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Skin friction from wind load for buildings

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> -----Original Message-----
>
> From: Keith Fix <kefix(--nospam--at)yahoo.com>
> Subject: Skin friction from wind load for buildings
> To: seaint(--nospam--at)seaint.org
>
> Where can I find skin friction values for design wind loads (ASCE 7-95,
> 98 or equiv.): _not_ force perpindicular to surface, but parallel?
>
> Skin friction related to wind speed or velocity (speed with direction)
> would also be an acceptable format.  The more building materials
> covered, the better.
>
> Keith Fix
> Cromwell Architects Engineers


Dear Keith:

For most buildings, normal pressures dominate the wind loads, and surface
friction drag is well below the "noise level" of the building code provisions. 
However, for some structures, such as free-standing roofs and canopies, surface
drag can become significant because the projected area normal to the wind may be
very small compared to the plan area,  For example, consider a corrugated sheet
steel roof sitting on pipe columns.  In general, the friction force F may be
calculated by the formula F = qCA, where A is the surface area, C is the drag
coefficient, and q is the stagnation pressure of the wind, including all of the
adjustments for exposure, height, terrain, gust effects, importance factor,
phase of the moon, etc.  For engineers familiar with building code wind
provisions, the only real question is what to use for the drag coefficient C.

>From about 1993 to 1997, the SEAs of Washington, Oregon, and California
developed a wind code proposal for UBC-97.  At that time, the issue of surface
drag on free-standing roofs was addressed by Dr. Dale Perry, who provided us
with the following values:

C  =  0.01  for wind on smooth surfaces or parallel to corrugations.

C  =  0.03  for wind perpendicular to corrugations.

The value of 0.03 was incorporated into the Tri-States Wind Code Proposal for
all unenclosed structures and canopies.

On the other hand, the Australian Wind Code AS 1170.2-1989 (96 pages of wind
provisions) provides the following values:

C  =  0.01  for smooth surfaces without corrugations or ribs,
             or where wind is parallel to corrugations or ribs.

C  =  0.02  where wind is across corrugations.

C  =  0.04  where wind is across ribs.

The friction drag is additive to the pressure drag calculated in accordance with
the building code.  More "precise" information is available in the literature,
but it is certainly overkill for ordinary building design.  If you think you
need additional information for a more refined analysis, contact me privately
for a counseling session on the accuracy of building code load calculations.

Best regards,

John V. Loscheider, P.E., S.E.
Loscheider Engineering Company
P.O. Box 2440
Renton, WA  98056
(425) 255-0126  phone
(425) 228-9798  fax
jvl(--nospam--at)loscheider.com