Cantilevered glass wall design 1

[Fatboycowen]

I am the Structural Engineer for a Lobby Renovation project in a large building.

The architect intends to build a (roughly) 5 to 6 foot tall by roughly 11 feet wide.

The glass will likely be back lit, and may be etched. It will be a artistic sign display. It will be in front of an old unused staircase, indoors. People will be able to walk right up to it, but probably not behind it.

The architect is suggesting that we design this from scratch. This will be my first real structural use of glass. So, i need some guidance.

Loads: What should i use? Should i treat this as a cantilevered railing?

Bottom Support: We have an existing concrete slab on terra cotta arch, below a marble tile. The idea is to bolt some steel angles to the floor, which the glass is sandwiched between. Does this sound reasonable? Can anyone comment on the detailing of that bottom support?

Glass Design: I would assume we need at least tempered glass.

The other question is, should I recommend that we hire a specialty engineer consultant to design the glass and support system?

Thanks in advance.
Jon

 

[boo1]

For Safety/Tempered Glazing requirements, Chapter 24 applies. Specifically, Section 2406 "Safety Glazing". That is where listing the locations where safety/tempered glazing is required. It follows the concept of "Hazardous Locations". If the glazing is located at hazardous locations, then safety glazing is required.

Designing like it is a railing be appropriate.

 

[Fatboycowen]

Thank you for this. Does anyone have a detail for the connection at the base of a cantilevered railing or baluster?

 

[Michel60]

Good morning Fatboycowen,

Structural glass design is a real specialty. It's not not something most of us are trained in or covered very well in references or codes other than the provision provided by "boo1". ASTM E1300 will also provide some guidance on selection of the glass type and thickness. Tempered glass may not be your best option depending on the conditions. Heat strengthen and/or laminated could be a better choice. Note also that a FS=4 design for glass does not mean the same as reducing the allowable stress by a factor of 4 (although that will work, but it's excessively conservative). Glass is "designed" based upon a statistical probability of breakage (Pb). Typically this is 8 lites in 1000 (Pb=0.008) for normal glass design. You'll be looking at Pb=0.002. This probability does not have a direct constant linear variation with stress. Dropping the stress by 10% or 15% may be all it takes to get the FS=4.

The base connection you describe is reasonable, I've done something similar with even a taller panel. However having the glass in direct contact with the steel angles must be avoided (it will cause the glass to fracture) by using hard rubber or plastic shims.

Finally the base anchorage will also need to be designed with the FS=4. Actually this could be the most difficult part in terms of getting it all tied down if the floor slab is not thick enough or of sufficient strength to get good anchor values. Chapter D is a miserable experience under the best of conditions. Through in an added safety factor and you'll be bald in no time.....

With all that said I would encourage you to find a specialty designer at least to review your particular design.

regards,
Michel

 

[Fatboycowen]

I met with the architect, and convinced them to use a glass company to design the glass. I will be handling the attachment at the base.

For loading, what would you suggest?

 

[steellion]

That's good, you really don't want to be responsible to design something you're totally unfamiliar with. I'd use the standard 5 PSF lateral load for an interior wall, plus any loads you can think of as far as things that may be attached to the wall, or people pushing against it, hanging on it, etc.

 

[connectegr]

Good luck with that (delegating to the manufacturer). We frequently design glass partitions and handrail systems, since they are becoming more popular with architects. They use to use a cap rail and base shoe, such that the glass was simply infill and not structural. But we have seen more and more cantilever conditions like you describe. A base shoe anchored down the center, with self supporting glass rail. Most glass manufacturers will verify their product up to 36" or 42", basically handrail height. Above this you will need to provide calculations or find a specialty engineer. I have even seen loading tests done, when an architect persisted with an application that we could not calculate. Julius Blum has several examples of the base support you are looking for. If you can provide a similar support at the top you will be better off. Bending is no friend of glass.

http://www.FerrellEngineering.com

Providing fabrication and erection efficient structural design of connections. Consulting services for structural welding and bolting.

 

[Fatboycowen]

I fully anticipate reviewing their calculations, and requiring a PE stamp from them.

The architect is contacting the glass company they have worked with before to find out how they can help. We'll see what they say.

The architect is dead set against a frame around the glass. Given the location of the glass, side or top support is almost impossible. The only possibility of a tie back at the sides would be to existing solid marble (HUGE) historic newel posts. These are historic items. Drilling into them would make this installation much more permanent.

Luckily, the building owners are well aware of the safety concerns, as they own many buildings with glass, and have seen them break and cause injuries. However, the architect doesn't seem to to be too concerned.

I'll keep this updated. The info here is excellent. Thank you!

 

[Michel60]

Sounds like it's getting pretty well sorted. Again, I think the glass is probably the "simple" part and the base design being what will take some effort. There might be reason to consider the element as more as signage or a partition/divider and avoid the added FS in the anchorage design. In this case the controlling loads could be the normal partition loads or seismic loads and consider individual 200# point load at about handrail height as "steellion" suggests just as a "what if?".

It's really unlikely that they will come up with an off the shelf base shoe for this one. It wasn't clear whether the base fixture is in your court, but you might want it to be or have some input on the configuration to find places for all the anchors you're going to need.

regards,
Michel

 

[Fatboycowen]

The base will need to be custom, i'm assuming. And, i anticipate it will be in my court.

There are some thick tiles, which we can hide a wider base plate in. We can probably go as much as 10 or 12 inches wide, which will spread the anchors out a bit.

I'm hoping the shoe can be fabricated from steel plates.

 

[Michel60]

It should all be possible. The base fabrication will need some stiffeners on the vertical legs. The architect may want to design some sort of base enclosure to hide the "ugly" parts. On one that I did it also conveniently became the housing for the lighting of the panel.

Fixing the panel to the base can be done initially with rubber setting blocks and wedges. Once it's all aligned a typical installation method is to "pour-rock" the gap with grout. Check with your installer to see what sort of clear space they'll be needing.

As an aside cement grouted bases should only be used in indoor applications when combined with laminated glass. In an outdoor application the grout will trap moisture on the bottom edge of the glass and cause the laminate inter-layer to cloud.

regards,
Michel

 

[Fatboycowen]

Michel60 - thank you! This is great info. Is this some kind of flexible grout?

On the same job site, years back, we checked the mounting for short glass walls (maybe 3 feet) used to surround the elevators (so people are forced to walk through turn-styles). I didn't design the glass or mount, i just checked the fastening for the building owner.

Today, the glass is somewhat loosely fit into the base, with what appears to be something flexible, like rubber. The glass is able to flex in the mount. With just a finger, you can move the cantilevered pains of glass back and forth 1/4 inch or so. Clearly, those panels were not grouted in.

 

[Michel60]

Actually the "pour-rock" type grouted base is rigid cementitious material...pretty much base plate grout with just fine sand as aggregate. You'll need something like this to hold the glazing solidly and limit deflection of the tall panel. It is appropriate to have this rigid material in contact with the glass (although the installer may still use thin fiber gasket on the face of the glass) since it conforms to the glass surface while fluid and avoids creating stress concentrations which could result from having the glass in direct contact with metal. There is also a base clamped system where the base fittings would be clamped with HS bolts through the glass panel (also using a fiber gasket as an isolator). But I would think it would be more difficult to design and in the end not really appropriate.

The application like the one you describe sounds like "wet set" glazing with a silicone caulk. Kind of unusual for a cantilever glass railing. There are mechanical wedge type systems that will fix the glazing panel and then the remainder of the space is just caulked in place to seal the gap but these types of systems are usually more rigid than what you have. The wedges may have just released themselves over time.....might want to get your installer to have a look at that one too.

regards,
Michel