ASCE 7-05 Figure 6-23 Trussed Towers

[bjb]

I have a trussed tower that is square in cross section. The tower is a vertical tube that will support external stairs. The ratio of soild area to gross area is 0.2.

For calculating wind loads I am using ASCE 7-05 Figure 6-23. My initial thought was that the wind load is applied to leading tower face and the trailing tower face-without shielding. After studying this figure more, I am now wondering if the wind load only gets applied to the leading face.

What is your opinion?

 

[msquared48]

Based on the tower geometry, it will most likely be applied to the rear face members too, even considering the shielding effects.

The critical words here, in note #1, in my opinion, are "for the tower segment under consideration", implying you have to consider the leading face as well as the trailing face.

Also look at Figure 6-22 where it talks about latticed framework for signs, which is often three dimensional too. See note #2 where it says "all exposed members and elements". To me, the impolication is clear here, and the thinking directly related to Figure 6-23 also.

Mike McCann
MMC Engineering

http://mmcengineering.tripod.com

 

[IsaacStructural]

This is a question many have asked about this particular section.

You might also look at ASCE's guide to petrochemical facilities

http://www.amazon.com/Loads-Petrochemical-Other-Industrial-Facilities/dp/0784411808

It contains an alternate method for calculating wind loads on open framing, and as I recall, the results can be wildly divergent from one another.

M.S. Structural Engineering
Licensed Structural Engineer and Licensed Professional Engineer (Illinois)

 

[bjb]

I was just looking at TIA/EIA 222 rev G and it seems to indicate that the area is just one face. The Cf factor that I calculate using Fig 6-23 seems to be a lot higher than I would expect, so I wonder if the intent of fig 6-23 is to apply wind to the leading face, with an increased Cf to account for the fact that 2 faces will actually be loaded.

I see what you are saying about Fig 6-22, but the Cf factors are very different when compared to Fig 6-23. using Fig 6-23 my Cf factor is 2.98, but using Fig 6-22 my Cf is 1.8. That suggests to me that different areas may be under consideration.

 

[IsaacStructural]

This is the conclusion I came to as well bjb, but I'm not aware of an official source that clarifies the issue.

M.S. Structural Engineering
Licensed Structural Engineer and Licensed Professional Engineer (Illinois)

 

[msquared48]

I see what you mean in TIA-222-G, section 2.6.9.1.1 where they consistently say "in one face".

The only thing I can think of is that the effect of any structure to the rear of the front face must be hidden in the design coefficients, and generated by model testing.

Mike McCann
MMC Engineering

http://mmcengineering.tripod.com

 

[bjb]

msquared: I think that explains why the Cf factor is significantly higher in Fig 6-23.

I wish that ASCE 7 was more clear. I sometimes find it very hard to understand. It would be nice if it came with solved examples that illustrate its use. We have something like that from the ICC for the IBC 2000.

 

[IsaacStructural]

I agree bjb. Like I said, do consider looking into the petrochemical and industrial facilities, it has a different method for open framing. It deals with shielding, including multiple frames (more than 2). It brings its own issues, particularly when the frames are spaced at irregular intervals. If you do this work with regularity, it is worth looking into. On the other hand, if this is one shot deal, then i guess you have to use your best judgement about how to interpret ASCE.

Finding example problems is always hard, and when they do one, they love to take the easiest possible scenario and ignore special or remotely ambiguous cases.

M.S. Structural Engineering
Licensed Structural Engineer and Licensed Professional Engineer (Illinois)

 

[ARS97]

I believe when you use the force coefficient for such a structure, the TOTAL wind load is calculated. I believe it's the first exposed face. Technically, that wind load probably gets divided amongst the faces, but it seems to me that the force coefficient is intended to produce the total load acting on the structure.

Also, read note #1. it states "solid area of a tower face projected on a plane of that face". To me, that means the projected area of one side.

 

[chichuck]

bjb

The following quote from the indicated publication answers your question about Fig. 6.23 in ASCE 7-05. THis is from the guide to the wind load provisions in ASCE 7-02, but the table in ASCE 7-05 is the same.



Guide to the Use uf the WindLoad ?ruvisions of ASCE7-O2


Kishor C. Mehta
James Delahay

In the section that includes FAQ:

23. In calculating the wind forces acting on a trussed tower of square cross
section (see Figure 6-22), should the force coefficient, Cf, be applied to
both the front and the back (windward and leeward) faces of the tower?

No. The calculated wind forces are the total forces acting on the tower.
The force coefficients given in Figure 6-22 include the contributions of
both front and back faces of the tower, as well as the shielding effect of the
front face on the back face




regards,


chichudk

 

[RFreund]

Just to clarify -

When using the trussed tower method you would project any shape on the back face that is not "covered" by the front face in your area calculations, correct?

Also if you use wind loads for lattice frame work and compare them to trussed tower you will see that lattice frame work is about half of the trussed tower. It is my understanding that lattice frame work should be applied to both faces. Therefore the wind loads work out to be similar which makes sense

EIT

http://www.HowToEngineer.com

 

[chichuck]

Rfreund,

1) I would only figure the exposed area based on the windward face members and not consider any of the "unshielded" members on the leeward face.

2) Your point about applying wind to the leeward face of a structure comprised of 2 lattice frames is conceptually correct. However, there is some shielding of the rear trussed fram that occurs. Refer to Wind Forces on Structures, ASCE, Final Report of the Task COmmitte on WInd Forces of the Committee on Loads and Stresses of the Structural Divison, Paper No. 3269, 1961. Look at the section heades "Pairs of Trusses".

regards,


chichuck

 

[RFreund]

@chichuck -

1.) Interesting, it would seem then these "unshielded" members go unaccounted for. Unless it is made up for somewhere else in the factors which I could understand as this is a pretty specific type of structure and wind load calculation.

2.) Thanks for the reference.

EIT

http://www.HowToEngineer.com

 

[bjb]

It would be nice if ASCE added some clarification to this figure.