Wind loads on mutispan gable roofs

[wikidcool]

I've been asked to analyze a large greenhouse. It's approximately 360 ft long x 240 ft wide. The design is basically 12, 30 ft wide x 240 ft long gable roof frames stacked side by side (See attached elevation). ASCE 7-10 gives roof wind pressures for gable roofs, but doesn't address this particular configuration. The governing ASCE7 loads would apply the same positive pressure to every windward roof face, and the same negative pressure to every leeward face. I suspect the roof wind loads normal to the ridge behave differently on a building like this. I'm wondering if they get the same effect a gable roof does with wind parallel to the ridge, where the force decreases as you move further away from the windward edge. If so, applying the code loads to every surface might be excessively conservative?

I'm having trouble locating any up to date guidance about it. Does anyone have experience with this subject they can share?

Thanks in advance.

 

[EngineeringEric]

Looks similar to a sawtooth roof, i believe ASCE 7-10 has a section on this, the C&C section is figure 30.4-4 and shows the effects for wind on your structure. I believe there is a section for MWFRS but honestly you can see what happens to C&C and apply that to MWFRS.

 

[Settingsun]

If the answer from EngineeringEric doesn't provide what you need, Australian Standard AS 1170.2 gives guidance on this case (I'm assuming this is an enclosed building, not just a roof with no walls).

AS 1170.2 says to use the following loads:
- first gable upwind slope: use the same pressure as for a single-gable roof. (see letter B in the diagram below)

- first gable downwind slope: ditto (letter C in the diagram)

- second gable upwind slope: same pressure as for first gable downwind surface (letter C in the diagram)

- second gable downwind slope and all subsequent roof surfaces: use a coefficient of either 0.5 (suction, load act upward) or 0.3 (pressure, load acts downward). (letter M in the diagram). Pressure = [0.5 or 0.3]*air density*(wind speed^2)/2. This is external pressure for main structural members; purlins and sheeting are subject to higher loads near roof edges and the ridge.

[Edited: text diagram didn't work. I'll try the proper diagram straight from the standard.]

http://files.engineering.com/getfil...b7e-8d43-f1f36f7ae35d&file=multispan_roof.png

 

[wikidcool]

EngineeringEric: The C&C provisions do address a multi-span gable roof. I wasn't aware of that, so thank you. It's odd that they address this roof type here but not under the MWFRS section. But it also appears to just apply the loads the same as a single span. Either the loads behave the same either way (which I doubt), or they are just being conservative.

Steveh: Yes. It's enclosed. Thank you very much for that info. That backs up my idea of how it seems like the loading would work. I wish there was a US-based study on this.