effective wind area for parapets 1

[zrck99]

When designing a balloon framed stud wall with a parapet what span lengths do you use for the effective wind area = (span length)*(span length/3)? For example, if you have a 15' tall wall with a 5' parapet and you are designing the stud, would you use one single effective wind area of (15')(15'/3) for the back span as well as the parapet or would you use a separate (5')(5'/3) for the wind portions on the parapet itself? When you start looking at the zone 2/3 negative GCp values for the rear side of the Windward parapet, it can make a pretty big difference in the end results of the design.

 

[JAE]

By the letter of the code - the definition of Effective Wind Area given in Chapter 26 states that the area is the span length x the width.

So for that you'd use the span length of the lower stud for wind on it and the cantilever span for the parapet for the parapet wind loads.
The continuity doesn't affect the wind as the wind "doesn't know" that they are connected.

Having said that - the "letter of the law" here does not seem very intuitive and I would guess that the vast majority of "reasonable engineers" out there may use the entire length of the stud...which is less conservative.

You could probably defend your decision in a court of law either way as it is a bit nebulous.

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[zrck99]

JAE,

Thanks for the response. I agree as far as the letter of the law not being very intuitive. I've always been slightly bothered by whether or not it's logical to apply wind load as a patterned loading in cases like this as well. As you minimize your parapet load (or if checking as a patterned loading, remove it entirely) you can have pretty drastic effects on the moment in the backspan as well as the end reactions of the stud. I think per the code looking at a balloon framed stud with patterned wind loading is overkill but I still sometimes wonder about the possibility of wind hitting up at the parapet only or low in the backspan only. Have you ever seen a discussion of that?

ZRC

 

[JAE]

Have you ever seen a discussion of that?

Nope.

I might be inclined to look at 100% parapet with 50% lower stud wind or visa versa as a rational approach.
That approach isn't in the IBC/ASCE 7 other than the general concept with alternating live loads, though.

Engineering judgement is what is called for here along with perhaps some further research on the source of the loads. I would bet, however, that this balloon framed stud/parapet condition has not specifically been addressed or studied by the ASCE committee.

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