Cantilevered glass wall design
Cantilevered glass wall design
(OP)
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
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






RE: Cantilevered glass wall design
Designing like it is a railing be appropriate.
RE: Cantilevered glass wall design
RE: Cantilevered glass wall design
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
RE: Cantilevered glass wall design
For loading, what would you suggest?
RE: Cantilevered glass wall design
RE: Cantilevered glass wall design
www.FerrellEngineering.com
Providing fabrication and erection efficient structural design of connections. Consulting services for structural welding and bolting.
RE: Cantilevered glass wall design
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!
RE: Cantilevered glass wall design
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
RE: Cantilevered glass wall design
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.
RE: Cantilevered glass wall design
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
RE: Cantilevered glass wall design
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.
RE: Cantilevered glass wall design
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