Seismic Retrofit Argument 9

[bookowski]

Probably no b&w answer here, just looking for opinions for and against the following argument.

I've got a client looking at a 3 story conc building that is approximately 100yrs old and was originally a manufacturing plant. Based on the column and beam sizes it was intended for very heavy use. Given the age there would not have been any explicit LFRS that would comply with current codes. This is in a relatively high seismic area.

Here's the catch. There are photos and documents that clearly show that this was originally a 4 story building and for whatever reason the top floor was removed at some point. Based on the photos the top floor appeared to be less heavy duty, still the same beam and slab construction but smaller members and a shorter story height - maybe offices for the factory. The project only makes sense for the client to purchase if they can add back the 4th floor. My first opinion was that they'd have to do a seismic upgrade but after some cajoling from them I'm considering if there's a valid argument to be made to the local bldg dept that we are not increasing the seismic demands. The EBC states that when evaluating increased demand in lateral loads "For purposes of this exception, comparisons of demand-capacity ratios and calculation of design lateral loads, forces and capacities shall account for the cumulative effects of additions and alterations since original construction." Usually this would happen in the reverse direction but in this case the cumulative alterations have reduced the lateral demands and we'd only be restoring them. Either way I've told them that they need to get the ahj to opine, it's too atypical for me to guess how they'd treat it. But looking for any opinions here on how reasonable this argument sounds to an engineers ears.

 

[hardbutmild]

Design life isn’t reached because codes change.

I'm confused, could you explain this?

 

[MJB315]

So, I wouldn't "ah, shucks" this one.

Three things:
1. AHJ Questions
2. Risk
3. Mitigation

AHJ Questions

If I was the AHJ, I'd ask you three questions (in the USA, based on IBC/ASCE approach):

1. Is the fourth floor classified as an "addition?" Yes, it's an extension or increase in floor area, number of stories, or height of a building or structure per IBEC-202.

2. Is the addition structurally independent of the existing structure? No, clearly not if it's vertical addition making use of the existing building below in someway.

Therefore, per IEBC 1103.2 the existing structure and its addition acting together as a single structure shall meet the requirements of 1609 and 1613 of the IBC using full seismic forces. So, now the final question is:

3. Please document how the existing structure and its addition acting together as a single structure shall meet the requirements of 1609 and 1613 of the IBC using full seismic forces. Link.

If you can do that and the approach makes sense to me, go for it! If you can't do that, you can't plop on the fourth floor. Sorry, not sorry!

Risk

I'm looking at one of the OP's comments on here, and I'm not sure I agree with this interpretation of risk:

To come full circle. I see both sides of the argument, which is why I was questioning whether I should even be presenting this to the AHJ for their opinion, i.e. should I be so against this philosophically that I don't want to do it even if they OK it. But it looks like I'm definitely nowhere near as strident as most on this one. There is clearly a valid, and code recognized, acceptance of risks with existing (and new) buildings and that certain levels of alteration don't warrant imposing significant upgrades. It is not nearly so black and white as thinking that if the story is restored the building will clearly fail, or even if it did that it wouldn't have anyway. Under a modern analysis would this work on paper? No definitely not, but it also doesn't work on paper as is and we accept that risk. Where the exact cutoff should be seems like less of an engineering decision than a society/financial one, or a mix of the two. So in this case I'm looking at the letter of the code and it seems to comply.

Society doesn't happily accept risk - not really and not if it can help it. In fact, as soon as an Owner chooses to alter their structure in a significant way, the IEBC requires them to remove risk from the building and align it closer to modern standards. Sure, you can't re-detail the rebar in the concrete. But you can add systems. You can do things. Sometime, the code forces you (any unreinforced masonry parapets on your building, perhaps?)

Also, it's you that accepts a whole lot of that risk. Just remember that. There will be one name on the permit set of drawings and that one person will be, well, the first person called out if the situation turns tragic. No one is perfect and you need to be a faithful agent for your client - but you also need to meet the standard of care of the general engineering community and, four-times-out-of-five, I feel like "show me the numbers" crowd will demand to understand how you justified cramming a multi-family residential project into a century old manufacturing facility with, a, belt-buckle-grab style of analysis and an sermon on societal risk. That's not the side of the line to be on, in my humble opinion. The "Under a modern analysis would this work on paper? No definitely not" assessment doesn't really meet the engineering standard of care either.

Mitigation

So, not knowing the details of the project, I'd say that if the client was looking to take on a former manufacturing facility, and alter and add onto it, they will also [begrudgingly] take on adding in a few reinforced concrete shear walls to lock their new purchase together with a coherent lateral system - especially if the structure is in a high-seismic area. Sure, it'll cost money. No, one one wants to pay money for shear walls. No, this won't be the first or last time it's required. Yes, you can design it and they can price it up and the project can move forward. Yes, the AHJ will want to come out and inspect it. Yes, that's all how it's supposed to work.

Now, having knowledge that there was a fourth floor in the past may make it more likely that the gravity system is capable of supporting the fourth floor. So that's good, but still - need some numbers. (What happened to that upper floor - did it burn down? Who takes away a story after construction, by the way?)

Many of us have been in a situation like this (and some of us have assisted AHJs reviewing projects like this)and my recommendation is to plan to reinforce the lateral force resisting system using a rational, code-supported approach. Every building carries some risk but society will look more favorable on you and your client if you do.

 

[MJB315]

Also, I'd like to pull on this thread a bit more too:

Quote above:
"I'm definitely not in favor of yearly assessments for buildings. If this is multiple choice and I have to pick one of your options then I'd choose the until it falls down one."

New York State (which operates differently, for some reason, than NYC) passed a law requiring periodic inspections of parking garages. Why? One simply fell down after years of neglect.

The result? Huge swaths of New York's parking garages being demolished (or shored awaiting repairs). Now, is that a good thing?

[ul]
[li]For some, it's a disaster. These owners did not anticipate the cost of needing to repair or replace their parking garages.[/li]
[li]For others, it's a no-brainer. Many owners have been dutifully maintaining their garages so their garage assessments passed without much trouble.[/li]
[li]For society, it's also a no-brainer - tear down unsafe structures before they harm the living.[/li]
[/ul]

You're also seeing this unfold in Florida. Buildings beyond a certain age are requiring recertifications and, unsurprisingly, are failing.

Now, is that a good thing?

[ul]
[li]For some, it's a disaster. Many families cannot afford the cost to repair their buildings and are loosing their homes.[/li]
[li]For others, it's a no-brainer. Why live in a several-million-pound house-of-cards with a ticking clock?[/li]
[li]For society, it's also a no-brainer. Are we protecting the public against foreseeable hazards (either natural, or man-made) or aren't we?[/li]
[/ul]

So while I suspect there are many different viewpoints on the "inspect and recertify existing structure" question - I believe that we should.

Physics is physics and hazards are hazards - whether we care to look at it or not.