17 June 2018

Welsh Bridges: 6. Llanilar Suspension Bridge

I recently spent a day visiting a selection of bridges in Wales, so here is the first in a series of posts.

The first bridge is an unusual footbridge at Llanilar, 6 miles south-east of Aberystwyth. It spans the Ystwyth River.

The Ystwyth foot and cycle trail runs alongside the river here, following the line of the old Manchester to Milford Railway Line. Llanilar Suspension Bridge connects the two river banks, providing access to and from a smaller path on the other side of the river.

The bridge has two structural systems superimposed. It seems that the suspension bridge arrangement is the original, and was supplemented later by cable stays.

The bridge deck is unstiffened, and even with the use of two support systems is prone to significant movement under load, as can be seen in my video below. The stay cable system is not strongly tensioned, which means it is probably contributing very little to stiffness.

The bridge masts are slender joists, held in position laterally by only a single crossbar. The function of this bar seems to be only partly as transverse bracing, but mainly to support pulley wheels, over which the main suspension cables are slung.

I haven’t seen this arrangement on a suspension bridge before, and I don’t know whether it is unique in the UK, or just highly unusual. It's not a good design for such slender masts: the cables provide no longitudinal restraint to the masts, and the bridge will move more under thermal expansion and contraction of the cables than if the cables were clamped to conventional mast saddles.

The deck is suspended from the main cables by hangers comprising flat plates, incorporating a twist to allow them to double as the fence posts. These are attached to the cable with simple clamps, and to channel-section deck transoms via bolted connection plates. In the middle part of the bridge, every fence post is also a hanger, but towards the ends only every second fence post is a hanger.

The timber deck planks bear directly on the metal transoms – there is not even the most minimal of metal edge members. The structural integrity of the bridge is therefore dependent on what should be a secondary element, the deck planks.

The stay cables are attached to the main masts with shackles and cable hooks, and to the deck with shackles and threaded eye-bolts. The jumbled array of attachments makes it very clear that these are an improvised addition to the bridge.

Cable-stayed bridges normally require a stiff deck to carry the compression forces which balance the longitudinal tension in the stay cables. The deck of this bridge cannot carry any substantial compression force, so an alternative arrangement has been devised, which connects the three places where stay cables meet the deck with two pairs of horizontal cables.

The tension in the stays is therefore balanced by tension in these additional cables. The balance cables can be seen to be sagging considerably, indicating that the stay cables have little tension in them. The diagram above shows the cable arrangement, with the suspension system in blue and the stay system in orange.

Within the main span, there are two pairs of stays on each side of the span, one directly connected to the deck, and one connected to the deck via an intermediate connection to the main suspension cables. This is a very odd detail. The stays are connected to a small strut, which sits on a steel angle. The angle is clamped to the main suspension cable, and each one seems to have rotated round over time - I believe the angles were originally situated so that the strut was vertical.

I have found no information on when this structure was designed or built, or by whom. I'd love to learn more about its history. If it's old enough, it should be Listed, in view of the highly unusual detailing.


Further information:


Bill Harvey said...

There used to be a bridge with cables supported on pulleys over the Exe about a mile north of Bickley. I haven’t had chance to check it out in 15years or more. There the cables were very flat. So much so that the hangers were just wrought iron bars hooked over the cables. The pulley arrangement recognises some of the issues with cables (like thermal and elastic movement in the back stay but... These backstays come down particularly steeply and generate massive (relative to the masts) horizontal forces at the top. Assume therefore that the secondary back stays to resist that are either part of the original design or fitted very shortly after when the masts bent unnervingly.

I saw a cable stayed bridge with timber deck in New Zealand which had, I think, seven complete cables from tower to tower each pulled down to a cross beam at a different point in the span. So no net horizontal force in the deck but some pretty tricky assembly.

The optimistic tack welded threadbar is a bit alarming. Great fun though. Well done for finding it. Must now check bridgemeister’s catalogues for the one on the Exe.

The Happy Pontist said...

Is this the Bickleigh one? http://www.geograph.org.uk/photo/2128752

crisb said...

It appears to be close to the old railway station, so could have provided access from the north. The "towers" look as though they might be made from bull head rail and something like an old railway axle. It's possible the "pulleys" weren't intended to turn so the ropes could have been held by friction similar to a conventional suspension bridge.
Old 6" OS maps online show the footbridge in 1904 but not in 1886 so this may date the original "design".

Unknown said...

I am searching for good references on early wire suspensions bridges in the U.S. Historical documents state that there was a "160 ft wire suspension bridge" across the Esopus Creek at Hurley, NY, but no one has ever seen a drawing or picture of it. It was built in 1856 and replaced in 1888. I am hoping to determine what style of bridge would have most likely been built in that year. I would love to get a drawing or photo of one that was close to the same length as the Hurley bridge and built within a few years of 1856. Thank you in advance for any assistance you can provide!