LATERAL DEFLECTION FOR GLASS

[BSVBD]

What horizontal deflection criteria do you use for glass / windows?

IBC 2009, 2403.3 Framing. To be considered firmly supported, the framing members for each individual pane of glass shall be designed so the deflection of the edge of the glass perpendicu1ar to the glass pane shall not exceed L/175 of the glass edge length or 3/4 inch (19.1 mm), whichever is less, when subjected
to the larger of the positive or negative load where loads are combined as specified in Section 1605.

L/175??? I’ve always considered glass to be one of the more “brittle” materials that should require L/360 or L/600.

The Weyerhauser “U.S. Wall Guide” also recommends L/175, which, of course, is referenced back to IBC.

Here is what I’ve used in the past: (Please comment)

HORIZONTAL DEFLECTION CRITERIA
L/600 = Lat. wind on vertical element w/glass or brick
L/360 = Lat. wind on vertical element w/glass or brittle finish
L/240 = Lat. wind on vertical element w/semi- brittle finish
L/120 = Lat. wind on vertical element w/flexible finish

Thank you!

 

[KootK]

The limits you quoted are common for the mullions. Min (L/60, 1") is common for the glass itself. Brittle diesn't necessarily mean deflection sensitive.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[Nor Cal SE]

I suppose glass is not a "one-size-fits-all" material. Depending on the chemistry and properties, some glass might be very ductile and others might be very brittle. I presume L/175 is fine for some. But with any particular glass, I would research the particular manufacturer's data, contacting them directly if necessary, and if they have stricter requirements than L/175, go with the stricter requirements.

I'm not an expert on glass, so that's all I can think to do. Your listed criteria for the other categories of finishes appear solid and similar to what I've generally used.

 

[Yao1989]

Interesting post. I have seen L/175 or 19mm from icc as well.

Glass is brittle, but it also has about 1/3rd young's modulus of steel.

 

[klaus.]

Well .....allowable deflection depends on the usage ...and the manufacture warranty
Codes usually do not say what the allowable deformation is
You can even go below L/100 ....
Stress must be check separate





best regards
Klaus

 

[brut3]

I believe the deflection criteria come from the American Architectural Manufacturers Association (AAMA) voluntary testing standard AAMA/WDMA 101/I.S.2 which incorporates ASTM Testing standard E1300. These testing standards have been adopted by the IBC/IRC and is why L/175 is referenced. Below is a link providing the AAMA test standards as well as some background information of the appropriateness of the L/175 deflection criteria.

http://industry.glass.com/USGlass/2001/0106/deflection.html

 

[MrHershey]

L/175 (or max 3/4") is for the supports and framing themselves, not the glass. And technically that's not a hard cap, it's the cap for the glass to be considered 'firmly supported'. You can go above those deflections, but if you do someone needs to make sure glass and/or attachments are engineered and detailed for the higher deflection.

Would say unless you've got a curtain wall or glass consultant telling you otherwise that it would be prudent to stick to those limits for the supports though. I would imagine the vast majority of off-the-shelf glass products assume they're fully supported on all four sides.

 

[Trenno]

Gee, I must be pretty strict when it comes to glazing deflection limits. I usually stick to L/500 or L/400 if I'm feeling generous...

 

[klaus.]

oh....we go below L/100 if necessary ....
Big facade glass panels ...



best regards
Klaus