jeffhed
Structural
- Mar 23, 2007
- 286
I have always been taught (and required for plan review) to use skip loading on roof members, especially cantilevers. But I have always wondered if it was really required as I can't find anywhere in the code(s) that specifically state that pattern loading is required on roofs. All I can find where it actually says to use skip loading on roofs is IBC 1607.12.1. which reads as follows:
1607.12.1 Distribution of roof loads. Where uniform roof live loads are reduced to less than 20 psf (0.96 kN/m2)
in accordance with Section 1607.12.2.1 and are applied to the design of structural members arranged so as to create
continuity, the reduced roof live load shall be applied to adjacent spans or to alternate spans, whichever produces
the most unfavorable load effect. See Section 1607.12.2 for reductions in minimum roof live loads and Section 7.5
of ASCE 7 for partial snow loading.
When I read that section, it seems to imply that if I reduce the roof live load to less than 20 psf then I must consider skip loading and I should refer to ASCE 7 section 7.5 and apply the skip loading similar to how I would snow loads. It seems to me when I read it, that skip loading is ONLY required if you reduce live loads to less than 20 psf. If this was always required, then why would section 1607.12.1 talk about reduced roof live loads at all? Wouldn't that section just say the same thing but without the reference to reduced live loads? Thinking about it logically, if I reduced the roof live load, it would make sense that the skip loading should be looked into because the skip loading can control the design and where you have reduced the live loads to lower than the 20 psf, it becomes even more critical. But if you don't reduce the roof live load then you can ignore the pattern loading because you have designed for the higher load. Am I interpreting this incorrectly? Applying the roof skip loading can make huge difference. Where snow loads are concerned, skip loading is a must because of drifting, etc. Bur for just roof live load it seems like it isn't necessarily required in the building code.
1607.12.1 Distribution of roof loads. Where uniform roof live loads are reduced to less than 20 psf (0.96 kN/m2)
in accordance with Section 1607.12.2.1 and are applied to the design of structural members arranged so as to create
continuity, the reduced roof live load shall be applied to adjacent spans or to alternate spans, whichever produces
the most unfavorable load effect. See Section 1607.12.2 for reductions in minimum roof live loads and Section 7.5
of ASCE 7 for partial snow loading.
When I read that section, it seems to imply that if I reduce the roof live load to less than 20 psf then I must consider skip loading and I should refer to ASCE 7 section 7.5 and apply the skip loading similar to how I would snow loads. It seems to me when I read it, that skip loading is ONLY required if you reduce live loads to less than 20 psf. If this was always required, then why would section 1607.12.1 talk about reduced roof live loads at all? Wouldn't that section just say the same thing but without the reference to reduced live loads? Thinking about it logically, if I reduced the roof live load, it would make sense that the skip loading should be looked into because the skip loading can control the design and where you have reduced the live loads to lower than the 20 psf, it becomes even more critical. But if you don't reduce the roof live load then you can ignore the pattern loading because you have designed for the higher load. Am I interpreting this incorrectly? Applying the roof skip loading can make huge difference. Where snow loads are concerned, skip loading is a must because of drifting, etc. Bur for just roof live load it seems like it isn't necessarily required in the building code.
