ASCE 7 Wind - C&C - Pos and Neg combinations for open roof

[JAE]

In ASCE 7 it offers various C factors for calculating wind in zones 1, 2 and 3 of an open pitched roof. I'm talking about components and cladding wind here (C&C).

There are actually two sets of factors - a positive set of values and a negative set of values.
The footnotes below the tables indicate that both combinations must be checked but don't go into much detail as to actually defining how this is done.

Which is correct?:

1. The positive values of the wind zones on a windward side of a pitched roof should be only combined with the positive values on the leeward side. Likewise the negative with the negative.
OR
2. The positive values on the windward side should be combined with the negative values on the leeward side and visa versa.

If you do option 1 above, there is no net horizontal force on the roof - say you were checking a single truss which is spanning parallel to the wind and has a windward side and a leeward side with a peak at midspan. And you are designing specifically the end connection (hold-down/support connection).

With option 1 you would only get a vertical uplift since the windward and leeward horizontals cancel each other out.
This seems counter-intuitive but using option 2 also seems counter-intuitive to the way ASCE 7 has typically applied the sign conventions.

With option 2 there is a net horizontal force on the truss connections.

 

[Ron]

JAE...I've typically used Option 2. I've done lots of canopies like this and looked at it variety of different ways, parallel and perpendicular to the ridge.

 

[UcfSE]

My first reaction is to use option one for C&C, since the horizontal force will be captured by the MW coefficients which have a positive on the windward side and a negative on the leeward side.

 

[JAE]

Hmmm - two options and two preferences. We've looked through books and guides on the use of ASCE 7 and none of the examples or FAQ's appear to address this.

 

[wannabeSE]

Interesting question. I have only designed monoslope open structures. If the roof has a diaphragm, I would neglect the C&C horizontal reactions since the diaphragm will distribute the load to the MWFRS. If there is no diaphragm, I would probably consider both cases (+windward and +leeward; +windward and -leeward; -windward and +leeward;.-windward and -leeward)

 

[haynewp]

From comparing the MWFRS coefficients for open pitched roofs from Fig 6-18B and 6-18D (7-05), I would check both cases for C&C in 6-19B. Since it isn't stating otherwise, I assume longitudinal wind could similarly cause all positives or all negative pressures and transverse give the mixed positive and negative.