IBC 2018 vs. ASCE 7-16 Load Combinations

[Futzin]

The load combinations presented in Section 1605.2 and 1605.3 of IBC 2018 for LRFD and ASD, respectively, are generally equivalent to those presented in ASCE 7-16 Sections 2.3 and 2.4 for strength design and ASD, respectively, albeit arranged differently. When examining them in detail, however, there are some small discrepancies. In the instances where the LCs conflict with one another, which do y'all take as the prevailing code? Intuitively I think it should be IBC, but it's further complicated that AISC 360-16 indicates that ASCE 7 combinations shall be used directly whilst ACI 318-14 effectively reproduces the ASCE 7 combos therein.

An example of the discrepancies is in Section 1605.3.1 of IBC 2018 which indicates flat roof snow loads of 30 psf or less need not be combined with seismic loads. This exception is only noted for ASD load combos and looks like a shoddy re-write of Section 12.7.2 of ASCE 7-16 where flat roof snow loads of 30 psf or less need not be considered in the effective seismic weight but doesn't exclude it from the seismic load combos entirely.

Another example (also snow-related) is the f[sub]2[/sub] factor in the IBC LRFD combos that differs between 0.2 and 0.7 depending on the snow shedding characteristics of the roof. In the equivalent ASCE 7 load combo, this factor does not vary and is always taken as 0.2.

Any thoughts?

 

[phamENG]

Are you asking legally, or the best engineering choice?

Legally, it's the IBC as adopted and edited by your local/state authority. For example, in Virginia we have the Virginia Uniform Statewide Building Code, which is just the 2015 IBC with some minor edits here and there. That is what is called out in State Law. That, in turn, references the various supporting documents (ASCE 7 among them), but in a limited way. So if the IBC (as edited) tells you what load combinations to use, you must legally use that as a minimum. There is no prohibition on using loading that creates a more critical load effect.

That last part leads to the best engineering choice. If a load combination in the IBC leads to 2x10 rafters in a roof but the ASCE 7 load combo leads you to 2x12, you'd be in compliance with the code if you spec'd 2x10s. Though if something were to fail, you could get caught against the standard of care if all other engineers in your area are using ASCE 7 load combos and would have spec'd 2x12s. So if there's a difference, I'd use the more restrictive.

 

[JoshPlumSE]

It's the whatever the local building code requires you to use. That will generally be the IBC. However, some jurisdictions in high snow areas have modifications of the building code specifically related to snow load requirements. If so, this trumps the IBC.