Bridge Bash with a Difference - M20 in Kent, 27 Aug 2016

[whole diary]

[Gordon the Blue Engine]

The piece that fell was a separately-made beam that was dropped in place on bearing ledges (at the "clean breaks" that were clearly visible e.g. on Street View). Thus (as noted earlier) it was just pushed off its ledge and fell.
 

Stuving is right - good idea looking at Google Earth street view.  Although the bridge is cantilevered above the eastbound carriageway over the pier on the north side of the motorway, the beam over the westbound carriageway does appear to be a simple beam supported at each end (ie at one end by the cantilevered beam above the central reservation, and at the other by the pier behind the south side hard shoulder.  Which of course explains why it was relatively easy for it to be knocked off following an impact.

Whether the bridge was too low or the lorry too high remains an unknown.

[ellendune]

The bridge sloped across the road and may have been a little lower over the hard shoulder (though it's not supposed to be and probably wasn't when it was new either).

That does happen sometimes, but the section over the hard shoulder still should be the minimum height.

But even if a roadful of structures are all supposed to be a single regulation height, you'll find one is the lowest and that's the one that gets hit first.

That is my experience from 35 years ago too.  The bridge was rebuilt to the same height as the (vehicle's) insurers would not pay for improvement. The second time the Highway Authority stumped up the extra cost of raising it.  

Currently the gantry and that footbridge would have higher minimum clearances than solid bridges, specifically to avoid the additional hazard of bits falling on the road. I'm not sure that was the case when the footbridge was designed.

I agree, the gantry may have been high enough as it was built much later, possibly after the 2005 standards was published.

From their signage, the contractors were working on the parapets, so presumably not the bridge itself.

Yes as it was said further up the thread ...

Is it a fairly new foot bridge? looked to have construction fencing on it

I was also wondering about it being a new footbridge for the same reason but reading about it online they are/were working on it to make the barriers either side higher to stop people being able to throw things over onto the motorway.. So it can't be that new then

This bridge appears to consist of 2 cantilevered sections which meet in the middle: from the pictures the intact section looks undamaged so presumably, from the structural point of view, could remain in place for the time being.  However, it may be distracting for motorists approaching it!

It may look undamaged, but I would be surprised if that was the case. At the very least the bearings will need a check.  I suggest they will probably lift it off and if it is undamaged put it back later.  This might allow them to put it back slightly higher!

The low loader with the digger on it was presumably travelling at quite low speed as it was on the hard shoulder, and the digger does not seem to have moved much on the trailer.  The impact was close to the strongest part of the bridge.  I’m just surprised that the bridge came of worst in this impact – I wonder whether the quality of construction may turn out to be an issue.

The bridge may be strongest in vertical bending at that point, but the failure mode was horizontal shear at the bearings.  I would be very surprised if there was a quality of construction issue.  

[ChrisB]

One doesn't know what the diggers arm was doing/angle at which it collided with the bridge.....

[stuving]

It's a confusing thing to look at in the Street View stills, isn't it? I find the eye insists that it is built on the skew, where really it's almost at right angles. It's the false perspective due to the taper and slope that does it.

If you look up at it, you can see a lip running along the sides of the span. I suspect that the digger arm was pushed up and caught under that, and perhaps the whole machine rocked backwards but was well tied to the trailer. That would give a sustained force rather than an impact, and one that lifted the bridge off its bearing as well as propelling it forwards.

There must have been some positive location mechanism at the bearing, though only to prevent creep due to vibration, thermal cycling, etc. Could that have been a pin and socket? Any lifting would overcome that at once. Once the span had got going as fast as the (now decelerated) lorry, the digger would rock forwards and pitch it onto the poor little dumper truck in front of it, and other things.

As to reuse, I think we were saying the standing cantilever looks OK. As to the fallen span, wouldn't that impact crack it? It may have been a soft-ish landing on the lorries, but my recollection is that concrete is really bad at impact resistance. And I imagine a big crack, even if it doesn't weaken it initially, is a BAD THING.

[ellendune]

This is the sort of thing I would expect for the bridge bearing.  I am assuming that the expansion is taken up in the central bearing.

(for more information see http://www.bridgebearings.org/product/disc-bridge-bearing.html

Yes it does have a shear pin but as stuving suggested, if the excavator arm lifted the beam, it would soon lift the pin out. 

[ellendune]

I see from the bbc that they have inspected the other half, but have put sensors on it to monitor it.  They are going to leave it in place for now. 

[stuving]

For completeness: when the span fell it broke its back over the dumper truck it landed on. So there's no question of just popping it back in place.

I wonder whether it will ever be replaced. It was part of a footpath along what was a lane, but the next bridge either side is not far away, so it only serves a few houses on either side. There's no obvious destination apart from those houses, so it's not clear why that right of way had to be preserved.

[ellendune]

But Highways England will probably not be concerned about the cost because normally the Hauliers insurance will have to pay for the reinstatement of the bridge.

[FarWestJohn]

When the footbridge across the dual carriageway at Plymouth station got knocked down in a similar way it took years and years before it was replaced.

[IndustryInsider]

More Bank Holiday 'chaos and misery' on the roads with a crash closing the M5 for hours earlier today:

http://www.mirror.co.uk/news/uk-news/m5-closed-both-directions-after-8728509#rlabs=2%20rt$category%20p$1

[TonyK]

One doesn't know what the diggers arm was doing/angle at which it collided with the bridge.....

It should be securely fastened to the flat bed of the transporting lorry, and in the position recommended by the  manufacturer. That is made clear in Chapter 13 of the DfT^» (Department for Transport - about) Code of Practice for the Safety of Loads on Vehicles.

13.7 All moveable assemblies such as jibs, brackets, booms and cabs etc. must be left in the position recommended for transportation by the manufacturer and must be secured to prevent movement relative to the main body of the machine.

That also says in another paragraph:

13.4 High loads may endanger bridges etc. over roads, so when these are carried it is essential that the driver know the exact height of the load, and the width of the load at that height.

This is a Code of Practice rather than Act of Parliament, but driver training will always draw attention to it, and the Courts will find it persuasive.

Transporting of heavy plant is normally done using vehicles designed for the job, and by drivers with specific training in the work. Sometimes it is done by people without such training, making export runs of vehicles obtained by methods other than the traditional offer-acceptance-consideration route.

[ellendune]

After all the Highway Code is only a code of Practice and failing to observe that does not go down well in Court in the event of an accident. 

[TonyK]

After all the Highway Code is only a code of Practice and failing to observe that does not go down well in Court in the event of an accident. 

True only in part, ellendunne. The Highway Code tells you, in the introduction:

Many of the rules in the Code are legal requirements, and if you disobey these rules you are committing a criminal offence.

In such cases, the wording signifies a mandatory requirement by saying "MUST / MUST NOT" and having a reference to the chapter and verse of the law. The other bits, where you are told you "SHOULD / SHOULD NOT / MAY / MAY NOT" are not backed up directly by specific legislation. That will not stop the Court from having regard to them when deciding if a driver's conduct amounted to careless, reckless, or dangerous driving. When the code proves to be inadequate, the law is normally strengthened - an obvious example is the prohibition of use of mobile phones when driving.

Ignorance of the Code is not going to get you far as a defence. All new drivers are effectively tested on it in the driving theory test, and the presumption is that they will keep themselves appraised of any changes, as any reasonably competent lay driver would. The man on the Clapham omnibus is the yardstick, not the man driving it.

[ellendune]

The Grauniad reports that the M20 will be closed next weekend so they can remove the remainder of the footbridge for safety reasons.  There is a 50 mph speed limit until then.

Highway England’s chief highway engineer, Catherine Brookes, said: “Safety remains Highways England’s top priority. We worked hard last weekend to reopen the M20 as soon as it was safe to do so and I would like to thank drivers for their patience while we did this. The remaining section of the bridge has been assessed and is safe for traffic to pass underneath with a temporary 50mph speed limit.

“We naturally need to remove it under safe controlled conditions this weekend. We will use the closures to carry out as much work as possible, including barrier repairs, resurfacing and litter-picking. We will start planning the replacement in due course.”

[Bmblbzzz]

For completeness: when the span fell it broke its back over the dumper truck it landed on. So there's no question of just popping it back in place.

I wonder whether it will ever be replaced. It was part of a footpath along what was a lane, but the next bridge either side is not far away, so it only serves a few houses on either side. There's no obvious destination apart from those houses, so it's not clear why that right of way had to be preserved.

When a motorway interrupts another right of way, whether that's a road or a footpath, there is a presumption that the right of way be maintained, whereas when any other road interrupts another right of way, there is no such presumption. This is because the road is itself a right of way, whereas a motorway is not, being a Special Road. AIUI^▸ (as I understand it).