Bristol Bridges: 4. Meads Reach Bridge

This bridge was another early morning visit, to an award-winning span which is probably unique in Britain.

Built in 2009 for £2.4m, the 55m long bridge is a rare all-stainless steel structure. It was designed by Price & Myers with Níall McLaughlin Architects, and built by M-Tec. You'll see it described on the internet as a "stressed skin" bridge, to make it sound like it's using aircraft technology, but to bridge engineers it's simply a twin half-through box girder bridge, conventional enough except for the geometry and material.

The two edge girders hold up the bridge and also double as parapets. They consist of continuously welded, perforated Duplex 2250 stainless steel plate (supplied by Outokumpu), wrapped around triangular diaphragms. A series of cross-beams and bracing frames are hidden within the deck, which is surfaces with perforated stainless steel planks (of which, more later). The entire bridge weighs about 75 tonnes, and sits on four hidden stub "legs", one at each corner.

The extensive use of stainless steel is a bold, and presumably expensive, choice. The bridge's skin is punctured with thousands of small holes, arranged in size according to the calculated level of stress at each point of the girder surface. The process of analysing the bridge, determining acceptable levels of perforation, and translating that back into a fabrication model, will have been complex and demanding, and is an impressive achievement.

Luminaires are located within the girders and below the bridge's floor, and the perforations allow the structure to glow from within at night.

The bridge won the IStructE Structural Award for pedestrian bridges in 2010, and I think justifiably so, as it is highly innovative and geometrically challenging. I haven't seen it illuminated at night, but in daylight, sunlight reflects off the skin in a very attractive manner. However, the bridge is not without its critics.

The main complaint appears to relate to the deck panels, which are clearly very slippery, to the extent that the bridge is disfigured by prominent warning signs at each end advising users to cross with care. Reportedly, bridge users who do slip and fall have frequently been injured by the "cheesegrater" quality of the decking. In one month, over 600 people signed a petition asking for the bridge decking to be replaced.

However, as with the nearby Valentine Bridge, this is a public amenity which is privately owned. Although the decking is clearly not fit for purpose, the public may find it hard to force change.

Further information:

• Google maps / Bing maps
• Exploded diagram
• Mondo ARC
• Meads Reach Footbridge, Bristol (BDOnline, 2010)
• Oasys Software case study

Bristol Bridges: 3. Valentine Bridge

I'm nearly caught up on a backlog of bridge visits. The next two are both in Bristol, where I've previously covered Clevedon Pier (yes, I know that stretches the definition of "Bristol" and "bridges") and Pero's Bridge.

Valentine Bridge was built in 2000 by Alfred McAlpine, to a design by Atkins. The steel cable-stayed bridge carries pedestrians and cyclists across Bristol's Floating Harbour close to Temple Meads railway station.

As with the nearby Meads Reach footbridge (for which, see the next post), the bridge was privately funded and is privately owned.

The structure is S-shaped in plan, with a triangular cross-section framed truss deck. The upper and lower chords are circular steel tubes, connected by I-section struts in a Vierendeel arrangement.

This is not a good bridge.

It seems evident that the client must have wanted a "landmark" bridge of some sort, hence the mast, cables, and general layout. However, little effort seems to have been put into creating an efficient or well balanced structure. Of the four back-stay cables, two are so slack as to appear un-stressed, presumably a result of a cable layout which appears baffling when viewed from overhead. A balanced layout could have been achieved with fewer cables working more effectively.

Some of the cable detailing also looks a little awkward, perhaps cheap, especially the anchor detail to the deck.

The curved stainless steel parapets seem to be trying a little too hard, with an over-pronounced arc to the parapet posts. The abutment at one end, a mix of bold brick and prison-fencing (to guard the bearings), is quite unfriendly, as are the (pointless) anti-cycling barriers which block the walkway entrance.

Most annoying of all is the decking, which seems to have been fixed incorrectly, making a pronounced clattering sound each time someone crosses the bridge. If you watch the video below carefully, you can see the deck planks actually bouncing up and down.

Further information:

• Google maps / Bing maps
• Structurae

Bristol Bridges: 2. Pero's Bridge

I've badged this and the previous post as "Bristol Bridges", as if you're going to get a whole series of bridges in the area that I've visited. Well, actually only the two, for now anyway. But I do plan to make a return visit to the area and cover a few more.

Pero's Bridge is a relatively recent addition to Bristol's scenery, having opened in 1999. It spans an inlet of Bristol Harbour, and undeniably improves pedestrian access in a very busy part of the city. Designed by Arup with artist Eilis O'Connell, it features an 11-metre long bascule span, flanked by two fixed spans.

Visually, the most obvious feature of the bridge is the two large "trumpets" forming the rear half of the lifting deck. These provide a counterweight mass to balance the main deck, so that minimum force is required to open or close it.

The bridge is unusual in being a modern implementation of the rolling bascule, which rocks back onto its supports like the exaggerated motion of a rocking chair. Teeth are provided on the support beams to ensure it does not move out of tolerance when it does so. Bridges of this type were once common: I've visited those at Queensferry and Liverpool previously. One advantage of the form is that the centre of gravity of the rolling span can be positioned so that it lies to the span-side of the centre of rotation when the bridge is down, but so that it moves to the rear-side of the centre of rotation when the bridge is up; in both cases, it holds the bridge secure in the rest position.

The bridge is operated by a hydraulic piston below deck level, which remains above the water level at all times (the level being constant in Bristol Harbour). In the photograph on the right, it can be seen that the deck is a box girder with strutted cantilevers. Much of the detailing below deck level is visually awkward, particularly the main pier supports for the roll path, including their uneven relationship with the other pier.

I find the bridge unconvincing overall. It's a reasonable engineering solution, but the counterweight trumpets seem awkward to me. To some extent their shape is compelled by the engineering demands - the bulk of the weight has to be high up so as to balance the main deck properly. Other elements of the bridge are over-fussy, such as the parapets, which with their complex array of posts, bars, rails, and mesh panels, offer little in the way of elegance.

The bridge is named for an African slave, Pero Jones, brought to Bristol in 1783 by a local merchant.

Further information:

• Google maps / Bing maps
• Wikipedia
• Structurae
• National Archives (information about Pero)

Bristol Bridges: 1. Clevedon Pier

The pier at Clevedon is one of my favourite bridges to nowhere. Built in 1869, its elegant iron arches make me think of the Pont des Arts in Paris. There are eight spans, each 100 feet in length, comprising a wrought-iron superstructure supported on cast iron screw piles. It seems impossible that such spindly supports could both carry crowds of Victorian pleasure-seekers and withstand the force of storm-tossed waves.

However, other than various minor alterations, they did indeed withstand them, and the collapse in 1970 of the seventh and eight spans from the shore was man-made rather than due to the forces of nature. The bridge was being load-tested for insurance reasons, with water tanks providing a load of 2 kPa, and while the first six spans passed the test, the seventh failed, dragging the eighth down with it. This seems a very low load capacity when you consider that a modern pedestrian bridge would be designed for 5 kPa loading, plus a safety factor (so, typically 7.5 kPa), but this simply highlights the conservatism of current codes.

Another aspect of the pier which would not comply with current codes is the presence all along both edges of continuous bench seating. This is prohibited on any modern footbridge because a child can stand on the seating and no longer has safe containment. At Clevedon, a sign simply asks visitors to supervise their children; it should also be noted that the pier is a private space, with a toll paid for access, rather than a public “crossing”.

When built, the pier took advantage of the availability of “top hat” section Barlow rails, surplus stock from the South Wales railway. These were riveted back to back to form columns and longitudinal arches, supporting wrought iron plate girders along the edge of the deck. They're easily visible in the photograph on the right, showing the original pier construction.

Although the reconstructed spans match the general elevation of the original spans, both the girders and support framework are in modern welded steel, and to my eye look out of place on a Grade I Listed structure. If you compare the photograph on the left with the previous one, you can see some of the differences: the column and arch sections are cleaner, with no rivets and less interesting shapes.

Another photograph of the original construction shows further differences. The trestle supports have different horizontals at mid-height, with a straight bar on the new sections (above left) in place of lenticular struts on the old sections (right). The new trestles have a horizontal cross-beam at the top, while the old sections are arched at the top. Coupled with the loss of detail in the main deck girders, I find the pier reconstruction disappointing in its lack of adherence to the original construction. You wouldn't realise this from most websites or texts, which give the impression the pier was rebuilt exactly in its original form.

As with Southport Pier, after the load testing collapse the local council at Clevedon proposed to demolish the pier, but public outcry eventually won a reprieve. Some of the funding for rebuilding, and further refurbishment, was provided by many individual donations, commemorated with plaques inlaid into the deck planks and mounted on the rear face of the benches. This crowd-funding was supplemented by grants, and I think it's a great way of raising money and demonstrating popular support for a project.

Although this is a pier, not a bridge, it clearly has many similarities. I wanted to feature it here because it's such a supremely elegant structure, with a slenderness and loveliness of form that few such structures can match. Poet Sir John Betjeman described it as "the most beautiful pier in England", and it recently won the "Pier of the Year Award" for 2013. Money is currently being raised to fund construction of a new visitors centre.

Further information:

• Google maps / Bing maps
• Wikipedia
• Structurae
• Engineering Timelines
• British Listed Buildings
• Clevedon Pier and Heritage Trust
• National Piers Society
• The Heritage Trail
• Civil Engineering Heritage: Wales and Western England (Cragg)