Wind Load Ext. Pressure in ASCE 7-02 1

[DaveVikingPE]

I'm confused re: Figure 6-6.

For a particular roof pitch, there may be a positive and negative Cp. According to Note 1, "plus and minus signs signify pressure acting toward and away from the surfaces, respectively." OK, but there are several cases of Cp values for a given pitch or h/L and both values of Cp are negative, one significantly larger than the other, it seems.

For example, I have a roof pitch of 15 degrees and an h/L of .5. There are two values of windward-acting Cp, e.g., -0.7 and -0.18. Thus, both are negative and, based on note 1, indicate pressure acting away from the roof surface.

 

[FSS]

I would say it is a matter of aerodynamics. The shallow pitch roofs are much closer to flat and result in uplift type pressures across the whole surface. As the pitch increases, you get more of a horizontal component as the "sail" area increases.

 

[DaveVikingPE]

Here's what I've looked at:

7-95 and 7-98 do not have dual negative values in Figure 6-6. (In 7-95 it's Fig. 6-3; in 7-98 it's Fig. 6-3).

In the case of my 15 degree pitch roof above and wind direction normal to the ridge, if I use 7-95, Cp = -0.7; if I use 7-98, Cp = -0.7; if I use 7-02, Cp = -0.7 AND -0.18. Why bother with Cp = -0.18?

If the roof pitch changes to 25 degrees (for whatever reason, the architect changes the roof, etc.), Cp 7-95 = -0.3 and +0.2; Cp 7-98 = -0.3 and +0.2; Cp 7-02 = -0.3 and +0.2. OK, no problem because in each instance note 3 says "Where two values of Cp are listed, this indicates that the windward roof slope is subjected to either positive OR negative pressures and the roof structure shall be designed for both conditions." (My emphasis.) Great, that's intuitive and with a minus and plus value of Cp I'll design accordingly. My issue that that 7-02 lists TWO minus values for a pitch angle wherease previous editions listed a single minus value.

I don't see the point in including the additional negative values of Cp. All this does is add a load case to the design that's going to be superceded by the greater wind load computed using the larger (in magniture) value of Cp.

 

[JAE]

Dave:
How about this point: You have two conditions of wind that are:

1. Windward roof is very negative (-.7) and
2. Windward roof is somewhat negative (-.18)

For a typical building I think you are right that for the Primary Wind Resistant System, the -.7 wouldn't really control as the less negative (or more positive) value on the windward wall usually creates a larger total/net lateral force on the system.

However, this is a code and it must be anal. (quote by me)

There may be non-typical structures where this would apply...for instance, if you had a building with an expansion joint along the ridge line, you would essentially have two structures, two wind resistant systems, and the MORE negative value on the windward (-.7) would then POSSIBLY control the design on the windward half of the structure.

My point is that for your current applicable structure, it does seem non-relevant to include both negative values, but there could be cases where they must be checked and the code must be generic/universal in its provisions.

 

[ajh1]

I would tend to agree with you. The only area where it might (unlikely) be an issue is if you are working with a rigid frame support system. In that scenario, you are interested in the interaction of the coefficients on all 4 surfaces (2 roof, 2 wall) at the same time. The reduced windward suction might have a changed effect on the leeward suction effect and/or the forces from the walls. The -0.7 has somewhat of a balancing effect on the net overall shear, whereas the -0.18 will allow a larger net horizontal shear coupled with a reduced effect on that roof surface. The dual numbers for parallel to ridge make even less sense than the normal to ridge numbers.
Keep in mind that the research community is the prime driver for most codes and standards. So what if you need to create 47 additional load combinations to get the answer 1% more accurate, we all have computers, right?
Good luck!

 

[DaveVikingPE]

JAE, ajh1,

1. I agree with your points re: differing uplift pressures (i.e., negative and somewhat negative). That's initially how I thought to approach the problem.

2. BUT, the notes in Fig. 6-6 (ASCE 7-02) indicate to me that in the case of two values for Cp, one will be minus, the other plus. The two negative values, then, don't match what's described in the table.

3. Further, I've checked back through ASCE 7-93. In all the standards the additional negative value of Cp is absent, yet the note I discuss in 2. above is present. I think the Cp = -0.18 (BTW, it's -0.18 in every case, odd) is a typo.

I really would like someone from the committee to comment.

 

[chichuck]

Check the commentary on page 286.

"Observations in wind tunnel tests show that areas of very low negative pressure and even slightly positive pressure can occur in all roof structures, particularly as the distance from the windward edge increases and the wind streams reattach to the surface. These pressures can occur even for relatively flat or low-slope roof structures. Experience and judgement from wind-tunnel studies have been used to specify either zero or slightly negative pressures (-0.18) depending on the negative pressure coefficient. These new values require the designer to consider a zero or slightly positive net wind pressure in the load combinations of Section 2."

So what does this mean? When you combine these coefficients with the internal pressure coefficients from Figure 6-5 ,you get:

for h/L = 0.5, theta = 10 degrees

for enclosed structures:

-0.9 - (-0.18) = -0.72

-0.9 - (+0.18) = -1.08

-0.18 - (-0.18) = 0.00 (new load case, the zero net pressure mentioned above)

-0.18 - (+0.18) = -0.36


For partially enclosed structures

-0.9 - (-0.55) = -1.45

-0.9 - (+0.55) = -0.35

-0.18 - (-0.55) = -0.73

-0.18 - (+0.55) = +0.37 (new load case, the slightly positve net pressure mentioned above)



chichuck

 

[chichuck]

My apologies! I transformed the figures, corrected numbers are shown below.


For partially enclosed structures

-0.9 - (-0.55) = -.35

-0.9 - (+0.55) = -1.45

-0.18 - (-0.55) = +0.37 (new load case, the slightly positve net pressure mentioned above)


-0.18 - (+0.55) = -0.73


chichuck