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Allowable deflection of steel posts providing lateral support to glass balustrade panels 1

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ajk1

ajk1

Structural
Apr 22, 2011
1,791
1. Does anyone know of any published requirement, or recommendation, for the maximum allowable horizontal deflection of steel posts that are to provide lateral support to glass balustrade panels? Posts are about 1.5 m on centres.

2. We calculate the posts will deflect horizontally h/80 (1070 mm / 80 = 13 mm) under the Code specified horizontal loading (since the posts are cantilevers, this would correspond the about L/160 for a single span member). Is that ok, or should the post be stiffer to avoid cracking the glass?
 
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ASTM E985 Standard Specification for Permanent Metal Railing Systems and Rails for Buildings requires a horizontal load of 200 lbf with a maximum deflection of h/12 of metal handrail systems...not sure if that is applicable for glass panels.
 
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Ask your glass panel manufacturer!

Most glass panels attach through drilled bolt holes in the glass lined with a plastic or soft rubber grommet, or are inserted into a liner-edge of metal. The liner edge (a u-shaped strip) has a inner liner of rubber of plastic, sometimes a felt-like material that wraps around the glass edge and provides movement, some expansion allowance, and an softer impact surface agianst low velocity accidents.

Your manufacturer will tell what movement is allowed in their system, and how to mount the posts for their system - usually at 4 ft intervals. They have to meet the codes listed above = there minimum standards solve your question.
 
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To Ingenuity and racookpe1978. Thanks for your comments and help. We were aware of ASTM E985 and its h/12 limitation, but that seems so vastly more generous than any Code limitation for other applications, as well as the general practice of stringently limiting deflection where brittle materials are involved, as well as that if the balustrade really deflected h/12=90 mm (3.5") then anyone leaning against it would panic (I would). So we thought that we must be misinterpreting the ASTM intent.
- Are you saying that the glass connection details allow for such gross deflections?
- Do you know how the h/12 limit came about?
- Is a metal handrail the same as a balustrade guarding a drop?
- Where does the 4 foot usual spacing come from? Any publication?
 
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As far as I know, the 4ft spacing typically used is to optimize load to a post - the US code says post need to be designed for 50 plf on the handrail or 200 lbf, in any direction, so 4 ft makes those equal.

I have never really understood the large allowable deflection per the ASTM.
 
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To struct10 - thanks for the explanation for the 4 foot spacing; makes sense. The corresponding numbers from the National Building Code of Canada, after conversion from the metric units, are 51 plf and 225 pounds. so maybe that is how they got to 5 feet on this project, after a little rounding up.

But I am still interested if anyone knows how the h/12 deflection limit was derived.
 
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Like you say, it does not really explain anything, but thanks for trying. I am still wondering where h/12 comes from, and who the heck would be using that. It seems like a dangerous limit, but maybe I am missing something.
 
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I have no references to share but h/12 makes sense to me. It's a limit on a life safety item. The intent should be to prevent gross deflection while not unduly impinging upon architectural creativity.

Most glass guards that I've encountered would be relatively insensitive to post deflection perpenducular to the guarded edge. They are attached at two vertical elevations on the post and, as such, would see only rigid body rotation as a result of post deflection. The curvature of the posts would not be transmitted to the glass.

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.
 
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KootK - I agree with your approach towards the deflection and typically make use of this line of thinking; Even thought the glass panels will rotate as a whole and move outwards, they will not crack.

Great spirits have always encountered violent opposition from mediocre minds - Albert Einstein
 
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I don't think deflection of the railing is a life safety only issue, it's for serviceability. H/12 = 3.5" on a 42" high rail. That would be a very uncomfortable railing. Even if you took 25% of the 200# as a service load you are almost at 1". That seems unlikely to inspire confidence.
 
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@Bookowski: I didn't say that deflection is a life safety only issue. I said that H/12 is a likely a life safety deflection criteria.

It's also important to remember that the code loads for handrails and guard rails are not serviceability level loads. True serviceability level loads would be much less and are not specified in codes.

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.
 
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Hi Kootk - I had thought that the code specified loadings for pedestrian guards were true serviceability loadings, but perhaps I was wrong. What is your reference document for saying that they are not?
 
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ASTM E1300 Glass and Glazing in Buildings specifies a deflection limit of L/175 for simple spans of mullions for all of its glass design tables to be valid. If you used 2L/175 it would meet the intent of this. AAMA standards have similar guidelines.

In reality, 2L/175 could be quite conservative depending on your glazing detail.
 
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@ajk1: as I mentioned in my first post, I don't have any references to share. What I've shared is simply my interpretation.

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.
 
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For comparison, Standards Australia advise that a horizontal deflection of h/60 + l/240 "may be considered acceptable".
 
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To apsix - that is very useful information. Thanks very much. Seems the Australian Standards sometimes have information that is hard to find elsewhere. It would be interesting to know what European standards say, but never seems to be anyone on here from Europe. Apsix, would it be too much trouble, if you ever have some spare time, to scan that page of the Australian Standard and attach it? Is it the National Standard of Australia? Thanks again apsix. Very much appreciated.
 
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To apsix - does Standards Australia say that the deflection limit of H/60 + L/240 is ok to use where the infill panels of the guard are glass?
 
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