How is the structural integrity of very old buildings, bridges, etc. tested to ensure suitable use by people, cars, etc?
In: Engineering
Structural engineer with experience in retrofitting and historic preservation: For low level stuff like houses you normally don’t outside of very unusual jurisdictions. These are basically grandfathered in and providing you don’t make any significant renovations or the building doesn’t get damaged they’re allowed to continue in use on the basis they’ve been there for a loooong time, even if they wouldn’t meet current code.
For bigger things – train stations, bridges, large multi story buildings: Almost all of the things like this built since the industrial revolution were engineered to an extent and often drawings are available. For the most part they keep getting used as-is until there’s a change in use that warrants an engineering assessment. In the US this is if there’s more than a 5% increase in gravity loads or 10% increase in lateral loads (code proscribed). So say you want to convert an old apartment building into office space – the code loads are higher so you’ll need to do an assessment. There are a fair number of codes and industry documents around that give guidance on how to approach these evaluations in a manner consistent with current practice.
To do the analysis you find whatever existing drawings you can – these can be surprisingly detailed and will even give material strengths. If there are gaps in the drawings, missing drawings or there have been renovations since you do field verification of the structure, including material testing to determine the strength of the brick, concrete, steel, iron, timber etc that makes it up. There is also a decent volume of information available to help with this – eg – for older steel in the US you can get a publication that lists the exact section sizes each major mill produced (they used to all vary) based on the construction year, along with the documented strength of steel used – you can go look at a beam until you find a mill mark (they’re frequent) then take the construction year and you’ll be able to looo up fairly precise member properties. Then you take the information and analyze it and determine the code load rating, and then strengthen if necessary.
A lot of the time buildings built during this era have a fair amount of spare capacity as the loads and designs used were conservative, but once in a while you find something that’s an edge case.
Bridges: If the use changes – you want to permit bigger trucks, or it gets damaged/modified you do the same, generally with a lot of field investigation as the structure is often exposed, which means it’s more subject to corrosion or other damage.
For older stuff – pre industrial era – you’re normally looking at some combination of masonry and timber only. These are well understood materials. Original drawings are rare, but most of the time the buildings are structurally fairly simple (things like churches and other monumental buildings aside) so can be investigated fairly easy making a few informed assumptions with knowledge of the construction practices of the time. These types of construction though are normally inherently very robust unless you’re facing earthquakes and it’s mostly just floors that need review unless you’re doing something massive to the building.
TL;DR – you (normally) don’t unless you’re changing something or carrying out structural repairs, and then you have an engineer investigate and analyze it.
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