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

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

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

(OP)
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!

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

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.

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

(OP)
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.

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

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?

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

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.

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

bmagdalena,

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

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

(OP)
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.

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

(OP)
Thanks for the attachment file.  I understand it now.

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

(OP)
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)?

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

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

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

(OP)
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?

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

Yes.

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

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?

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

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.

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

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?
 

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

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.

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

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.
 

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

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.

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

ok thanks

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

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?
 

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

Since this question never was answered, I am giving it a bump

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=0.8, values that are givien in the table are for h/L<0.5 or h/L>1.

What does one do in this case?

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

I believe you are to interpolate as noted in footnote #2

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