How is the structural integrity of very old buildings, bridges, etc. tested to ensure suitable use by people, cars, etc?
In: Engineering
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Civil engineer here. For old buildings, if they have the architectural plans then they can usually derive all the mathematical info they need and account for decayed members. If they don’t, they’ll try to generate one and usually schedule renovation. That’s mainly for old buildings like houses. Bridges can be tested for a variety of factors – they use a lot of deflection analysis, so like seeing how much a bridge will sag under a given load, or how much it’s cables (assuming suspension bridge) extend under that load. A lot of visual analysis occurs as well – cracks, rusty rebar, split timbers can all indicate the current state of the structure and can be accounted for in projections of how long it will last based on modeling. Other methods can also be used to look for wear *inside* a structure, like using electrical pulses or vibration feedback on concrete. Also, because building codes are so meticulous and regulated, a good inspector can actually refer to old building manuals and practically determine from those what kind of design life a structure was built for. It’s a fine-tuned skill and one of the aspects of civil engineering that is highly valued because of the experience it requires.
My timber design professor was such a knowledgeable guy that he was brought in to inspect why a frame holding a very expensive painting fell, and it didn’t even take him long to figure it out.
Honestly it’s pretty cool. Good question!
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The most basic is looking for deterioration, looking for where corrosion has occurred, bricks fallen mortar cracked, and can get a general idea of the condition. Tension cables can be tested for how much tension they’re under, and can have electrical current run through to check for corrosion.
On the back of this, are they periodically checked? A bridge in Nottingham has been declared unsafe as they found rust in the support s or something, I’d like to know how and why they found out.
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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.
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.
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.
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.
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