Weight limits on bridges

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DavidAll
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Weight limits on bridges

Post by DavidAll »

Hello all. This is my first post so please forgive me if it is in the wrong forum.

Network Rail and Surrey County Council are proposing to close for 5 weeks the bridge at Westhumble which has been surveyed as weaker than the current 7.5 tonne restriction. The bridge has cast iron troughs in the deck overlayed with concrete.
I believe that the only mandated sign other than 7.5T is 3T.
SCC are proposing to remove 100mm of the surface and install very substantial width restrictors at 2m with an inset of 450mm either side of the base.
They say that the bridge will then be safe at 7.5T.

I have asked for the calculations and if it shows that the weight capacity is greater than 3T then with a Temporary Traffic Notice/Order a greater weight limit than 3T could be posted and the road surface could be left intact.

With a 2m width restriction are there many vehicles that could get through that weigh more than say 4T?

Many thanks for your thoughts.
AndyB
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Re: Weight limits on bridges

Post by AndyB »

2m would rule out most Luton vans, which conventionally have a maximum gross weight of 3.5t so they can be driven on any Cat B licence.

On the other hand, a normal Mercedes Sprinter would fit if driven carefully.

All that said, I think the intention is to actively discourage use of the bridge.
DavidAll
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Re: Weight limits on bridges

Post by DavidAll »

Thanks AndyB.

Would a Luton have a width across the wheels greater than 2m? The actual width restrictors they are putting in are 2m at wheel level and then set back 450mm either side at body height in order to let ambulances through. So 2.9m at body height.
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jervi
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Re: Weight limits on bridges

Post by jervi »

While 3 and 7.5 are usual weights to sign, I think other intermediate weights can be signed if needed.
For example 7T here https://www.google.co.uk/maps/@50.81929 ... 312!8i6656
Ronnie
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Re: Weight limits on bridges

Post by Ronnie »

I saw a 2T limit somwhere in Aberdeenshire the other day (just have to remember where). It stood out to me as it’s so low.
WHBM
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Re: Weight limits on bridges

Post by WHBM »

It's more complex than just an individual vehicle weight. Depending on the nature of the weakness (beams; side buttresses; ground conditions; already moving or not) it can be not just one vehicle but the cumulative effect over time. This is how you can do an "except regular buses" which may be over the posted limit, but not many of them. Width restrictions also deter gross overweights by loaded mainstream HGVs, which are probably the most serious issue; even Tower Bridge was damaged by not one but two substantially overweight HGVs running nose-to-tail.

The calculations are available in a range of civil engineering handbooks, which the engineers will be using. One doesn't just make this stuff up.
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KeithW
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Re: Weight limits on bridges

Post by KeithW »

There is a 3 ton over the Crinan Canal here
https://www.google.co.uk/maps/@56.07361 ... 6656?hl=en

There was another with a 1.5 ton limit which has now been closed as dangerous.
https://www.google.co.uk/maps/@56.06366 ... 6656?hl=en

There is also a section of road that has a Weak Road sign and an 18 ton limit which is I think quite rare.
https://www.google.co.uk/maps/@56.06110 ... 6656?hl=en
Fenlander
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Re: Weight limits on bridges

Post by Fenlander »

The thing that a lot of people miss is that these weight limits refer to the vehicles plated limit, not its actual running weight so an empty 3.5ton van would fall foul of the 3 ton limit. Something like a Mercedes Sprinter comes in a variety of plated weights too to further complicate things, including higher than 3.5ton which needs more than a standard driving licence.
WHBM
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Re: Weight limits on bridges

Post by WHBM »

It's quite reasonable to base the limit on Gross Vehicle Weight, as it's then very straightforward to know if it's allowed or not, for both user and enforcement. Otherwise it would be necessary to send any violation that was detected to a weighbridge. The calculations for the bridge limit take this into account knowing the general percentages of vehicles that are up to the limit, or not, in assessing how much wear will result.
DB617
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Re: Weight limits on bridges

Post by DB617 »

DavidAll wrote: Sun Jun 06, 2021 20:18 Thanks AndyB.

Would a Luton have a width across the wheels greater than 2m? The actual width restrictors they are putting in are 2m at wheel level and then set back 450mm either side at body height in order to let ambulances through. So 2.9m at body height.
If Ambos fit, then a minimum sized Luton should fit. They're practically the same dimensions.

Regarding weight limits and MGW vs current weight, to my memory most MGW limits are provided for engineering reasons - as you say, for weak structures - indeed my local one is a 3t limit at M32 J2 where the Tesco fire compromised the NB offslip structure. Several of our (Merc Luton) drivers have been known to come off there anyway. As a civil engineering student I can't quite bring myself to follow... I would have serious egg on my face were I caught or worse, caused damage. :shock:

Really the only use for conventional weight restrictions (the ubiquitous 7.5t lorry diagram comes to mind) is for the purpose of restricting traffic types and funneling heavy vehicles out of or onto certain routes.
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Vierwielen
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Re: Weight limits on bridges

Post by Vierwielen »

NATO has its own set of weight limits for bridges etc. A number of them can be seen here. Some of them allow heavier vehicles to proceed in one direction if none are coming in the other direction. These signs are very common in Germany, though I have seen one here. This sign, on a road in Aldershot that is owned by the MoD, allows vehicles of 50 tonnes to cross the Basingstoke Canal as long as the traffic is in one direction only. As soon as traffic is in running in two directions, the weight limit per vehicle drops to 30 tonnes.
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Chris5156
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Re: Weight limits on bridges

Post by Chris5156 »

Vierwielen wrote: Wed Jun 09, 2021 20:56 NATO has its own set of weight limits for bridges etc. A number of them can be seen here. Some of them allow heavier vehicles to proceed in one direction if none are coming in the other direction. These signs are very common in Germany, though I have seen one here. This sign, on a road in Aldershot that is owned by the MoD, allows vehicles of 50 tonnes to cross the Basingstoke Canal as long as the traffic is in one direction only. As soon as traffic is in running in two directions, the weight limit per vehicle drops to 30 tonnes.
You can tell it's a copy of a sign normally used mainland Europe because the two-way arrows indicate driving on the right :?
2 Sheds
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Re: Weight limits on bridges

Post by 2 Sheds »

WHBM wrote: Tue Jun 08, 2021 08:15 It's more complex than just an individual vehicle weight. Depending on the nature of the weakness (beams; side buttresses; ground conditions; already moving or not) it can be not just one vehicle but the cumulative effect over time. This is how you can do an "except regular buses" which may be over the posted limit, but not many of them. Width restrictions also deter gross overweights by loaded mainstream HGVs, which are probably the most serious issue; even Tower Bridge was damaged by not one but two substantially overweight HGVs running nose-to-tail.

The calculations are available in a range of civil engineering handbooks, which the engineers will be using. One doesn't just make this stuff up.
Retired bridge engineer here. This is correct. In assessing which limit to apply an engineer might also consider that cast iron is prone to sudden brittle failure, whereas brick arches usually fail very gradually over many years as bricks and mortar drop out. The only official weight limit demarcation below 7.5 tonnes is 3 tonnes and that has been the case for 20+ years. I expect the variants quoted above have either been in place for many years, or are not backed up by a legal traffic order.
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ravenbluemoon
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Re: Weight limits on bridges

Post by ravenbluemoon »

It's been a while since I've dabbled with structural calcs, but I'm wondering if the width restriction is to slow traffic down over the bridge. The lower the speed, the less stresses on things like bearings.

Also, it would filter traffic onto the bridge, as it would be single file. Provided it isn't too busy so that traffic queues onto the bridge!
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