Glass railing wind load at balcony 2

[johndeng]

Glass railing wind load at balcony is supposed to be the same as the parapet wind load design (both faces have load, positive from face of wall plus negative from roof/wall, say 100psf c&c).
But the railing has 6" gap at bottom, is the wind load condition still the same as parapet? That's the parapet has continuous opening at base. I feel the wind load is the same 100psf c&c, just want to confirm.

Thanks

 

[XR250]

Are you even allowed to have a 6" gap at the bottom?

 

[johndeng]

Yes glass railings have gap top and bottom, even the pickles railing has gap under bottom rail.

 

[Aesur]

I think what XR250 is referring too here is the largest gap is 4" per code.

As for loading, there is no study that I'm aware of that reduces wind for this, I would use the parapet loading. The only thing I could even think of, which doesn't apply here, is in the commentary there is something about mechanical screens are allowed to use lower forces if they are 6' or more from the parapet/roof edge or out of edge and corner zones.

 

[Eng16080]

I'm not sure how applicable this would be, but there are provisions for "Solid Freestanding Walls and Solid Signs" per ASCE 7-16, Section 29.3. Based on the diagrams in that section, it appears you could account for the gap at the bottom. I might guess that using this method could be conservative (although I don't really know).

 

[rowingengineer]

Rofail and man's 2003 windtech consultants is an Australian research but maybe of help.

 

[johndeng]

@Eng16080 I dont think you can use "Solid Freestanding Walls and Solid Signs" per ASCE 7-16 since it's a main wind force resisting system, not c&c.
C&C elements with tributary areas greater than 700 ft2 (65 m2) shall be permitted to be designed using the provisions for main wind force resisting systems (MWFRS).

 

[phamENG]

Have you run both to see what sort of loads they give you? How do they compare?

It's important to remember that these wind loads come from fluid dynamics. Air is a fluid moving around and interacting with objects, resulting in forces on those objects (our wind loads). The simplified calculations in ASCE 7 (and I do mean all of them - they are all very much simplified when compared to implicit functions of aerodynamics, drag, etc.) are based on certain circumstances and conditions - if you read through ASCE 7 and the commentary you can get a good idea of what they are, and if you still need more then there are several books that explain the development of those equations. Once you understand how the air is flowing around a building to create those loads, you can determine what to use for your situation. But, remember, MOST conditions aren't actually covered by ASCE 7 directly. You either have to use judgement to apply a similar but conservative loading to it, or you have to put it in a wind tunnel. Now, unless you're an academic or working on a high rise, very few people ever go the wind tunnel route, but it's important to understand the limitations of the tools you're working with.

So the answer to your question: it depends. Is the railing on the edge of a roof? Parapet. Is it on the edge of a large, expansive balcony with no roof? Parapet. Is it on the edge of a small balcony with a roof over it and walls around it? I'd be leaning more towards the wall C&C load since that space behind the railing will be at the same pressure or very similar pressure as the outside of the railing.

EDIT: also, these wind loads are approximations. Don't get caught measuring with a micrometer, marking with a crayon, and cutting with an axe.

 

[johndeng]

@phamENG to your wall c&c option, you mean only winward load to the railing, almost 0 leaward load on the back of railing? That's what I need to know.
From code, I only know if the sign is hanging on the wall, the gap from the sign to the wall is small, only wall c&c is considered.for the sign wind

 

[phamENG]

Essentially. I found a couple of resources that back up my stance and provide a little more information to help quantify it:

Structure Magazine

Video - Wind Load FAQ (she wrote that bit for Structure, but the video has a little more insight)

 

[johndeng]

@phamENG You are genius! I have the question in mind for long, now it's solved. Thanks

 

[phamENG]

You're most welcome. Glad I could help.

 

[rowingengineer]

https://windtech.com.au/wp-content/uploads/2012/08/07.pdf

I don't get why everyone isn't reviewing the test data, let's preference extrapolation from a code for 2d analysis to what is a 3d wind flow problem. In other words think fence vs building wall, they have very different wind behaviours.

 

[phamENG]

rowingengineer - thanks for sharing that. I didn't see your earlier post and it never came up in any of my searches.

Two things -

1) I believe everyone else in this thread is practicing in North America, and I for one have no familiarity with Australian or European standards. I'm not sure how C_p is used in those standards and whether or not it can be dropped right into the equations we're familiar with. Do you have a reference document that could get us up to speed quickly and easily?

2) Assuming it is a one-for-one swap, it's consistent with what I posted. That research paper uses a scale depth of 2m for the balcony, which is right about 6 feet. The information I posted suggests switching to parapet wind loading when the depth of the balcony is 2 to 4 times the guardrail height. That's 6 to 12 feet. Parapet wind loading in ASCE is 2.25x the wall loading. If we break that down to our C_p, 0.8x2.25=1.8. The largest C_p reported in that paper is 1.82.

 

[johndeng]

@rowingengineer Thanks

@phamENG great you got a summary here, however, I dont understand what the conclusion is. Maybe it only tested one condition, i.e. when the balcony is 2m, and the building is high enough to have big wind speed, it should be designed as parapet. Correct me if I'm wrong. It didnt test the smaller balcony case. But you mentioned Cp is 1.8 which confused me a bit. Positive GCp is 0.9 (Cp 1.06), negative has two load case, bigger GCp is 2.3 (Cp 2.7).

 

[lexpatrie]

This is for a wood guard, but there's calculations for wind that might be useful for the front end of your calculations.

Wind Load Determination for Residential Decks, Lyman, Bender, Wood Design Focus, Summer 2013.

The guard and lattice are incidental to what they are interested in, but there's calculations you can refer to.

Regards,
Brian

 

[johndeng]

@lexpatrie very useful, thanks! but it's for deck floor I imagine, not the open railing

 

[lexpatrie]

They do the "lattice" for the railing.