Equation for wind calculation External Pressure Coefficient ASCE-7 pg. 335 3

[Iasonasx]

I wonder if there are equations for the GCP external pressure coefficient. I am not really fond of the idea of using a graph like this one:

 

[mike20793]

There are logarithmic equations, but you have to buy the guide to ASCE 7 wind loading to access them. I think they used to be in the commentary in ASCE 7-02, but I'm not for sure.

 

[XR250]

What is wrong with using the graph?

 

[Archie264]

>>>What is wrong with using the graph?<<<

It's difficult to program into a spreadsheet.

Iasonasx,

For G see equations 26.9-6, 26.9-7, 26.9-8, 26.9-9, 26.9-10, 26.9-11, 26.9-12, 26.9-13, 26.9-14, 26.9-15a, 26.9-15b and 26.9-16.

Values for Cp are taken from various tables, for example the one in Figure 27.4-1 for the Main Wind Force Resisting System for walls and roofs. Not sure where the components and cladding one is of the top of my head but it's probably in there somewhere.

If I were to try to present any one of those equations referenced above you'd think I had lost my mind. I hope the wind knows it needs to be raised to the .63 power, multiplied by .63, square-rooted, multiplied by 1.7 and other factors including one of them raised to the 1/6 power and multiplied by .925 along with other factors I left out.

At some point it might become simpler and cheaper to just build a model and pay for the wind tunnel testing.

 

[mike20793]

Wind loading is incredibly easy to program into a spreadsheet, if you have the pressure coefficient equations. If you want to build a spreadsheet then just buy the guide to wind loading so you have them. It's an amazing resource with very good examples of wind load calculations.

 

[wannabeSE]

This should work for the sloping part. Just interpolate using the logarithm of the areas.
GCp = GCp·min + [GCp·max - GCp·min] * [LN(A) - LN(A·min)] / [LN(A·max) - LN(A·min)]

A = the area
A·min = Area corresponding to the minimum GCp on the sloping section
A·max = Area corresponding to the maximum GCp on the sloping section

 

[Iasonasx]

WannabeSE, I did not realize this was a proper logarithmic scale. Your formula makes perfect sense. I will try it tomorrow. Thank you.

 

[spats]

Works great wannabeSE! However, there is a little subtlety. GCp max should be GCp for A max, and GCp min should be GCp for A min, in order for it to work out for positive or negative GCp, no matter which way the line slopes.

 

[Iasonasx]

Here's how I set it up in the spreadsheet and it worked with the numbers I had:
=IF(Y9=4;IF(L9<10;1.1;IF(L9>500;0.8;1.1-((1.1-0.8)*(LN(L9)-LN(10))/(LN(500)-LN(10)))));IF(L9<10;1.4;IF(L9>500;0.8;1.4-((1.4-0.8)*(LN(L9)-LN(10))/(LN(500)-LN(10))))))
It works fine. thanks!