Northumbrian Bridges: 4. Cragside Bridge

It's been about 2 months since I posted anything from the IABSE study tour of North East England. I've previously covered the bridges we saw on Teesside and in Durham, on the first day of the tour. The second day begun with a detour north from our Newcastle base, to the Cragside estate at Rothbury, in Northumberland.

This bridge seems to be overlooked in various books about bridges of the region, which is odd, as it is one of the finest in the area.

Cragside was built in the late 19th century for Lord Armstrong, a pioneering industrialist. It incorporated a number of engineering innovations, such as hydroelectric power and electric lighting. Not a great deal is known about the beautiful bridge in its grounds, reported to have been designed in 1864, and built some time between 1870 and 1875.

Over the years, there has been considerable speculation that it was a very early steel bridge (although it is generally named the "Iron Bridge").

The bridge was extensively refurbished, winning a Historic Bridge and Infrastructure Award in 2009, and the description of this work states that it comprises wrought iron. On our visit, we were lucky enough to meet the National Trust engineer responsible for the site, who showed us a number of interesting photographs of the refurbishment works. The bridge was strengthened by the addition of a number of very discrete steel plates, rivets replaced, and new handrail posts installed.

Loads on the handrail posts have been reduced by tensioning the parapet wires between bollards at each end - this allows the posts to remain much slimmer than would otherwise be the case. Keeping the parapets minimal must have been essential for a bridge like this, to avoid them detracting from the elegance of the rest of the structure.

The main structural system is charming yet structurally illogical. The bridge is supported on a single arch in its main span, with two half-arches to either side, acting as much like a cantilever bridge as a genuine arch structure. The arch ribs are attractively slender in elevation.

Each spandrel is infilled with two intersecting arches: this is the aspect of the bridge which defies structural logic, as what looks at first glance to be a nicely layered set of structural forms results in the application of significant concentrated loads transverse to the arch ribs, introducing a bending moment which greatly diminishes material efficiency. However, a "correct" geometry for this system would introduce kinks into the main arch ribs, which would clearly be visually unpleasant. The structural eccentricity is therefore easily justified, and the only result is that the bridge's overall load capacity is greatly reduced. Given its location, that doesn't seem a major problem.

Viewed from below, along the axis of the deck, the bridge's most striking feature consists of the rather massive plates linking the arch legs together, and providing stability against transverse sway. While these have a somewhat weighty look when set against the more filigree elevations, the curves and echo of the main arch forms makes them appear at least half-reasonable.

As is often the case, the structural engineering quibbles are irrelevant when the bridge is considered as a whole. I can think of few comparable structures - it has some similarities to the celebrated Pont des Arts in Paris, but I think the Cragside bridge is the more attractive of the two. Within the United Kingdom, it's easily one of the most beautiful historic footbridges that we have.

Further information:

• Google maps / Bing maps
• Structurae
• Engineering Timelines
• British Listed Buildings
• DMJ Engineering Ltd (refurbishment details)
• National Trust Book of Bridges (Richards, 1984)
• Civil Engineering Heritage: Northern England (Rennison, Thomas Telford Publishing, 1996)
• An Encyclopaedia of Britain's Bridges (McFetrich, Priory Ash, 2010)

Northumbrian Bridges: 3. Heatherslaw Mill Bridge

The Union Bridge and Twizel Bridge are amongst the best known of Britain's historic bridges. Heatherslaw Mill Bridge is definitely not - I don't believe it features in any of the guides to British bridges that I covered recently, nor is it in "Civil Engineering Heritage: Northern England".

Nonetheless, it is a Grade II Listed Building, and a type of bridge that I always find interesting.

It provides road access across the River Till to a Victorian corn mill on the Ford & Etal Estate. It was built in 1877, supposedly as a temporary access in place of an existing river ford, by the Glasgow firm of A & J Main & Co. They were at one time better known for their wrought iron fence posts and gates, although they became increasingly involved in structural metalwork as time went on.

The bridge is described at SINE as consisting of iron with a steel lattice parapet, and at Scottish Ironwork as a wrought iron lattice girder.

The lattice girder has obvious attractions to the designer, in combining the functions of the parapet and support beam in one (this wouldn't be permitted on modern road bridge, due to the risk of vehicle impact leading to the collapse of the whole bridge). From one perspective, it overlaps multiple triangular trusses, allowing the shear force effects to be shared by smaller members which are easier to fabricate. Seen another way, it's a plate girder with the web perforated, allowing the largely redundant material in most girder webs to be pared away.

At Heatherslaw, the two sets of lattice members are noticeably eccentric to the girder centreline - I presume that any tendency of this to twist the main girder flanges in plan is reduced because the lattice members apply roughly equal opposing forces at any nodal point.

The bridge originally had a timber floor, which was replaced in 1951 with the War Surplus decking units now present. It's not a type of flooring I can recall seeing previously.

It's an unassuming bridge design, a reminder from the past that modesty can still be a virtue.

Further information:

• Google maps / Bing maps
• SINE
• Scottish Ironwork
• A&J Main & Co
• Bob's Bridges: Jottings from a Northumbrian Foreman's Diaries (Robson, 1998)

Northumbrian Bridges: 2. Twizel Bridge

Twizel Bridge sits alongside the A698 where it passes over the River Till, between Cornhill and Norham. The roadway which previously crossed over this narrow (4.6m wide) medaeval arch span has long since been rerouted onto a modern structure.

Along with the Devil's Bridge at Kirkby Lonsdale, Twizel Bridge is one of northern England's great medaeval spans. It's date of construction is not known for sure, although it was certainly there in 1513 when troops crossed it on their way to the battle at nearby Flodden.

It spans 27m, which was reportedly the largest stone arch span in England for three centuries. John Leland described it as "of stone one bow but greate and stronge".

What makes it particularly attractive are its five chamfered ribs, an element which make any masonry arch bridge much more interesting. The dentilated string course is also a nice feature.

Bob Robson's book, linked below, describes a number of repairs carried out to the bridge in the 1960s and 1970s, when it was clearly unsuitable for the traffic on it. Damage to the parapets required repair, a wing wall needed to be propped, and the spandrel walls were reinforced with tie rods (with their anchor plates reportedly hidden behind the masonry facing, although there are further tie rods visible in my photos which perhaps were added later by a less sympathetic engineer).

Further information:

• Google maps / Bing maps
• Structurae
• Engineering Timelines
• SINE
• The Ancient Bridges of the North of England (Jervoise, 1931)
• A heritage of bridges between Edinburgh, Kelso and Berwick (Paxton & Ruddock, 1980)
• Civil Engineering Heritage: Northern England (Rennison, 1996)
• Bob's Bridges: Jottings from a Northumbrian Foreman's Diaries (Robson, 1998)

Northumbrian Bridges: 1. Union Chain Bridge

I recently visited the English/Scottish Borders area, and visited one or two interesting bridges while there, all historic structures.

I'm calling the first of these bridges a "Northumbrian" bridge, although it's every bit as much a "Scottish" bridge, given that it spans the River Tweed across the border between England and Scotland.

The Union Bridge was the first suspension bridge in Britain to carry vehicles, and when opened in 1820, it was the longest suspension bridge in the world. It cost £6,449 to build.

The deck spans 110m and is 5.5m wide. The bridge has three chains of iron links on each edge, spanning 129m. The chains are vertically above each other, with the links staggered so that the hangers are evenly spaced. The chains pass over a masonry pier at the Scottish end, while at the English end, an abutment is built into a cliff face, with the chains anchored by a masonry arch buried within the rock. The difference in spans is because the deck reaches ground some way in front of this abutment.

Its survival to the present day is remarkable. Its designer, Captain Samuel Brown, mainly got into suspension bridge works as a way of exploiting the iron chain technology he had developed while in the Navy, and his very slender, unstiffened bridge decks were prone to oscillation in the wind. Most of his bridges have long since been destroyed.

The Scottish tower looks particularly large and robust to my modern eyes, but that can be blamed not on Brown, but on John Rennie, who had helped the Berwick Turnpike Trustees to appoint Brown. Rennie described Brown's tower as "clumsy, ill arranged and overloaded with ornament", and suggested that the tower be redesigned as larger and with tapering sides.

The Union Bridge has been refurbished and strengthened on many occasions (the paper by Miller linked below gives ample details, as does Bob Robson's book), most obviously including the addition of a wire rope cable above the original three suspension chains.

Even then, the bridge's load capacity is very limited, and in addition to a 2-tonne weight limit, only one vehicle is allowed to cross at a time (this is probably to avoid the risk of damage to the cables, which are not protected by parapets of modern standard). Walking across it, the vibration from a large 4x4 was noticeable, and it was fairly responsive even to a bit of jumping up and down.

I especially like Robert Stevenson's contemporary description of the bridge in the Edinburgh Philosophical Journal (linked below):

"The general effect of the Union Bridge, which we have now endeavoured to describe, is interesting and curious; and such is the extent, and its light and elegant appearance, that it has not inaptly been compared to an inverted rainbow. Those who visit this undertaking, as affording much novelty to the scenery of this part of the banks of the Tweed, will not be disappointed in their expectations; while, in a national point of view, as a great improvement, it deserves the most particular consideration of the country at large."

Further information:

• Google maps / Bing maps
• Structurae
• Wikipedia
• SINE
• RCAHMS
• Engineering Timelines
• Description of Bridges of Suspension (Robert Stevenson, Edinburgh Philosophical Journal, 1821)
• A Memoir of Suspension Bridges (Charles Stewart Drewry, 1832)
• A heritage of bridges between Edinburgh, Kelso and Berwick (Paxton & Ruddock, 1980)
• Civil Engineering Heritage: Northern England (Rennison, 1996)
• Bob's Bridges: Jottings from a Northumbrian Foreman's Diaries (Robson, 1998)
• Union Chain Bridge: linking engineering (Miller, Proc. ICE, 2006)
• Civil Engineering Heritage Scotland: Lowlands and Borders (Paxton & Shipway, 2007)
• Union Chain Bridge blog