Wind load for multi-span roof

[AlexWong122]

Hi all,

I am calculating the wind load for a multi-span greenhouse structure. In the building code (i.e. National Building Code of Canada) there is only information regarding the wind load for single-span roof and I am wondering what is the proper way to deal with the multi-span roof (the CpCg Value for primary structure action).Or alternatively is there a software that can calculate that?

Attached is sketch of the shape of the structure.

Thanks in advance for any inputs.

Alex

 

[SAIL3]

at first glance, I would try a conservative upper-bound solution...

Case 1....assume no interference from one span to the other...that is, using values for one-span on each span simultaneously.....this would give max lateral wind load..

Case 2....assume suction on entire multi-span roof...this would give max uplift if it is greater than the uplift obtained from overturning in case 1....at the moment, not sure what suction value to use and would require more thought..

above applies to a closed bldg....being a greenhouse, the envelope may be delicate and subject breaching.....if that is a possibility, then it would radically change the problem....

 

[Ron]

Do not consider "shading" as Sail3 noted....treat each gable section independently, then collectively with full loads applied. Refer to ASCE 7 for sawtooth roofs.

Consider the wind in lateral, longitudinal and oblique directions. The oblique direction might give your worst case.

 

[ajh1]

See NBCC 2010 Commentaries Figure I-12 or NBCC 2015 Figure 4.1.7.6.-F
They don't appear to make a distinction between the first gable and successive gables in the direction of the wind flow.

 

[Guest262653]

See Eurocode 1-1-4 (EN1991-1-4), clause 7.2.7 - Multispan roofs and Figure 7.10.
There is a 40% reduction of wind pressures for some of the leeward spans.

 

[AlexWong122]

Thanks, all.

Ajh1, Figure 4.1.7.6.-F of NBCC 2015 doesn't show if it is windward or leeward. How do we know which number is which? Also seems that it is only for structural components and cladding. What about if it is for primary action?

Thanks,

 

[ajh1]

Alex, you are correct. What I gave you is for Components and Cladding. For C&C, the values are simply pressure and suction perpendicular to the surface. This figure is for roof surfaces. Figure 4.1.7.6.-B is for walls. You would use either Section 4.1.7.5 or Figure 4.1.7.6.-A to determine the primary action forces. Neither of these is going to give you any special solution for the multi-gable condition, so I guess we are back to your original question.