assuming metal roof deck rigid diaphragm

huangyhg

assuming metal roof deck rigid diaphragm
i believe i read somewhere that when you use a heavier gage of metal roof deck you can assume the roof diaphragm to be rigid. has anyone else found this provision. i believe the cut-off between rigid and flexible is 18 gage.
tkil10:
someone correct if i am wrong, but i do not believe you can make a blanket statement about one gage of deck being 'rigid' and another being 'flexible'. the term is dependant on a diapragms displacement for a given span and width.
sdi, in the diaphragm design manual defines this as g' with units of k/in of dispalcement.
flexible diaphragms have a g' of 6.67 to 14.3
semi-flexible = 14.3 to 100
semi-rigid = 100 to 1000
rigid = greater than 1000
you need to calculate the diaphragm midspan deflection using the sdi formulae combining shear and flexural deflections. the metal deck diaphragm stiffness is a function of the deck profile, the deck gage, the deck span, the deck attachment to supporting   
sundale's reference to the ibc drift vs. diaphragm deflection is the code defined line between rigid and flexible.
from ibc 2003 - 1617.5.3:
"diaphragms constructed of untopped steel decking or wood structural panels or similar light framed construction are permitted to be considered as flexible."  
in ibc 2006, see 1613.6.1