Glass Balustrade Design

[ajk1]

Has anyone designed a glass balustrade for a stair in a building? Since we are in Ontario, the design should conform to the Ontario Building Code 2012 and the CGSB-12.20 - Structural Design of Glass for Buildings. CGSB requires, in clause A5.1, for glass guards and balustrades that "...alternate lights be assumed failed in the strength determination, and a rigid continuous guard over two or more lights".

My questions are:

1. What should be the deflection limit? Would it be L/360 (but since this is a cantilever, it becomes the cantilever length /180)? Or is larger deflection permissible? My preliminary calculation indicates that deflection may govern rather than strength, and that the thickness required to meet deflection criterion of height/180 is 3/4".

2. Does anyone have the details for the track that restrains the glass at the base? Its shape, dimensions, and thickness?

3. The architect has marked "1/2 inch tempered glass" on his drawings. I don't think this will work for deflection limit of height/180. I am using E=70,000 MPa (10 million psi) as per CGSB in the deflection calculation and deflection limit of height/180.

4. Is tempered glass the right thing to use? Tempered glass shatters into many tiny pieces when it breaks, so someone leaning on the glass will fall to the ground if it breaks. Would laminated glass comprising both tempered and heat strengthened layers be better? What is usually done for balustrades, in this regard?

 

[CELinOttawa]

Be very careful here; The new code (OBC 2012) has adopted a number of recommendations from a Toronto working group due to the glass barrier breakages that city has been experiencing.

Tempered is very hard to make work under the current code, and effectively it must be heat soaked tempered in those few (and restrictive) instances where it is allowed.

I have some experience in this area, and I doubt your architect will have given you geometry which you can make work for heat soaked tempered. You'LL likely need to go to laminated.

 

[Canuck67]

ajk1,

2. I normally source information on glass rail hardware (tracks, base shoes, posts, etc..) from C.R. Laurence. They can provide you all the details for their products. There are other suppliers as well.

4. The purpose of the top rail is to prevent a fall during the temporary condition when the glass shatters. This is why the top rail or handrail must be continuous over several panels so that the load can be transferred to the unbroken, adjacent glass panels. Laminated is a good option too.

 

[ajk1]

Two very helpful replies from kowledgeable people. I am grateful.

CELinOttawa (Structural) - thank you very much for this. I had forgotten about the latest glass recommendations. I will look that up. Thanks again, I owe you.

Canuck67 (Structural) - thank you very much for this source of information on glass hardware. I will pursue it.

For the laminated glass, what would you use? Would it be one lamination of tempered glass and one lamination of heat strengthened glass?

 

[boo1]

Design the cantilever glass panel for flexural strength and deflection. In the US the primary references are the Glass Association of North America and IBC Section 1607.7, 2406, and 2407. Consider using laminated fully-tempered glass or laminated heat strengthened glass.

You can see tracks using clamps or wedges or cemented

http://www.glassguides.com/digital/2013/MayJun2013.pdf

USA References
1. IBC International Building Code
2. ASCE Standard ASCE/SEI 7-05
3. ASTM C1048 “Standard Specification for Heat Treated Flat Glass”
4. ASTM C1172 “Standard Specification for Laminated Architectural Glass”
5. ASTM E 1300-2003 “Standard Practice for Determining Load Resistance of Glass in
Buildings”
6. ASTM C1036 “Standard Specification for Flat Glass”
7. CPSC 16 CFR Part 1201 “Safety Standard for Architectural Glazing material”
8. GANA, Glass Association of North America “Glazing Manual” and GANA LD 09-0513

 

[glass99]

ajk - Glass balustrades are a whole thing. Its hard to answer in one post. Classically in the US, glass balustrades are made from 1/2" tempered glass without lamination. In Europe they are typically 2*3/8" tempered. The rules are changing in the US, and the design requirements are significantly different based on which rules you are working with.
- There is no deflection limit give in the code, and no glass balustrade will come close to meeting L/360. OSHA specifies H/12.
- The new IBC requires laminated glass, though this is a cost bump and may or may not be in force.
- Handrails, top caps and panel lengths are something to consider

 

[ajk1]

glass99 (Structural) - thanks for your comments. They are very interesting and helpful, especially about the deflection limit. I suspect that the latest Ontario Building Code requires laminated panels in light of all the balcony guard glass failures in the Toronto area over the last few years (glass falling to street level), but I have to read up on it. My somewhat limited knowledge of glass is that you would not want to use a single layer of tempered glass because if it breaks it essentially disintegrates into tiny pieces and falls out, so anyone leaning against the glass below the top rail (such as a child) will be flying thru the air to his death. Do you agree?

 

[glass99]

There are some folks who very rationally believe that laminated glass should be a requirement for balustrades. There are however thousands of miles of unlaminated balustrade installed largely without incident. The new 2015 IBC requires lamination, but is not in force anywhere.

 

[ajk1]

ok, I appreciate your comments. It is hard to argue with success, although sometimes failures are covered up or at least not publicized, so perhaps things are not always what they may seem. But I appreciate your letting me know what the industry practice has been and that it seems to have ben successful (although perhaps not so successful in the Toronto area...). Thanks again. Much appreciated.

 

[countervail]

UK based:

Glass post failure is the most important consideration.

1st check: bending stress less than allowable stress (toughened, tempered in the US, has higher allowable hence us generally used in the UK)

2nd check: deflection l/180 with an absolute limit of 20-25mm at the tip.

3rd check: broken behaviour. Laminated check for a single leaf broken with a higher allowable stress for the remaining leaf (escape stress). Un-laminated use hand rail to span to adjacent panes and check these for increased loading. No check for deflection.

4th Check: clamping angles for deflection, bending, she's, fixings etc. There are general rules of thumb for clamping angle bite on glass based on thickness of glass but I can't remember those off the top of my head (i.e don't use a 50mm bite on 30mm glass even if the numbers work its just not right).

 

[ajk1]

Since my last post here, I have reviewd the latest Ontario provincial standard "Supplementary Standard SB-13 Glass In Guards", June 12, 2012. These regulations came about following a number of glass failures in recent years in high rises in Toronto, where the glass fell into the street. By these new regulations, for glass located within 50 mm (2") of a floor edge, only heat strengthened laminated glass is permitted (1/2" tempered glass would not be permitted).

Based on what my limited investigation indicates has been designed recently in the Toronto area to meet the new regulations, I believe the appropriate glass balustrade would be one layer of 10 mm heat strenghtened glass lamination, a thin (1.8 mm?) interlayer of polyvinyl or similar material and 10 mm heat strenghened glass lamination.

Now I have to produce some calculation to show that this can resist a 0.75 kN/m horizonmtal force (and not less than a 1 kN concentrated force) acting at the top of the 1070 mm high cantlivered glass panel times a load factor of 1.5. Since I am using 2 layers of glass, each layer would take half the load (and moment), as I believe it is standard practice not to consider the 2 layers to act monolithically.

Can I check the strength by using a rational analysis based on CGSB 12.20 "Structural Design of Glass for Buildings", which would give a permissible factored flexural stress of 2 x 20 MPa = 40 MPa for heat strengthended glass, and find the ultimate resisting moment of the two laminations as 2 x the elastic section modulus of one lamination, times the permissible factored flexural stress? Does this have to be multiplied by a material resistance factor phi? If so, what should phi be? If I take phi=0.9, then this assembly is OK, but if phi is less can't really say it has adeqaute strength (and anyway it says in CGSB in my environmental conditions that is what shoudl be done).

But I have to consider that alternate glass panels may fail and the top rail distributes the load to the remaining panels. Under this condition, the remainig panels have insufficient strength. Am I doing something wrong? How do I get over that issue?

In calcualting the deflection of the cantilevered glass balustrade, is it correct to use the usual small deflection theory, or should large deflection theory be used?

Any comment on any of the above that anyone who has experience in glass design can help me with?

Thanks.

 

[ajk1]

One more item- Considering the two laminations to act non-compositely, I get the deflection to be 26 mm which corresponds to height / 41 for 2 laminations of heat strenghtened glass 10 mm thick per lamination and E = 70,000 MPa. Is that deflection ratio ok? What would be my basis of saying it is ok? OSHA? If OSHA says L/12 is the limit, do they also give an absolute maximum value?

 

[ajk1]

...Well I think I have found something that helps with all this. It is the DuPont Sentry Glaa Plus calculator.