Quartering Wind Loads on a Free Standing Tank

[Bwnarrow3]

Hi,

I am currently designing a foundation for a substation breaker, which is basically a rectangular tank with cylindrical bushings on top of it. When developing wind loads on the tank portion specifically, we currently consider the breaker a solid sign, with the legs supporting it above the ground, and develop the Cf according to "solid sign" criteria.

My question relates to the quartering wind case. Would the Cf value be different because it now acts on the tank along the diagonal? Figure 6-21 in ASCE 7-05 notes a few Cf values for tanks, but it mentions in the commentary that these are specifically for rooftop structures, which is not applicable to this scenario. Figure 6-20 appears to include loads that are both normal and oblique to the sign for case B (and case C), but this isn't explicitly mentioned in the notes below the figure.

Would the Cf be the same as the normal case but instead the wind pressure adjusted for each orthogonal direction by multiplying by sin(45) & cos(45)? Would you anticipate this resulting in a larger base shear than a normal wind case?

Thanks,

 

[JStephen]

I'd suggest referring to a newer version of ASCE 7 (-16 or -21).
In ASCE 7-16, Section 29.4 applies to "other structures (chimneys, tanks, open signs...), whether ground or roof mounted..."

Then in Figure 29.4-1, Note 1, "The design wind force shall be calculated based on the area of the structure projected on a vertical plane normal to the wind direction. The force shall be assumed to act parallel to the wind direction."
I don't have ASCE 7-05 in front of me to compare, but I think that is more or less the comparable section to what you're referring to.

There are additional sections in ASCE 7-16 for round tanks, round tanks on legs, grouped round tanks, etc., so make sure there is no other section applicable to the item.

Also, I will note that while the ASCE 7 seems fairly detailed on some of this, there is actually a fair bit of uncertainty as to how some of the provisions should be applied in practice.

 

[Bwnarrow3]

Thank you for your response.