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RE: Canopy Roof Wind Load

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Lew -
  Prior to receiving your post, I reviewed "Wind Engineering" by Henry
Liu(1991). Pages 81, 82 and 83 address the effects of wind across
free-standing roofs. Four conditions are addressed: 1. Roof plane horizontal
- not much uplift; however, large pressure fluctuations occur above and
below roof(due to development of turbulent boundary layer). May cause
vibration. 2. Roof is leaning in leeward direction - pressure differential
results in downward thrust. 3. Roof is leaning in windward direction -
Pressure differential results in upward thrust. 3. Gabled roof - Combination
of downward thrust on windward side roof and upward thrust on leeward side
roof results in counterclockwise torque on roof.

  The above tends to support your suggestion about heeding note #2 of Table
6-6. However, if i were to apply a wind load at an angle of -10 degrees to
the flat roof, could a significant increase in uplift be expected?

> ----------
> From: 	Lew Midlam[SMTP:Lew.Midlam(--nospam--at)]
> Reply To: 	seaoc(--nospam--at)
> Sent: 	Sunday, May 17, 1998 6:22 PM
> To: 	seaoc(--nospam--at)
> Subject: 	Re: Canopy Roof Wind Load
> For this type of roof you need to look at ASCE 7-95,  Table 6-6, and pay
> particular attention to note 2.
> Lew Midlam, PE
> =======================================
> -----Original Message-----
> From: CanitzCF <CanitzCF(--nospam--at)>
> To: seaoc(--nospam--at) <seaoc(--nospam--at)>
> Date: Sunday, May 17, 1998 12:50 AM
> Subject: Canopy Roof Wind Load
> >I'm presently working on a project for which I'm trying to
> >dtermine the wind uplift pressure on a standing seam metal
> >roof of a free standing entry canopy structure. The structure
> >is free standing and is approximately 73'x46'x25' high. The roof
> >is monoslope(approximately 10 degree slope). All four sides of
> >the structure are open. Per ASCE 7-95, I'm designing for cladding
> >of a monoslope open structure. The wind speed is 90 mph, Exposure
> >Category C and an Importance Factor of 1.00. The surrounding
> >terrain is not a factor.
> >
> >When I calculate the roof wind load, the value is only 10 psf(based
> >upon p = qz*G*Cf). This implies very little, if any, uplift. Based on
> past
> >experience, this seems unrealistic. Therefore, my question is the roof
> >of this type of structure normally subjected to very little uplift due to
> the
> >absence of walls?
> >
> >Thanks in advance.
> >
> >Charlie Canitz, PE
> >Bel Air, MD
> >
> >
> >