Wind pressure on an open roof

[CANPRO]

I'm working on a project involving a roof that is about 80'x60' and about 22' tall at its peak. The roof is being built on the ground and then lifted onto the main structure which is about 40' above the ground. Right now I'm trying to determine how the wind is going to act on the roof. I followed the procedure in the National Building Code of Canada to find the pressures acting on the top of the roof but this procedure is for a roof sitting on an enclosed building, and when the roof is being lifted it is going to be more like a canopy. Any suggestions on how I would go about calculating the uplift on the roof while it is being lifted? Any suggestions would be helpful.

Thanks

 

[msquared48]

You might look at ASCE7 and use the coefficients for an open structure with the proper pitch.

I would design to a specific wind velocity and not lift if the velocity is excedded or forecast for the time of lift. You do not have to design for the highest wind load for your area if you are just lifting. However, once in place, the structure, to include the roof, can and will see the recommended design wind loads for the area.

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

 

[csd72]

Not sure why you are worried about wind when it is being lifted, usually lifts for these types of things are limited to less than 7m/s which is much lower than the wind code.

After it is lifted in then you need to treat it as a double sloped free roof (most codes have a coefficient for this).

You may find that the critical case is between the initial andf the final state where it is partially blocked under.

 

[BAretired]

How many cranes will do the lifting? Do you get any calm days in Nova Scotia?

BA

 

[CANPRO]

We are limiting the wind speed to 25 km/hr (7 m/s) for the day of the lift. Nova Scotia, particularly this area, is quite windy so there is a decent chance the lift is going to happen in moderate winds. The lift is being done with 2 100' cranes and 2 spreader beams. I looked into ASCE7, but that only deals with single sloped canopys, this roof is a hip roof with a few dormers. Considering the deadload of the roof and the spreader beams, and the wind acting on the top of the roof I figure there will be about 5' of sway. This is not including the effects from the wind passing underneath the roof, which I have a feeling will be significant. Overall the reactions from the wind are going to be managable, I would just like to have a ball park of what I'll be dealing with if the wind gets up to 25 km/h.

Thanks for your input so far.

 

[csd72]

The BS6339 part2 coefficients for a duopitch free roof of 30 degree pitch are:

Overall: +0.9 or -1.2
Or in 4 zones:
A +1.3/-1.4
B +1.9/-1.9
C +1.6/-1.4
D +0.7/-2.0

with negative being upward.

B,C, and D are at the ends, eaves and ridge respectively and extending 10% of the length of the respective roof dimension.