Flat roofs on which water may accumulate may require analysis to ensure that they are stable under ponding conditions. A flat roof may be considered stable and an analysis does not need to be made if both of the following two equations are satisfied:
C p + 0.9 C s £ 0.25I d ³ 25S 4 / 10 6
Where C p = 32 L s L 4 p / 10 7 I p
C s = 32 SL 4 s / 10 7 s
L p = length, ft (m), of primary member or girder
L s = length, ft (m), of secondary member or purlin
S = spacing, ft (m), of secondary members
I p = moment of inertia of primary member, in 4
(mm 4 )
I s = moment of inertia of secondary member, in 4
(mm 4 )
I d = moment of inertia of steel deck supported on secondary members, in 4 /ft (mm 4 /m)
For trusses and other open-web members, I s should be decreased 15 percent. The total bending stress due to dead loads, gravity live loads, and ponding should not exceed 0.80F y , where F y is the minimum specified yield stress for the steel.
