ASCE 7-10 Wind Load Questions
ASCE 7-10 Wind Load Questions
Say you have a trussed tower and want to use either Fig. 29.5-2 for lattice framework or 29.5-3 for trussed towers. The Cf factors come out to be very different. From studying the figures and footnotes I’m under the impression that the lattice framework wind load gets applied to both faces while the trussed tower wind load gets applied to a single face. I believe the footnotes are trying to explain this, but it’s still not 100.00% clear to me so I’m hoping someone can provide clarification or at least tell me what they’ve seen commonly practiced.
I ran some sample calculations comparing the figures for a square tower. For ‘ratio of solid area to gross area’ of 0.05 to 0.30, the wind pressure for trussed towers comes out roughly 60-90% higher than lattice framework, this difference seems way too high for them to be applied the same way. If you apply lattice framework to both faces and trussed tower to a single face, then the overall wind force from lattice framework comes out to 5-25% larger than the trussed tower, a much more reasonable difference with lattice framework being a little bit more conservative.
To clarify, I’m using a single face for determining the ‘ratio of solid area to gross area’ in each case so the ‘ratio of solid area to gross area’ is the same for both figures. The difference is when the wind load is applied, the lattice framework would be applied to both faces while trussed tower would be applied to a single face.
In accordance with posting policies, I did look up this topic in the forum prior to posting and yes, this topic has been brought up multiple times in the past, but I found that addressing this question is a necessary “pre-requisite” for my following questions to make sense. Generally speaking, these past posts have mostly come to similar conclusions to what I’ve presented, but I’m all ears for fresh perspectives. I’ve included links to some of those previous threads below if interested:
Regarding Fig. 29-5.3 Footnote 4: “when the wind is directed along a tower diagonal” is referring to a wind direction such as a North-East wind on a square plan cross section, right? If so, then how would this diagonal wind force be applied to the tower? Would you apply it to the two most exposed faces? In other words, how would you calculate the value for Af as it’s used in eq. 29.5-1 for calculating F? Also, would the ‘ratio of solid area to gross area’ change? For this figure specifically the full definition is ‘ratio of solid area to gross area of one tower face for the segment under consideration’.
When designing a trussed bridge such as a conveyor or utility bridge (not AASHTO) that has very similar construction to a trussed tower except that it’s horizontal instead of vertical (i.e. 4 wide flange chords, angles for web members, etc.), would you consider calculating wind loads per Fig.29.5-3 an appropriate alternative to calculating wind loads per Fig.29.5-2? Please provide reasoning behind why you think one way or the other.
Does anyone know if there’s a place on ASCE’s website or an ASCE contact that you can submit technical questions to? i.e. AISC offers their “Steel Solutions Center” which lets you submit technical questions and they’re very responsive and helpful, I wish other codes had something like that.
Sorry for such a long post, please feel free to respond to any part of it individually.