ASCE 7 Wind Loads Force Location 3

[wijgeng]

Hi,

I am determining the wind load on a "trussed tower" using Chapter 29 of ASCE 7. Equation 29.5-1 provides a resultant wind force in pounds, but I was expecting a pressure. Furthermore, it states that qz is the velocity pressure evaluated at height z as defined in Section 29.3, of the centroid of area Af.

What does the definition of qz mean? Is z the total height of my structure, but the resultant wind force acts on the centroid of the area? Or is z a location at any height, which in this case should be the height of the structure centroid?

Am I correct that this procedure treats the wind force as a point load acting at the centroid of my structure?

Note that I will be reviewing this with a senior engineer, but I am interested in other people's input so I can be more prepared for my design review.

Thanks!

 

[mike20793]

Think about it.. you're applying a pressure over a fixed area so where would the resultant need to be? At the centroid of the area. If you have a diaphragm and a floor above, then to calculate the force at the level, you would use the tributary width/height of half the floor above and half the floor below. No for your case, you have a cantilever trussed tower, so the tributary area for the load at the base would be the entire structure projected area.

 

[JAE]

The presssure, qz, varies with height (see Table 29.3-1).
So for a vertical trussed tower, the pressure will increase as you go up.

The equation 29.5-1 provides you with the variation of pressure (F/Af = qzGCf)
As mike states above - the pressure applies the area.

For a trussed tower, you wouldn't, im my view, apply a large single force, F, at the mid-height of the tower, but rather a more uniform load (plf) on the members, increasing in magnitude as you get higher per the Kz table.

Check out Eng-Tips Forum's Policies here:
faq731-376

 

[wijgeng]

Thank you for your replies. I agree with both of you.

So it is common practice to calculate varying pressures across the height of my structure?

 

[WinelandV]

wijgeng,

These trussed towers are a common design item where I work. We have a spreadsheet set up to calculate the wind loads at 10' intervals up the tower (the towers we design have struts every 10'). You absolutely want to calculate the varying pressure. If you were to "worst-case" the pressure for the tower, you'd be using the pressure at the top for every level, which would be very conservative. Though, depending on fabricator and installer quality, and how the end user plans on using it, maybe that level of conservatism is warranted.