20 June 2018

Welsh Bridges: 7. Craig Goch Dam

Craig Goch Dam is the uppermost of a series of dams built in the Elan Valley from 1893 to 1904. Most of these dams are simple spillways, and this is the only one with an exposed spillway and a viaduct across the crest. Together, this Grade II* Listed dam and bridge are both spectacular and very beautiful.

The other Elan Valley dams are also well worth visiting, but in the absence of bridges I won't be featuring them here. Someone else will have to start their own blog. "Weir Weirdos", I suggest.

The Elan Valley dams and reservoirs were constructed under the direction of engineer James Mansergh on behalf of the Corporation of Birmingham, a city some 75 miles away. Water carried from the Elan reservoirs apparently takes about a day and a half to reach Birmingham, all by gravity flow.

The dam is some 156m long, 36m tall, and 32m thick at its base, a masonry giant surmounted by 13 segmental arch spans. Note that I've taken the 156m figure from an information board nearby - the plaque picture above gives a shorter length for the "length of weir".

It's a beautiful and very well-put-together structure. The viaduct piers merge smoothly into the curved profile of the dam spillway, and the contrast between the spillway's smooth stone facing and the viaduct's rough stone facing is effective (although I think this is the effect of many years of water flow, not an original feature).

The detailing of the viaduct is admirable in pretty much every respect, and doubly so on the lovely octagonal valve tower.

The viaduct is masonry faced on its elevations, but with a narrow band of brickwork forming the arch barrels in between. There are drain holes at every span in the roadway, and surface water drains out through the piers directly onto the dam spillway.

The view downstream from the dam is spectacular, and the road journey from here westwards through the beautiful hills to Cymystwyth is also recommended (I had come that way after visiting Llanilar Suspension Bridge).

The Coflein website has aerial photos, as well as some photographs of the dam under construction.

Further information:

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:

15 May 2018

Yorkshire Bridges: 32. Manchester Road Footbridge, Bradford

This is the last bridge in my current series of bridges in Yorkshire. This one is not in Leeds, where most were, but in nearby Bradford.

I wrote about this design in September 2010, before it was built:
To me, the whole effort is uninspired and unattractive, a step backwards visually, and certainly a lost opportunity to exploit the opportunities of structural engineering.
The bridge was designed in sympathy with the Manchester Road bus shelters project, with the same vocabulary of slightly disjointed bright red structural steel. The bridge takes pedestrians and cyclists over the busy main road, connecting to a wider network of pathways.

The bridge was designed by Bradford Council's in-house engineering team, and built by Eric Wright Group. I don't know who the steelwork subcontractor was, and I'm not aware of an architect. It won a Certificate of Excellence from the ICE in 2013.

Visiting the bridge "in-the-flesh", I certainly had to revise my original opinion. Whatever was I thinking?

The Big Red Bridge (as it is apparently known), is big, and red, and more than a little absurd. Obviously, the colour makes it stand out from its surroundings, but I like how it signposts itself so well. You want to go there, from here? The path is obvious.

Like the bus shelters, it is something of a jumble. The bridge supports come in at least three different flavours, but everything is related. I like the way the steel H-sections in the supports are connected together with stiffened intersections.

The central tower, which seemed to be nothing but an impertinent appurtenance, makes sense as an extension of its support, and as a waymarker that anchors the whole assembly visually.

Even the corrugated iron parapet infills lose something of their inherent naffness when painted red and united with the rest of the bridge.

I think it's a very interesting and surprisingly visually attractive bridge, with a bold and very self-consistent sensibility, that fits well into its site.

Further information:

13 May 2018

Yorkshire Bridges: 31. Newlay Bridge, Leeds

Okay, this is the penultimate structure in my current sequence of posts. For this one, I headed west out of Leeds to Horsforth, to discover a real gem of a bridge.

Spanning the River Aire, Newlay Bridge was built in 1819, and is one of the oldest surviving cast iron bridges in Yorkshire. It was built by the Bradford-based iron foundry Aydon and Elwell, who also built bridges which survive at Sowerby Bridge (1816) and Walton Hall (1828). Cast iron bridges in Yorkshire date back, of course, to one of the first iron bridges in the world, built at Kirklees in 1769 (yes, ten years before that other one).

The bridge was originally constructed for £1500 at the request of landowner John Pollard, replacing a previous structure built in 1783. A halfpenny toll was charged, earning Pollard £600 per year.

Different accounts vary on what happened to the bridge in later years: Civil Engineering Heritage reports that the bridge was taken over by the Midland Railway and Horsforth Council in 1880, providing access to Newlay and Horsforth Railway Station. The Newlay Conservation Society mention that the Midland Railway built a new footbridge in 1886, which would seem odd if they had just acquired this bridge. Photos suggest the second bridge to have been just downstream, closer to the river weir.

The Pastscape website has a hugely detailed description of the bridge, so I'll not repeat that here. The bridge comprises four cast iron ribs, separated by cast iron bracing members. It spans approximately 25m.

The structure is very well apportioned. The layout and spacing of the bridge spandrel members is surprisingly modern. The edges of the main ribs, and also the centreline of the spandrel members are reinforced with small outstands, which also provide a degree of visual definition to what would otherwise be quite plain.

The stringcourse fascia is subdivided into repeating panels, with the smaller panels coinciding with the main parapet posts above. These posts are an ornate ironwork design, with plain vertical infill bars in the sections in between. Panels on the parapet credit Pollard, Aydon and Elwell.

Leeds Council are planning to refurbish the bridge in summer 2018, although I thought it looked in pretty good condition already. The parapets were repainted as recently as February 2017, although the underside of the bridge was not repainted at that time. As at Leeds Bridge, the bridge will be repainted using a High Ratio Co-Polymerised Calcium Sulfonate Alkyd paint system, specially sourced from North America. The driver seems to be the bridge's bicentennial anniversary, which falls in Spring 2019.

This is a well-preserved, interesting and historic bridge, in a very attractive setting, and well worth a visit if you are in the area.

Further information:

10 May 2018

Yorkshire Bridges: 30. Riverside Way Bridge, Leeds

This footbridge was opened in 2007, to a design by Capita Symonds and Carey Jones Architects. Also known as the Whitehall Bridge, it cost £1m to build.

It's a classic modern tilted-arch design. I'm not sure who first designed one of these, but it's a typology that has definitely caught on. I've designed one myself.

Structurally it's fairly straightforward. The bridge deck is curved in plan, so tends to tilt away from its supports. The arch is also curved in plan, so tilts the other way. Connecting the two with cables allows the weight of the arch to balance the weight of the deck and any pedestrians. In addition, the stiffness of the deck helps to brace the arch against buckling.

Riverside Way Bridge is mildly decrepit, with substantial grime disfiguring the metalwork, and one of the parapet cables having snapped and come loose. This could have been guarded against by using shorter lengths of cables, rather than wires which run the full bridge length.

I think the arch looks pretty good from most angles, and I like the detailing where the parapets intersect the arch rib.

Further information: