ASCE 7-10 - Wind Load on Box Truss - Fig 29.5-2 or 29.5-3?

[ARS97]

Just as the title of the thread suggests. I've always used Fig 29.5-3 since I figured that a "trussed tower" sounded more appropriate for a box truss. However, I suppose "lattice framework" sounds similar too (Fig 29.5-2), plus it gives lower Cf values. Unfortunately, ASCE doesn't give much of an explanation of the intent of these figures, so I'm in debate. I'm just looking for what is most appropriate......any opinions?

 

[briancpotter]

Try this thread:

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

The consensus seems to be that a lattice framework is a 2D surface, whereas a trussed tower is a 3D structure. Therefore, the Cf coefficients on the tower are increased to account for load on additional faces. This should help guide which designation is more appropriate in this case.

Brian C Potter, PE
Simple Supports - The history and practice of structural engineering.
ConstructionPic - Send annotated jobsite photos.

 

[ARS97]

briancpotter - that's exactly what I thought. Thanks for the link......

 

[aggman]

We use the trussed tower design methods. The lattice framework is appropriate for single sided frames, not multiple frames. The trussed towers accounts for member shielding on the back side. Back some years ago we were always debating this in the office (and with some other consultants). Some people would use trussed towers, some would use the lattice frameworks. I found some information on the matter in "Wind Loads on Petrochemical & Other Facilities" and then further information leading all the way back to the ANSI A58 document (with references). The petrochemical document takes the research and consolidates it into a useful design method. The nuts and bolts of it are that you can more or less use the towers information and while not exact it will be pretty close depending on how your frames are built. (I think the towers information assumes a 1:1 aspect ratio for shielding which may or may not be true). You can conservatively (very conservatively) use lattice frameworks, but if you do you should apply the 1.6, 1.8, 2.0 to both leeward and windward frames independently in full non reduced load.

 

[ARS97]

aggman - thanks......that was my gut feeling on it, but I never traced it back to the original reference.