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

 

[JLNJ]

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?)

 

[sandman21]

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.

 

[JAE]

C&C for individual studs.

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[Ron]

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.

 

[structSU10]

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.

 

[mike20793]

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?

 

[sandman21]

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

 

[medeek]

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

http://www.medeek.com

 

[jayrod12]

My vote is c&c loads

 

[JAE]

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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[medeek]

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/SHEARWALL_CALCULATOR.pdf

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

http://www.medeek.com

 

[sandman21]

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

 

[cnorvell]

"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.

 

[JAE]

cnorvell is perfectly correct (and perfectly stated). (sandman as well!)

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[medeek]

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

http://www.medeek.com

 

[medeek]

Here is a related thread from 2 years ago.

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/getfil...sc=212727627.17.1469148696562&__hsfp=72877508

Am I misinterpreting this paper?

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

http://www.medeek.com

 

[medeek]

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

[pre]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).[/pre]


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

http://www.medeek.com

 

[JAE]

I guess I totally disagree with that white paper.

This comment:

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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[sandman21]

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.