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Wind load question

Wind load question

Wind load question

(OP)
thread507-203133: ASCE 7-05 Wind Load Methods # 1 & # 2

I would like to re-open this thread. I'm trying to compare the base shear forces with the ASCE7-05 simplified and analytical method.

NJEngineer said that that the simplified procedure cames from the analytical method, but there's no sin/cos function to obtain the correct wind direction forces.

He said:

"It appears to me, something is amiss in the ASCE 7-05. Figure 6-10 should state definitively whether the pressure shown is perpendicular to the surface (therefore horizontal & vertical components of the pressure should be calc'd using sine & cosines of the roof/surface angle) or it should state the contrary. As it reads right now, the figure is vague.

This is an email I recently sent to Mehta & L. Griffis, who are both on the ASCE 7 Wind Committee. I have yet to receive a reply.

It appears that Fig 6-10 (External Pressure Coeff) can be used to calculate the columns of Fig 6-2 using the givens on pg 283 of Commentary as shown below:

Where (1) GCpi= positive. (2) The positive direction is in the direction of coefficient 1 and up.

horizontal loads
p.A = qh[(GCpf.1E-GCpi)+(-GCpf.4E+GCpi)]
p.B = qh[(GCpf.2E-GCpi)+(-GCpf.3E+GCpi)]
p.C = qh[(GCpf.1-GCpi)+(-GCpf.4+GCpi)]
p.D = qh[(GCpf.2-GCpi)+(-GCpf.3+GCpi)]
vertical loads
p.E = qh[(GCpf.2E-GCpi)]
p.F = qh[(GCpf.3E-GCpi)]
p.G = qh[(GCpf.2-GCpi)]
p.H = qh[(GCpf.3-GCpi)]

However, where as the pressures found in fig 6-2 are based on horizontal and vertical projected surfaces, fig 6-10 appears to be based on pressure coefficients normal to each surface (as indicated in your Guide to the use...of ASCE 7-98 Fig 3.8.2 etc..). The later would suggest that the vertical and horizontal components of pressure coefficents should be used as follows:

horizontal loads
p.A = qh[(GCpf.1E-GCpi)+(-GCpf.4E+GCpi)]
p.B = qh[(GCpf.2E-GCpi)+(-GCpf.3E+GCpi)]sin(roof angle)
p.C = qh[(GCpf.1-GCpi)+(-GCpf.4+GCpi)]
p.D = qh[(GCpf.2-GCpi)+(-GCpf.3+GCpi)]sin(roof angle)
vertical loads
p.E = qh[(GCpf.2E-GCpi)]cos(roof angle)
p.F = qh[(GCpf.3E-GCpi)]cos(roof angle)
p.G = qh[(GCpf.2-GCpi)]cos(roof angle)
p.H = qh[(GCpf.3-GCpi)]cos(roof angle)"


The ASCE takes directly the surface pressure without a sin/cos function from the method #2 to obtain the method #1 and that's what I don't understand.


 

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