AWWA D100 vs. API 650 Roof Design (Specifically unbalanced snow loading)

[jimmyhutmacher]

Hoping the experts can clear something up for me. I think we can agree AWWA D100 is generally a more conservative design standard when compared to API 650. So I'm wondering why there's such a large difference in roof design with API 650 being more conservative. API 650 requires roofs to be designed for unbalanced snow loading while AWWA D100 does not. This leads to a significant difference in rafter count/size and compression ring size for tanks with a roof >10 deg. Thoughts?

Also, is anyone aware of the reasoning for adding the unbalanced snow loading to API 650? Was there a slew of roof failures due to unaccounted snow drifting? Are there any statistics on these failures?

There are other differences in roof design including the method to the find roof snow load and the rafter spacing equation itself, but I'm particularly curious about the difference in using balanced vs. unbalanced snow loading.

 

[IFRs]

Note also that AWWA used to not allow credit for higher strength rafters unless the live load was very high, specifically you have to use A36 even though most of the rafter shapes are made in 50ksi steel. I'm not sure if that is still in the standard.

 

[jimmyhutmacher]

That's still part of the latest AWWA D100-21 and I've always wondered why they have that limitation. Wish they gave background on some of these rules.

 

[LittleInch]

Probably because it gives you more strength to resist the snow loads...

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[IFRs]

My guess is both have the same modulus of elasticity which governs deformations in bending, stronger material would be a lighter section with less moment of inertia and then for the same factor of safety on yield the higher strength member would bend more, possibly ponding leading to higher ice loads.

 

[JStephen]

The major issues for water tank rafters are corrosion and elastic stability, and increasing the material strength doesn't really help in either case.