[keithpeter]

“We had inspections before the work began, and found mortar that had crumbled, but the stones hadn’t moved. It was truly overengineered. It is so strong.”

UK: I use bridges and viaducts of this vintage everyday - train bridges, canal towpaths, road bridges. They over-engineered because of the fear of failure.

[stevekemp]

There is something to what you say, but it is also an example of survivor bias.

Yes you use things that haven't failed, because the ones that have failed have failed. We've always been prone to over-engineering, because of mistakes like this:

http://en.wikipedia.org/wiki/Tay_Bridge_disaster

[bcgraham]

Not sure about UK, but in the US, they're occasionally overengineered because materials suppliers have a lot of say in determining the safety factor (think smoky rooms with bags of cash).

[nickff]

Do you have a citation for this? I studied mechanical engineering, and learned that there are tables of standard safety factor for most types of engineering projects (including bridges). The material suppliers can under-specify their products, in the hopes that more will be used, but this generally does not occur, as the engineers will select other suppliers with better specifications, or the under-speccing may result in design changes to eliminate that material. Before there were standardized safety factor tables, the project engineers determined appropriate safety factors, and were thereafter responsible for the results of their decisions (as they still are).

[crististm]

"In one famous case, a manufacturer of wire rope sold inferior material to the bridge company. The shady contractor, J. Lloyd Haigh, escaped prosecution. But the bad wire he sold is still in the bridge, as it couldn’t be removed once it was worked into the cables. Washington Roebling compensated for its presence, ensuring the inferior material wouldn’t affect the strength of the bridge."

http://history1800s.about.com/od/bridgebuilding/a/brooklynbr...

[JoeAltmaier]

I know a local man, fired as an inspector, because at a concrete-culvert installation the crane dropped the casing. It broke, revealing almost no rebar in the thing. He stopped the installation and started an investigation. Result: lost his job. Everybody but him was in the deal apparently.