ASCE7 Wind Load on Lattice Framework and Trussed Tower

[RFreund]

I posted this in the wrong forum 'group' last time and wanted to try it here instead.

In regards to ASCE7 -05 or -10 do I have the following correct?

Lattice Framework:
Ratio of Solid Area to Gross Area - Use area of of the face of the elevation (or plane) that you are analyzing. Meaning that if you had a square section tower the solid are would only count the windward face of the front member. You would not count the front and back windward member face areas.
Area Af - This area includes both the windward face area of the front and back members.

Trussed Tower:
Ratio of Solid Area to Gross Area - Use area of of the windward face of the elevation (or plane) that you are analyzing. Meaning that if you had a square section tower the solid are would only count the windward face of the front member. You would not add the front and back member face areas.
Area Af - This area includes the windward face area of the front members ONLY.

So the difference is that Trussed towers deal with the projected area on a plane for both the ratio and the force (almost as if shielding is already considered) where as lattice frame work uses the projected area for the solid ratio but then applies the wind to all members windward face.
Thanks in advance!

EIT

http://www.HowToEngineer.com

 

[RFreund]

One Response:

RF...a lattice framework is assumed to exist in one plane. A trussed tower has multiple planes, each contributing (to some extent) to the projected load. So yes, your assessment is correct.

EIT

http://www.HowToEngineer.com

 

[RFreund]

My response:

Ron -
Response is leading me to beleive that I have it backwards. Let me explain further.
Lattice Framework:
Ratio of Solid Area to Gross Area - Use area of of the face of the elevation (or plane) that you are analyzing. Meaning that if you had a square section tower the solid are would only count the windward face of the front member. Update: Meaning if there is an angle on the front face you have the area of the windward face of this angle. The angle that is on the back face (say 18" back) and is covered by this angle does not contribute to the solid area. You would not count the front and back windward member face areas.

Area Af - This area includes both the windward face area of the front and back members. Update: Meaning the angle on the front face receives wind pressure and the angle on the back face receives pressure even though its is 'covered' by the front face angle.

Trussed tower:
Ratio of Solid Area to Gross Area - same as Lattice frame work.

Area Af - Use the same solid area as used in the Ratio above. Do not apply wind load to members 'covered' by members on the front face of the tower.

This is still correct or do I have this backwards?

Thanks again!

EIT

http://www.HowToEngineer.com

 

[transmissiontowers]

Lattice transmission towers and wind is an interesting topic. We use ASCE 74 which refers to ASCE 7 for maps. We also use NESC which dictates the wind on structure. Take a look at NESC-2007 Rule 252B2c:

c. Latticed structures

Wind loads on square or rectangular latticed structures or components shall be computed using a
force coefficient (shape factor) of 3.2 on the sum of the projected areas of the members of the
front face if structural members are flat surfaced or 2.0 if structural surfaces are cylindrical.
The total, however, need not exceed the load that would occur on a solid structure of the same
outside dimension.

EXCEPTION: The force coefficient (shape factor) listed under Rules 252B2a, 252B2b, and 252B2c may
be reduced if wind tunnel tests or a qualified engineering study justifies a reduction.​

The NESC uses a 1.6 factor for angles and does not consider shielding for the back leeward face because they just double the 1.6 for the wind on the front and back. The 2 side faces are considered to be shielded by the legs when the wind is normal to the front face. If you use a yawed wind, the wind will blow on all members.

If you use PLS-CADD for line design, there is a wind on tower method that uses ASCE-74 to compute wind and they use a fluid mechanics based method to apply wind on every member in the tower, which is much better in my opinion for accounting for the oblique or yawed wind. You just have to account for the missing redundant members by either putting them in the model or adjusting the section drag factors to account for them.

I'm assuming that you use PLS-CADD software for the T-Line analysis since they have about 95% of the market. If you don't use it, I would highly recommend that you get it.

_____________________________________
I have been called "A storehouse of worthless information" many times.

 

[RFreund]

TT - Thanks for the info. We actually don't do T-Line's but we were analyzing a roof top steel structure that supports communication antennas. It resembled a 'Trussed Tower' or "Lattice Tower' of sorts but it was a pretty 'beefy' structure and the replacement of antennas would increase the load but only marginally. I wanted to show that the increase in wind force was less than 5% of the total wind force.

EIT

http://www.HowToEngineer.com

 

[transmissiontowers]

OK, now I see. I thought since you came to this forum it probably had wires on it. In your case, you probably need to look at the EIA/TIA 222 Rev G which deals with wind on lattice structures that support antennas. They adapt the ASCE 7 wind maps for use with their structures and you may have to deal with twist and sway if you have microwave dishes that point to another antenna tower. This is probably the reason for some of the beef.

We have a few of these free standing towers that I analyze. We also put antennas on our transmission towers and I have to analyze them to see if they overload the tower.

_____________________________________
I have been called "A storehouse of worthless information" many times.