Masonry Wall Design: C&C or MWFRS 2

[1SEngineer]

Hello SE's:

Question: When designing masonry walls to resist wind forces, are you using C&C pressures ... or MWFRS pressures?

From day one as and SE, I was told to use C&C pressures. Since then ... I have always used C&C pressures ... but according to the definition for MWFRS in ASCE7 ... it seems that MWFRS might work also? I feel like my masonry designs may be a bit conservative ... and just wondering if I'm falling behind with my knowledge base.

Are you also using C&C ... or MWFRS?

Thanks in advance!

 

[focuseng]

Both -depending on the function and scale of the element/component.

MAP

 

[JAE]

When you are designing the wall as a shearwall - and it receives wind from > 700 s.f. areas, then it would use MWFRS.

When you are designing the wall for perpendicular wind forces, and you use a tributary area of (1/3 x height) x height and this area is less than 700 s.f., then you would use C&C wind.
If the area is > 700 s.f. you would use MWFRS.

 

[JedClampett]

I'm not sure I understand your question. The names describe them pretty well. MWFRS is the main wind force resisting system, like bracing and/or shear walls. C & C is for the individual pieces, like walls and windows. The larger the area, the lower the overall load. For instance, it's very unlikely a whole building will see a large wind gust. But one wall region might.
For the use of the walls as shear walls, where they act as the lateral force resisting system, the MWFRS analysis applies. For the "local" effects, like spanning from floor to diaphragm level, the C & C forces apply.
If you have to combine them, you're on your own.

 

[ash060]

I also will design using only MWFRS loads for combined uplift and perpendicular to wall forces. I am in FL we have lots of uplift.

 

[DST148]

@1SEngineer - Assuming wall is part of MWFRS and on the exterior of the building ----
Case A - Wind in the direction of the plane of wall - Design wall for loads using MWFRS pressures, inplane loads (as a shear wall) and for loads due to wind pressures on the sides
Case B - Wind perpendicular to wall - Design wall for loads using MWFRS pressures, out-of-plane loads (as a shear wall - due to torsion if any) and windward / leeward wall pressures
Case C - Wind perpendicular to wall - Identify location, either interior(zone 4) or corner (zone 5); design wall using C & C pressures for windward / leeward wall.

 

[1SEngineer]

Thanks for the responses thus far.

If a masonry wall is used purely as a shear wall then obviously it is MWFRS. However ... say we are designing a box building like a home depot ... the walls are simultaneously receiving axial and in and out-of-plane loads. This is MW as well as CC ... How are you designing this scenario?

Thanks.

 

[fancypants]

Masonry Walls will all receive out-of-plane loads even if it is an interior partition wall. See the code for interior wall pressures.

Out of plane bending will receive higher wind pressures base on CC. Use the MWFRS for shear on the walls.

 

[ash060]

Typically for the MWFRS loads you have the main case of three loads. You have the uplift loads that are received from the roof framing. Second, the sidewall pressure which is creating out-of-plane loads on the wall. Lastly, the windward and leeward wall pressures that are applied to the wall as in-plane loads. All of these loads act at the same time on the wall.

One, good way of noting the different loads is to look at the diagrams. For MWFRS the diagrams show wind load applied to the roof and all walls simultaneously. For C&C loads the diagrams only show loads applied to a specific component, like the walls or the roof.

In most cases for me the amount of reinforcing in the wall is controlled by the C&C loads. I usually take the reinforcing I get from the C&C numbers and check the wall with the MWFRS loads and verify that the amount of reinforcing works, if not I add as needed so that it does.

 

[charliealphabravo]

I haven't seen the 2010 ASCE-7, but if the definitions for C&C and MWFRS have not changed then neither has my opinion. I have pasted my response to a previous post.

A structural element should be designed for MWFRS pressures if it is part of the MWFR system and for C&C pressures if it is not. This determination is made using the definitions of MWFRS and C&C given in the Standard (ASCE 7). Note that these definitions do not mention anything about 700sf or Effective Wind Area. The language about 700sf is an exception that allows C&C to be designed using MWFRS pressures if the Effective Wind Area of the C&C is 700sf or more. This provision should not be interpreted to mean that no elements in the MWFRS will have wind areas less than 700sf. In fact, Effective Wind Area is not really relevant to MWFRS analysis. The tabulated pressures for MWFRS are not a function of Effective Wind Area; nor are Effective Wind Areas the same as tributary areas.

Also note that, to be precise, the definitions of MWFRS and C&C are mutually exclusive. That is to say, the way the definitions are written, an element cannot be both MWFRS and C&C. I am not aware of any provision in the Standard that speaks to the design of structural elements to both MWFRS and C&C pressures; or any language in the Standard that requires small MWFRS to be designed for C&C pressures. You can find language that recommends this in Kishor’s Guide to ASCE7 and you might possibly infer it from language in the Standard Commentary. It is left to your professional engineering judgment to determine if MWFRS pressures or C&C pressures or both must be used. It is left to your professional engineering judgment to determine whether a structural element is C&C or part of the MWFRS system; notwithstanding the lists in the Commentary that would seem to imply that a certain structural element (a truss for example) is both C&C and MWFRS. This cannot be determined without knowing how the structural element is loaded which is why this language is in the Commentary and not in the Standard.

With all due respect, it does not take any twisting of the definitions to arrive at this conclusion....In fact, I must confess that my opinion is based almost entirely on what the Standard says. I don’t have extensive knowledge of the inner workings of the Committee or of how the numbers were derived, except what I have learned from Kishor, Minor, McDonald and others, and this is not always encouraging. This really is an issue that the ASCE7 Wind Load Committee should take up. If the Committee believes that Effective Wind Area should be part of the MWFRS and C&C definitions they should explicitly make it so.​