Components and Cladding Wind loads

[sda97]

The Guide to the Use of the Wind Provisions of ASCE 7-02 indicates (in FAQ) that flat roof trusses should be designed as C&C. If these trusses(or joists) bear on masonry walls or beams, can the walls and beams and anchorages be designed for the MWFRS loads? The FAQ on gable trussses would seem to indicate that this is the case. Any comments?

It would seem like a flat truss and a gable truss serve the same function - that of a MWFRS. The Guide, however, treats them differently.

 

[DaveAtkins]

When I design hurricane tiedowns for roof trusses, I use C & C loads. I believe MWFRS loads are only to be used for elements of the lateral load resisting system, such as shear walls, braced frames, rigid frames, etc.

DaveAtkins

 

[JAE]

DaveAtkins is essentially correct in my opinion. But keep in mind that the C&C loads vs. the MWFRS loads have a lot to do with the influence area on the particular piece you are designing.

The concept is that wind pressures on any building vary wildly in magnitude over small areas. Thus, for a piece of structure that deals with a small area (such as a single stud in a wall, or an edge fastener on roofing) the magnitude of the pressure in this tiny area can be quite high...thus, the C&C loads are higher.

As the influence area gets larger, the effective pressure on the piece becomes the AVERAGE of all the variations of wind pressures along that piece. So as the area gets larger, the C&C pressure gets smaller until at some point (I believe its 1000sf in ASCE 7) you dump the C&C pressures and go to the MWFRS pressures.

 

[Ron]

The process outlined by JAE and DaveAtkins is as I use it also. The inference in ASCE 7 is to use the more conservative approach to developing loads, and as JAE pointed out, the influence or tributary area is key.

 

[sda97]

It just makes sense that the roof structure is part of the the main wind force-resisting system and should be designed as such. It resists uplift, resists lateral load from the walls and serves as a diaphram for the structure. The ASCE-7 Standard seems to be very conservative in there analysis methods and treatment of indidual stucture components. Just a comment.

 

[UcfSE]

I agree with Dave and JAE.

MWFRS is for lateral resisting elements such as shear walls and moment or braced frames, not individual members such as trusses or footings holding up pinned-end columns. The other engineers in my office don't agree with me but that's another story. Other engineers I know, friends and such, say the same as Dave. If the element contributes to the overall lateral stability of the structure, then it's MWFRS as stated in ASCE 7. Some say that if it receives wind from more than one surface then its MW but that's a guideline in ASCE, not a rule or definition.

 

[TFL]

I think the concept is being confused here. You need to look at what function the element is serving and then design for the appropriate loads. Roof structures are serving multiple purposes.

1. they provide lateral stability thru diaphram action.
2. they keep the wind and such out as a cladding.
3. they keep snow and the like out for gravity loads

Therfore a roof would need to be designed as both MWFRS and compomenets and cladding and gravity. The diaghpram loads would be calced using MWFRS and the uplift forces on the roof would need to be calculated using C+C loads.

Also the tributary are of a roof joist is typically no that large. As far a i undersatnd it when doing C+C loads the area is that of the component NOT the Structure. Therefore you can not use the argument that you have a huge area to design for and can reduce the loads. no matter how big the roof is the influence are is only of the actual component(joist etc).

 

[JAE]

tfl - I agree with your points - but there are some elements of a structure that DO have very large tributary areas...such as a major truss girder supporting mulitple roof joists. If those major elements have trib. areas greater than 1000 sf, then the MWFRS wind loads would be appropriate.

Of course diaphragms should be designed for MWFRS as the wind applied is generally across the entire width of the structure. And uplift forces on a deck are definitely Component and Cladding wind as the area is very small (spacing of fasteners).

 

[doejohn]

Following up on tfl's lead--which I think is right--would it be fair to say that:
1. If the wind pressure acts directly on an element/connnection, it must resist CC loads.
2. If the element's resistance to, or distribution of, wind forces is part of the wind force-resisting system, it must resist MWFRS loads.
3. These are not mutually-exclusive categories! A given element may function in both ways, and if so must be analyzed/designed accordingly.

A proposed rough rule of thumb...whadya think?

Back on sda97's original post, I haven't looked at the FAQ's he mentions, but above approach would mean that the connection between wall/beam and roof truss is load path for:

1. Vertical forces originating as out-of-plane wind loads on the roof (wall/beam supporting roof). Greater of MWFRS loads, or CC loads to an area equal to the wall/beam's tributary roof area.
2. Lateral forces originating as out-of-plane wind loads on the wall/beam (roof supporting wall/beam). Again, greater of MWFRS loads, or CC loads to an area equal to the wall/beam's tributary wall area.
3. Lateral forces. acting in-plane of both roof and wall/beam, required to collect or resist roof diaphram shear. MWFRS loads.

Design loads for wall/beam and roof trusses include effects of these load mechanisms, regardless of whether roof truss is flat or gabled.