Internal Wind Pressure on Roof Beam

[Woody1515]

Hi everyone!

I have a question from a structural course I took a while ago (I am starting to get into residential design). The question is as follows: a load bearing wall is to be removed in a single storey house. The roof is stick-built, so we were given a snow load to design for (along with dead load). However, they made no mention of accounting for the internal wind pressure. Should this be accounted for in the design? In the case the internal wind pressure is acting "downwards" (same direction as snow and dead load), this will increase the loads acting on the beam so I figure it should be accounted for.

Thanks in advance!

 

[JAE]

Yes - wind pressure both internal and external would be required on a beam that supports an exterior roof. Load combinations are given in the building codes that include some combos with wind.

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[Woody1515]

Thank you for the response. What about the case where it is not supporting roof load, just dead and live load from the attic (let's say there are roof trusses)? In this case, would there still be internal wind pressure and no external pressure?

 

[Woody1515]

Perhaps my attachment will clarify my question. For beams 1 and 2, is there internal wind pressure?

 

[JAE]

We usually don't apply internal wind pressure to an interior beam of the type you show.



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[Woody1515]

That's what I figured, I haven't seen an example of that before. But could you explain why this is? I'm having troubles determining why we don't consider it.

 

[jimstructures]

Woody 1515,

Beam No. 1: No internal pressure as it cancels out.

Beam No. 2: If it is a truss framed roof, yes there would be internal pressure, applied in both directions as pressure and suction, as separate cases. If it is a stick framed roof the internal pressure would be applied to the underside of the rafters, in both directions as pressure and suction, as separate cases.

Assuming you are talking about Main Frame Wind Resisting Systems (MFWRS) and you are using ASCE 7 - XX.

Jim

 

[Woody1515]

Hi Jim,

Interesting, why the difference between the trusses and the rafters? And I am actually in Canada so I use the NBCC but I'm sure the principles are similar.

 

[jimstructures]

I have always understood the structure is like a balloon that contains the internal pressure or suction. So the wind load is applied at the inside face of the external structure. I.E. the inside face of the rafter or the underside of the truss roof frame. The answer might be moot as the global force will be the same in either case.

Jim

 

[BAretired]

Location and size of openings in exterior walls can cause internal pressure differences across floors or partitions but for small residential structures, it is not normally considered.

BA

 

[Woody1515]

In the case of the trusses, would it not just be internal pressure on the beam as the rafters will take the external pressure?

 

[BAretired]

If the attic space is vented, which it has to be by code, internal pressure occurs there as well. Internal pressure can be positive or negative, depending on size and location of openings as well as wind direction.

BA

 

[Woody1515]

Would you account for this or neglect it?

 

[BAretired]

For a single family residence, I would neglect it.
For a building with significant openings, I would make allowances for it.

BA

 

[Contraflexure74]

Yes I agree with previous response, if there are no large openings in the building then internal pressure will not be an issue and suction is normally counter balanced by dead load of roof. If roof structure is very light then suction maybe a critical load case.