Navigating the Complexities of Historic Structures

[MagicFarmer]

Good afternoon,

I have found myself in yet another interesting place in the world of structural design. I recently have been assigned the superstructure replacement and substructure re-rehabilitation of a historic structure. I have been reading all of the associated documentation and have come up against something I have never encountered before... the Standards and Guidelines for Conservation of Historic Places documentation.

The condition of the superstructure merits total replacement. The structure has been identified as a cultural resource, which means that, among many other things, "Where character-defining elements are too severely deteriorated to repair, and where sufficient physical evidence exists, replace them with new elements that match the forms, materials, and detailing of sound versions of the same elements." I have also been introduced to the concept of "Context Sensitive Design..."

As such, the national authority has identified many features which they wish to retain, and in doing so, requires contravention of certain code clauses.

I have been told that we are dealing with an "Informed Client," and that, if these deficiencies are discussed and detailed in a report, that the structure can be upgraded, yet not totally, while still preserving the national heritage value and context of the structure. Arguments being made to bolster this approach include the fact that the new structure will be safer than before, or that the old, deficient structure, has performed adequately for over a century. Some of these arguments I would be more comfortable with, if I were looking at a minor renovation, and not a replacement. The terms "replacement-in-kind" (I have also seen replacement-of-kind) keep appearing.

Has anyone worked on historical structures before? This, at first glance, seems to be littered with red flags. I am unsure how I am supposed to stamp something that lacks total code compliance, simply to satisfy a national standard/guideline. I cannot see a defense, in front of a disciplinary board, where a directive from an "informed client" would hold up.

I am just starting to dig into the complexities of this project but wanted to start a dialogue and see what others have experienced in this realm.

As always, thank you in advance.
-MF

 

[P205]

Have you read through Commentary L (User's guide - NBC 2010 Structural Commentaries)? It gives information for meeting only up to a certain level of current code forces/loads depending on if there is or isn't a change in use and occupancy.

See table below. This could be used to defend your stance that, the design of any upgrade cannot use the loads and material standards from when the original building was built. But if there is no change in use or occupancy, then you can use the loads and material standards from the time it was built to evaluate the structure.

 

[TLHS]

There are no national codes in Canada, to my knowledge, that have codified requirements for existing structures. There are guidelines (Appendix L), and municipalities or other permitting authorities may have requirements, but you're firmly in 'Engineering Judgement' territory here.

Maintaining existing items that are not code compliant is not in contravention of building code. However, that doesn't mean that you aren't expected to follow good industry practices as another competent engineer would... Those best practices would definitely be guided by code.

I would definitely suggest reviewing Appendix L, but I'd look at the newest version, not the 2010 version. The latest one has some significant revisions to it.

This is also a situation where looking at the intent statements in the code is important. If you think you're compromising on something where there are life safety implications in the intent statements, are there ways you can mitigate the problem with alternate solutions?

Whether something is replacement in kind, and can reasonably treated as maintenance of an existing condition, is often a judgement call.

Are you changing occupancies or usage? If you aren't, you end up being in a logical conflict if you say that items must be brought to current code, but that decision makes upgrade impossible, so instead no work is done. The solution is worse than the problem, because the bad existing condition remains instead of a mitigated improved condition.

None of this is to say that there aren't unsafe situations where you should put your foot down, or that you shouldn't hold work to a reasonable standard. Public safety is primary. However, it is definitely not a black and white question of code compliance.

It's also likely worth reading through the International Existing Building Code. It's got some opinions on how to approach these problems as well.

 

[MagicFarmer]

Thank you both for the comments. I feel as though I should clarify that this is in fact a bridge, and that it would fall under CSA S6.

 

[BridgeSmith]

What aspects of the replacement bridge are you having difficulty making code compliant and visually similar to the existing? Generally, when faced with a situation like this, we've been able to keep the same shape, dimensions, and appearance, while using newer, stronger materials. Yeah, it definitely isn't the most economical solution, but most aspects can be made acceptable.

That said, the one place where the historic preservation people have had to give a little on the aesthetics (and they have) in the interest of public safety, is the traffic railings. There's no getting around that cars are bigger, heavier and faster than they used to be. Making a railing system of a similar size to the existing, that meets the current standards for impact resistance, is often not feasible without the use of extraordinarily expensive materials.

 

[TLHS]

Oh yeah, bridges are a different thing. There are requirements for evaluation of existing bridges in the code. I don't know what the federal enabling legislation says, but the provincial ones I've looked basically say that you need to follow the code, with some specific outs for specific regulations written by the provincial MoT.

So the authority you're working for might not have the authority to deviate.

 

[dik]

This is where Ontario, Canada is a head and shoulders above... They have a Part 11 to their code, which substantially addresses the issue of renovation of buildings in excess of 5 years old and code compliance. It removes a substantial part of the AHJs discretionary and often capricious actions. Essentially, if the structure after renovation is 'better than what it was', the renovation is OK (paraphrased).

Dik

 

[dik]

TLHS: The Ontario Building Code has specific requirements.

Dik

 

[BridgeEI]

We’ve done this quite a bit before and the majority of the time we end up designing what we need, then add a facade that mimics the original elements.

If the feds are involved, then there’s a 4f document that goes through all of the situations of replacing, rehabbing, etc and the justification. Normally it’s not in the best interest of the DOT to keep a historic structure since it’s not as economical.

Are you preparing the plans for this structural or are you having to do the justification for the 4f document?

 

[MagicFarmer]

A lot of the code compliance issues stem from the TAC manual and are geometric issues, where arguments are being made for reduced speed limits and focusing on the fact that this is a 'low volume road,' which is where a case is being made by the company's roads safety officer that the road and bridge has been operating without undue risk to the public for a long time, and some of the changes (reduced speeds and a curb)are an improvement to safety and therefor justifiable if the feds want to maintain the historical value of the bridge.

There are also areas that I am now looking into and educating myself when it comes to redundancy.

As I will be the "checker" stamp on this structure, I am reaching out to get a feel for others' experiences when it comes to "using good engineering judgement" when it comes to making accommodations for a unique situation/structure. Obviously public safety is paramount. There is no question there. I was just hoping to hear about anyone's experience in this area that requires, perhaps, a little more thought and fineness than simply grinding through the codes letter for letter. The area where 'better than it was' over laps with 'the best it could be,' with historical value/engineering judgement being the two moderators. I have to find my own answer and comfort here, just looking for any wisdom from those that have walked the path before me.

Thanks again,
- MF

 

[BridgeSmith]

Perhaps this will help with your comfort level: Since you are not the governing authority, your responsibility is only to provide the highest safety possible within the aesthetic and economic limits agreed upon by that governing authority and the historic preservation people. Any solutions you can propose that can integrate and enhance those generally competing objectives, is good engineering practice.

 

[MagicFarmer]

@HotRod10, thank you. That helps a lot. That might be the best piece of advice I've been given to date in my career. Much appreciated.

-MF

 

[TLHS]

Honestly, I think existing experience at a location is a better indication of performance than a set of generalized road geometric requirements.