ASCE 7-02 Wind Calcs - Method 2: All Heights Method 1

[aokamura]

Hi,

What are the values of Cp that I can use to analyze the leeward roof (wind direction parallel to ridge) for method 2 MWFRS? Figure 6-6 does not have a table under the leeward column... I'm a structural engineer intern, and this is the first time I've encountered something like this.

Oh, and fyi... h/L < 0.5 with horizontal distance from windward edge between h/2 and h.

Thanks!

 

[tadeng]

It is in figure 6-6 in the row for parallel to ridge for all (theta). For the horizontal distance from windward edge, >2h is -0.3, -0.18. So, anything greater distance from the windward edge than twice the height you would use these values. Similar values are given h to 2h, etc.

 

[aokamura]

tadeng,

Thanks for your reply... but what I was asking was, what values of Cp can I use for the leeward roof. I know the windward roof values are given but I'm not sure what to use for the leeward roof.

 

[bmagdalena]

tadeng - what you worte is for windward, not for leeward. but if wind is in parallel direction isnt that you dont have leeward side of roof?

 

[tadeng]

See attachment. I would consider anything past the midpoint as "leeward." The value of Cp you use depends on the distance you are from windward edge with respect to h. So, it will not necessarily be the values I stated earlier.

 

[tadeng]

bmagdalena,

You are correct in that leeward is a relative term in this case.

 

[aokamura]

bmagdalena,

Thanks for the reply. I don't see why there wouldn't be a leeward side, regardless of wind direction? I don't understand why they don't have the Cp values for leeward direction on the roof for wind blowing parallel to the ridge.

 

[aokamura]

Thanks for the attachment file. I understand it now.

 

[aokamura]

I had one last question regarding this topic...

So, if the total horizontal distance from the windward edge is h and the roof is symmetric (equal area on both halves), then the windward and leeward roof pressures will cancel out? Or, do I calculate for the worst combination (i.e. use -0.9 for windward half, -0.18 for leeward half or vice versa)?

 

[Ron]

When considering wind loads parallel to the ridge, there is no leeward side.

 

[aokamura]

Hi Ron,

So would that mean the wind pressure parallel to the ridge would only have one value? Wind pressure normal to the ridge had two values to calculate the total pressure, but the parallel direction has only one pressure acting on it?

 

[Ron]

Yes.

 

[bmagdalena]

how can you have one value of wind pressure if Cp is based on horizontal distance from windward edge. what is that horizontal distance? wouldnt you have to evaluate whole roof just like is shown on tadeng sketch?

 

[Ron]

bmagdalena...read the footnotes in the Cp table...it tells you where to apply the load. You can also distribute the load over an area.

 

[bmagdalena]

Thanks Ron

But just to confirm that I understand this correctly. Horizontal distance from windward edge is the length of the building in parallel direction (based on ASCE notes "L"). Since leeward does not exist, we would analyze roof for the same pressure acting on whole roof and select Cp base on total length of the roof that is parallel to the wind direction. Is that correct?

 

[tadeng]

Per ASCE, you would have a different Cp depending on the length away from the windward edge (shown in my sketch above). For practicality and ease, you could analyze the entire roof for one pressure.

 

[bmagdalena]

Tadeng
what you are saying is that you should divide the roof into sections. so that is different from what Ron was saying on having one pressure on the whole roof. I guess that would make more sense since bigger value are closer to the windward side, and if you would analyze based on whole length then you would have lower value that would not represent condition of whole roof.
Im trying to get this correct since as you can see, everyone is comming up with different interpretation of the code.

 

[tadeng]

From a code standpoint, I disagree with Ron in that you can have one pressure for the entire roof. There will be different Cp based on length from windward edge. However, it can be cumbersome to have different pressures on the roof. It would be conservative (by 3 times) to use the windward pressure on the leeward end of the roof for >2h.

 

[bmagdalena]

ok thanks

 

[bmagdalena]

I have new question regarding Cp for roof in wind direction normal to ridge for 0 degree angle and for wind parallel to rige.

I have ratio h/L=0.87, values that are givien in the table are for h/L<0.5 or h/L>1. So Im in between and what now?