Understanding a footbridge (and making it into a road bridge)

[47137]

I want to make a very minimal road over-bridge for my layout. By this I mean a carriageway barely 8 feet wide and with a 3-tonne weight limit. Something clearly ancient but somehow still just about safe to use.

 

Yesterday evening I was looking at this footbridge, this is over a weir on the Chelmer and Blackwater Navigation at Beeleigh:

 

This seems a pretty massively-constructed thing. The girders are about 6 in wide, 14 in deep. The path is 4 ft wide, surfaced with an inch or two of tarmac on top of concrete. The concrete finishes 3 in below the tops of the girders. The girders extend 4 ft over the abutments. I am puzzled as to why this is so strongly made - perhaps to survive floods. 

 

But really, what I want to do is use this as ideas for a prototype for my model road bridge with a span of c.12 feet. Does this sound plausible? Also, please could someone tell me, would we expect the concrete decking to go down all the way down inside between the girders? What might the underneath of the deck look like? I'm not sure if it would be bare concrete (held by shuttering during the pour) or some corrugated metal?

 

Many thanks.

 

- Richard.

[DCB]

The problem is many big impressive railway bridges carrying a road over the railway, an "Overbridge",  had carriageways barely 8 feet wide and a 3 ton weight limit. The one up the road from my house was one of them,  See part of Malcolm Mitchell Pic.    The lack of material over the arch, shallow dimension from arch to top of wing wall was a give away.  Many now carry 44 tonnes without any real strengthening

[47137]

Possibly I have posted this in the wrong section of the forum, but what I want to do is to use the essence of the deck of the Beeleigh footbridge as the root or inspiration) of a prototype for a model road bridge. This would have a 12 ft span, and a 3-tonne weight limit.

 

To make a start, I'd like to understand why the deck here seems to be so massively constructed.

 

(The software for calculating beam sizes is not free, so I need to copy or adapt a prototype rather than make the design calculations from scratch).

 

- Richard.

[Nearholmer]

Yes, feasible as the basis of a road bridge, possibly with the spans between piers shortened (that plus the number and size of spanning members is what sets the load-carrying capacity.

 

You’d need to add heavy kerb timbers to prevent vehicles going through the balustrade and into the water, though.

 

There is a road bridge like that about five miles from where I live, but I can’t readily find a picture.

 

construction will hugely depend upon when it was designed/built. Modern construction might use precast reinforced planks, over RSJs, but an older construction might well be entirely timber.

Edited May 16, 2019 by Nearholmer

[DCB]

The bridge in the picture is the top of a weir. It has to be massive to withstand the water pressure when the river rises and the water reaches the deck, that's why the beams are so deep and the deck is sandwiched between the beams.  Normally a bridge deck is designed to be high above the water level or not be over water at all, so the surface is on top of the strength members and not between. I think you are looking at post steam era construction or an early 1900s light railway for a prototype.

[47137]

14 hours ago, Nearholmer said:

...

construction will hugely depend upon when it was designed/built. Modern construction might use precast reinforced planks, over RSJs, but an older construction might well be entirely timber.

 

The notes and inspiration are all out there online ... as usual the starting point is knowing what to look for and in this case "precast reinforced planks" is a whole lot better than my imagined "concrete beams" which takes me to the supporting structure not the decking. 

 

Anyway - from here I have worked my way through to timber decking over a steel frame and this seems right for my layout for this road bridge and for a rail over rail bridge nearby. This chap has something of a showcase on his web site:

http://www.geofffreedman.co.uk/gallery-vehicle-bridges/

 

I could even make my road bridge as if it is getting a new deck.

 

- Richard.

[Nearholmer]

The other thing you'll need to ponder is the nature of the abutments and piers that support the span(s).

 

Again, a swift google will reveal all sorts of options, depending upon date of construction, nature of the ground, the durability that is being sought, etc.

 

A type of small bridge that has always fascinated me is the ones used on the very extensive 3ft gauge bog railways in Ireland, which uses timber piles below ground level to support otherwise concrete and steel structures. The use of timber seems counter-intuitive, but it lasts for centuries in bog-country, I think because the soil and water are very acidic. Raised timber walkways, built on timber piles, immersed as water levels rose, thousands of years old have been found in bogs, in perfectly good condition. The wood is like dark-brown iron.

 

For beam dimensions, a useful visual trick is to follow a rule of thumb, which says that the ratio of the depth of beam to the span of the beam should be about 1:6 if the load to be carried is high, down to about 1:8 for a lightly loaded road or railway bridge, and even 1:10 for a footbridge. In reality, of course, the load-carrying capacity in the case of simple beam bridges depends upon the number of beams in parallel as well, but on a model this is rarely visible (and usually not modelled).

 

The other option is to use some sort of more complex structure, to get strength with lightness ......... how about making the bridge on your layout as a Bailey Bridge on Military Trestles? Visually very interesting, and there are innumerable examples of such bridges having been installed 'temporarily' and lasted many decades.

 

 

[47137]

7 hours ago, Nearholmer said:

The other thing you'll need to ponder is the nature of the abutments and piers that support the span(s).

...

 

 

As it happens I bought some Wills moulded stone sheets a few days ago and these are a perfect height to make my bridge abutments without joins or waste.

 

I have two bridges to make. One road over rail and one rail over rail.

 

The road bridge has a railway running "through" it as well as "below" it, but I'll ignore this for the visible side of the model bridge:

 

The rail bridge is a skew bridge with both tracks curved and on gradients:

 

The setting here is imaginary, but I would like to think of the abutments as being remnants of a former narrow gauge railway and quite old - say, late 19th century. These old masonry structures now having more modern spans added on top.

 

My present thinking is to have the masonry abutments capped with modern concrete and bearings, and holding fabricated steel spans. The road bridge to be surfaced with timber, and the rail bridge to be surfaced with chequer plate. It might be more interesting for the road bridge to actually have two spans: a narrow roadway and a footbridge beside it. Both resting on the same pair of abutments.

 

For the spans I have some Plastruct girders to hand and I can do either 1:8 or 1:6. In fact if the roadway stays as 5mm foam board and I attach 7mm Plastruct to this (easy to do), I'll have the 1:6 ratio and the bridge I expected to have notional 3 tonne weight limit (in the first post) will actually look sturdy enough to carry a lorry.

 

Thank you for the idea of the Irish bog bridges but I am getting fairly hooked on the Wills sheets. I want to define a local vernacular for the layout and these sheets will be good for older remants, with concrete and steel for the modern parts.

 

Edit: I like the idea of a bailey bridge too, I remember one near Tywyn many years ago, but I think this must wait for another layout.

 

- Richard.

Edited May 17, 2019 by 47137