Flexible Diaphragm Deflection
Flexible Diaphragm Deflection
(OP)
Along a similar vein as my previous thread is the question of diaphragm deflection between lateral supports.
SDI has a procedure and tons of tables to determine the deflection of a metal roof deck between supports.
In the past, I've considered this carefully in the design and spaced lateral frames to keep this deflection reasonable (2" +/-, depending on the specifics)
Often, this means some sort of lateral restraint at every 120' or so (typically 40'x40' bays in industrial structures).
Recently, I've seen several buildings that utilize braced frames at exterior bays only. For a 300' wide building, this means that there would be significant (8" +/-) deflection between the bays, which strikes me a grossly unacceptable.
Am I mis-interpreting SJI's deflection calculations? It seems like this is significant when you think about the exterior beams/columns deflection with the deck this much and all the second order effects that it would create.
As always, thanks in advance.
SDI has a procedure and tons of tables to determine the deflection of a metal roof deck between supports.
In the past, I've considered this carefully in the design and spaced lateral frames to keep this deflection reasonable (2" +/-, depending on the specifics)
Often, this means some sort of lateral restraint at every 120' or so (typically 40'x40' bays in industrial structures).
Recently, I've seen several buildings that utilize braced frames at exterior bays only. For a 300' wide building, this means that there would be significant (8" +/-) deflection between the bays, which strikes me a grossly unacceptable.
Am I mis-interpreting SJI's deflection calculations? It seems like this is significant when you think about the exterior beams/columns deflection with the deck this much and all the second order effects that it would create.
As always, thanks in advance.






RE: Flexible Diaphragm Deflection
RE: Flexible Diaphragm Deflection
Does the code weigh in on how much deflection is permissible before the eccentricity must be specifically addressed? I don't image that ever 1/4" deflection needs to get 5 pages of calculations, but is this a judgement call or is it specifically addressed somewhere?
RE: Flexible Diaphragm Deflection
RE: Flexible Diaphragm Deflection
Maybe the designers of these buildings you have seen are performing their chord calculation in a different manner. Not sure what your exterior wall is in this situation, but there are several elements that can be considered for chord action and stiffness that many designers overlook. Such as the ledger, the wall itself, all of the rebar in the wall and parapet etc... Maybe these buildings you have seen have a large chord element you're not seeing.
Just a thought.
And yes, I agree, as long as you have checked deflection compatibility of your gravity carrying elements which includes both forced deflections and accompanying p-delta effects, I don't see an issue with it.
RE: Flexible Diaphragm Deflection
When you say 'columns' are you referring to all columns or only columns that participate in the LFRS?
EIT
www.HowToEngineer.com
RE: Flexible Diaphragm Deflection
Also, I typically use 0.70 x the service load for wind for deflection calculations to get to a 10-year recurrence interval. Make sure you're using service loads, not strength loads.
RE: Flexible Diaphragm Deflection
Is that allowed by code somewhere to reduce the interval? (I'm aware of the IBC allowing this for components and cladding but not for mwfrs)
RE: Flexible Diaphragm Deflection
Going hand in hand with my previous post, this question was raised by the review of drawings by others. "What they're using as chords" would need to be another thread all together and is an excellent question.
The general concensus above seems to be that anything is ok (from a strength standpoint) and long as the p-delta effects are included in the design. How does this typically work it's way into the work flow for a typical warehouse design? I`m not aware of a way to include it in a ram Model, and checking each column individually could be cumbersome on a large building. Is there an efficient way to check a number of columns simultaneously?
RE: Flexible Diaphragm Deflection
RE: Flexible Diaphragm Deflection
But yes as JAE describes is what I would expect. If you are designing with steel, then the Direct Analysis Method already deals with this using Notional loads. The idea of the DAM could be applied to other situations as well.
EIT
www.HowToEngineer.com
RE: Flexible Diaphragm Deflection
RE: Flexible Diaphragm Deflection
RE: Flexible Diaphragm Deflection
RE: Flexible Diaphragm Deflection
Table 1604A.3 attached, footnote f.
RE: Flexible Diaphragm Deflection
RE: Flexible Diaphragm Deflection
For MWFRS drift, our company standard is to limit story drift at the center or rigidity to H/400 and at any point to H/300 under 0.70 x ASCE7-05 Wind (50-year recurrence service-level wind).