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Where could I go about finding the maximum safe span of a pane of glass for a specific width?

I.e. say you had two rafters, X meters apart, with glazing bars, supporting a pane of toughened glass (arbitrary height) 4mm thick along two edges; how would I go about finding, or finding the information to help me calculate, an appropriate maximum value of X?

(In order to determine whether a given design provides sufficient support.)

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I'm confused - is the glass sheet lying flat? – Doresoom Apr 21 '11 at 17:43
@Doresoom, sorry no pane is on a 1/3 (33deg.) pitch roof. – sebf Apr 21 '11 at 18:03
Time to use Mohr's Circle! Do you know what type of tempered glass you're planning on using? Physical properties of the glass are going to play a big part in this. Also, what material are you using for glazing bars, and how will they be attached? – Doresoom Apr 21 '11 at 18:07
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Except the real answer is whatever the building code requires - it doesn't usually have much to do with physics – mgb Apr 22 '11 at 5:11
@Doresoom - I contacted a supplier but they didn't have any technical information on the glass. The response I got implied that they recieved "standard" clear glass (they cut and toughen it) from a couple of suppliers. While I obviously don't think theres such a thing as standard glass it is more believeable that for many common small building projects (conservatories, etc), glass with similar properties is used. I know this isn't a very good question but was hoping that there was an equation, liked the one you linked, that I could use to get a couple of specific values to request from the... – sebf Apr 23 '11 at 19:46
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1 Answer

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You still have too many unknowns to make such a calculation, e.g.

  • Wind load needs to be taken into account.
  • Snow load needs to be taken into account.

The above loads will totally depend on whereabouts in the world you are installing said glass.

For total peace of mind (and most likely to cover yourself legally) you really should hire the services of a a qualified structural engineer to perform such calculations.

Rule of thumb, back of a cigarette packet calculations tend to only get people into trouble further down the road.

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In the UK we have the BS 5516 which is the code of practice for design and installation of patent glazing (which is the edge supported glass roofing I was interested in). This includes maps and tables for determining things such as snow and wind loading, and example calculations for using these and other criteria for determining the load on the glass, and then selecting an appropriate thickness for the loads and span using the included graphs. I knew someone who worked for a university who got me a copy from Athens (which provides access to things like this to different institutions) – sebf Jul 3 '11 at 10:00
unfortunately I'm not sure how much use this will be to anyone else as without access through something like that a copy costs a couple hundred pounds! – sebf Jul 3 '11 at 10:01
@sebf, no offence to yourself or anyone else that might read this, but the average Joe on the street is not in the right position (have the necessary education/experience) to fully comprehend such a document and everything else that is associated with it. If an average Joe used said document to design a piece of glass and got it wrong (because they don't fully understand what is truly involved) the ramification could be very! damaging. That is why I fully stand by my initial answer/suggestion that a qualified structural engineer should be employed to perform such a professional task. – Mike Perry Jul 3 '11 at 18:35
Its always tricky when answering questions like this, I know. Some of the 'average joes' I know couldn't design a dog house that stayed up overnight but others' capabilities far outweigh some of the many professionals in the many trades I have dealt with. I hope my response did not seem terse, I was simply stating there was an appropriate resource that could be used for the task; I don't intend to appear confrontational, but you don't know anything about me, or many others who may read this (I really don't, but its very very hard to say that without sounding it :D) – sebf Jul 4 '11 at 22:22
The thing is, unlike UltimateHandyman or Diynot, the Q&A format of the Stackexchange sites doesn't lend itself well for indepth conversations, in which the provider of assistance can get to know and gauge the capabilities of the receiver over a long conversation. The whole site is designed to be about brief, fact based, generic questions and comprehensive but to the point answers, which leave us with a choice: answer a question and risk encouraging someone of suspect competance to do a job that may result in disaster later on, or respond with 'Call a pro'(TM). I see why the first is scary, – sebf Jul 4 '11 at 22:23
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