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MWFRS vs. C&C

MWFRS vs. C&C

MWFRS vs. C&C

(OP)
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/STUDWA...

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
www.medeek.com

RE: MWFRS vs. C&C

I would tend to us the C&C load to be conservative (just did so for some masonry a few minutes ago), but you might be right to use the MWFRS pressures.

It's not exactly in the code, but I think of it as a continuum with higher pressures for a few square feet on one end to lower pressures approaching the MWFRS values when the tributary area gets to be large. It seems reasonable to use your judgment based on relative gravity and wind element tributary areas. (I.e. how likely are you to get max axial and max wind simultaneously?)




RE: MWFRS vs. C&C

Components and cladding should be designed using Ch. 30 of ASCE7-10. When a component or cladding has a tributary greater than 700ft you can use the MWFRS provisions, 30.2.3. The check of an individual stud for gravity and wind should be using C&C loading.

RE: MWFRS vs. C&C

Agree with JAE....also, for rigid wall surfacing (stucco) check differential deflection between anchor studs (attached to walls) and adjacent studs as well as double or triple studs at windows relative to single adjacent studs....can be an issue with rigid claddings.

RE: MWFRS vs. C&C

As I understand it C&C loading is the way to take into account the localized variations in wind pressure a building experiences (there is a good diagram of this in the commentary of ASCE 7). Since the area of an entire structure is so large (generally), the localized pressures +/- are smoothed to an overall wind pressure. When you 'zoom in' to an individual member with small tributary area, those localized higher pressures become more important to consider.

You can use stud height * stud height/3 as tributary for a stud per ASCE 7 to reduce the pressure a bit too.

RE: MWFRS vs. C&C

I think there's more going on than just a simple stud analysis. The OP is asking about combined axial and bending from wind load. I assume he is referring to an uplift or downpressure on the studs from something above in addition to the lateral wind load on the wall itself. Is that correct? medeek, do you know the thread where this was discussed originally?

RE: MWFRS vs. C&C

He is talking about checking a stud for out of plane wind load, if you follow his link.

RE: MWFRS vs. C&C

(OP)
Checking the stud wall involves a combined check. Usually the axial loads are some combination of Dead, Live Floor and Snow. The bending load on the stud is usually the wind. The question is when I do the combined check do I use the C&C or MWFRS wind values? When I do just a bending check I already use the C&C value however my understanding has been where you have combinations of various load types they you should use the MWFRS wind values. I should probably dredge up the prior thread where this was discussed ad nauseam.

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

RE: MWFRS vs. C&C

My vote is c&c loads

RE: MWFRS vs. C&C

Quote (medeek)

however my understanding has been where you have combinations of various load types they you should use the MWFRS wind values.

That understanding is incorrect unless the stud effective area is greater than 700 s.f. Then you could use the MWFRS per ASCE 7.

It is a single component of the structure and has nothing to do with the main wind force resisting system.

Now if that stud was part of an end post in a shear wall, then you'd have to check both - C&C and MWFRS for each of the conditions (axial+out-of-plane loads with C&C and axial+out-of-plane loads with MWFRS when the shear wall is engaged as a shear wall.)

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RE: MWFRS vs. C&C

(OP)
How is the shear wall example really any different from a stud loaded axially with various gravity loads from the rest of the structure?

You do bring up an interesting point though with studs in shearwalls. I updated my shearwall calculator about a year ago to include checks for the bearing, tension and compression in the shearwall end posts (chord).

http://design.medeek.com/resources/shearwall/SHEAR...

However it does not do a combined check for bending of the stud due to wind loads (I'm assuming C&C now) and axial loads from the MWFRS wind loads via the shear walls (and also other axial loads D, L, S etc...). I guess I probably need to add that check in as well. Most shearwall end posts are often at the wall corners, but not all, so the appropriate braced length will become an important variable in the analysis.

If the shear wall is an interior shear wall then the wind is not a factor for bending. Also my current spreadsheet takes the larger of the seismic or wind axial load and runs the compression, bearing, tension check on that value. I can see now that I need to do the combined check for wind on the end post regardless if the seismic axial load is greater, the combined wind may fail and thereby govern.

I just finished a small residence and the report was a 139 pages and that did not include about half of the supporting docs. The amount of checks even with a small residence can become staggering.

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

RE: MWFRS vs. C&C

It's C&C loads unless a component or cladding has a tributary greater than 700ft you can use the MWFRS provisions, 30.2.3

RE: MWFRS vs. C&C

"How is the shear wall example really any different from a stud loaded axially with various gravity loads from the rest of the structure?"

When checking a stud for a combination of axial gravity loads and out-of-plane wind loads, you are checking it for the critical wind load that a particular stud could be subject to, which is the C&C load. Since a single stud is not an element of the main wind force resisting system, it doesn't make sense to use MWFRS loads unless its tributary area is larger than the 700 sqft code threshold.

When checking a stud as a component of a shear wall, with a combination of axial gravity loads and in-plane wind loads, you are checking it as a element of the main wind force resisting system using the MWFRS loads.

RE: MWFRS vs. C&C

(OP)
I agree with the in plane loading of the shearwall using the MWFRS values. However if you were to check the end post of the shearwall for out of plane wind loads whilst it is also loaded axially by MWFRS loads from the shearwall would you use the C&C wind value for the load causing the bending in the stud?

In this common scenario the end post (stud or double stud, 2-2x6) is loaded in compression from the MWFRS shearwall load and also by the dead, live floor and snow load from the roof and floors above. Let's assume the shear wall terminates at one end next to a window or door (ie. the stud is not braced out of plane against the winward wall forces). In this situation the end post is subject to axial compression and bending due to the out of plane wind forces. Based on the previous comments the appropriate value to use for the out of plane wind load would be the C&C wind value and the axial loads from the shear wall would come from the MWFRS.

I just read through the Forte help in some detail and they use C&C Zone 4 for their stud wall calculations, Zone 5 would be more conservative.


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

RE: MWFRS vs. C&C

(OP)
In particular page 7 and 8 of this white paper caught my attention:

The final step in design of the studs is to choose a member which has sufficient design
capacity to resist the induced loads tabulated above. Stud walls are a hybrid system in wind
engineering terminology. Studs should be designed using MWFRS pressures when considering
the combined interactions of axial and bending stresses; and designed using C&C pressures
when considering axial or bending stresses individually. This interpretation was developed
because only MWFRS pressures provide loads which have been temporally and spatially
averaged for different surfaces (MWFRS loads are considered to be time-dependent loads).
Since C&C loads attempt to address a “worst case” loading on a particular element during the
wind event, these loads are not intended for use when considering the interaction of loads from
multiple surfaces (C&C loads are not considered to be time-dependent loads) In the above
example, stud design is limited by the C&C load case. This is not uncommon and in most cases
can be considered the controlling limit in wind design of loadbearing and non-loadbearing
exterior studs. However, until sufficient boundary conditions are placed on this simplification,
both MWFRS and C&C load cases should be considered. These assumptions were also used in
the development of the Wood Frame Construction Manual for One- and Two-Family Dwellings,
1995 High Wind Edition (WFCM-SBC). 

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

RE: MWFRS vs. C&C

I guess I totally disagree with that white paper.

This comment:

Quote (White paper)

Studs should be designed using MWFRS pressures when considering
the combined interactions of axial and bending stresses; and designed using C&C pressures
when considering axial or bending stresses individually.

is just stupid done well. (yes there are whiffs of stupid inhabiting our codes these days).

Or you could at least say the sentence is misleading and confusing. The use of C&C vs. MWFRS doesn't depend on whether there is bending, axial, etc.
It depends on whether you are designed the stud in its individual state as a component with a small wind tributary area (i.e. high C&C winds) or if you are
checking it as a part of a shear wall with wind coming in on it from the overall building-shear wall interaction.

If you have a stud in an exterior wall that is also part of a shear wall in that wall plane, you have to check it for both types of wind.
What we do is this:
Part A
1. Use C&C wind along with ALL the other applicable loads on that stud, be it Dead, Live, Roof Live, Snow, Rain, etc.
2. Use the ASCE 7 load combinations of these loads.
3. Design the stud.
Part B
4. Now analyze the overall structure and determine the load in the shear wall using MWFRS wind.
5. MWFRS wind does include side wall suction forces which should be included with this analysis to create bending in the stud.
6. Determine the axial forces on the stud from the shear wall analysis (i.e. end post axial in tension or compression)
7. Use the axial, shear and bending forces from this MWFRS analysis along with ALL the applicable axial, shear and bending forces from dead, live, roof live, snow, etc. in the ASCE 7 load combinations.
8. Design the stud.
Solution
9. The applicable stud size is the larger size from item 3 or item 8 above.

Note that in both Part A and Part B there could very well be axial, shear, and bending in both. Thus the white paper is misleading.



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RE: MWFRS vs. C&C

Nearly everyone in the thread you linked says to use C&C with all other loads. One paper covering ASCE 7-98 says otherwise, they provide little justification for their selection.

Also no one answered your question regarding why truss manufacturers use MWFRS loading, the trib area is greater than 700sf thus section 30.2.3 kicks in.

RE: MWFRS vs. C&C

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.

RE: MWFRS vs. C&C

(OP)
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
www.medeek.com

RE: MWFRS vs. C&C

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.


RE: MWFRS vs. C&C

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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RE: MWFRS vs. C&C

(OP)
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
www.medeek.com

RE: MWFRS vs. C&C

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.

RE: MWFRS vs. C&C

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.

RE: MWFRS vs. C&C

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

RE: MWFRS vs. C&C

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.

RE: MWFRS vs. C&C

(OP)
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
www.medeek.com

RE: MWFRS vs. C&C

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

RE: MWFRS vs. C&C

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.

RE: MWFRS vs. C&C

Correct me if I'm wrong, but I get two things from that FAQ relevant to this discussion:
  1. 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.
  2. 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.
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.

RE: MWFRS vs. C&C

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.

RE: MWFRS vs. C&C

Quote:

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.

Quote (ASCE 7-10, C26.2)

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, studs, 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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