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C&C External Pressure Coefficient in ASCE
3

C&C External Pressure Coefficient in ASCE

C&C External Pressure Coefficient in ASCE

(OP)
Welp I'm a moron. Made a wind spreadsheet a while ago and failed to realize that the GCp is on a logarithmic scale. Makes sense, diminished returns bla bla bla...

Anyways, I can't find any equation for the Gcp other than interpolating the graph. Is there at least any data points other than 10sf and 500sf (the end points)?

Like for Figure 30.4-1, For Zone for 10sf = +1.0 and 500sf = +0.7. 20sf looks like 0.95. But could be 0.96 or 0.94. Is there anywhere that tells exactly what these points are. Looked but couldn't find.



Thanks.

RE: C&C External Pressure Coefficient in ASCE

It's a lizard leg multiplied by a newt nose. At least, that wouldn't be any less confusing.

G can be determined by Eq. 26.9-6 but I can only find a few tabular references for Cp that include Components and Cladding (C&C). There are considerably more tables of Cp for the Main Windforce Resisting System (MWFRS) but it's unclear to me if they can also be applied to C&C. In any case the code explicitly states that when GCp is listed together they should not be separated lest you incur the wrath of those who don't want their system figured out, apparently. I think the bottom line is just to take the values they give you and reverse engineer your spreadsheet to match them. Sorry.

RE: C&C External Pressure Coefficient in ASCE

If you plot the 2 end points in excel and fit a logarithmic trendline, the equation that you'll get gives almost exactly the same results as the charts in ASCE7.

RE: C&C External Pressure Coefficient in ASCE

I put them into mathCad with a linear interpolation that seems to work well. Say the factor varies from 1.0 at 10 SF to 0.7 at 500 SF. For the transition I did 1.0 - (0.3/(ln(500)-ln(10)) * (ln(A)-ln(10)) where A is the effective area you are interested in. plugging in 20 SF from your example it comes to 0.947.

It is cumbersome to program with all the cases, but seems to work well.

RE: C&C External Pressure Coefficient in ASCE

(OP)
ahh. I tried 1.0 - 0.3*ln(A-10)/ln(490) and got something goofy. Guess I don't know my LOG very well.

RE: C&C External Pressure Coefficient in ASCE

2
The equations are posted in the Guide to Wind Loads of ASCE 7 (any year). I'll list the walls for h<=60 ft:

Positive Zones 4 and 5
GCp = 1.0 for A = 10 sq ft
GCp = 1.1766-0.1766logA for 10 sq ft < A <= 500 sq ft
GCp = 0.7 for A > 500 sq ft

Negative Zone 4
GCp = -1.1 for A = 10 sq ft
GCp = -1.2766+0.1766logA for 10 < A <= 500 sq ft
GCp = -0.8 for A > 500 sq ft

Negative Zone 5
GCp = -1.4 for A = 10 sq ft
GCp = -1.7532 + 0.3532logA for 10 < A <= 500 sq ft
GCP = -0.8 for A > 500 sq ft

RE: C&C External Pressure Coefficient in ASCE

Those are the equations for GCp. They show the equation for G but they don't show equations for Cp and they disallow us from trying to separate out the two coefficients when they are listed together. It's not that I care, mind you, I just find it weird.

RE: C&C External Pressure Coefficient in ASCE

(OP)
Even better! Found the one from 1998 laying around my office and they are listed there. Thanks!

RE: C&C External Pressure Coefficient in ASCE

Jerehmy: Would you post the equations you found?

Jim H

RE: C&C External Pressure Coefficient in ASCE

In Excel, try:
=GCpAmin+(LN(MIN(MAX(Amin,B5),Amax))-LN(Amin))*(GCpAmax-GCpAmin)/(LN(Amax)-LN(Amin))

Where
Area is the cell with the area in question
Amin is the cell or value with the smaller limit (eg 10ft2)
Amax is the cell or value with the larger limit (eg 500ft2)
GCpAmin is the value with cell of the GCp corresponding to the smallest area
GCpAmax is the value or cell with the GCp corresponding to the largest area

RE: C&C External Pressure Coefficient in ASCE

(OP)
There's a lot, so I'm just gong to post the line equation(s). You know the end points.
I doubled checked the numbers after inputting, even though I reaaalllly didnt want to lol. So they're
accurate per the book.

Gable roof < 10 deg
Positive with and without overhang: Zones 1, 2, and 3
0.4 - 0.1*LOG(A)

Negative w/ out overhang: Zone 1
-1.1 +0.1*LOG(A)

Negative w/ out overhang: Zone 2
-2.5+0.7*LOG(A)

Negative w/ out overhang: Zone 3
-4.5+1.7*LOG(A)

Negative with overhang: Zones 1 and 2
-1.8 + 0.1*LOG(A) for 10 to 100
-3.0307 + 0.7153*LOG(A) for 100 to 500

Negative with overhang: Zone 3
-4.8 + 2.0*LOG(A)

10 deg < Gable roof <= 30 deg
Positive with and without overhang: Zones 1, 2, and 3
0.7 - 0.2*LOG(A)

Negative with and without overhang: Zone 1
-1.0 + 0.1*LOG(A)

Positive without overhang: Zones 2, and 3
-2.8 + 0.7*LOG(A)

Negative with overhang: Zone 2
-2.2

Negative with overhang: Zone 3
-4.9 + 1.2*LOG(A)

30 deg < Gable roof <= 45 deg
Positive with and without overhang: Zones 1, 2, and 3
1.0 - 0.1*LOG(A)

Negative with and without overhang: Zone 1
-1.2 + 0.2*LOG(A)

Negative without overhang: Zones 2 and 3
-1.4 + 0.2*LOG(A)

Negative with overhang: Zones 2 and 3
-2.2 + 0.2*LOG(A)

10 deg < Multispan gabled roofs <= 30 deg
Positive: Zones 1, 2, and 3
0.8 - 0.2*LOG(A)

Negative: Zone 1
-1.8 + 0.2*LOG(A)

Negative: Zone 2
-2.7 + 0.5*LOG(A)

Negative: Zone 3
-3.7 + 1.0*LOG(A)

30 deg < Multispan gabled roofs <= 45 deg
Positive: Zones 1, 2, and 3
1.2 - 0.2*LOG(A)

Negative: Zone 1
-2.9 + 0.9*LOG(A)

Negative: Zone 2
-3.3 + 0.8*LOG(A)

Negative: Zone 3
-3.5 + 0.9*LOG(A)


3 deg < Monoslope roofs <= 10 deg
Positive: All Zones
0.4 - 0.1*LOG(A)

Negative: Zone 2
-1.4 + 0.1*LOG(A)

Negative: Zone 2'
-1.7 + 0.1*LOG(A)

Negative: Zone 3
-2.4 + 0.6*LOG(A)

Negative: Zone 3'
-3.6 + 1.0*LOG(A)

10 deg < Monoslope roofs <= 30 deg
Positive: All Zones
0.5 - 0.1*LOG(A)

Negative: Zone 1
-1.5 + 0.2*LOG(A)

Negative: Zone 2
-2.0 + 0.4*LOG(A)

Negative: Zone 3
-3.8 + 0.9*LOG(A)

Sawtooth roofs >= 10 deg
Positive: Zone 1
0.8766 - 0.1766*LOG(A)

Positive: Zone 2
1.4 - 0.3*LOG(A)

Positive: Zone 3
0.9 - 0.1*LOG(A)

Negative: Zone 1
-2.8474 + 0.6474*LOG(A)

Negative: Zone 2
-4.1418 + 0.9418*LOG(A)

Negative: Zone 3 (SPAN A)
-4.5 + 0.4*LOG(A) for 10 to 100
-8.2782 + 2.2891*LOG(A) for 100 to 500

Negative: Zone 3 (SPANS B, C, and D)
-4.6030 + 1.0015*LOG(A)

Buildings with H > 60ft and Roofs <= 10 deg
Negative: Zone 1
-1.6943 + 0.2943*LOG(A)

Negative: Zone 2
-2.7120 + 0.4120*LOG(A)

Negative: Zone 3
-3.7297 + 0.5297*LOG(A)

Walls All Theta
Positive: Zones 4 and 5
1.1792 - 0.2146*LOG(A)

Negative: Zone 4
-1.0861 + 0.1431*LOG(A)

Negative: Zone 5
-2.5445 + 0.5723*LOG(A)

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