Design Criteria for Glass Elevator Shaft Frame 2

[KootK]

I'm designing my first ever elevator shaft where the shaft will be a stand alone element not really integrated into the building. It will be quite similar in concept to the photo below. I have some questions:

1) For these kinds of elevators, is it normal to have them tied into the second floor? Or are they truly free standing and have an expansion joint at the second level door? Tying into the second floor certainly would help matters structurally but would require connections that pass through the glass. But then the L2 elevation seems as though it would be a natural place for a glass joint (elevator door sill).

2) What kind of drift limitation would apply here? Elevator shafts integral with buildings obviously are designed for h/500 typically so I would consider that to be an upper bound. Could I relax that some?

3) From a stability standpoint, I see the situation as I've shown it in the sketch below. The elevator cab is essentially a concentrated mass that needs to be stabilized by the shaft framing. I would estimate these stability load and then add them to the seismic rail loads provided by the elevator supplier. Is that reasonable?

4) What should I do about the hoist beam? Many of the examples that I've found online have roof framing. However, that may just be to support a glass roof. Could the hoist beam be a roof beam of the main structure above? Or would that be unsafe? Could the hoist beam be a temporary thing used just for the install and stored elsewhere? If I design a permanent hoist beam into the shaft framing, should I expect it to get beat up pretty good during the installation?

Naturally, any advice in addition to these questions would be welcome. For what it's worth, there is an elevator consultant involved in the project. Unfortunately, their ability to advise me has proven quite limited for a number of reasons, not least of which is the fact that the elevators are to be a tenant improvement item in the future.

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.

 

[KootK]

Help. No need to be a glass expert. It's really more about elevator knowledge.

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.

 

[jayrod12]

1) I would be tying it into the second floor. Depending what your second floor construction is, it shouldn't be too difficult to provide something the glass people can work with.

2) How many floors? I could see there being an argument for H/400 but it also may depend on the glass.

3) This is well outside my realm (I've never even looked at seismic besides calculating S(0.2) to ensure I don't have to design for it). And I've also never designed a free standing shaft. Although I would be inclined to think that the elevator shop drawings seismic loads would account for maximum occupancy no? When they give me a max load from the hydraulic piston at the bottom I don't add the elevator's capacity onto that.

4) I've never seen the hoist beam be a integral part of the roof system. I have however, seen removable hoist beams with bolted plate connections at each end to allow for it's removal.

 

[glass99]

KootK: You should get your hands on a copy of the ASME A17.1 elevator code. There are a bunch of loads which are specific to elevators, especially on the rails and doors that are not in ASCE07 or regular building codes. Otherwise the elevator cab is a lumped mass subject to component seismic loads.

From a glass point of view, a conservative approach is the ASTM E1300 deflection limit of L/175 (ish). Is it point supported glazing? In any event, the glazing designer would ideally be provided with your movements and design accordingly.

In respect to story drift, I think H/500 is very conservative. There are lots of buildings in California with elastic seismic story drift in the range of H/275, and inelastic story drift of H/50. I don't know of a specific story drift limit for elevators however.

 

[KootK]

Now we're getting somewhere...

@Jayrod: it's two floors, just like the photo. I would also think that occupancy would be included. The rail loads are seismic loads for the most part, right? As discussed below, I'm getting conflicting information on that.

@Glass99: it turns out that I have the elevator code, just under the Canadian B44 designation. Working through it today. The shaft will be point supported glass. I like the H/275 recommendation. Thanks for that. You must have done one of these at some point. Do you know if it's normal to tie the shaft frame back to the floor slab?

I just spoke with an elevator consultant that used to work for Otis. I got a little more info this time:

1) He's only ever installed systems where the shafts are tied to the floor decks. Apparently the differential movement at the door sill cannot exceed 1.25"

2) Lead time on a glass elevator in western Canada is about 18 months.

3) Installers like hoist beams fairly near to the top of the shaft vertically. Up in space at the main roof is undesirable.

4) A temporary hoist beam is an option as Jayrod had mentioned.

5) Only one rail sees load at any given time. This makes sense to me for side to side loads but not front to back.

6) Supposedly, the 2kN rail loads supplied to me have nothing to do with seismic. Rather, they are a consequence of imbalance in occupancy. Apparently, in my jurisdiction, we ignore seismic altogether for elevators. If I can't find a code requirement for seismic, I'll just make up one of my own.

7) Apparently shafts take a beating during installation. The consultant recommended not applying finish to the shaft framing until all of the rail support brackets etc are in place. Could I specify a finish that would be shop applied but touch up-able?






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.

 

[glass99]

I would imagine you should connect the top of the hoistway to the upper floors. The only reason for not doing so would be if the anticipated primary structure story drift was really high.

We completed a fancy glass elevator cab and hoistway for a museum in LA recently. The hoistway was a cylinder of glass cantilevered from the slab using a structural silicone shoe. The decision to cantilever the hoistway was all about architecture and nothing to do with movements etc. Our focus was the structural glass design, and we didn't really look at the rails and supporting structure for the rails, but I have some idea how it works too.

There are some surprisingly high forces in the ASME document. Hoistway door retainers have a 5kN load. Human impact loads are relatively high too.