ASCE 7-10 Wind Load Questions

[Steel5]

I have a number of questions regarding ASCE 7-10 wind loads.

1.

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:

http://www.eng-tips.com/viewthread.cfm?qid=257769

http://www.eng-tips.com/viewthread.cfm?qid=327871

http://www.eng-tips.com/viewthread.cfm?qid=327745

http://www.eng-tips.com/viewthread.cfm?qid=335846

http://www.eng-tips.com/viewthread.cfm?qid=349391

http://www.eng-tips.com/viewthread.cfm?qid=349390

2.

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’.

3.

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.

4.

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.

 

[ENGINEER92]

1. The trussed tower force coefficients, in Figure 29.5-3, is a simplified method. It being simplified typically means it is a conservative approach. The TIA-222 tower codes has more in depth procedures to get lower force coefficients for trussed towers. And if you are using figure 29.5-3 I believe you still apply wind pressure to all faces. If a member is four times the width of the shielding member away it does not experience any shielding.

2. Yes, it is referring to wind hitting the corner of the tower. Not sure what you mean by how would you apply that. And yes, the ratio of solid to gross would potentially change for the corner wind force.

3. Yes, I would use Figure 29.5-3. Values are based off of wind tunneling testing and the orientation of truss should not change the force coefficients.

4. Not that I know of. They typically host webinars that address typical questions about the ASCE.

 

[Steel5]

Thanks for the response. I respectfully disagree that you would apply the wind pressure to all faces for Fig.29.5-3. If that were the case, then Fig.29.5-3 would always give larger overall wind force vs. Fig.29.5-2, and by a very significant margin (60-90% difference for e=0.05-0.30 as outlined in the original post). A trussed tower could be considered a lattice framework, so upon that realization everyone would always use Fig.29.5-2 for towers and Fig.29.5-3 would be redundant and pointless.

That’s why I think the difference comes down to how these wind pressures are applied. Fig. 29.5-2 applied to two faces vs. Fig. 29.5-3 applied to one comes out much closer to each other, and depending on the specific nuances of the structure either one could control in different cases. That seems to make more logical sense to me. And it seems that most people came to a similar conclusion in those other threads that I linked.

At the end of the day I can’t prove it one way or the other. I think the footnotes are trying to address this but it’s still not 100% clear, which is why I asked the question. I could very well be wrong, so I appreciate the perspective.

Regarding question 2, I understand your confusion on what I’m asking, it only really makes sense under my interpretation of question 1. (That very well could be an argument by itself in favor of your interpretation of question 1.)

 

[WARose]

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.

In 29.5-2, you apply that to every member. The numbers you come out with for 29.5-3, it's just projected against one face of a tower. (That is facing the wind.)

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.

If I understand you correctly: No. That's kind of a catch-22 with 29.5-3: it is not appropriate if we are talking more than about 2-3 bays behind the windward face. At that point either you can use 29.5-2 (i.e. load each individual member).....or you can use some methods in Chapter 5 of 'Wind Loads For Petrochemical and other Industrial Facilities' (by ASCE).