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MWFRS, Components & Cladding, etc...

MWFRS, Components & Cladding, etc...

MWFRS, Components & Cladding, etc...

At Thread507-59761 I received very helpful and interesting comments from Lutfi, CTSeng and jheidt2543.  Thank you!

Inthat thread, I had asked why I should design the entire building, to include Interior Zones, for End Zone, or worse case pressures.  I accept the answers and comments.

But, to get started, what pressures do we use?  Do we design for MWFRS Horizontal Pressures, MWFRS Vertical Pressures, MWFRS Overhangs, Components and Cladding, Components and Cladding Roof Overhang, or worse case of them all?  (What happened to the pre-IBC days of using one wind pressure over the entire building?)

What is the difference between "Components and Cladding" and "Overhangs"?  

I read the definition for "Components and Cladding".  Do not "Overhangs" fall under "Components and Cladding"?

Since I have been advised to apply End Zone pressures to Interior Zones, shall we also apply Components and Cladding Roof Overhang Zone 3 pressures to the MWFRS Interior Zones?

Do any of you actually apply ALL these different pressures toward ALL applicable portions of your structures?  Or do you simplify by employing the worse case of Components and Cladding Roof Overhang Zone 3, which can be extremely high?

Am I overlooking or overcomplicating?

Thank you all!

RE: MWFRS, Components & Cladding, etc...

I agree that ASCE 7-98 is more complicated than UBC.  However, even in 1997 UBC there were different pressures used for Primary Frames and Systems and Elements and Components.  
Wind pressures for overhangs are only specifically defined for the component and cladding level design.  If you want to be conservative, you can include the overhang in your MWFRS calculation using the appropriate roof pressure.

RE: MWFRS, Components & Cladding, etc...

Things use to be simple when you just used a wind load of 20, 25 or 30 psf over the whole exposed area.  However, if you run the calculations you'll find that the wind loads under the new code, while numerous, are usually considerably less, near 1/2 the old one size fits all amount.  So, do you want it easy and conservative or lengthy and economical?  Engineering like life is full of trade-offs.  But, the engineering time is usually less expensive than the field time and ninety percent of the cost savings available in a construction project are found during the design stage.

Personally, I liked it better simple.

RE: MWFRS, Components & Cladding, etc...

Let me see if I can answer your questions and clarify some key points, first let me start with Components and Cladding.
Componenets and Cladding. ELEMENTS of the building envelope that do not qualify as part of the Main Wind Force-Resisting System (ASCE 7).

The cladding of a building recieves wind load directly. Examples of cladding include wall and roof sheathing, windows and doors. Components receive wind loading from the cladding and transfer the load to the main wind-force-resisting system.Components include purlins, studs, fasteners and roof trusses. Some elements, such as roof trusses and sheathing, may also form part of the MWFRS and must be designed for both conditions.

Because of local turbulence, which may occur over small areas and at ridges and corners of buildings, components and cladding are designed for higher wind pressures than the MWFRS.

The effective wind area is used to determine the external pressure coefficients.

Because of the local turbulence at corners, walls are divided into 2 zones and roofs are divided into three zones with a different wind pressure coefficient assigned to each. In addition, different values are applicable for buildings with mean roof height not exceeding 60 feet and for buildings exceeding 60 feet in height.

The concept of effective wind area, defined in ASCE 7-98 section 6.2 is unique to determination of wind loads on components and cladding. The effective wind area is used to select the gust/pressure coefficient, (GCp), to be used to calculate the magnitude of the design wind pressures, whereas the tributary area is the area over which the calculated wind pressure is applied for that specific C&C designed element.

A roof overhang can be evaluated as C&C, with high negative pressure on the windward side and neglect the small pressure difference of the leeward side.

It can seem quite complicated, however, it is really not too difficult once understood. A helpful guide published by ASCE is Guide to the use of the Wind Load Provisions of
ASCE 7-98 (or 7-02) by Kishor C. Meta and Dale C. Perry, available through ASCE website: WWW.asce.org.

RE: MWFRS, Components & Cladding, etc...

Good comments -

Just to summarize - MWFRS loads do include provisions for overhangs, etc. and these should ALL be included at once (for the appropriate direction and interior pressure) for design of the entire MWFRS.

For example - there is, in ASCE 7-02, a Primary Wind (MWFRS) pressure for overhangs.  While overhangs are a component, the MWFRS wind on that component is included for the bracing design, but not for the design of the component iteself.  Make sense?

For the overhang design itself, you would use the C&C pressure fr overhangs.

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