CMU Wall Design C&C Load Combo Questions

[structuresguy]

I am reviewing the calcs done by an engineer we recently let go where he has designed an 8" CMU wall, 32 feet tall to the roof joists. The project is in central Florida area. I do not believe it will work for out of plane bending, but his calcs show it works based on the load combos he used.

Specifically he used only DL+LL and 0.6DL+W. But in the wind case, he only used MWFRS loads for the out of plane wind.

My feeling is that C&C loads should have been used for the .6DL+W case, and another case of DL+.75LL+.75W should also have been used. MWFRS loads should only be used for in-plane load combos.

I would like to get some other opinions to support (or refute) my position.

Thanks much.

 

[DaveAtkins]

I agree--use C & C WL for out of plane bending.

Remember that the tributary area = 32'*32'/3 = 341 sf.

DaveAtkins

 

[JAE]

I agree with DaveAtkins - since the tributary area of the wall is based upon 1/3 the height, the area is less than 700 s.f. (per ASCE 7) and thus, C&C loads are required.

 

[UcfSE]

Agree with using C&C. Note that in FL you can use the 1/3 stress increase provided you don't combine it with the 0.75 reduction in the load combinations with LL and WL acting concurrently.

 

[RareBugTX]

Dave Atkins:

Is the 1/3 tributary rule found on ASCE 530, I thought we should consider a 1' foot strip. for out of plane.

 

[haynewp]

It's in ASCE 7.

 

[RareBugTX]

Thanks Haynewp!!!

 

[structuresguy]

Thanks for the consensus. We have been seeing some odd things since this engineer left. Just wanted to make sure I was for sure correct before telling the owner we need to increase the wall to 12" CMU. fortunately, we have a chance now as they made some last minutes changes.

 

[hokie66]

Not familiar with your code, but don't you have any maximum height/thickness limits? H/t=48 would not comply with robustness requirements of the Australian standard.

 

[chichuck]

I also agree, for the out of plane loads, you should use C&C coefficients. The reason is as follows:

C&C coefficients apply when the component (the block wall) receives wind from only one surface. For desgning the windward or leeward walls, they receive load due to wind only on that wall itself (yes, both inside face and outside face) but only from the wall itself. The roof diaphragm, or steel braces loads from both the windward and the leeward walls.

For designing the block shear walls, it is more complex. The in-plane loads come from the roof diaphragm or braces, so those loads come from more than one surface (the windward wall and the leesard wall.) So those loads should be evaluated using MWFRS coefficients. At the same time, there is suction on the sidewalls (both internal face and external face). Those loads should be calculated using C&C coefficients and combined with the in-plane shear loads calculated as above.


regards,


chichuck

 

[JAE]

chichuck,

Not to start an argument (there's another thread here in Eng-tips where we really hashed this out big-time) - but the assertion that the number of surfaces that the wind comes in on determines whether you use MWFRS or C&C isn't directly specified in ASCE 7. It does show up in the commentary as a way to envision the difference, but it is not really the code applied means.

MWFRS and C&C differ only in that the MWFRS is a lower value due to the fact that those MWF systems usually receive wind over a LARGER AREA and the statistical variations of wind pressures average down to a lower value due to the large areas involved.

C&C wind applies to elements that take wind from SMALLER AREAS and thus the variations over those small areas tend to average higher.

One way to look at this is to envision a wall system comprised of a metal panel supported by horizontal cross hat channels, supported by vertical studs, and then finally the studs supported by horizontal girts which are connected to vertical columns. Now this system probably never existed anywhere but just as an example, what would you design the vertical columns for?

If the tributary wind area on the column was less than 700 s.f. then you would use C&C loading, despite the fact that it gets wind via four "layers" of structure.

Also - in your last paragraph, you never combine C&C wind (your orthogonal suction) with MWFRS wind (in plane on a shearwall). There are already defined MWFRS orthogonal pressures that you do combine with MWFRS in-plane shear.

 

[vincentpa]

JAE,
I disagree with this statement,

"Also - in your last paragraph, you never combine C&C wind (your orthogonal suction) with MWFRS wind (in plane on a shearwall). There are already defined MWFRS orthogonal pressures that you do combine with MWFRS in-plane shear."

In ASCE's guide book "Guide to use of Wind Load Provisions to ASCE 7", they specifically state you should use C&C and MWFRS for precisely this circumstance. In fact they give an example of the design of an eave strut designed for bending (suction) using C&C and axial for MWFRS in the same load combination. This makes sense because while the area for MWFRS is large, the suction could spike over the small area supported by the eave strut out of plane (suction). The case of the CMU wall is the same almost. I would use C&C out of plane and MWFRS in plane.

The suction pressure for MWFRS is calculated for MWFRS only and will only have effect on the MWFRS if the geometry of the building is such that the suction will impact the MWFRS. What they are trying to get at is that suction will not always cancel out and must be considered in the design of the MWFRS. The only time MWFRS is used for individual member design is when the member is acting as the MWFRS element for that direction and/or when the area is larger than 700 sq. ft.

 

[asixth]

I can't comment on the design code because I practice in another country. However, you let this engineer go, they obviously got laid off for a reason, I would be looking over there work very carefully and if I don't agree with anything they have done I would definitley make changes.

 

[structuresguy]

asixth: That is exactly what we are doing right now. However, some projects have already started construction. So I need to weigh "proper" design against code minimum design, and even consider how much initial overstress we may allow in some cases.

Here is another case when a mix of C&C and MWFRS loads are used (at least in my opinion). Roof deck design for combined uplift and diaphragm shear. I design the deck and fasteners for C&C uplift and MWF diaphragm shear simultaneously. The SDI Deck Design Manual gives the equations, and shows examples, but never addresses whether the loads they are using are C&C or MWF. So I use a combination of both. I think this is appropriate since the diaphragm would be average over large areas, and the uplift would be highly localized for the individual fasteners.

 

[vincentpa]

STRUCTURESGUY,
YOU ARE CORRECT. C&C FOR UPLIFT AND MWFRS FOR SHEAR. Sorry for all caps. I was picking up marks in CADD. It seems like overkill but that is their intent. USD has charts for interaction.

 

[structuresguy]

USD also now has an online combined uplift and shear calculator on their website. Works great. It was used to calculate their interaction charts. It takes all combinations of deck type, welds, screws, pins, etc...

THe USD charts for us are not very useful, as they only go up to 50 psf uplift. We frequently have well over 100 psf uplift on our roof decks here in Florida.