How is the structural integrity of very old buildings, bridges, etc. tested to ensure suitable use by people, cars, etc?
In: Engineering
What about huge buildings like the John Hancock in Chicago? What is the lifespan of a place like that?
I used to inspect bridges!
Short answer: We look for cracks.
Cracks, and “efflorescence,” which looks like concrete icicles or milk stains under concrete structures. (rain + chemical reaction = salt leaking out of concrete: makes it weaker)
The *underwater* portion is what made my team pretty unique. We would get a guy in scuba gear and have him check the underwater portion of the pilings (pile-ings)/pillars. The water (and anything floating in it) wears on those piles, so we keep an eye on that wear.
The scuba bro sent back information via built in headset radio and his buddy in a small boat (me!) tends his line and takes notes on his findings. Hope that helps 🙂
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.
It depends on your country’s level of development. In the US a civil engineer will calculate load density maximums using an algorithmic formula based on the structures building materials and overall design. They will also use a tool called a dinglemomiter to ‘ding’ the structures main support beams. The dinglemomiter will read out important data that allows them to estimate the core tensile strength of the supporting structures material. In the end it is all an educated guess, so they typically leave tons of room for error.
In other places around the world, they pick the most gullible person nearby to get in a heavy truck and test it the old fashion way.
If you read the first paragraph and beleived it, then I’ve got the keys for you.
Survivor bias: If they are still around then you can expect them to be overbuilt. All the crappy stuff fell down decades ago.
“Okay Joe bring one more elephant up here” crash. “Yup that bridge was only 4 elephants strong”.
When I visited Hagia Sophia in Istanbul the guides showed us a section of wall that had glass slats mortared in. They explained that the building’s stewards would check the slats after any earthquakes to see which walls were shifted and where to make repairs based on which of the glass slats broke. The Hagia Sophia was built in 537. When you consider the cost of glass in relative prices for the time, this safety measure was a significant investment.
There was a Pbs special on it, discussing modern approaches for stabilizing the building. If you can find it they talk about all the modern engineering improvements they’re trying to bring in to the structure without compromising its antiquity too much.
My job deals with bridges. We instrument the bridge to record it’s behavior then we drive trucks over the bridge with different weights. Then the engineers look at the data and can decide on a load rating for the bridge and it’s health. We’ll do this for any bridge and also long term monitoring on some structures to watch for degradation.
I’m a Civil Engineer too from Brazil, let me just mention something.
It’s very usual here to have old buildings which you haven’t got the structural plans to, and you can’t guarantee the original builder followed the codes of the time (they don’t even follow it now.)
So one technique that’s particularly interesting to test the strength of the concrete is that you can cut a cilinder out of a few columns (there are places in the column that you can do this safely) and test that cilinder’s strength on a concrete press. This tells you how strong the concrete is, and you can close the hole with stronger concrete later.
This is called destructive testing, and there are many non-destructive methods available, but this is a fairly straightforward as many labs would have a concrete press.
Of course this doesn’t cover the steel reinforcement, but as was already said that can be evaluated by other techniques.
Buildings are designed to signal you that it will be on the end side of its usefulness. For example in concrete, The best one to see it is by crack. Engineers will observe it by how the cracks looks like. Cracks doesn’t mean painting cracks though, but on a different kind. There are various kinds of it and it will say if the building is failing and we have checklist for that as ASCE dictates(or whatever governing codes on your area).
A 12inch of ruler is an example. If you bend it too much in the middle, notice the crack. That may how it looks like in the reality. Then try to break the first one inch of it, that’s what we call shear failure. It’s hard to bend the ruler if you use the deeper side than the thinner side right? We call it stiffness and its important in deflection.
There’s a local code in your area that will tell you when are you going to need an evaluation for this. If its historical building, retrofitting is a hundred percent needed.
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