ASCE 7-05 Chapter 6 Figure 18A
ASCE 7-05 Chapter 6 Figure 18A
(OP)
2 questions:
1. Does anyone know the meaning of Load Cases A & B, for which pressure coefficients are listed in Figure 18A?
2. Should the pressure coefficients contained in Figure 18A be multiplied to the projected frontal area of the roof i.e. A*sin(theta) to get the load, or the roof area ie. A? I know All other MWFRSs methods use the frontal area but the diagram at the top of figure 18 makes me wonder if they mean the full roof area. Besides, why would they bother giving pressure coefficients for theta=0 when the frontal area is 0?!
Thanks
1. Does anyone know the meaning of Load Cases A & B, for which pressure coefficients are listed in Figure 18A?
2. Should the pressure coefficients contained in Figure 18A be multiplied to the projected frontal area of the roof i.e. A*sin(theta) to get the load, or the roof area ie. A? I know All other MWFRSs methods use the frontal area but the diagram at the top of figure 18 makes me wonder if they mean the full roof area. Besides, why would they bother giving pressure coefficients for theta=0 when the frontal area is 0?!
Thanks






RE: ASCE 7-05 Chapter 6 Figure 18A
Regarding 2, due to the direction of the Cnw and Cnl coefficients, I interpret the load, or pressure to be normal to the roof slope, independent of the vertidcal exposure.
Regarding 3, on page V of the manual the statement is made that "Errata as they are found are posted to www.SEInstitute.org". You might check that out. Maybe someone else has already asked the same question.
Good luck.
Mike McCann
MMC Engineering
RE: ASCE 7-05 Chapter 6 Figure 18A
RE: ASCE 7-05 Chapter 6 Figure 18A
That said, if you apply this pressure to the elevatin of the roof(as if it were a perpendicular surface) then you will get the horizontal component which can be used in overturning.
You will also have a vertical component which can be calculated by the plan area of the roof.
RE: ASCE 7-05 Chapter 6 Figure 18A
I have checked the errata and, although there are lots of other corrections to Chapter 6, there is nothing that answers my questions.
For the time being I'm going to assume the Load Cases represent worst case lift and suction, although from an aerodynamicists point of view I don't see how a flat plate at small positive angles of attack within the ground effect could ever create downforce, and far less a nose down pitching moment.
I will apply the pressure to the roof area since this seems logical.
RE: ASCE 7-05 Chapter 6 Figure 18A
Did you ever find a definitive answer to your question? I have pondered this for many days now, and can't seem to get a straight answer anywhere. I have even called ASCE and asked the technical support department - from which I have received no answer yet. I am having difficulty with the relation of this load case to diaphragm shear calculations for a hip-roofed gazebo structure. Thanks
RE: ASCE 7-05 Chapter 6 Figure 18A
RE: ASCE 7-05 Chapter 6 Figure 18A
RE: ASCE 7-05 Chapter 6 Figure 18A
RE: ASCE 7-05 Chapter 6 Figure 18A
2. Pressure are normal to the surface.
RE: ASCE 7-05 Chapter 6 Figure 18A
RE: ASCE 7-05 Chapter 6 Figure 18A
Let's say you're checking an open roof with the figure we've been referencing, 6-18A. You'll determine the roof pitch first, say 15°. Go to the table and pick out your pressure coefficients for a roof angle of 15 degrees for both γ=0° and γ=180°, for both load case A and B.
Now use those coefficients with the appropriate equation earlier on in the standard, for both values of γ, and both load case A and B. This means for figure 6-18A you'll typically have 4 sets of pressures to check, and use the one producing the worst load effect.
RE: ASCE 7-05 Chapter 6 Figure 18A
IV
RE: ASCE 7-05 Chapter 6 Figure 18A