Do we need expansion joint between main building and elevator tower

[jdengos]

Wondering whether we should place an expansion joint between main building and elevator tower due to seismic load. Please see attached plan, 4-story building. Even this building may not need since we have shear walls in the building, deflection is small. But for moment framed building, or for PT building, do we need expansion joint?
Thanks

 

[Greenalleycat]

Koot, I agree with what you've said. I'm interested to your thoughts for loading in the other direction, as that is what I was more concerned about
In that direction there will be a large primary shear trying to be forced through the link slab between the external core and the main part of the structure

 

[KootK]

In that direction there will be a large primary shear trying to be forced through the link slab between the external core and the main part of the structure

Same fundamental answer really:

So design it for one of the following:

1) The forces that would be generated by the two structures remaining contiguous OR;

2) The displacement capacity that would be required of the elevator shaft for it to ride along with the main building.

Following path #2, for shits and giggles, it would look like the sketch below. And, unless the main building drift is a lot more than I'm imagining it to be, designing the elevator bits for the imposed displacement ought not be too challenging. Heck, it might just need to rock up on its foundations.

 

[milkshakelake]

@Greenalleycat In the horizontal direction, the slab will be along for the ride and the outward wall will be entirely free to deflect, except for some incidental minor axis moment restraint through the north-south beams. I think the outward wall horizontal wall needs some bracing/moment frame, or engage the CMU infill.

 

[Greenalleycat]

@Milkshakelake, the point I was trying to make is that I'm not sure it's that simple
It sounds like that external core is a rather stiff masonry infill structure, so it may want to take a decent amount of load
Quantifying and designing for those forces is important - I'm not convinced that it will just 'go along for the ride'

 

[Tomfh]

There’s already a big core at that end, so what’s the point of releasing the elevator shaft?

 

[jdengos]

Both big or small structures have minimal drift, less than 0.4" on top. I am ok without expansion, only worried about possible cracks in the link slab or elevator beams/columns. The concern in the case is only because we did nothing to model and design them together.

My question is, for any kind of similar 5 to 6-story buildings with the same layout, before you start your design, will you place expansion joint between them. If not, I would need to model them together in the future.

 

[milkshakelake]

@Greenalleycat I agree that if it's masonry infill, it needs to be properly detailed for flexibility or stiffness as required by the design. No need to leave it up to chance.

@jdengos I think almost every commenter said at some point that it should be modelled and taken into account. If you connect it, it definitely 100% needs to be calculated together.

 

[KootK]

My question is, for any kind of similar 5 to 6-story buildings with the same layout, before you start your design, will you place expansion joint between them

For a six story version of the same layout, I would not introduce an expansion joint.

Rather than seeking hard and fast rules on this, I would encourage you to approach each individual situation by first considering the fundamentals:

1) Is there a plausible demand for differential movement that needs to be addressed?

2) Is there an important design reason to isolate the two structures (seismic etc)?

Let the answers to such questions be your guide.

 

[BAretired]

No separation joint:

 

[jdengos]

@BAretired, that's way beyond my head. For a beam/column frame with infill wall pinned T&B, how to make it rigid or flexible, there is no slab in the elevator. I can only model them together as one diaphram, otherwise I have no idea how to proceed.
I wish I can treat the link slab and beams as collector/drag strut, so I can still separately analysis each. Bu it's not a typical collector.

 

[BAretired]

Make the wall rigid either by using cross bracing or shear wall.