Wind Pressure on Trussed Towers - Distribution Type?

[jbuening]

We are designing agriculture support towers with a square footprint in RISA. We have calculated our wind pressure (67psf for this 6'x6' footprint) from the formulas in ASCE 7-10 and utilized the Trussed Tower Force Coefficient from Fig 29.5-3. I am a little confused on if the wind pressure is to be distributed as if it were fully cladded (two way/one way distribution in RISA), or just that pressure applied to the exposed area of the column (Open Structure distribution in RISA). The two way/one way seems a bit overkill in my opinion. When I apply an area pressure to one side of the tower using Open Structure, the resulting transient loads end up being a line load of 26plf to all four columns (strange since I would expect it to be just that plane, or only two columns). These are HSS4x4 columns.

Let me know your thoughts on this please.

 

[JoshPlumSE]

The open structure area loads in RISA do apply to all members equally, not just to members in that plane. And, the back frame will get the same loading as the front frame... meaning no reduction for shielding.

When we implemented this feature, there was some wording in ASCE-7 that specifically prohibited the consideration of shielding. The rational being that a small angle to the wind load would mean that the back frames get essentially the same load as the front frames.

I'm not sure that this is how engineers who work with trussed towers and such really do their design. But, that's the code provision we were looking at when this was implemented.

For what it's worth, we have a "user request" to change this or to add an option to only consider the current plane. I'll add your name to the list of users who have requested this enhancement.

 

[jbuening]

Thanks Josh. It was my understanding that one either uses the Open Signs/Lattice Framework coefficient and apply it to leeward and windward columns (coefficient is lower and thus smaller pressures), or use the Trussed Tower coefficient (which results in higher pressures) and only apply it to the windward face.

 

[JoshPlumSE]

I'm not an expert enough on the wind codes to give you guidance. I'd have to pull open the codes to review them. My experience is with the following:
1) Open structure using much, much older codes (ASCE 7-1995)
2) Building Structures using the current ASCE codes
3) Lattice communication towers design to the TIA-G standard

Based on my experience I would guess that your understanding is correct conceptually. But, ultimately you'll have to pull open the code and review it yourself to make sure that it is correct. If I recall correctly, the SEAoW has a design guides / commentary / manual on the wind code that might be useful to you. Haven't looked through it in years. But, when I did, I found it useful.