ASCE 7 Wind: Lattice Framework vs. Trussed Tower 1

[Gumpmaster]

I'm calculating wind loads using ASCE 7-05 for an industrial facility. I have lots of open stair towers and platforms, similar to a petrochemical facility.

ASCE 7 has two categories that may seem to fit the bill, Lattice Frameworks and Trussed Towers. There doesn't appear to be any defenition of the two and it doesn't look like the commentary sheds any light on the subject. The two categories yield very different results for the same structure.

Example: I have a 48ft tall stair tower that access the top of a tank, made up of mostly W12x26 members. Wind speed is 85mph. The Lattice Framework category gives me 22.5 psf while the Trussed Tower category gives me 39.2 psf.

Which should be used? I'm strongly leaning towards the Lattice Framework category, because 39.2 psf seems way to high (the shear on my structure is 80 kips!!!, no cladding or roof or anything, just bare members!!!).

Comments? Suggestions? Time for a more clear wind code?

 

[JedClampett]

I believe that the difference is due to the multiple members that can simultaneously see wind in the trussed tower. In other words two members in different planes can see wind at the same time. I picture the lattice as a two dimensional surface and the trussed tower as three dimensional one.
I would be careful in using the lower value. As I always say, I wouldn't want to be sitting across from a lawyer explaining I used the lower value of wind pressure.

 

[paddingtongreen]

I'm in agreement with JedClampett, by definition, the lattice has one face, the tower has three or four. If you used the lattice load, you must apply it to both faces of the tower that face the wind. That would be 22.5psf twice, worse than 39.2psf once. Unless the faces are identical, you do have to check the wind perpendicular to each face for the tower.

Michael.
Timing has a lot to do with the outcome of a rain dance.

 

[Gumpmaster]

I've been applying it to both faces of the tower. My tower is 15ftx70ftx48ft tall, so I don't think I really have much shading. Both faces are definately going to feel the wind.

I think I'm going to use the Lattice Framework load. I just can't fathom how a minimally open framed structure like this could have the same wind load as if the whole thing was cladded (39.2 psf gives the same load as if the tower was cladded and a 24psf load was applied).

Thanks for the suggestions.

 

[JoshPlumSE]

Where in ASCE 7 - 2005 are these terms used? I'm trying to find it so that I can look up that section in the SEAW commentary on wind load provisions.

The best I can tess is the "other structure" figures 6-22 and 6-23. Is this correct?

If so, then the SEAW commentary attributes some of these provisions to the Swiss standards, specfically research jakob Acheret.

The only thing SEAW says about lattice framework is "thin, long members and low velocities produce higher drag froce coefficients than short, streamlined members."

The SEAW commentary has a whole section devoted to "truss towers" because it says they have unique characteristics. They devote this section mainly to tall, slender radio and television towers (such as those designed to the TIA standard). They essentially say that the trussed tower method assumes that only one face is taking the wind load. Therefore, the loads on this face are increased a bit to account for the wind drag on the members in other faces of the tower.

 

[Gumpmaster]

Thanks Josh. You're right. Figures 6-22 and 6-23. ASCE should really explain a little more about those. Maybe a picture in the commentary, a defenition, something.

It sounds like maybe a lattice framework is the way to go, because my structure looks nothing like a radio tower (That's what I figured they were talking about).

There's an old ASCE publication about wind loads in petrochemical facilities. It's a little outdated, but I'm sure it has some good insights to these types of structures. Rumor is they're going to update it someday.

Thanks for the info! Those Swiss are bright ones.

 

[paddingtongreen]

If you use the tower pressure it is to the windward face only.
The higher factors are to cover for the trailing face(s). That is why the factors are different for square and triangular towers.

Michael.
Timing has a lot to do with the outcome of a rain dance.

 

[Gumpmaster]

I could go either way. For this particular structure, my answer comes out roughly the same. If you have an entire facility of the same construction (3 or 4 bays in the wind direction), then the Trussed Tower category would give you a much lower answer.

Seems like its just a shortcoming of the ASCE 7.

I read through the SEAW commentary. It's pretty in depth on the trussed towers section. It even specifically mentions that you could apply the same methodology to pipe racks or similar structures. At some point though, the analogy between a trussed tower and an open facility breaks down.