Downward roof wind pressures

[IngeIvan]

Per strength level combos, there is a load combination (1.2D+1.6Lr+0.5W) where the wind load is always additive to your worst case gravity loading on your roof. This wind load per components and cladding (i.e. for a joist or purlin) is always going to have a value that is around 0.2*wind load and end up with a higher vertical loading that cannot be ignored.

Has anyone thought about this? How are you guys handling this load case in your gravity design?

 

[JAE]

We just include it in our design along with the other load combinations.

Check out Eng-Tips Forum's Policies here:
faq731-376

 

[klaus.]

You have to consider all loads and combinations which are relevant for design
Codes just give a guideline .... thats all
If you need more combinations or other combinations...do it ...




best regards
Klaus

 

[DaveAtkins]

I can't say I have ever designed for that combination. With a relatively flat roof, won't wind load always be upward (suction)?

DaveAtkins

 

[JAE]

For flat roofs there is a component wind downward (from ASCE 7-05 - 7-10 similar)

Check out Eng-Tips Forum's Policies here:
faq731-376

 

[IngeIvan]

DaveAtkins,

JAE is right, that is the 0.2 factor I was talking about. For typical wind speeds, that 0.2 factor can become to be around 7-10 psf of additional load on your purlins. But then, you don't need to apply those loads on your girders as their tributary areas will most likely exceed the 700 sq ft components and cladding limit and can be designed using MWFRS, for which there is no downward pressures for flat roofs.

I just wanted to get a feeling of how you guys are handling this problem when designing in your software of preference.

Thanks

 

[T_Bat]

This issue has come up recently in our office and to be honest I hadn't thought of it before. Older and wiser engineers than me suggested that positive C&C should be included in the combination IngeIvan points out. I tend to agree with them. How you apply them in your software depends on how your software treats the loads and combinations. For example, in Ram Structural we make a separate positive C&C wind model and will apply an equivalent roof live load that includes the live load and positive C&C load with appropriate factors. If this controls most of the roof members (which it often does) we set the sizes in this model then proceed with our typical analysis separately.

One issue I have often run into is ASCE 7 prescribes minimum wind pressures to be used for positive C&C pressures. These are typically higher than what you would calculate using the pressure coefficients. For example, using ASCE 7-10 for a 30' tall flat roof building in a 115 MPH wind zone you may calculate a positive wind pressure of around 11 psf. However per ASCE 7-10 30.2.2 the minimum pressure would be 16 psf. I don't have the strongest background on the origin of this minimum pressure but you seem to be unnecessarily penalized using these minimums.

 

[DaveAtkins]

I suppose I have never considered it, because snow typically controls over roof live load on my projects, and the load combination which includes snow and wind allows you to multiply both by 0.75 (ASD). So full snow would typically still control.

DaveAtkins

 

[UcfSE]

I typically size roof members for gravity and C&C by hand, and enter those sizes in the analysis model. I have always checked this combination as well as the others. Most of my work is in hurricane areas so the positive wind is regularly higher than the minimum.