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.
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There are a few around like this one on the M11
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.Phil wrote: ↑Wed Nov 24, 2021 16:14Because 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.Bryn666 wrote: ↑Tue Nov 23, 2021 18:01https://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?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.
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.
Look a bit closer - they both have ladders and walkways and are most defiantly not 'simple structures'KeithW wrote: ↑Sun Nov 28, 2021 11:12There 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.
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.Phil wrote: ↑Sun Nov 28, 2021 18:03Look a bit closer - they both have ladders and walkways and are most defiantly not 'simple structures'KeithW wrote: ↑Sun Nov 28, 2021 11:12There 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.
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.