ASCE 7-05 Chapter 6 Figure 18A

[AeroGuy83]

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

 

[msquared48]

You're right on 1, there is no indication I can find anywhere, even in the commentary to explain the load cases. Only in other areas pertaqining to free standing walls do I see a load case A and B mentioned. Sounds like this might be an Errata for ASCE7-05. It affects Figures 18A through 18D.

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

http://www.SEInstitute.org".

You might check that out. Maybe someone else has already asked the same question.

Good luck.

Mike McCann
MMC Engineering

 

[ajh1]

Can't say as I had given item 1 a lot of thought in the past, but I would assume the rationale is similar to the two sets of coefficients in Figure 6-6. Many building codes show two sets of coefficients for the same slopes, one generally a pressure case and the other generally a suction case.

 

[csd72]

All pressure act perpendicular to the surface, this is part of the definition of a pressure.

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.

 

[AeroGuy83]

Thanks for your comments.

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.

 

[ryanac]

AeroGuy83,
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

 

[dgkhan]

It is very obvious. Case A when gamma = 0 and Case B when gamma =180

 

[dgkhan]

sorry I am wrong

 

[dgkhan]

I used this lately to design solar panels support. We should check the complete system or frame with Case A and B values. There might be some reason which we can find out with wind tunnel test but this arrangement might caused two kinds of pressure and hence we should check for both.

 

[UcfSE]

1. Case A and B represent two sets of coefficients to be used when checking design pressures. They represent two different possibilities of pressure coefficients that could occur on a given roof. You check case A and get pressures, then case B and get pressures, and use the ones with the worst load effect.

2. Pressure are normal to the surface.

 

[ryanac]

I know they are two different cases, but what is Case A and what is Case B. There is no definition of the difference or any given load case co-efficients that I have ever seen.?. Thanks

 

[UcfSE]

They just represent two physical possibilities for values of the pressure coefficients. The letters don't mean anything as far as load combinations or anything else; it's just a designaiton chosen. They could have said load cases red and blue instead of A and B and it wouldn't be any different. They are meant to be kept together also, not mixed; you wouldn't use a C_NW value from load case A with a C_NL value from load case B at the same time.

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.

 

[cdi12]

I have asked similar question earlier and one of answers was "Case A & B looks to be maximum variation in loading due to gusts at different times". Hope this helped.
IV

 

[ryanac]

Got it. I won't say it makes perfect sense, but I understand it. I have a tendency to be a definitive answer kind of guy, which unfortunately is usually not the case in wind engineering. Thanks to everyone for the responses.