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10 PSF Minimum Wind Load

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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