Components and Cladding

[smokiibear]

Curious about Components and Cladding. As I read the code and several different design manuals, articles, etc, I find the discussion quite confusing. Does component and cladding on say, a residential wood framed structure, apply to all parts of the exterior of the structure? Would I design my entire structure for MWFRS and then design individual parts (i.e. Trusses, Beams, Columns, Studs, Footings) for C&C in both positive and negative vertical direction as well as horizontal?

There is a lot of language saying that when the component is part of the MWFRS, then C&C loading is not required. Others say that once a component has tharansfered it's load to the MWFRS, that is the end of load path.

I also realize that not all conditions of C&C will govern a design that has wind from MWFRS....

What is the load path for loads for C&C? In other words, when trusses are required to be designed for C&C vertical, does that load need to transfer to the foundation, albeit with different areas resulting in different loads? Would a stud have both vertical and horizontal C&C loads at the same time? Does. A footing ever require C&C loads to be applied?

Thanks,

 

[WesternJeb]

Components and Cladding should be considered a local max. force. A wind gust could have a localized maximum force of 60 psf near an edge/corner. But the overall profile on an entire wall (MWFRS forces) might only be 20 psf. C&C is meant to account for the unpredictable spikes in wind speed.

There are calculations in chapter 30 (ASCE 7-10 for me) for the components and cladding wind loads based on square footage. So, if you have a wall stud @ 16" o.c. and 10' tall. You would design for the C&C loads based on 13.33 sf. If you have a truss @ 4'-0" o.c. and a 30' span, you would design it for 120 sf from your C&C wind calculations.

So the MWFRS is for the entire building, but each individual member should be designed for C&C loading based on "effective wind area" in chapter 30. This usually controls door jambs, just something to look out for.

 

[XR250]

I believe for trusses (long and narrow trib members) you are allowed to use span * span/3 for the trib.. At least that is how it used to be.

 

[smokiibear]

Thanks @eit in training. I added a tad to the OP, but do you have understanding of the several other questions asked?

 

[WesternJeb]

There are a lot of factors that govern those questions and they should be considered on a case by case basis. The connections of members should be designed for C&C loading, but the load path after is up to engineering judgement. For a long stud wall or masonry wall, C&C loading will likely not go to the foundation and you should use MWFRS pressures as the C&C loading will "even out". If you're looking at a steel column with a small tributary area then I would likely take the load into the foundation for overturning checks. Again, it is just a case by case basis and up to engineering judgement.

As far as designing for both vertical and horizontal at the same time, I haven't looked into that too much. Since it is based on tributary area, it seems like your tributary area would most often reach an amount where MWFRS pressures would apply and to only design for one or the other. Again, this might end up being a case by case basis call based on building geometry.

 

[KootK]

Does component and cladding on say, a residential wood framed structure, apply to all parts of the exterior of the structure?

Yes, pretty much. That which is not MWFRS is C&C. And, sometimes, parts of your MWFRS is also C&C.

Would I design my entire structure for MWFRS and then design individual parts (i.e. Trusses, Beams, Columns, Studs, Footings) for C&C in both positive and negative vertical direction as well as horizontal?

My opinion is that your MWFRS loaded system is the VLFRS and all of the stuff that aggregates wind applied to the various building surfaces and transports it over to your VLFRS. So shear walls, diaphragms, chords, collector... etc.

I don't believe that one needs to design any particular element for both it's MWFRS and C&C loads concurrently. Where both loads would apply to an element, they are to be considered separately.

What is the load path for loads for C&C? In other words, when trusses are required to be designed for C&C vertical, does that load need to transfer to the foundation, albeit with different areas resulting in different loads?

Strictly speaking, the C&C loads do need to be designed to be transferred throughout the entire structure as you've proposed. In practice, this is seldom done. The reasons for that being some combination of thoughtful engineering judgment and negligence. A lot of C&C design tends to end at the attachment of the component to something beefier in my experience.

Would a stud have both vertical and horizontal C&C loads at the same time? Does. A footing ever require C&C loads to be applied?

This part is murkier and, unfortunately, more open to interpretation.

I feel that, strictly speaking, the answer is yes. That said, I've never seen anyone design a stud as you've proposed.

One could make the argument -- and many have -- that the answer is no because this situation would normally involve the member being designed to receive loads from multiple building planes which triggers MWFRS loading.

 

[KootK]

One could make the argument -- and many have -- that the answer is no because this situation would normally involve the member being designed to receive loads from multiple building planes which triggers MWFRS loading.

Both CFSEI and SBCA have position papers on whether or not their products should be designed to MWFRS or C&C loads:

Link
Link

I don't agree with their positions although I do agree that, based on the strictly applied wording in ASCE7, there is an argument to be made in support of their stances.

They go as far as to suggest that a truss overhang ought to be designed to MWFRS loads because it receives wind from more than one surface (top and bottom). I don't agree with that and feel that it amounts to special interest groups exploiting unintended loopholes.

 

[Klitor]

FWIW I was involved recently with large PV power plant...those structures are basically canopies, and wind tunnel testing was done for the project layout.
Larger "c&c" wind forces were by testing "limited" to happen on 3mx3m area possible to be placed anywhere on the structure.
The problem was that 3mx3m placed anywhere was acctually enough to encompas one full purlin, one full beam, and even column tributary areas.

Designer chose to design only purlins and panel attachement for those larger loads.

Could be kind of a vague area as others have mantioned..