Wind pressure on lattice frameworks ASCE 7-02

[PXC]

I would appreciate some help to clarify the definition of Af and E for design of a lattice framework, Figure 6-21.

The area Af is defined as the solid area projected normal to the wind direction.

Does this mean that I add up the projected areas of all members, even those that are hidden behind another? If so, this means that the total area Af could be much larger than overall projected area of the structure, calculated as its width x height.

E seems to be defined as the ratio of solid area to gross area for the "face" of a lattice structure. In other words, I need only look at the leading face, or perhaps a representative slice within the structure if it seems more appropriate for a particular structure.

Have I got this right or not?

thanks

 

[Lutfi]

Go to this link:

http://www.c-conceptsinc.com/files/example1.pdf

and follow their example. I think it will be better than explaining.

Regards,
Lutfi

Lutfi

http://www.cdeco.com

 

[JAE]

And consider this...

For lattice structures, there is always the distinct possibility that the lattice might be covered with a fabric or other material that would close off the openings and present a sign-type condition.

I was once given a project of supporting a large space-frame grid. The space frame grid was comprised of pipe shapes joined with a ball-type connection - a proprietary system. Initially, the design criteria called for a lattice wind load - on the projected areas of the pipes. But after inquiring I found that the owner desired to occasionally use the grid as a base for large plastic banners - essentially creating a more solid surface that would drastically change the wind load...so consider the possible uses of the lattice.