Assuming Metal Roof Deck Rigid Diaphragm

[TKIL10]

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.

 

[lkjh345]

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

 

[sundale]

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 members and the deck sidelaps.

If the diaphragm deflects 200% of the average story drift (i.e. deflection of the vertical lateral system) then it is a "flexible" diaphragm per the IBC. If not, then it's "rigid" diaphragm.

Most metal deck diaphragms deflect very little and are thus "rigid" in my experience.

 

[JAE]

sundale's reference to the IBC drift vs. diaphragm deflection is the code defined line between rigid and flexible.

 

[rsaunders]

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