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RE: 10 PSF Minimum Wind Load

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It never occurred to me from reading the Code that there were 2 load cases to check.  One with the loads of the zones shown in the tables and one with 10 PSF over the entire structure.  I have not seen this illustrated in any design examples in text books.  Does anyone have a published example of this?

 

For my particular design situation it looks like the 10 psf over the entire area would control since the roof area is so large and the difference between the zone loads and the 10 psf is so great.

 

Are there any other opinions to this interpretation?

 

Rich

 

 


From: Jason Christensen [mailto:jason.christensen(--nospam--at)es2eng.com]
Sent: Wednesday, April 12, 2006 9:33 AM
To: seaint(--nospam--at)seaint.org
Subject: RE: 10 PSF Minimum Wind Load

 

My understanding, and someone correct me if I am wrong, but the pressure that you show in Zones A, B, C, D are one load case.  You check that load case then you check to see if 10 psf over the entire structure and take the larger of the two cases.

 

Jason

-----Original Message-----
From: Rich Lewis [mailto:seaint03(--nospam--at)lewisengineering.com]
Sent: Wednesday, April 12, 2006 7:30 AM
To: seaint(--nospam--at)seaint.org
Subject: 10 PSF Minimum Wind Load

 

I have a question regarding common practice of minimum wind load for a roof for the MWFRS.  IBC 1609.1.2 and ASCE 6.1.4.1 state the minimum wind force is to be “not less than 10 PSF multiplied by the area of the building or structure projected on a vertical plane normal to the direction of the wind”.

 

My question has to do with a sloped roof, such as a gable or hip roof.  I’m looking at designing a shear wall system for a bank building.  As I interpret this the minimum wind pressure on the roof “vertical projected plane” is 10 psf, even thought the calculated wind pressure may be significantly less.

 

Let me illustrate this.  90 MPH Wind.  Roof slope is 25 degrees.  ASCE 7-02 Figure 6-2 and IBC 2003 Table 1609.2.1 have the following wind pressures:

Zone A – 16.1 PSF

Zone B – 2.6  PSF

Zone C – 11.7 PSF

Zone D – 2.7  PSF

 

Zones A and C are wall zones and greater than 10 PSF so the minimum requirement is met.  Zones B and D are roof zones and less than 10 PSF so the wind pressure for design of the shear wall has to be increased to 10 PSF minimum for shear wall design.

 

Am I interpreting this correctly?  This makes a dramatic increase in the shear wall design.  The roof is much taller than the walls and all this extra shear is added directly to the shear wall.  I haven’t found any good examples in my literature regarding applying this minimum wind pressure to the roof.

 

I would appreciate any other insight into this interpretation.

 

Rich