ASCE 7-05, Figure 6-18b

[mnovicki]

I'm working on an open gable structure. When I read Figure 6-18b in ASCE 7-05, for a 45 degree pitched roof, open structure, Case A, Clear wind flow, I get Cn of 1.1 and Cnl of .90. In the notes below, note 4 says that "plus and minus signs signify pressures acting towards and away from the top roof surface, respectively". This would imply a net lateral pressure of .2 (1.1 toward surface leeward side and .9 toward surface leeward side). Doesn't feel right. I want to change the .9 to negative on the leeward side. Anyone have any experience with this?

Thanks

 

[steve1]

Read Note 4 again.

+ indicates toward the windward and - indicates away from the leeward(thus both are acting in the same direction from a global perspective)

therefore your net is 1.1 + .9 = 2

 

[mnovicki]

RIGHT, MINUS SIGNIFIES PRESSURE AWAY FROM ROOF SURFACE. CNL IS POSITIVE, WHICH WOULD BE TOWARD ROOF SURFACE ON LEEWARD SIDE. I'M STILL GOING WITH 1.1+.9=2, BUT TO ME THIS STILL READS INCORRECTLY. THANKS FOR YOUR COMMENT.

 

[msquared48]

Cn of 1.1 and Cnl of .90

This means 1.1 on the WINDWARD surface and .9 on the leeward. I believe you mistyped when you said ".2 (1.1 toward surface leeward side and .9 toward surface leeward side)."

Seems OK to me.

Mike McCann
MMC Engineering

 

[steve1]

OK I see your point. I looked at "Guide to the Use of the Wind Load Provisions of ASCE 7-02" (I don't have the 05 edition). It presents an example of the loading on a building with a pitched roof. For the roof both windward and leeward wind is depicted as away from the roof. These loads are considered to act normal to the plane of the roof, thus their vertical components are additive and their horizontal components are subtractive.

The wind on the walls of the building would be additive in the global sense.

 

[BAretired]

I do not have the document in question, but in the National Building of Canada, a positive sign represents inward pressure. A negative sign represents outward or suction pressure. The arrows in the attached diagram indicate positive pressure.

Cp is the pressure coefficient and Cg is the gust factor. In this chart, they are combined.

When wind blows generally perpendicular to the ridge, Zone 2 and 3 have coefficients of 0.4 and -0.8 respectively for a slope of 30 to 45 degrees. The horizontal component of pressure would be additive.

When wind blows generally parallel to the ridge, Zone 2 and 3 have coefficients of -1.3 and -0.7 respectively. The horizontal components oppose each other.

BA

 

[NITTANYRAY]

I was glad to see this section added to to 05 edition because the 02 edition only covered mono-sloped open roofs.
I agree that 1.1 and .9 should be added together because the wind pressure on the leeward side is actually acting primarily acting on the underside of the leeward surface much like a sail. I have seen a picnic shelter with a 45 deg. roof that failed during a storm with 45 MPH winds. The 2.0 Cn factor predicts a failure but using a .2 factor the structure would not have failed.

Hopefully they will fix this obvious error in the next revision of ASCE 7. I hope most engineers have enough feel for their designs that they will come to the same conclusion that mnovicki came to.