Full Glass Railing for Balcony 2

[kmart30]

I am trying to do calculations for full sheets of tempered/laminated glass (1/2" thick) for railings along a balcony. See attached picture... The balcony is constructed with wood trusses and the glass will be side mounted to a couple of 2x SYP band boards that are fastened to the trusses. Code says I need to use a F.S. of 4 which is really increasing my loadings. Glass is 42" tall above the deck and I am using a concentrated load of 200lbs per code. Manufacture has 2 holes in glass every 2' o.c. and I was trying to make 1/2" lags work but I am not even close with a F.S of 4. Anybody come across a similar situation? Maybe through bolts with some good sized washers? Any help here would be appreciated...

 

[oldestguy]

Could you set the glass in a channel of sorts that can take tension or compression perpendicular to it. Might allow for lower bolt loads.

 

[kmart30]

No I dont believe so... the architect/owner has a specific product and look in mind. They are going for the seamless look.

 

[Ron]

Thru-bolting is likely your only option. Lags into sides of 2x lumber cannot develop a lot of capacity.

For yours, you have tension in the upper bolt of about 9600 lbf (including safety factor). For 1/2" bolt, that puts your stress at over 80ksi. Not good.

Also, for exposed glass like this, you have to accommodate other stresses such as thermal, so you have to use flexible washers under the bolt heads and provide oversize holes, preferably with neoprene shoulder washers.

 

[kmart30]

Very true on all those points Ron, the loads/stress get up there quick with the F.O.S...

This is what I have so far...need to check band boards to trusses as well. Not feeling the warm and fuzzies about this one.

 

[NorthCivil]

I've done loads of these designs. a lot of engineers "make it work", ignore the standards, and sign off on guards exactly as you describe. 1/2" single pane tempered will not work. but you will see it all over the place, a lot of dangerous guards around.

because there are engineers who sign off on this, contractors expect it. Doesn't make it right. It's all fine and dandy until you meet someone who has fallen through an inadequate guardrail and wound up with a broken neck (been there).

Consider:

1. check your glass design standard. you likely need a redundant guardrail should a pane break. this means, either a fully continuous structural top cap securely fastened at ends to take that 200lb load, or, a laminated sheet of glass that will take that 200lb load should one side of that lite break. look into dupont SGP interlayers.

2. most standards require a top cap, to give the top edge protection. no consumer wants a top cap interrupting their line of sight. if you specify a light duty cheap-o top cap, very likely it will get removed immediately after compliance inspections are done with.

I don't sign off on these anymore. but when I did, I took a pragmatic approach. To make a design like this work to the letter of the code, you will likely need 10on10 tempered laminated, or, 19mm tempered glass with a fully structural top cap. Not many contractors will hire you again if you force them down this road. they may even drop you and find another engineer, as your solution is too expensive.

I would relax my requirements a little bit, and sign off on a guard that was slightly less than code compliant. because if i didnt, my client would go down the road and hire an engineer who would sign off on something downright dangerous. This method makes a real good topic for ethical debate.

PS recommend pushing for 6" spacing between bolt fixings to reduce the moment, original design likely calls for 4"

 

[nonplussed]

Based on my experience certifying these balustrades, I would recommend that your first avenue is to contact the balustrade suppliers and see if they have testing reports of balustrades for your configuration. Most of these have been tested in a lab to show compliance and not necessarily with hand calculations. The reports I have seen indicate 12mm tempered glass is fine for the edge of balcony loadings we have here (not necessarily the case for your location) using pairs of stand-offs at 500mm average centres. However, these are typically using a continuous handrail which reduces risk in the case of failure and helps redistribute load. I would certainly not recommend non-laminated glass where there is no continuous hand rail.

Looking at your calcs, 3 inch bolt centres is very low; I would use min 4 inches or even 6 inch as NorthCivil says. Also, I would assume the load will distribute to other stand-offs in the glass pane instead of just going to one bolt couple. I would tend to make a quick FEA model of this to work out the load distribution if you don't have the testing data.

 

[klaus.]

What country is that ??
The problem will/can be the glass...not the bolts

 

[kmart30]

This is in the US (florida)....the glass needs to meet a certain spec per the building code and they have verified that it does.

 

[BridgeSmith]

Obviously, they want to avoid having a rail at the top, and I have no idea how you manage that one. It would make me fairly nervous. However, would a rail or beam along the top row of bolts be something the arch. would go for? If it were stiff enough, that would give you continuous, rather than discrete, support for the glass, which should help with that aspect.

 

[jayrod12]

We did a glass guard without a top cap but as Northcivil indicated, we designed the thickness required as if a single lite of the laminated glass broke and relied solely on a single lite. It wasn't cheap but the client in this case did not care about the cost (within reason).

 

[hetgen]

@kmart

Most design codes ( if not all) require impact load assessment ( see attached table). The static load requirement should also be appropriate to the type of occupancy, the loading requirement for public areas is very high as compared to residential applications.

 

[KootK]

@NorthCivil: if you've not recently checked your LinkedIn messages, please do.

 

[JLNJ]

The glass manufacturer may have test data which allows for some redistribution of the load without the top rail.

Last one of these I did was for a loge level in an arena and I was surprised how strong the glass was. It seems to me that the glass calcs allowed for considerable load sharing, even if the load was applied at the edge of one panel. My anchorage was into concrete, not wood, and I remember that it was troublesome.

 

[NorthCivil]

KootK,

My linkedin mailbox is empty, which means I've succesfully remained anonymous!

Appreciate the reaching out though.