MWFRS vs. C&C 2

[medeek]

Per a lengthy thread on calculation of stud lateral loads my current stud calculator uses both the C&C and MWFRS wind loads (winward wall) to look at the deflection and biaxial loading on wall studs.

http://design.medeek.com/resources/studwall/STUDWALL_CALCULATOR.pdf

When calculating the combined stress of the axial load and the lateral wind load I use the MWFRS value rather than the C&C value for the wind load. Looking at Forte's output it appears their software is using the C&C value when computing the combined stress ratio. Am I doing this wrong? Should I be using the C&C wind load instead of the MWFRS.

There is numerous threads on this subject already, and I've spent considerable time in the recent past thinking about this but when I see someone or something that is giving values that are more conservative than my own it always gives me pause.

A confused student is a good student.
Nathaniel P. Wilkerson, PE

http://www.medeek.com

 

[cnorvell]

I've always done what JAE describes. Conceptually, I could see a justification for analyzing the stud for a combination of gravity loads, axial loads from in-plane shear wall overturning using MWFRS loads, and out of plane bending using C&C loads. But I haven't see this done in practice.

 

[medeek]

As you can see I mulled this over quite heavily a couple years back and then at the advice of the white paper I decided that MWFRS was the way to go. Forte's output is what brought this up as an issue again for me. Bottom line I like to have things done "right". I'm now of the opinion that the C&C value is the correct load to apply as the out of plane force on the stud, column etc...

I appreciate everyone for sorting me out on this one and for the well thought out responses, it seems I rarely have a consensus on any given question submitted to this forum but I always get my eyes opened to various ways of looking at same the problem and this is never a bad thing.

A confused student is a good student.
Nathaniel P. Wilkerson, PE

http://www.medeek.com

 

[Michel60]

Good afternoon all,

I don't mean to upset the consensus but according to the "Guide to Wind Load Provisions of ASCE7-10" C&C loads are only applicable to those elements that receive loads directly from one cladding surface and are to be combined only with loads that are transmitted directly through that same cladding surface, e.g. roof sheathing transmits loads to the roof rafters along with dead and live loads applied to that and only that same cladding surface. However, for something like out of plane wall loading C&C loads would NOT be combined with axial loads in the studs that are tributary to other surfaces such as a floor above. For that combined loading case MWFRS wall loads would be used along with the floor tributary axial loads. C&C loading on the stud is treated as an independent load case for the stud flexural strength and connections.

 

[JAE]

I just disagree with the commentary. That commentary has been badly worded and confusing to engineers for multiple iterations and totally ignores the source reason for use of wind loads based on smaller area (and thus higher variability & peak pressures).

The wind pressure that a structural member receives does not "know" whether it came through one or twenty surfaces. Its magnitude is only affected by the size of the tributary (effective) area on that member.

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[medeek]

According to Michel60 my method of looking at the C&C bending separately is correct, and then using the MWFRS for the combined bending and axial.

A confused student is a good student.
Nathaniel P. Wilkerson, PE

http://www.medeek.com

 

[cnorvell]

The idea of checking bending in a stud without gravity axial loads doesn't make physical sense to me, regardless of whether MWFRS or C&C wind loads are used.

 

[Michel60]

I don't disagree that the Commentary often leads to more questions than it provides, but the separate Guide to the Provision that ASCE publishes is pretty clear as to the intended requirements for C&C loads. They are intended for use with loads resulting from application to a single surface.

And as to what is real or not, that's a much different discussion. C&C loads are a simplification of wind effects that doesn't require evaluation of the global structure and those effects that might result. To include them with C&C loading, while not wrong, just isn't necessary.

 

[sandman21]

Provide the justification for not using C&C loads with any other loads. IBC chapter 16 section? ASCE 30?

 

[Hokie93]

Please cite the page/s and/or examples in the ASCE Guide to the Wind Load Provisions of ASCE 7-10. I looked in the Guide briefly before leaving the office and did not find a statement in support of your position. I agree with JAE and cnorvell.

 

[medeek]

We've been talking about studs but this same issue also involves biaxial loading of headers and in wall columns when you think about it.

A confused student is a good student.
Nathaniel P. Wilkerson, PE

http://www.medeek.com

 

[XR250]

We've been talking about studs but this same issue also involves biaxial loading of headers and in wall columns when you think about it.

Quit trying to complicate my job!
I have never checked biaxial on residential. Alway check vertical. Sometimes check out-of-plane on longer ones.

 

[Michel60]

To be clear my statement was that C&C loads are not to be combined with loads applied to other surfaces... "C&C loads are only applicable to those elements that receive loads directly from one cladding surface and are to be combined only with loads that are transmitted directly through that same cladding surface, e.g. roof sheathing transmits loads to the roof rafters along with dead and live loads applied to that and only that same cladding surface"...

Please find copy of the FAQ from the Guide.

 

[Michel60]

Sorry let's try that again....

 

[Michel60]

...good lord the world is conspiring against me this morning. And again.

 

[cnorvell]

Correct me if I'm wrong, but I get two things from that FAQ relevant to this discussion:
[ol 1]
[li]An exterior shear wall would be analyzed using MWFRS loads for both in-plane and out-of-plane simultaneous actions. I think that was the consensus anyway.[/li]
[li]More generally, ASCE or the commentary's authors intend that analysis of a component that receives wind from more than one direction/surface be analyzed using MWFRS loads.[/li]
[/ol]

But what I'm not seeing is the idea that gravity loads need not be considered when looking at a component under C&C loads.

 

[Michel60]

Good morning cnorvell,

It's not that they are not considered its when. For a roof rafter or joist the gravity loads are applied to the same surface as the C&C loads and are then to be included in the loading combinations. The pivot is if the gravity load is applied to a surface other than the C&C surface, as for the example being considered on a wall stud supporting a floor and/or roof. The stud uses a MWFRS wall load on the sheathing surface in combination with tributary gravity loads. And just to make it more complicated the sheathing and it's attachment on that same stud is designed for just the C&C load applied to the sheathing surface and continuing to the stud and it's connections to the greater structure.

 

[JAE]

The pivot is if the gravity load is applied to a surface other than the C&C surface, as for the example being considered on a wall stud supporting a floor and/or roof....

Sorry - I'm not buying that and never will. As I said above - no structural element "knows" whether the wind load is coming from a C&C surface or a MWFRS surface.

If a stud takes only a very small area of wall wind (i.e. 8 ft. x 16") then C&C wind - with its higher peak pressures - should be used in conjunction with the gravity loads placed on that stud. Even if I have furring channels, or stand-offs separating the exterior "cladding" from the interior stud I will use C&C wind, not MWFRS wind. A stud is NOT a MWFRS element.

Look at the definitions of C&C and MWFRS in AASCE 7.

COMPONENTS AND CLADDING - Components receive wind loads directly from cladding and transfer the load to the MWFRS. Cladding receives wind loads directly. Examples of components include fasteners, purlins, girts, [red]studs[/red], roof decking and roof trusses. Components can be part of the MWFRS when they act as shear walls or roof diaphragms but they may also be loaded as individual components. The engineer needs to use appropriate loadings for design of components, which may require certain components to be designed for more than one type of loading, for example, long span roof trusses should be designed for loads associated with MWFRS, and individual members of trusses should also be designed for component and cladding loads.

So "long span roof trusses" which have very large effective areas can use MWFRS wind BECAUSE the effective area is large - and the individual elements making up that truss, with smaller effective areas, would ALSO have to be checked using C&C wind.

So this is very similar to studs in shear walls - the shear wall - and its overall shear from the MWFRS winds - should be designed as a unit taking shear and all other gravity loads using MWFRS wind. Then the individual stud would be designed using C&C along with all other gravity loads in the various combinations. These are two separate checks of the same element based on very different functions.

If the stud was not part of a shear wall, then C&C ONLY would be used with the gravity loads in the combinations.

So the "pivot" is not the nature of the surface (despite what the confusing guides may suggest) but the effective wind area that the element is affected by for its various functions. This is much more consistent than dealing with "surfaces" in that the wind pressure is directly affected by area - not by surface type or proximity.



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