With the collapse of a few bridges oop north,how does one o about testing a bridge,with usual engineering stuff, loads and forces are usually applied at various points, with a bridge, the load is not constant,and posibly a significant part is under water.

So hows it done.

deja vu all over again? 😉

It'll be tested in scale in a lab, but ultimately you have to rely on theory and FOS. I've not seen any of the bridge failures, I presume you're talking about with the flooding? I suspect most bridges are designed with only a certain level of flooding in mind and some of the more recent deluges have simply been outside the spec of the bridge – they're not designed to resist the sorts of forces they're being put under and their foundations are not going to like being soaked for days then heavily loaded.

and their foundations are not going to like being soaked for days then heavily loaded.

one might suppose the foundations of a bridge would be designed to be soaked for their entire operating life

unless a bridge has pre-established marker points there'll be nothing useful to measure and the only testing I can think of would be to explore the foundations for signs of scouring. I suppose you could fix a laser reflector in the middle and then gently reverse a full laden artic onto the bridge (close to the maximum rated load) and measure the deflection…

In a previous railway bridge collapse it was found that the excessive speed of the train may have caused the bridge to fail,due to the creep of the train pushing onto the rails and moving the structure forward.

and moving the structure forward.

backward unless it was braking

>their foundations are not going to like being soaked for days

River bridge design FAIL

sfb/allthepies – the foundations of the bridge side supports do not have to be soaked for life, only any piers in the middle of the span if hey exist. I'm considering the sort of small road bridge you get where large embankments generally sit mostly out of the water, either side of a river, but in times of flooding are completely submerged. Short duration submerging isn't an issue, but long duration I suppose could weaken the structure unless the foundations can be driven into bedrock.

For example, look at this:

It is usually only about 6ft wide and maybe 2m lower than the current water height, normally the concrete is not even close to the water and there's a good few feet of land either side of the river before the bridge supports. The water was ~1ft from the underside of the bridge 2 days ago, causing massive loading on the back face of the supports AND with both of them fully submerged. I was fairly quick to get off it.

And also….

backward unless it was braking

Nope, not necessarily, it depends on the dynamic loading of the bridge and how the "impact" of the moving vehicle propagates a wave of momentum along the bridge, it's not quite as simple as the reaction forces at the wheels in this case.

Ohhh, there's loads of ways of testing structures in situ. I've done some of the simpler ones myself, but I'd imagine there's a lot of examining the structure for movement, cracks damage etc.
Visual stuff backed up by, ohh I dunno, X-rays, density testing, taking core samples, surveying, all sorts of stuff like that.

CK – see where you're coming from 🙂

Concrete is quite happy to be kept wet as well. There's additives from people like SIKA that take care of that.

Concrete is quite happy to be kept wet as well. There's additives from people like SIKA that take care of that.

It's not the concrete I'd be worried about, its what's supporting it both vertically and cross-ways. If they've just built the bridge onto a floating concrete bed then high levels of flow and saturation might cause the surrounding materials to be washed out, but even more simply – was the bridge designed to take full-height water loading for days on end with saturated supporting soils? You'd hope so, but there must be some failures even if theory was followed.

*edit – just a side note, I'm not a civil eng so I'm going off engineering intuition rather than specific knowledge here. As PP says, there's loads of testing mechanisms once a bridge is in place to see if it's shifting or being over-stressed, but they're not realtime monitoring usually and I'm not sure how any of that falls into the scope of the bridge failures mentioned.

Arches have been arround in construction since before christians would like to believe the wrold even existed, as a resultt he modes of failiure are fairly well known.

That one they keep showing on the news looks like the foundations arround the central/southern supourt had been washed out and the bridge was collapseing around that point. As far as testing goes, I'm guessing its a case of looking at the foundations and checking for any movement in the bridge (presumably they get surveyed on a regular basis?)

I think a few of the bridge failures were due to the flow rate going through the bridge, which undermined support piers and collapsed the bridge. Nowt you can do with old bridges if the river bed has been scoured away. They're thinking of putting caged rocks in around certain bridge piers, to reduce the risk.

Visual stuff backed up by, ohh I dunno, X-rays, density testing, taking core samples, surveying, all sorts of stuff like that.

A nice handwaving explanation Peter 🙂 Is it a good idea to test integrity by drilling holes something ? I'd be worried about setting off Xrays strong enough to penetrate metres of masonry anywhere near living creatures…

Is it a good idea to test integrity by drilling holes something ?

Well it should be non-destructive and have little effect unless the bridge is teetering on destruction already, in which case you'd like to know 😀

Re the xray testing – I suspect he's thinking about the metalwork in rather smaller quantities and thicknesses lol.