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:

• Google maps
• Bridgemeister
• Coflein