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# RE: arched roof wind theory

• To: seaint(--nospam--at)seaint.org
• Subject: RE: arched roof wind theory
• From: Scott Maxwell <smaxwell(--nospam--at)engin.umich.edu>
• Date: Tue, 25 Jan 2005 23:39:06 -0500 (EST)

```Andrew:

I am not sure I agree with the "...MWFRS loads could be used rather then C
and C" statement.  To my understanding, the "functional" difference
between MWFRS wind loads and C&C loads is the tributary area used.  As I
see it, for all intents and purposes, MWFRS loads are in essence very
similar to C&C loads, it is just that you use a "default, maximum"
tributary area and as a result, don't have a direct factor that you
determine based upon trib area...that factor is the same for ALL MWFRS
calculations and is "built in".

In other words, having an arched roof should have nothing to do with
whether you use MWFRS or C&C loads.  That decision is basically "governed"
by the size of the element for which you are determining the wind
pressure.  If it is the whole building OR an element of a building (which
would normally be considered a component or cladding) which has a trib
area less than 700 sq ft (see section 6.5.12.1.3 of ASCE 7-02), then you
can use MWFRS pressures.  If it is an element that has a trib area less
than 700 sq ft, then you use C&C pressures.  This is independent of the
type of roof configuration.

Besides, I will point out that there appear to be provisions in ASCE 7-02
for arched roofs.  Figure 6-8 is described as "Main Wind Force Res.
Sys./Comp and Clad. - Method 2" for "Arched Roofs".  The figure gives
Csupp values for MWFRS, but Note 4 of that figure gives instructions on
calculations.

HTH,

Scott

On Tue, 25 Jan 2005, akester wrote:

> My theory (no research):
> As wind passes over an arched roof, beyond the perimiter, there are no edges or sudden changes in pitch, nor obstructions, that would cause a localized pressure difference. This is compared to a roof with a ridge or hip. In Bernoulli-esque terms, this would be uniform or undisturbed flow over the surface of the roof, so that it would recieve a lower, more constant uplift pressure, so that MWFRS loads could be used rather then C and C.
>
> Also, due to the small percentage of these types of roofs being constructed (in the US), there has probably been little demand for more research and testing on this type of roof system, versus gable and ridge roofs, and hip roofs, which are on 98% (out of my a\$\$) of houses and other residential type structures.
>
> Now, I have said this before, but can I vote "YES" now on more funding for canopy roofs and overhang roofs, like those on every gas station, strip mall, grocery store, school, etc. in the friggin US???
>
> Side note: Down near the east coast, central FL, a rest area off I-95 had little noticeable hurricance damage, except for the pre-fab aluminium canopies which many had no roof and had racked. Some soffit damage to the main building with 16' CMU walls, that is about it (standing seam roof). Ok, now I will pat myself on the back because I did the structural eng on the main buildings, but that is not my point... Do you think anyone will take that company and their engineer to task on the design? As far as I know we had no winds down there near the design speed of 130 mph.....
>
>
> Andrew Kester, PE
> Structural Engineering Consultant
> Lake Mary, FL
> C 407-324-6255
>

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