huangyhg

direct design method - aisc
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 rigid vs. flexiblefff"> 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 pδ lateral thrust forces that will further distort the deck and the braces. this further distortion will cause additional pδ 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.

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hmm, interesting question. i am just starting to get into the ddm from the new manual. (i confess that i still use the green book most of the time). will be interesting to see how other people feel about this.
like you, i am working on an in-house procedure and presentation on the topic. unfortunately, i have not been to any seminars on the topic. have you found any good resources on the web regarding ddm?
thanks
i found this quote at gostructural.com:
"this difference highlights the fact that low-rise structures with high gravity-to-lateral load ratios are often more stability sensitive than multi-story structures and warrant careful assessment of second-order effects."
this sort of confirms my worry/question about this issue.

since deck distortion is part of the nonlinear contribution, i think that they expect this flexibility to be input into the model so that it will be included in the p-delta/delta analysis. i know that doesn't help much, but that is what i anticipate the intention is (3d model analysis seems to be required).
i have not seen any papers on using an increased initial notional load to account for diaphragm flexibility.
haynewp - i'd agree with you - it's just that including correct diaphragm stiffness characteristics is tough to do in 3d models.
and for large warehouse buildings, a 3d model isn't always that necessary - although with revit, bim, etc. we probably will in the future anyway.

before you say it, i know that making a 3d model is really not practical for most single story box buildings. i am not really sure what i would do about including the diaphragm flexibility effects.
you posted that one right as i was typing (again). lol
woo... this esp thing has got me scared...

but you use the larger of the lateral loads whether applied or notional.
so in your example about the warehouse the applied lateral loads would control the design of the lateral force resisting columns not the notional loads.
in the seminar i went to they said use one or the other, and that notional load was just considered a minimum unless you had very large second order displacements compared to the first-order displacements, in which case you add them together.

i think you apply the notional loads always. this is to estimate initial structure out-of-plumbness.
i know what you are confusing, you can either put in the notional loads or model in the initial out-of-plumbess directly.

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