Why is the M20 signed as "Dartford/Maidstone (M20)" on the M25 and not "Dover (M20)"?

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KeithW
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Re: Why is the M20 signed as "Dartford/Maidstone (M20)" on the M25 and not "Dover (M20)"?

Post by KeithW » Sun Nov 28, 2021 11:12

Phil wrote:
Wed Nov 24, 2021 16:14
Failing that, you do what the French do and make them very simple structures just supporting signs and not having walkways etc on them.
There are a few around like this one on the M11
https://www.google.co.uk/maps/@52.19699 ... 8192?hl=en

And here on the A1(M)
https://www.google.co.uk/maps/@54.11189 ... 8192?hl=en

They have pretty chunky foundations though.

2 Sheds
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Re: Why is the M20 signed as "Dartford/Maidstone (M20)" on the M25 and not "Dover (M20)"?

Post by 2 Sheds » Sun Nov 28, 2021 13:34

Phil wrote:
Wed Nov 24, 2021 16:14
Bryn666 wrote:
Tue Nov 23, 2021 18:01
WHBM wrote:
Tue Nov 23, 2021 17:34
For the last 12 months or more Heathrow, the Central Terminals 2 and 3, has had no signage from the M4 out of London at all. Just this at the half mile mark :

https://www.google.com/maps/@51.4935878 ... 384!8i8192

If we can't get this right, what chance the rest.
https://goo.gl/maps/rfjCJhvgWgcMNFsL6 at least it's got a massive concrete block awaiting the future sign. How have we ended up with foundation design this ugly, and critically, above ground?
Because NH / HE have a fetish for long Cantilever gantries interoperating walkways! On made up ground like embankments you need a huge heavy base (or deep piles) to stop such a structure failing in high winds or simply through its own weight.

A portal style gantry spanning the carriageway (or even one spanning the whole motorway) is inherently far more stable and the foundations can be smaller / shallower as a result.

Failing that, you do what the French do and make them very simple structures just supporting signs and not having walkways etc on them.
As a retired structural engineer I would say that the additional dead and live loadings associated with walkways is minimal compared to gantry weight and wind loading on these gantries.

I remember talking to the DOT engineer (I think it was DOT then but who can remember !) tasked with giving technical approval for the original designs of many of these gantries some 30 years ago, and him telling me what a headache they were.

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Re: Why is the M20 signed as "Dartford/Maidstone (M20)" on the M25 and not "Dover (M20)"?

Post by Phil » Sun Nov 28, 2021 18:03

KeithW wrote:
Sun Nov 28, 2021 11:12
Phil wrote:
Wed Nov 24, 2021 16:14
Failing that, you do what the French do and make them very simple structures just supporting signs and not having walkways etc on them.
There are a few around like this one on the M11
https://www.google.co.uk/maps/@52.19699 ... 8192?hl=en

And here on the A1(M)
https://www.google.co.uk/maps/@54.11189 ... 8192?hl=en

They have pretty chunky foundations though.
Look a bit closer - they both have ladders and walkways and are most defiantly not 'simple structures'

A simple structure would be something akin to a MS4 which can only be accessed by a Cherry Picker and requires much less chunky supports.

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Re: Why is the M20 signed as "Dartford/Maidstone (M20)" on the M25 and not "Dover (M20)"?

Post by Bryn666 » Mon Nov 29, 2021 23:47

Phil wrote:
Sun Nov 28, 2021 18:03
KeithW wrote:
Sun Nov 28, 2021 11:12
Phil wrote:
Wed Nov 24, 2021 16:14
Failing that, you do what the French do and make them very simple structures just supporting signs and not having walkways etc on them.
There are a few around like this one on the M11
https://www.google.co.uk/maps/@52.19699 ... 8192?hl=en

And here on the A1(M)
https://www.google.co.uk/maps/@54.11189 ... 8192?hl=en

They have pretty chunky foundations though.
Look a bit closer - they both have ladders and walkways and are most defiantly not 'simple structures'

A simple structure would be something akin to a MS4 which can only be accessed by a Cherry Picker and requires much less chunky supports.
Apparently the use of MEWPs and cherry pickers is deprecated on motorways. I had always thought the walkways and such were to reduce the need for lane closures but lanes still get closed for gantry works so I'm not sure what the advantages really are beyond contractor convenience.
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KeithW
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Re: Why is the M20 signed as "Dartford/Maidstone (M20)" on the M25 and not "Dover (M20)"?

Post by KeithW » Tue Nov 30, 2021 00:39

Phil wrote:
Sun Nov 28, 2021 18:03
Look a bit closer - they both have ladders and walkways and are most defiantly not 'simple structures'

A simple structure would be something akin to a MS4 which can only be accessed by a Cherry Picker and requires much less chunky supports.
Its not just a matter of access, to do anything meaningful you are going to be opening electronics cabinets to upgrade or replace components. If you have no walkway you are going to have to close lanes in case butter fingered Bernie drops something. From the structural point of view a cantilever is a simple as it gets. One fixing point a chunky concrete slab and a bunch of holding down bolts. In terms of the load the weight of a walkway and a light metal enclosure is a minor issue and the windload is barely any higher.

A simple double width electronics cabinet 2m high by 1.2m wide and 1 m deep is going to weigh around 150 kg before you load a single board into it. You need rather more than a 1m clearance to have enough space to pull a board and the last thing you want is to be trying to do this in the open with driving rain, that is a sure road to short circuit city. This is why in the power and chemicals industry we do NOT install such enclosures above grade outside a building without a walkway and weather proof shelter. That's before we consider HSE safety regulations.

Now if you are all fibre optic its a little simpler but you still need a weather proof power supply. A short circuit on a wet bank holiday weekend is not what you want in a safety critical system.

A typical VMS 3 line display weighs the best part of a tonne and requires a 2 kilowatt power supply. This is not trivial, 1 m wide Kennedy Grating for the walkway weighs in at about 21 kg per square metre so that soon adds up. Now I make no claim to expertise regarding road or rail design but instrument and sign enclosures are a different matter.

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