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Re: Wind Pressure Below Grade?

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Brian A. Peterson wrote:
     
<<I am looking for information and/or opinions on wind pressures at the 
below grade level. 
     
We install temporary containment structures in graving docks to 
blast/paint ship's structure.  The docks are 30 to 60 ft. deep adjacent to 
the water's edge, and ~100 ft. wide by ~1000 ft. long.  A containment may 
be installed in the base of the dock then rise above the "ground" level.  
     
For design, at above dock height I use standard wind load calculations 
from UBC and/or ASCE 7-95.  At dock height and below, for lack of other 
criteria, I have used 10 psf based on ASCE 7-95 "minimum design wind 
loading."  Depending on the depth below "ground" level, this may or may 
not be conservative.  To add to the problem, any ship in dock will 
undoubtedly have a channeling effect on the wind- but that's another 
story.
     
Barring major wind tunnel testing (not justified for temporary 
structures(?)), I would like an idea of whether or not I am on track with 
these assumptions.  Any help is appreciated.>>

*************************

Another way to look at this problem is to consider what happens as the wind 
stream hits the abrupt obstruction of a building.  The windward side 
experiences a positive pressure and the other sides and roof experience a 
negative pressure.  The wind going into your pit like a negative 
obstruction and is going to put both positive and negative pressures on 
your pit walls and any ship surfaces in the pit.

In this line of thinking, I would use the wall pressure coefficients from 
Figure 6-3 of ASCE 7-95, Cp = +0.8 to -0.7.  I would calculate qz based on 
60 feet and appropriate exposure category.  Running out the numbers will 
give you close to 10 psf for Exposure Category C.

Your number of 10 psf as a minimum seems reasonable.  Make sure you apply 
it both positive and negative.

Rick Drake, SE
Fluor Daniel, Irvine