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.
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
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
RE: MWFRS vs. C&C
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RE: MWFRS vs. C&C
RE: MWFRS vs. C&C
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
RE: MWFRS vs. C&C
RE: MWFRS vs. C&C
A confused student is a good student.
Nathaniel P. Wilkerson, PE
www.medeek.com
RE: MWFRS vs. C&C
RE: MWFRS vs. C&C
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
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
RE: MWFRS vs. C&C
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
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RE: MWFRS vs. C&C
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
http://www.eng-tips.com/viewthread.cfm?qid=373833
Here is the paper that has me calculating the stud wall out of plane loads with the MWFRS value instead of C&C:
http://files.engineering.com/getfile.aspx?folder=d...
Am I misinterpreting this paper?
A confused student is a good student.
Nathaniel P. Wilkerson, PE
www.medeek.com
RE: MWFRS vs. C&C
A confused student is a good student.
Nathaniel P. Wilkerson, PE
www.medeek.com
RE: MWFRS vs. C&C
This comment:
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
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
RE: MWFRS vs. C&C
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
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
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
A confused student is a good student.
Nathaniel P. Wilkerson, PE
www.medeek.com
RE: MWFRS vs. C&C
RE: MWFRS vs. C&C
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
RE: MWFRS vs. C&C
RE: MWFRS vs. C&C
A confused student is a good student.
Nathaniel P. Wilkerson, PE
www.medeek.com
RE: MWFRS vs. C&C
I have never checked biaxial on residential. Alway check vertical. Sometimes check out-of-plane on longer ones.
RE: MWFRS vs. C&C
Please find copy of the FAQ from the Guide.
RE: MWFRS vs. C&C
RE: MWFRS vs. C&C
RE: MWFRS vs. C&C
- 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.
- 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
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
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.
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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