NBCC Wind Load Calculations 2

[Woody1515]

Hi everyone,

When calculating wind loads, the national building code of Canada gives values for the external pressure coefficients “for primary structural actions” in Figure 4.1.7.6-A. It then gives different values for individual walls, roofs, etc in separate figures. When designing an individual tall wall, I am assuming I should be using Figure 4.1.7.6-B for the design of individual walls. When would you use Figure 4.1.7.6-A for the primary structural actions? Why are there different external pressure coefficients for the primary structural actions and all individual members?

Thanks in advance!

 

[Pmtottawa]

1st table is for bldg as a whole 2and table is components and cladding these have smaller trib areas.

 

[Woody1515]

I guess I am struggling with the meaning of “building as a whole”. If you model up your whole building, and apply the loads from Figure 4.1.7.6-A, the analysis will produce stresses, etc. acting on the building. How will these differ from looking at each structural element (wall stud, roof hoist, etc.) from the other figures? Will they not give different values?

 

[jayrod12]

I don't have any of my stuff handy, but I feel like somewhere it says something about more than 300 square feet and you can use the primary action numbers.

 

[skeletron]

Article 4.1.7.6(2):
For the design of the main structural system of the building, which is affected by wind pressures on more than one surface, the values for CpCg are provided in Fig 4.1.7.6-A

Article 4.1.7.6(3):
For the design of individual walls and wall cladding, the values of CpCg are provided in Fig 4.1.7.6-B



When you are modelling the building as a whole, you are interested in the shear and torsion that will be applied to your lateral system. You apply the forces to all your walls and follow through with the additional clauses that help induce torsion. These are your primary structural actions.

When you are looking at individual walls (such as the girt design) or cladding, you apply the B figure values. It appears that as you increase the tributary area, your coefficients converge to approximately windward+leeward of the coefficients in the A figure. Go through the comparison with a spreadsheet and you can see how and when the values will give you different answers.

 

[r13]

I think the OP is confused at, when a wall in the building is analyzed as a "whole" subjects to wind pressure X, then, it is analyzed individually, shouldn't it be subjected to the same wind pressure X, if not, why? Similarly the cladding attached to that same wall, what wind pressure it subjects to?

 

[Woody1515]

retired13 summed up my question well. For example, if you model a full building and apply the wind loads from Figure 4.1.7.6-A, you will get stresses developing in the building. How does this differ from looking at a portion of the building (ie, the walls) and using the values from Figure 4.1.7.6-B? If you are designing the walls of the buildings, do you use the values from Figure 4.1.7.6-A or Figure 4.1.7.6-B?

 

[Woody1515]

It also appears that for all individual structure portions (walls, roofs, etc.) the code says to add the internal pressure while the “whole building” pressure are not required to have the internals added? Is this because the internal pressures acting on the whole building cancel out?

 

[skeletron]

The internal pressure coefficients cancel out when you treat the building as a whole, so the net effect on the building as a whole is 0 since everything moves in or out. When you look at an individual wall or component, you need to apply that internal pressure as well. Similarly, when you look at the building as a whole you need to apply windward/leeward simultaneously.

As far as "shouldn't it be subjected to the same pressure": I believe you can verify this with the graph on Fig 4.1.7.6-B and consider the tributary area of the component.

The other point is that Fig B generally deals with facades and secondary components (girts). There is a different gust factor applied. I believe the intent is that they are the "first line of defence" whereas on Fig A the forces reflect the random fluctuations and smearing of the load as it gets into the LFRS.

 

[Woody1515]

Okay so then when would you use Figure 4.1.7.6-A? If you need to go through each building component and apply the external and internal coefficients, I don’t see a time when you would use the whole building values.

 

[skeletron]

My hypothetical approach:

Design of shear walls and diaphragm -----> Figure 4.1.7.6-A
Design of cross-bracing ------> Figure 4.1.7.6-A
Calculating building deflection ------> Figure 4.1.7.6-A

Design of cladding, wall studs, curtain walls, windows, girts, etc. ------> Figure 4.1.7.6-B

 

[jayrod12]

Agreed with Skeletron.

 

[Woody1515]

Thanks for the input. I am not sure why I am struggling with this concept so much. To clarify, if I apply the wind loads on the entire building using values from Fig 4.1.7.6-A, the bending moments, shear forces, etc. I get from this analysis can not be used when I’m designing the wall studs and roof rafters? I would need to add the internal pressure acting on the wall studs and roof rafters to design them properly?

 

[jayrod12]

Correct