Components and Cladding Wind Loading Question

[JD P.E.]

I am doing some self-learning of wind loadings and had a question regarding some outputs I am looking at. When it comes to the various roof/wall area loadings do you pick the highest (or most negative) worst case scenario value to apply to that area?

For example (see attached) for zone 3r, would the maximum design positive load (downward load) be 17.2 psf and the maximum negative design load (uplift) be -47.7 psf? Am I interpreting this correctly as far as choosing what to design to for C&C?

This is for self development not an actual design.

 

[JLNJ]

The short answer: yes.

It looks like your chart breaks it down into internal and external pressures. External pressure can be positive or negative. Internal pressures can also be positive or negative. So the first column is the max positive and min negative external pressure. The second column is the internal pressure (same magnitude everywhere could be either sign). The third column has those four possible results when you cross the 2 possible externals with the 2 possible internals.

 

[JLNJ]

One other thing. There is also a factor for tributary area not in the chart. The greater the trib area the lower the design pressure.

 

[JD P.E.]

JLNJ - So, are the third column min/max applied with the factor you are mentioning to get final design psf?

 

[JLNJ]

I've never seen this chart before but I read it:

Max Positive: +6.101+(+11.185) = +17.286
Max Negative: -36.607+(-11.185) = -47.792

Now, 5 significant digits for wind loading is foolishly comical, but here we are. Please round those up to whole numbers (or at least tenths)!

 

[JD P.E.]

JLNJ - That's how I read it as well. So then those would be the design pressures and looks like the wind uplift would govern the purlin/fasteners/R panel strength.

 

[driftLimiter]

Important to note two things:

1) The wind pressure coefficients for each component of the roof might be different, via the effective area. For example, a roof truss supports a larger area of wind than the overhang itself. ASCE code uses effective wind area of components to scale wind loads to that component. That means that you could potentially have different one of these charts for different components on the roof (there would be different pressures in each zone). The panel segments have a different effective wind area than the purlins, and so on.

2) We don't know that the wind uplift governs design of anything until we check the applicable limit states. There are load combinations that include downward loads (D, Lr, S) that get combined with wind loads, thats why we need max. positive pressure. It may well be that the strength requirements are governed by positive pressure for some elements, and the fasteners for example get governed by uplift load combo (0.6D+0.6W).

I really like the explanation of CC and MWFRS wind pressures that is used by this design guide. The quick review of how the pressures are developed, and how they are used to design different elements is very useful.

[URL unfurl="true"]https://vercodeck.com/wp-content/uploads/2021/04/Roof_Deck_Design_Guide_V2021.pdf[/url]

 

[JD P.E.]

driftLimiter- I see what you’re saying so I’ll have c&c tables for purlins, roofing, and then fasteners for a typical gable frame.

 

[driftLimiter]

In general yes, but the gable frame could be easily classified as MWFRS. it may have a large area, and it gets loaded by two different wind surfaces, check out that guide I posted.

 

[phamENG]

No, it's just one table...but it breaks it out based on tributary area. Here's an example of one of mine. Figure out the tributary area on your component (or the effective area for things like studs and joists), determine your zone, go to the table and get pressures.