] deck behavior.
With a flexible diaphragm, the [Δ] of the braces on either end of the diaphragm will induce leaning in the interior gravity-only columns. Also, the distortion of the metal deck under lateral loads will also add to that leaning of gravity columns. This combined leaning will create additional P[Δ] lateral thrust forces that will further distort the deck AND the braces. This further distortion will cause additional P[Δ] forces etc. until the building converges or collapses.
Again - the Direct Design Method doesn't explicitly address this other than in the commentary it mentions that Notional Loads are for the purpose of accounting for "...any other effects that could induce sway that is not explicitly considered in the analysis".
So my question is - If we first design the diaprhagm, and determine some lateral deflection - and use an INCREASED Notional load (i.e. higher than 0.002Yi) would we be OK? If so how much higher is appropriate?
Are there any papers or articles out there that address this issue? I've posted a question to AISC Technical staff and awaiting a reply but I thought I'd ask here.
