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ASCE7-05 Wind Load Fig 6-17 p65, External Pressure Coefficient Slope Line
2

ASCE7-05 Wind Load Fig 6-17 p65, External Pressure Coefficient Slope Line

ASCE7-05 Wind Load Fig 6-17 p65, External Pressure Coefficient Slope Line

(OP)
Hello Guys,

Can you guys help where can i find the equivalent equation of the line for the the external pressure coefficient in figure 6-17.

I really do not know how to determine the values of the pressure coefficient if my effective wind area is within that slope.

Thanks!



RE: ASCE7-05 Wind Load Fig 6-17 p65, External Pressure Coefficient Slope Line

As you can see on the graph and on the adjacent sketches, there are five different areas of the building. Areas 1,2 and 3 are for the roof only and will always be negative values. Areas 4 and 5 are for the walls and can be either positive or negative, depending on the side of the building. On the windward side, they are positive and on the leeward side they are negative.

To obtain your value for each of the areas, determine your tributary or influence area (effective wind area). Start at that value on the X axis and move up to the intersection of the appropriate line corresponding to the area of the building that is applicable. At the intersection of the line, turn horizontally and move to the left to get the appropriate GCp value. For example:

Consider that your effective wind area is 100 square feet and you want to know the uplift on the roof for area 2. Enter the graph at 100 square feet. Move up to the intersection of the 100 sf line and the sloped section of the "2" line. Turn left and go horizontally over the the Y-axis. Your horizontal line intersects the Y-axis at approximately -1.9.

A graphic example is attached

RE: ASCE7-05 Wind Load Fig 6-17 p65, External Pressure Coefficient Slope Line

For lines 3,2,1 suction, and 4/5 pressure in the figure, the equation is going to be:
result = (Value at 10sf)+((Value at 500 sqft)-(Value at 10 sqft))*(log10(act sf))-log10(10 sqft))/(log10(500 sq)-log10(10 sqft))
For line 3 at act sqft = 100, that would be (-3.2)+((-2.3)-(-3.2))*(log10(100)-log10(10))/(log10(500)-log19(10))
result = -3.2 + 0.9* (2-1)/(2.699-1.0) = -2.67
Line 4,5 suction will be the same process except use log10(20sqft) in lieu of log10(10).
This routine works for any line on a graph of this nature. If the x axis is normal scale rather than log10 scale, the process is the same except use the actual x values, not log10(x) values.

RE: ASCE7-05 Wind Load Fig 6-17 p65, External Pressure Coefficient Slope Line

ajh1....20sf should be the base for all 4,5, not just the suction.

RE: ASCE7-05 Wind Load Fig 6-17 p65, External Pressure Coefficient Slope Line

Do you mind if I ask where the project is located and what governing authority has jurisdiction? I ask because I notice this thread references ASCE 7-05, but the most recent version, ASCE 7-10, has been the applicable version in both IBC 2012 and now IBC 2015. Therefore, this would seem to be the standard unless for some reason your jurisdiction is still using IBC 2009 by law.

RE: ASCE7-05 Wind Load Fig 6-17 p65, External Pressure Coefficient Slope Line

Ron, you are correct. Obviously too early in the morning when I responded.

RE: ASCE7-05 Wind Load Fig 6-17 p65, External Pressure Coefficient Slope Line

NorCalSE....there can be a variety of reasons to use an older version of a code or standard. For renovation or corrective construction, it is not always necessary to use the most current version...many times you need only to comply with the version of the code in effect at the time of original design/construction.

Further, not all jurisdications adopt the latest version of the code. It is getting better, but 20 years ago, it was not uncommon for a jurisdiction to be a couple of code versions behind.

ajh1..lol....I have the same issue all too often!

RE: ASCE7-05 Wind Load Fig 6-17 p65, External Pressure Coefficient Slope Line

The specific equations are given in the ASCE 7 - Guide to Wind Loads books. I believe several have been scanned in for previous threads.

RE: ASCE7-05 Wind Load Fig 6-17 p65, External Pressure Coefficient Slope Line

Good points, Ron. And inevitably the wind load requirements have gotten more and more complex with every ASCE 7 edition, which is aggravating as a structural engineer.

RE: ASCE7-05 Wind Load Fig 6-17 p65, External Pressure Coefficient Slope Line

NorCalSE.....I agree....and unnecessarily so!

RE: ASCE7-05 Wind Load Fig 6-17 p65, External Pressure Coefficient Slope Line

(OP)
Thank you for the response guys, I found a template regarding the equation of the curve line, I'm still trying to figure it out how the equations were derive.

Still I appreciate all your responses bigsmile

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