Design Wind Pressure sign (+/-) convention using Coefficients GCpf and GCpi 1

[JAG-E]

I am designing the attachment for PV modules to a roof. I found a very helpful paper that explains how to use ASCE-7 for this purpose, though ASCE-7 is silent on the subject. The example is a near perfect match for my project except I am dealing with a higher wind velocity.

I follow the procedure and get the correct values when working through the example, except my values are positive (+) vs negative (-) as in the example.

The equation is p=q(GCpf-GCpi) q=19, => 19(0.3-(-.69)) = 18.81 but example shows -18.81
When solving for the Downward Pressure (last column) 19(-0.3-(-.69)) = 7.41, again the signs are reversed.

Am I supposed to just know which direction the resulting pressure is and apply the correct sign vs following algebraic rules?

Table 2 and the procedure can be found @

http://www.solarabcs.org/about/publications/reports/wind-load/pdfs/Wind_Load_blanksstudyreport3.pdf

Thanks
Joe

 

[BAretired]

You have reversed the terms GCpf and GCpi.
For uplift, you should have 19(-0.69 - 0.3) = -18.81
For downward pressure, it should be 19(-0.69 -(-0.3)) = -7.41

BA

 

[Agent666]

The sign convention most codes use is negative pressures act away from the surface (i.e. suction), this applies to internal and external pressures.

 

[JAG-E]

You made my night. The eyes are not were they once were. Obvious error on my part.
Now onto bolts and brackets I am familiar with.

Thanks again.

Joe

 

[JAG-E]

@BAretired
I just looked at the first example where I got the correct answer. The coefficient columns were in reverse order from each other.