One large (80000 SF footprint) RC bldg designed & built as 7 separate bldgs

[SIGrkr]

In 25 years of engineering I have never seen a building designed and built as separate buildings. This mostly completed reinforced concrete (RC) building is in a seismic area (Ss=1.30, S1=50), has an 80000 SF footprint, has no irregularities and is 5 stories. The design build contractor designed it as 7 separate buildings (units) with separations varying in width from 7 to 10 inches. Separation looks ok per code if you must have the separation, but I don't see why. All the interior walls are then doubled up so each separate building has "exterior" walls side by side amounting to just a lot of extra work and materials. The contractor says they did it for seismic reasons but I don't see the benefit of having two RC shear walls adjacent to each other. Has anybody designed this way or seen designs or construction done this way?
Thanks, SIGrkr

 

[racookpe1978]

Seems like everything (maintenance, paint, heating costs, cooling costs, movement, travel building to building), all services (power, phones, electronics, controls, sensors, etc) now have to go through exterior wall 1 to exterior wall 2 to the next building.

Construction isn't the only thing that this owner is going to be paying for.

Government? Union or contractor "payments" going properly and honestly?

 

[msquared48]

Does the owner plan to sell each building as a "commercial condo", each with its own utility systems?

Mike McCann
MMC Engineering

 

[jvvse]

Usually seismic joints are done to prevent any kind of seismic irregularity that would then increase the cost of construction. They are not uncommon in any sector. I've seen a RC building ~100 years old with seismic joints because the shape of the building had lots of appendages and re-entrant corners, but with joints it was just a bunch of rectangles on a common foundation.

When you say it has no irregularity, how do you know? Maybe the placement of the shear walls created torsional irregularities in the structure and the architect was unwilling to add more shear walls where needed. A seismic joint could get rid of that.

 

[SIGrkr2]

Thank you racookpe1978, msquared48 and jvvse. To racookpe, it is a government building and flexible joints etc have certainly incresed complexity and costs for the building and the Contractor has enjoyed those increases. Mike, the building is a single occupant and the systems, including HVAC have to cross these joints.
Jvvse, as you know, most of the irregularities in ASCE Tables 12.3-1 & 2 are observable. To your point regarding torsional irregularities, in modern construction why would the designer not address the irregularities by incresing the loads as allowed by code. It seems the choice to increase the loads vs doubling the shear walls and complicating design and construction would be automatic. When you say "not uncommon in any sector", are you referring to institutional, industrial, commercial, residential or material types? Not uncommon for a building to be designed as 7 separate buildings with separations varying in width from 7 to 10 inches? Are you saying not uncommon in the U.S.? If not, what Country is this not uncommon? If so, have you actually worked on one where this approach made sense?
SIGrkr

 

[jvvse]

Another reason is maybe the buildings were separate permits? Whether they were originally on one permit or not, smaller buildings will go through plan check faster and thus construction can start sooner.
You said it's a government building which puts it outside the norm. Government buildings typically have much higher standards and cost is less of an issue than with a privately owned building.

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Onto your questions:
Some irregularities are only observable if you have plans and I did not know if you had plans.

Some irregularities are not allowed by code. Extreme torsional irregularities for example are not allowed in high seismic zones per ASCE 7. If such an irregularity existed, the answer is usually to put in more shear walls (for example) which the architect/owner may not allow. Since this is a government building, the cheapest solution may not have been used.

I've seen seismic separations in all the sectors you mentioned except maybe industrial. Industrial warehouses, yes.

Seismic separations are not uncommon. I have seen and designed seismic separations and there are a lot of reasons for them. I did not say your case was not uncommon as we are trying to figure out why this building is separated so.

My experience is mostly Southern California but also some throughout the Western U.S.

 

[SIGrkr2]

jvvse: Thanks for the additional information in your thoughtful response.
I am looking at this building based on what the design build contractor said was their reason per my original post. They say it was for seismic reasons.
I'm licensed in CA, OR, WA and Alaska and I have never suggested to an owner that we should split one big rigid diaphragm up into some number of smaller rigid diaphragms. Knowing that the edges of all the smaller diaphragms would have to be supported. Other than base isolation, could you indulge me one last time with a few examples of the lots of reasons to design seismic separations? Not expansion joints but actual separations which then require a calculation for required distance between the "buildings".
SIGrkr

 

[jvvse]

Sorry for the late response, I've been busy. I don't think I have any more reasons than what I already mentioned. Summing up:

--Separate permits (building was split up to speed up plan-check, design, or construction time).
--Irregularities (you mentioned there were none, but unless the building is perfectly symmetrical in both directions there may have been some in the design).
--"Condo units" (mentioned by msquared48, if the government expects to be leasing some units out and wants their own units completely separate, utilities and all)
--Government buildings have different standards than other sectors. Usually cost is less of an issue and they like to overdesign for seismic so the building will last longer, if it isn't already an "essential" facility. If they are building their own structure there may be other requirements that you won't see anywhere but a government facility.

 

[SIGrkr2]

Thanks for your time jvvse! I appreciate everyone's input and the possibilities to consider.
SIGrkr