Allowable deflection of glass in NYC.

[Dmitri S]

Hello everyone, hoping to get some input from you on an issue I have. I am a glazing contractor in NYC and I was hired to install several insulating units that compose of ~1/2 outer layer (two layers of 1/4 tempered glass with .060 pvb interlayer) with 1/2" air space over 3/8 tempered glass inner layer. The units are being structurally glazed on all four sides into curtainwall frame.

The issue is largest unit is roughly 193" wide by 105" tall. My engineer did the calculations on it and determined that it will deflect by 1.6" per NYC code calling for 30psf. He is stating that to his understanding any deflection should conform to table 1604.3 of NYC BC, which limits deflection to L/(120x0.7=84) or 3/4" which ever is less. He is willing to accept 1" as maximum.

However the design engineer that designed the project is arguing that the glazed wall is not a structural element, but rather "infill glazing" and therefore the code does not apply to it. He is stating that it is safe to design glass with deflection well exceeding 1". He is also requesting for wind load calculations per ASCE-7, which is 23.9PSF vs 30 per NYC code. My engineer says that NYC code must prevail.

The bottom line is it seems that there really is no specific reference to allowable glass deflection anywhere in the code, and it comes down to the engineer's discretion. Both engineers are very accredited and well respected in the industry, but I am stuck between a rock and a hard place. I will much appreciate any input that can help me move on with this project. I certainly do not want to install this glass in bad faith, and risk potential liability if it breaks and someone gets hurt.

P.S. Another concern that I have, is it possible that two panes of glass will deflect and touch in the middle since there is only 1/2" of air space separating them? My engineer says that it is impossible and they should always deflect in the same direction, but I am worried about positive air pressure inside the building putting pressure on the inner lite vs a gust of wind on the outside pushing on the outer lite - if they touch in the middle that can certainly cause one or both layers to break.

 

[strucbells]

Interesting question. I have no experience in NYC or with structural glass, so can't really help you with most of your question.

However, generally local governing authorities adopt the IBC and modify them with local amendments. So usually (without specifically having familiarity with NYC code), NYC code would govern over ASCE7, especially if it is more conservative.

See if you can get user "glass99" to weigh in on this, think this is his specialty area from previous posts on this.

 

[jayrod12]

Does the glazing also need to protect people from going through it? Like a guardrail essentially? I'm not a glass designer, but there's a few people in our office who perform a fair bit of curtainwall work and they indicate on the larger lites of glass like that size, the fall arrest loads are fairly onerous.

 

[Aesur]

It's been a while since I designed Glass, however I believe ASTM E1300 has provisions for strength calculations and deflection requirements.

 

[MIStructE_IRE]

If the building designer is suggesting less stringent deflection criteria than your own designer then what’s the issue proceeding with your design if your design is code compliant. He’s suggesting you be less conservative... thanks, but no thanks.

For what its worth, I’d agree with your designer. I think a deflection limit is applicable. While its “non structural” as far as the building structure is concerned, it presumably Is still Subject to crowd loading (like a balustrade) to stop someone falling to their death... So I agree, code deflection limits apply.

Incidentally, over here we typically use L/175 from corner to corner measured diagonally - or 25mm.

 

[PT99]

NYC Building Code Section 2403.3
Max deflection for glass = L/175 or 3/4" whichever is less, with load combinations as specified in Section 1605 (the usual standard load combinations).

Wind load is also specified in the NYC Building Code, and is not subject to discussion.

Any less conservative deviation from the Building Code is not code compliant, period, so 1" glass deflection is not code compliant and should be rejected.

Note that when ASCE-7 is specified in the Code, refer to the Appendix in the Code to use the correct version year. The Code uses both 7-05 and 7-10 in different parts.

 

[NorthCivil]

maybe you will get input from glass99 who is a glass specialist

I'm also a bit of a glass specialist (what is an expert anyway?)

1. I'm not NYC based, so im not sure which code will govern for wind loading. though that should be feasible to resolve between the two engineers & potentially the project engineer.

2.I am worried about positive air pressure inside the building putting pressure on the inner lite vs a gust of wind on the outside pushing on the outer lite - if they touch in the middle that can certainly cause one or both layers to break.

I have seen 2 lites touch - yes this is feasible. I am not sure if it applies to your situation. what results is the two pieces of glass will touch in the middle of the lite and screech a little - it will be audible but wont really risk the lites breaking. as you are well aware, glass breaks usually start at the corners or the edges, the middle of the lights are very resistant to fracture.

this kind of loading can put a lot of pressure on the edge seals, as the glass is easily deflected on a large sheet, and the pressures vary a lot within the airspace. maybe this will result in edge seal failure prematurely.

3. out of all the things that give me the chills for large units like this, is the structural silicone sealant work. so critical to get this right. also, if the glass IGU edge seal fails, how is the outer lite held? structural sealant to the outer lite, or only weathering sealant?

 

[dik]

There was an incident in Toronto several decades ago where a lawyer ran against a curtainwall glazing to show how strong it was... it failed and he fell to his death. It should never have happened; the day was calm (relatively).

Dik