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jnichol (Structural)
13 Feb 07 17:58
Where do I find some good information (with a picture preferably) on

1. Building VLLR drift limits
2. Diaphragm deflection limits between the MFRS.

I have design guide 3, but that all seems greek. What do they mean drift perp to wall? Which wall, the shearwall or the wall which is perp to this wall. I know you always hear h/400 for the VLLR, but why. Where is that written in stone, and why can a prefab building drift so much more.

Thanks for any help.
twinnell (Structural)
13 Feb 07 18:38
Depends on what you are designing for.  There are seismic drift limits in the IBC code.  Design guide 3 is for serviceability and yes it is very hard to read.  Pre-Engineered metal buildings are allowed a limit of L/120 regardless of cladding, which I think is a political thing...lobbyists and all for the metal building association.  If you have masonry veneer, then you should use somewhere between L/600 and L/480.  A masonry association recommends L/730, but that gets a little crazy.  I would use design guide 3 except I would use the masonry veneer limit above.  Another thing you can consider is you wind speed.  It is based on a 50yr occurrence.  Who cares if your brick cracks a little in 50 yrs.  You can adjust your serviceability design wind speed by converting it to a 10 yr wind speed and reduce your wind speed by around 30%.  The conversion chart is in the back of ASCE 7.  Makes a pretty big difference if you can't meet your serviceability drift limit.  You still want to set your strength limit using the standard wind speed.

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