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

[whole diary]

[SandTEngineer]

Well for a change not a railway one........
http://www.bbc.co.uk/news/uk-england-kent-37204050

[IndustryInsider]

Crickey!  Lucky not to have caused more injuries or deaths.  Bet there was absolute havoc on the roads as a result.

[ellendune]

Except the Lorry in the picture is not the offender. That was a lorry carrying an excavator. That will be a very expensive insurance claim.  I think the driver may well have some explaining to do to Kent police as well. 

[stuving]

Except the Lorry in the picture is not the offender. That was a lorry carrying an excavator. That will be a very expensive insurance claim.  I think the driver may well have some explaining to do to Kent police as well. 

The white lorry in lane 1 was innocent but the artic carrying an excavator, beside it in the hard shoulder (and visible in some shots in that report), looks pretty guilty. What is surprising is that it appears to have suffered very little shock damage to trailer or load in the collision. It's as if it stopped and let the bridge fall on it, having hit that hard enough to bring it down (the BBC^» (British Broadcasting Corporation - home page) overhead pictures show that.)

I guess the beam that fell is pretty light (at least for reinforced concrete). It was only a footbridge, and tapered in depth from the middle of the road to the edge, though only to visually match the deeper section of the other half. It was pushed off its bearing at the landward end, and as it fell it slipped off its other bearing on the tip of the cantilever from the other side. So it looks as if the concern that that standing cantilever might fall was rather overdone.

Given its taper and slope across the road, it's very hard to judge in Street View what the clearance might be have been. There's no sign of any sign warning of it. Curiously, the hard shoulder on the other side is not being used. Before that it's taken over to extend a slip road, which runs out just before the bridge and the hard shoulder resumes just after it. So that looks like just a coincidence.

[LiskeardRich]

No warning signs of a height restriction that I can see on street view. Is it one of these smart motorways allowing hard shoulder running? The truck with excavator was in the hard shoulder.

[johoare]

I drive down that stretch of the M20 a fair amount and there is no smart motorway system there so if someone was  on the hard shoulder they should have been treating it as such. It does look like it happened just before the M26 turn off but again no reason for someone to have been in the hard shoulder really unless they had an issue with their vehicle.

[ellendune]

No warning signs of a height restriction that I can see on street view.

No warning sign is required if the height of the bridge is over (IIRC^▸ (if I recall/remember/read correctly)) 5.5m (16ft 6in).

I regret that bridge bashes from excavators in transit are a regular feature.  The reason is usually that the arm was not secured properly and moved during transit.  When in transit, the arm must be chained down. Failure to do this is the problem.  This is the driver's responsibility.  Given how close this came to a fatality I hope the authorities treat it with the severity it deserves.  Only then will hauliers take this matter seriously. 

I remember when I worked up North a footbridge on the M62 had only been restored a few weeks when is was bashed again.  The footbridge there was the first bridge after a major junction. 

The truck with excavator was in the hard shoulder.

It is not clear to me whether the excavator on the hard shoulder was the one in question or not. One photo appears to show it to have not passed under the bridge, but it is difficult to see.  The comentary says there were tow on the lorry so perhaps it was the one in question. 

Is it one of these smart motorways allowing hard shoulder running?

I drive down that stretch of the M20 a fair amount and there is no smart motorway system there so if someone was  on the hard shoulder they should have been treating it as such. It does look like it happened just before the M26 turn off but again no reason for someone to have been in the hard shoulder really unless they had an issue with their vehicle.

The heights regulations apply equally to the hard shoulder so it should have made no difference. 

[LiskeardRich]

The bbc reported it was being driven on the hard shoulder.

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

[johoare]

The bbc reported it was being driven on the hard shoulder.

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

[Billhere]

The BBC^» (British Broadcasting Corporation - home page) news showed the bucket of the digger pressing up against the side of the bridge, and the damaged trailer of the 'innocent' artic underneath it having been scalped by the bridge as it passed under.

Most shots showed the bridge from almost head on because it was obviously the most convenient, but the 6pm news had a shot from 3/4 rear which showed the digger bucket as described.

[stuving]

...
No warning sign is required if the height of the bridge is over (IIRC^▸ (if I recall/remember/read correctly)) 5.5m (16ft 6in).
...

There is (of course) more to it than that - and not just because 5.5 m isn't 16'6".

The quoted section was intended to give guidance on whether to allow a road surface under a bridge to be raised by resurfacing, but it says a lot obout minimum heights. It  is from "Prevention of Strikes on Bridges over Highways - A Protocol for Highway Managers and Bridge Owners", from DfT^» (Department for Transport - about)/NR^» (Network Rail - home page)/CSS^▸ (Cascading Style Sheets).
 

A.3 Unsigned bridges with 16’-6” (5.03m) or more headroom
A.3.1. Bridge deck is designed or has been assessed as able to resist vehicle impact forces,
current at the time of the surfacing works, as advised/agreed by the bridge owner:
(a) if existing headroom is 16’-6” (5.03m) or more allow reduction to 16’-9” if sought.
(b) if existing headroom is between 16’-9” (5.1m) and 16’-6” (5.03m) then no reduction
is allowed and efforts should be made to increase it to 16’-9”.
A.3.2 Lightweight structures not capable of resisting current impact forces as advised/
agreed by the bridge owner:
(a) if existing headroom is greater than 17’-9” (5.41m) allow reduction to 17’-9” if sought.
(b) if existing headroom is between 17’-9” (5.41m) and 16’-6” (5.03m) then no reduction
is allowed and efforts should be made to increase it to 17’-9” or as high as possible
and at least to 16’-9” if lower than this.
A.3.3 Relevance of different clearances:
A.3.3.1 Chapter 6 of TD 27/05* specifies headrooms, including additional provision for
sag curve compensation (S), for new construction and maintenance as follows:

• 16'-6” (5.03m) + S is the standard minimum maintained headroom below which all
  bridges should be signed.
• 17'-5” (5.30m) + S is the standard design headroom over the accessible highway
  for new bridges designed to resist current impact forces and for those built since
  the earlier version of TD 27 came into force in 1996.
  
• 18'-9” (5.7m) + S is the standard design headroom over the accessible highway
  for new lightweight structures, ie, those not designed to resist impact forces, and
  for those built since the earlier version of TD 27 came into force in 1996.
  
• 17'-9'’ (5.41m) + S is the equivalent minimum maintained headroom for new
  lightweight structures and those built since the earlier version of TD 27 came
  into force in 1996.
  
• Design headroom is provided for structures at construction and allows for
  subsequent limited surfacing overlays for maintaining the highway beneath.
  
• Maintained headroom is that minimum which must not be reduced at any time
  and which will consequently determine, along with other local issues, the
  possibility of overlaying.
  

A.3.3.2 The CSS suggests that highway authorities should over time aim to make 16'-9”
(5.10m) the minimum maintained headroom for all bridges prior to signing due to
confusion in drivers’ minds that 16’-6” (5.03m) represents the safe vehicle height for
unsigned bridges rather than the actual minimum headroom.

I can't see any difference in the rules for motorways. I would think this bridge counts among "lightweight structures, ie, those not designed to resist impact forces".

I'm pretty sure TD 27/05 is no longer in force, having been replaced by something or other, though I imagine its content is still valid.

*TD 27/05 is: ‘Cross Sections and Headrooms’ in DMRB Volume 6 ‘Road Geometry’, Section 1, Part 2, (February 2005) London: TSO^▸ (The Stationary Office (now OPSI)) for Highways Agency

[ellendune]

...
No warning sign is required if the height of the bridge is over (IIRC^▸ (if I recall/remember/read correctly)) 5.5m (16ft 6in).
...

- and not just because 5.5 m isn't 16'6".

Whoops  sorry.  I was going on memory from 35 years ago when I last did bridge engineering. 

I'm pretty sure TD 27/05 is no longer in force, having been replaced by something or other, though I imagine its content is still valid.

*TD 27/05 is: ‘Cross Sections and Headrooms’ in DMRB Volume 6 ‘Road Geometry’, Section 1, Part 2, (February 2005) London: TSO^▸ (The Stationary Office (now OPSI)) for Highways Agency

I can confirm that TD27/05 (DMRB Volume 6, Section 1, Part 2) is still in force. Though I have not read it. 

[Gordon the Blue Engine]

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!

I’m surprised that the collapsed section failed so easily where it did.  The thickest and strongest section is above the vertical support where there is the maximum bending moment, and would have been designed accordingly.

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.

[LiskeardRich]

I'm starting to have doubts it's actually hit.....

There is an overhead gantry at the same height a few yards earlier. Surely the excavator would have hit that first?

Also the breaks in the bridge look to be clean breaks, I would think being hit would give a broken area not clean.

Reported no arrests made, would this not be procedure for causing such an incident unless they thought the driver wasn't at fault.

this photo shows the details of a possible construction company carrying out work. Was there a known weakness in the bridge already to be undergoing work?

[stuving]

I'm starting to have doubts it's actually hit.....

There is an overhead gantry at the same height a few yards earlier. Surely the excavator would have hit that first?

Also the breaks in the bridge look to be clean breaks, I would think being hit would give a broken area not clean.

Reported no arrests made, would this not be procedure for causing such an incident unless they thought the driver wasn't at fault.

this photo shows the details of a possible construction company carrying out work. Was there a known weakness in the bridge already to be undergoing work?

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.

The gantry is, presumably, at least the regulation height for such things over roads, and looks much more recent than the bridge. 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). 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.

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.

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