Mid-Rise Modular Pod Buildings/Diaphragms/Details

[bookowski]

Anyone have any experience working the modular building systems that consist of steel hss frame "pods" that stack and connect (pre-finished interior)? Several of these have been done in NYC at fairly decent heights (20 story range), and presumably in lots of other cities. I am trying to find some info on how the diaphgrams work - the connections, load path, design basis, etc. but it seems to be well kept secret so far. I'm in the early days of looking at using this in a high seismic region and wondering how/if this can work. If anyone has come across any technical articles/info I'd appreciate the input.

 

[milkshakelake]

Probably not what you're looking for, but I've done these for industrial buildings/chemical factories a decade ago. They came with floors, piping, and equipment pre-installed. No finishes. They were designed both horizontally and vertically because they were shipped as towers to be stacked on top of one another and side by side, but lay flat during shipping. They acted as horizontal trusses when shipping and being lifted by crane, with braces designed to allow for doors. The lug plates were pretty massive and stuck out the sides so they wouldn't have to be removed, and they had cap plates when stacking. The lateral load paths were purely done through HSS bracing vertically and with angles horizontally. The width was the maximum allowable for shipping an oversized load. The drawings are under strict NDAs so I can't share the details and such, and didn't get to keep them (probably why you're having a hard time finding info about this stuff). If I recall correctly, the dimensions were about 16'x16'x60' tall per module, based on shipping restrictions (like length of trailer and allowable width/height of oversized load).

In most cases, the modules worked together as a combined lateral system. I don't remember exactly, but I think horizontal load transfer was done with plates butting each other and bolted. When they were separate buildings (like a cluster of modules), they were connected with a combination of long slotted holes and pinned smooth rods that allowed for rotation between walkways to avoid accidental transfer of lateral forces. Stairs came in their own modules. These were the easiest to design, because they barely had any loading. The modules were fully welded in shop (though they were designed as pinned) and shrink wrapped and boxed for transportation. When assembling them, they lowered them and installed anchor bolts into sleeves that allow for adjustment. And yes, this was done in high seismic zones. I designed about 30 of them but it became fairly routine after a while because the details were standardized, and my job was just to calculate member sizes, bracing, lateral, anchor bolts, etc. It wasn't really that much different from regular buildings when it came to design; only the details and methods were different.

Some components were dipped into huge galvanizing tubs. This probably doesn't apply for residential or commercial buildings. But the pictures of those tubs was something to behold.

 

[bookowski]

Thanks, Milkshake. I wasn't clear but I have found lots of details for the typical tube frame setup and standard connections, including the various truncated pyramid and other details used for setting alignment, custom tools and access holes, etc. etc. What I haven't been able to find is any examples of high demand details. I found an article on the hotel that went up in nyc that mentioned that they added diagonal ceiling bracing to each module to form a diaphragm but no more. I was hoping to find some discussion/info on high seismic detailing - what type of structural system detailing do they try to mimic, do they have special details to lay out discrete chords, collectors, etc. I'll keep digging. The tall examples typcially have a concrete core(s) that it all plugs into, so I'm wondering if they treat the rest like a steel frame w/conc core in high seismic and adapt the modules to follow those details, or there is something more clever.

 

[milkshakelake]

We did make "diaphragms" using horizontal diagonal bracing for industrial structures, but not for modules (see second paragraph). It was usually angles welded to the bottom of beams and connected to columns with horizontal gusset plates. I'm sure you could also pass an HSS through (interrupted at beams) if you don't want to lose headroom. I don't remember the collector and chord connections, I think it was bolted butt plates when it was from module to module, but that should work because it transfers axial forces. This would mimic an industrial roof diaphragm, where you consider the roof metal deck to be flexible.

If you have both horizontal and vertical bracing in a vertical truss, and it's all welded, would you even need a horizontal diaphragm? You would need it for the concrete core idea, but if each module has bracing, the lateral forces would be transmitted through the minor axis of beams and minor axis fixity through welds. That doesn't work if not every bay is braced, however.

If the modules come with a concrete slab installed (not something I've done, but I imagine it could be achieved with shear studs), diaphragm continuity between modules could be done using connections similar to precast concrete planks. Combined with chord continuity, it should all work. If the concrete is cast in place to reduce shipping weight, you have a continuous diaphragm anyway. I don't see how it's fundamentally different from a regular structure in a high seismic zone.

 

[bookowski]

I don't see how it's fundamentally different from a regular structure in a high seismic zone.

Maybe it isn't fundamentally different - I'm just getting into it and trying to see what others have done. On the surface it's definitely not like a "regular structure". It's hundreds of pre-fab tube frames that get stitched together with typically single bolt connections and little plates and shims, usually through access holes or sometimes specialized tools that reach deep into areas to tighten them. There definitely will not be concrete slabs so it'd be plan bracing. If you were to count on the modules for their own distributed lateral you'd need to detail them all as smrf's - doesn't seem right hence the concrete core (and that's what everyone else seems to be doing). These are in the 6 to 20 story range, 100k to 300k sq. ft., so the demands will be high. For the low rise ones, 6 story, maybe the modules could form the lateral - not sure yet.

I'm not looking to solve it here - was just hoping someone would say "check out the big modular they're doing in xyz california city" before I try to reinvent the wheel.

 

[milkshakelake]

Not SMRF's, I mean special moment resisting concentrically braced frames, or ordinary ones with R=3. You can't get much stiffness out of welded tubes without bracing. Align the doors with bracing. There's also another way with cold formed steel panels, which is a whole different thing than the steel tube frames.

I get what you mean about not wanting to reinvent the wheel. Hopefully someone with a specific project can chime in. In the meantime, this might or might not be what I meant earlier. Pictures there.

 

[bookowski]

Can't do ordinary or r-3 in SDC E.

This is the only thing I've found regarding high seismic/modular -

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC7386282/