Value of z for wind per ASCE 7

[jaydees]

In API-650 we can use ASCE 7 for determine the wind pressure. One of the factors is zg which uses a value of z. In ASCE 7-02 under "Other Structures" z was defined as the centroid of the area under consideration. In ASCE 7-05 it does not seem to say this. For a 40' high tank, should z be at the top of the tank (z = 40) or the centroid of the area (z = 20)?

 

[warrenw]

jaydees
I have always interpreted the meaning of (z) to be the height where you apply the wind force.
If you have a 40ft tank, your velocity pressure (qz) varies with height.
At 20ft (z) = 20, at 40ft (z) = 40.
This gives you a different velocity pressure exposure coefficient (Kz) at different heights.
I don't see where that has changed in ASCE 7-05

 

[jaydees]

In the 11th edition under 5.9.7.1 you calculate H1, the maximum distance between wind girders. Per paragraph d you have to calculate a divisor if your values specified for calculating the wind pressure are greater than what is shown in a-c. You are calculating one value of Kz for determining this. So what do you use for z to calculate Kz? Also, in 5.11.2 the wind load on the shell is at H/2 for uniform pressur on shell. If you do not use z as being H/2, how do you determine what value you do use?

 

[Elrae]

In ASCE 7-05, Table 6-3 gives the value for Kh and Kz based on the height above ground z. Table 6-2, the facing page, defines z as 0.6H or zmin, see the note.