CMU Elevator Tower/Shaft Design

[Croceng78]

Hi, I am trying to design a CMU elevator tower/shaft that is going to be built adjacent to an existing two-story building (located in seismic design category D) and was wondering if I would need to use chapter 15 of ASCE 7-16 for the design. Would it be considered a nonbuilding structure similar to a building or not similar to a building? Since there aren't any seismic coefficients for masonry structures in table 15.4-1, would I use the R values/seismic coefficients from table 15.4-2 for nonbuilding structures not similar to buildings for "All other reinforced masonry structures not similar to building detailed as intermediate reinforced masonry shear walls"? Any bit of information/guidance helps. Thanks in advance for your time!

 

[KootK]

So this is a shaft that is not laterally tied to any existing or new floor or roof diaphragms?

 

[Croceng78]

It will have a roof diaphragm and is going to be independent of the existing building.

 

[KootK]

And it will also not be connected to any new building outside the footprint of the shaft, correct?

 

[Croceng78]

There is a steel platform that will be located outside of the existing building where people will be able to walk on after taking the elevator up to the second floor level.

 

[KootK]

I feel that this should be designed as a building structure rather than a non-building structure. Use of the non-building structure provisions seems to be mostly a function of occupancy wherein non-building stuff is meant, in large measure, to serve / enclose mostly equipment and the people servicing that equipment.

Seismically, your shaft will be a bit odd as a conventional building structure in that it will be very difficult to create a design wherein a plastic hinge could ever form in the walls. Without any diaphragm masses contributing, the thing will just be wicked strong relative to the seismic loads it resists. As such, I would be inclined to detail the structure in accordance with the lowest ductility system allowed by code and then, also, do one of the following:

1) Design to R = 1.0 or.

2) Rely on foundation rocking for energy dissipation if appropriate.

 

[milkshakelake]

It sounds like a building to me because it has all the components of a building, like shear walls and a roof diaphragm. I design stuff like industrial equipment foundations, billboards, and storage tanks as non-building structures.

Some code here to elucidate the situation, from the commentary:

C11.1.3 Applicability
Industrial buildings may be classified as non-building structures in certain situations for the purposes of determining seismic design coefficients and factors, system limitations, height limits, and associated detailing requirements. Many industrial building structures have geometries and/or framing systems that are different from the broader class of occupied structures addressed by Chapter 12, and the limited nature of the occupancy associated with these buildings reduces the hazard associated with their performance in earthquakes. Therefore, when the occupancy is limited primarily to maintenance and monitoring operations, these structures may be designed in accordance with the provisions of Section 15.5 for nonbuilding structures similar to buildings.

Part of being a non-building is a reduced occupancy, which doesn't apply in this case.

 

[milkshakelake]

Without any diaphragm masses contributing, the thing will just be wicked strong relative to the seismic loads it resists.

With that being said, pretty much any design done to this shaft will certainly pass. So I'd just take the safe option and design it as a building. It's hard to imagine a shaft only supporting its own self weight and elevator equipment needing anything more than minimum reinforcement per intermediate masonry shear wall requirements.

 

[bones206]

Even when CMU shafts are located within a building, they are sometimes designed as standalone structures independent from the surrounding building. So whether it's located on the interior or exterior, it would be the same Chapter 12 seismic design approach with an independent LFRS for the shaft.

 

[Aesur]

I agree this is a building structure for seismic purposes. The difficulty I normally see with such structures is most elevator manufacturers want one full side open (lintel above) for the installation of the cab and their panels which means you essentially have a 3 sided structure structurally, which typically means you need to tie it back into the building.

 

[lexpatrie]

I've done something similar and it never occurred to me to consider it a non-building. We did attach to the structure, but there is enough 3-sided strength to support the elevator shaft independently. This was low-seismic (Williston, ND), however.

There might be some argument as to the Risk Category since it's an elevator and perhaps not LEGALLY on the egress path, however, people are in it, fundamentally it's a building. It's attached to a building and people travel through it to reach elevated floors. From the other view, this sounds a lot like a "canopy" that's above the egress from, say, a Hospital. You design the canopy as Risk Category III or IV. That's more of a non-building than a side-mounted elevator shaft that a) is intended to be water tight and b) keep out wind/snow.