ASCE 7-05 Open Building Gable Wind Loading 2

[waytsh]

Hello everyone,

I am working on a gabled portico which is going to be open on three sides and the fourth side is going to be against an existing building. The ridge runs perpedicular to the existing building which is attached at the rear of the portico.

To figure the lateral wind load normal to the ridge I took the horizontal component of the roof pressures as dictated by Figs 6-18A through 6-18D and equation 6-25.

I'm a little stumped as to the correct equation to use for calculating the wind pressure on the gable. As I mentioned before the front endwall is open except for the gable where the profile of the wood truss will be sheeted. The ASCE charts for an open structure and wind parallel to the ridge only gives values for pressure and suction on the roof. Should I just use the wall pressure for an enclosed structure and apply that to the area of the truss? Maybe treating it as a parapet or solid freestanding wall would be more accurate?

Thanks in advance for your thoughts!

~waytsh

 

[mjl23]

Which version of ASCE 7 are you referencing? I'm guessing 7-05 which I don't have a copy of at home.
Not knowing if 7-05 has something more specific now, I would have thought to use the partially enclosed structure provisions.

 

[waytsh]

ASCE 7-05

I suppose that is yet one more possibility. I had not really considered it because the bottom of the trusses are going to be sheeted over with soffit.

 

[mjl23]

Do you have a copy of the "guide to the use of the wind load provisions of asce 7-02"?
It doesn't address the gable end specifically, but there is an example where there's a porch open on two sides that connect to the main structure that has a roof overhead. Fairly similar to your description. The 'gable' for the porch is relatively small and the example doesn't include any load on that surface (doesn't include or not include, just kinds of omits that surface).
But, going along with that, the porch in the above example is attached to an enclosed structure, and the porch was designed using the enclosed structure equations, albeit modified due to it's open nature (no internal pressure, etc).
You don't say much about the existing building (enclosure, relative size, etc.). But that might be the way to approach it (design as enclosed or partially enclosed...no internal pressure, etc.).

 

[waytsh]

That's interesting. I don't have that reference but it sounds like it may be a good one to purchase. I was hoping they had something similar for '05 but I guess they haven't gotten around to writing one yet.

The exsiting building is enclosed and is much wider and higher. In fact the rear portion of this porticoo is going to be supported off of that structure.

Thanks so much for your help!

 

[mjl23]

Sure! Yeah, to me it sounds like the way to go. Sounds like you'll just have the positive external pressure for the windward direction against that new gable end.
That reference has been very helpful for me. I haven't heard of any updates for it to the updated asce 7.
Good luck.

 

[DaveAtkins]

I have typically called this type of structure an OPEN structure.

But the one thing that seems counterintuitive is that I seem to recall getting lower uplift on the roof than if I had assumed it was partially enclosed with internal pressure.

DaveAtkins

 

[UcfSE]

I would call this an open structure also. If it does not meet the definition of partially-enclosed, there's no need to use it. If it does, it does. I would use the open structure for the roof and check the gable itself with sign pressure or enclosed pressure, with and without GCP_i. With roofing and soffit enclosing the trusses, it is conceivable that some internal pressure can build up within the truss space. That, of course, depends on the materials and detailing and so forth.

Try to think of a few different realistic cases that your condition could meet, then use those to find a range of pressures you could use. Engineering judgment should take over from there. Note that you do have the option of referring to another code, such as the British code, if the ASCE 7 does not contain what you need. Also note the addition of components and cladding pressures for open structures in the ASCE 7-05 code.

 

[waytsh]

Thank you all very much for the input. One thing I did forget to mention is that I am primarily concerned with figuring the load on the MWFRS from a longintudinal wind load. After reading all your comments it does appear that this is a grey area in the code. So I am going to use the controlling pressure from the freestanding wall, signs, and enclosed structure equations. It is a small structure so following the old adage "When in doubt, make it stout" will not have a huge cost impact on the project.

If anyone has any further thoughts on the issue I would still be interested to hear them.

Thanks,

~waytsh