28 September 2013

London Bridges: 32. Hammersmith Flyover

Here's more concrete, this time going back to roughly a decade before London Bridge.

Designed by Maunsell and Partners, Hammersmith Flyover was opened in 1961. It was intended to relieve congestion on London's Great West Road, by elevating the through traffic and isolating it from local traffic junctions. The structure rests on relatively narrow central columns, minimising the impact on the road layout below.

It has sixteen spans for a total length of 622m, and is 18.6m wide. The deck is integral with the piers and consists of a single multi-cellular variable-depth post-tensioned concrete box girder. This was built from precast box segments each 2.59m long, alternating with 0.3m long precast "ribs", which cantilever beyond the box to carry the full width of the deck slab. The precast elements are stitched together with 75mm thick in-situ concrete joints. The whole system is explained in a diagram taken from the technical paper on the bridge design (see later in this post).

There is only a single expansion joint, near the middle of the bridge, pictured here. The piers sit on roller bearings below ground level, allowing for thermal expansion.

Hammersmith Flyover was certainly one of the earliest bridges to be built as precast, segmental and prestressed: Troyano's Bridge Engineering: A Global Perspective credits the first as being Nagatinsky Bridge, in Moscow, also completed in 1961 (although Structurae dates that bridge to 1966), but his generally excellent book omits the Hammersmith Flyover entirely. The Concrete Bridge Development Group claims, incorrectly, that the technology was first applied in France in 1962.

This is essentially the same form of construction that would be used again in London Bridge some ten years later. Indeed, segmental post-tensioned concrete construction has changed very little since it was introduced - the major difference if one of these bridges were built today would be the omission of the in-situ concrete stitches.

It's interesting to compare Hammersmith Flyover to a much more widely-lauded urban viaduct of the time, Pier Luigi Nervi's Corso di Francia Viaduct, completed in Rome just one year before. The Nervi bridge boasts more aesthetically shaped piers but is otherwise far less ambitious: it uses only simply supported precast beams, of much shorter span, and thus requires two rows of T-shaped piers. It lacks the smooth lines of the Hammersmith bridge, and its footprint at ground level is much greater. Nervi never made much use of prestressed concrete and was, it has to be said, never a great designer of bridges.

Many of the choices made in the bridge design have a positive aesthetic impact, but were made for other reasons. One of the key features which distinguishes the bridge from other structures of a similar type is the presence of the cantilever ribs. There's a section in Fritz Leonhardt's famous book Bridges which discusses alternative options for concrete viaducts (and, again, which omits any mention of the Hammersmith Flyover). This shows a variety of systems, almost all united by the smooth uniformity of their surfaces - they are largely texture-free. The Hammersmith ribs not only break up what would otherwise be massively monotonous concrete surfaces, they also create a visual rhythm, and make the bridge more comprehensible seen in perspective. However, their primary purpose was not aesthetic, but was simply to facilitate rapid erection of the deck slab while remaining within a construction footprint constrained to the width of the spine beam.

The curved bridge soffits also add to the sense of rhythm, and are more attractive than the constant-depth girder which would surely have been more economic. Again, aesthetics were not the primary driver in choosing this geometry - the curvature provides the necessary depth and strength over the piers, while maximising highway headroom over the roads which pass under the middle of each span.

The box girder webs are also curved, when seen in section. They are a constant width at the top, but the intersection of the web curvature with the soffit is such that the underside of the girder varies in width, being widest at midspan and narrower at the piers, as can be seen in the very first photograph above.

As with London Bridge, the papers in the ICE Proceedings are well worth seeking out and reading, they are far better than what tends to be published today, with copious diagrams and photographs. The assembly diagram shown below is one highlight.

The bridge was built for a cost of £1m (roughly £18m at today's prices). As well as being structurally innovative, the bridge incorporated roadway heating cables, intended to avoid the need for application of road salts. These ceased to work a long time ago, and presumably road salts were applied to clear ice ever since.

One contributor to the Discussion in the ICE Proceedings stated that "It was of great interest that the structural members at Hammersmith should not cost a halfpenny to maintain over the next hundred years, which was more than could be said for any similar structure erected on the site in steel". Clearly the contributor lacked any gift for prophecy, as it was announced in December 2011 that "serious structural defects" were to force the closure of the bridge.

It was reported that significant corrosion had been found in the prestressing tendons, in the vicinity of the piers. The prestressing layout for the bridge is both unusual and simplistic: the structure was built span-by-span, rather than by cantilevering from the piers, so there is significantly less prestressing at the top surface over the piers than might be expected in a continuous viaduct of this sort. Prestressing tendons are anchored to one side of the pier in the upper surface of the top slab, passing in ducts through the slab. They continue horizontally below the slab until they pass through the pier, and are then deflected downwards, adjacent to the webs. At midspan, they pass through ducts in the bottom slab, before being deflected back upwards towards the next pier in line. Where they are external to the precast concrete, they are protected by a concrete surround cast at a later stage. The diagram below shows the typical layout.

I haven't seen a detailed description of the areas which are most seriously corroded, but the presence of anchor points on the upper surface, and the ducting through the top slab, must both have been highly vulnerable to the ingress of water and road salts.

The solution proposed by TfL's consultant Amey was to add additional prestressing into the top slab, by providing new tendons symmetrically above and below the slab. These are capable of being restressed in the future to carry more of the load if and when the original prestressing deteriorates further. New concrete barriers were installed in the central reservation of the highway to protect these strengthening cables. A first phase of strengthening work was completed by Freyssinet for the five most seriously affected spans in 2012, with an estimated cost of £10m reported. In June this year, a further £60m contract was announced, to tackle the remaining spans, address underlying problems such as rewaterproofing and surfacing the deck, refurbishing the drainage, and other works such as bearing renewal. Hammersmith and Fulham Council were reported as hoping no further money would be lavished on "this monstrosity", favouring instead development of a longer-term tunnel scheme to replace the viaduct entirely.

A tunnel had in fact been considered before the viaduct was conceived, back in the 1950s, but was rejected due to the presence below ground of the London Underground railway, a large sewer, and a high water table, felt at the time to make the cost of tunnelling prohibitive. It's difficult to see how that will have changed, now with the added complication of building a tunnel directly below a massive flyover. One proposal is for a 3.7km long deep bore tunnel (well below the level of the aforementioned obstructions). The longest road tunnel in Britain at present is the 3.2km Queensway Tunnel in Liverpool, built in 1934 at a cost which would be roughly equivalent to £500m today.

Further information:

23 September 2013

London Bridges: 31. London Bridge

London Bridge is, with some justification, one of the most interesting bridges in Britain, with a fascinating history. There have been a number of bridges at the site, with the earliest permanent structures dating back to Roman times. Significant reconstructions took place in 1176-1209 (Peter of Colechurch's Old London Bridge), 1824-1831 (John Rennie's New London Bridge), and 1967-1972 (the present bridge, by Mott Hay and Anderson with William Holford and Partners). I see these as a little like Doctor Who's regenerations, with the present bridge very much the Peter Davidson of the series, not the Tom Baker or William Hartnell.

I will say very little for now about the earlier bridges - there would simply be too much to say, which will have to await another opportunity. I've similarly limited the references at the end of this post to only those which have something useful to say about the current structure.

John Rennie's bridge lasted for about 140 years. It comprised five granite arches, found to be slowly sinking into the ground even in the 1920s. It had been built alongside its predecessor, which remained in use throughout construction, but the same approach could not be taken again. Instead,when a decision was taken to replace the bridge, it had to be retained in use while the new bridge was built along the same alignment. The new bridge was designed for four lanes of traffic, two unusually wide footways, and even to support a proposed overhead walkway equipped with travelators.

What can be seen today offers little if any evidence of its exceptionally complex construction sequence, which is a shame, as this was a truly exceptional project. Peter Jackson's book, London Bridge: A Visual History has diagrams and photographs taken during construction, but the best document of what took place is Albert Yee's London Bridge Progress Drawings. Both books are well worth seeking out, and I've included a couple of Yee's marvellous sketches below to illustrate the construction work - click on any image to see it full size.

To build the new bridge, giant shafts were excavated below the existing structure, with workers hand-digging these some 25m below the riverbed. The balustrades of the Rennie bridge were taken down and a giant overhead truss erected at one edge of the bridge. This was used to assemble twin-celled precast concrete box units over the river. Gaps between the units were filled with in-situ concrete and the assembly progressively post-tensioned. Match-casting and glueing was considered but rejected. The prestressing is a mixture of internal tendons in the flanges, and external tendons adjacent to the webs.

Once one side of the bridge was complete, the second edge was rebuilt in the same way. This allowed foot and road traffic to be transferred onto the new bridge decks. Demolition of the original bridge could then commence, with the internal spandrel walls and infill removed first. The gantry truss supported temporary panels below the deck while the arch voussoirs were removed one by one, destined to be reused to clad a reinforced concrete replica bridge at Lake Havasu City in Arizona. Once this was complete, the gantry was used to assemble the final two box girders in between the first two deck sections.

Every part of this operation was an amazing piece of work in its own right, including the massive gantry structure, which weighed 1900 tonnes. This was designed both to support the weight of the existing structure during demolition, and the weight of the new precast units during construction, whichever gave the worst effect.

The geometry of the bridge was determined largely by navigational requirements. The choice of four box beams was made to facilitate the traffic management needs during demolition and construction. Although the bridge appears continuous, it is actually not. There is little sign of its articulation on the monolithic elevations, but two expansion joints are visible in the carriageway in the centre span. The bridge consists of two large cantilevers which support a short central suspended span, as shown in the illustration above, taken from the ICE Proceedings paper on the bridge design. The technical papers, I have to say, are well worth reading, a model of comprehensive, clear information, and the subsequent discussion in the Proceedings is a source of information regrettably generally absent today.

The bridge today is mainly characterised by a sense of corporate blankness, particularly apparent with the granite block parapets, which look like they could adorn the front of a City bank as happily as sit on the edge of this enormously wide bridge. The sheer width is essential from a practical point of view, particularly at rush hour. Nonetheless, it's hard not to think how much it could be improved by some simple planters, seating or even a few trees. The footways were originally equipped with heating elements to prevent ice formation, but these have long since ceased functioning.

I do like the underside of the bridge, where the concrete can be seen close up. This is the only place that a trace of its construction history is still apparent, and it shares the characteristics of the best brutalist London architecture of the 1970s. The diaphragms between the beams appear to be an oddity - Brown's paper on the bridge design states they were not needed for live load distribution, but only to cope with high concentrated loads from the proposed overhead walkway, which of course, was then never built.

Further information:

18 September 2013

Islington Place Footbridge, Birmingham

Birmingham is not blessed with contemporary bridges. I've reported on the rather unimpressive Mailbox Footbridge before, and the Selfridges Footbridge, which is only a little better.

The Islington Place Footbridge spans the Birmingham and Fazeley Canal at Farmer's Bridge Locks. I've been unable to find out who designed or built it, so if you know, please post in the comments.

My initial reaction to this bridge was simply that it was completely unnecessary. It's such a short span that you could almost cross it with a few short planks and a bit of sticky tape. This is clearly a bridge where its role as a sculpture is far more important than as a crossing.

However, I ended up feeling quite a bit more generous towards the bridge. The crossed hoops work very well as a visual device, looking very different from a variety of angles, and offering a landmark while still remaining small enough not to be overbearing.

The cable array doesn't look quite right to me - I think the ties between the two hoops should run much more horizontally, but they do seem to be actually doing something, which is not the case some bridges that I've seen.

The steelwork below the deck seems a little bit over-engineered to me, but this doesn't really matter given that it's tucked mostly out of sight.

Further information:

16 September 2013

Bridges news roundup

I'm preparing posts on a handful of bridges I recently visited in London and Birmingham. Until those are ready, have some news:

Plan for viewing platform at top of Forth Rail Bridge
Yes please!

A bridge from the future that never was
I reported positively on Calatrava's Katehaki Bridge, in Athens, back in May. Here's a gloomier appraisal: "The bridge has never really been loved."

The architect behind 'eyesore' Dresden bridge
Unesco's Most Wanted speaks out.

Canberra bridge wins prestigious design award
Landscape architecture award for masterplan including unpretentious little bridge.

Point Resolution Bridge / Warren & Mahoney
Slickly futuristic. Super-slender piers. Structurally irrelevant arch features, sorry "armatures". Good? Bad? Who knows. Many more photos here.

Høse Bridge / Rintala Eggertsson Architects
A very "industrial" looking bridge in a natural landscape. I'm unconvinced by the desire to enclose the bridge in a setting like this, but it's unfair to judge only by photos.

Craigieburn Bypass / Tonkin Zulaikha Greer + Taylor Cullity Lethlean
A very different setting, another enclosed bridge.

Amazing camouflaged bridge
Go on, you must be able to think of a few more bridges out there that could do with being turned "invisible".

11 September 2013

Cumbria Bridges: 11. Bridge House, Ambleside

This bridge is a curiosity more than anything else.

In Bridges in Britain, G Bernard Wood described the bridge as follows: "The tiny bridge-house is a waif of Time, stranded between road and car-park".

It was reportedly built over 300 years ago as an apple store for the nearby Ambleside Hall. It was apparently constructed over Stock Ghyll in order to avoid land taxes.

In 1926, it was purchased and passed to the care of National Trust, who still use it as an information centre and shop.

Further information:

10 September 2013

Reminder: Footbridge 2014 call for papers

Here's a reminder as the deadline is now approaching:

The ever-popular Footbridge conference has announced a call for papers for its 2014 event. The deadline to submit abstracts is 30th September 2013.

The conference theme is "Footbridges: Past, Present and Future", and papers have been invited addressing the following sub-themes:
  1. Historical and heritage structures
  2. Dynamic response and structural behaviour
  3. Inspirations in footbridge design
  4. Planning, design and construction of sustainable footbridges
  5. Advances in materials technology for footbridge construction
  6. Future directions in footbridge design and construction

08 September 2013

Cumbria Bridges: 10. Trevor Woodburn Bridge

Now, here's a quite remarkable little bridge, which I think is totally unknown beyond its immediate vicinity but which easily deserves to be better publicised.

It lies on a foot and cycle trail connecting Elterwater to Skelwith Bridge, near Ambleside in Cumbria. It spans the River Brathay a short distance upriver from the waterfall, Skelwith Force. The creation of the trail was proposed in 1998 by local man Trevor Woodburn, and the bridge is named after him. It was completed at the end of 2006, and officially opened in July 2007.

The bridge was designed and fabricated by specialist architectural and sculptural metalwork firm Chris Brammall Ltd, who have also been responsible for some highly sculpted bridge parapets at Staveley and Sunderland. It cost £225,000, spanning 20m and weighing 16 tonnes.

The bridge deck consists of metal plate on steel members, while the parapets comprise a series of steel flat posts supporting an oak handrail. According to the designer's website, the geometry of the posts was "inspired by the tectonic pressure patterns in the surrounding rock faces".

The posts each have a slightly different geometry, with a slight indentation in profile developing towards midspan into a pronounced kink. The effect is to create an intriguing sculptural surface which presents an attractive profile from almost any perspective, yet dissolves entirely on a closer view.

The inner and outer surfaces of the parapet are formed from the same geometry yet visually are very different.

I think it's a gorgeous little bridge, and a genuine enhancement to visually and environmentally surroundings.

Further information:

05 September 2013

Cumbria Bridges: 9. CKPR Bridge 75, Crozier Holme

Finally, here's the last of this set of disused railway bridges on the Cumbria, Keswick and Penrith railway lines.

Bridge 75 is an upright bowstring truss spanning 101 feet. For this bridge, I have a copy of a bridge record drawing, taken from John Rapley's book (see link at the end of the post). This shows the bridge before any strengthening was added in 1931-3. You can compare this against the photographs to see what was changed.

Of the various upright trusses on the line, this one has the most substantial overhead bracing. Otherwise, it is broadly similar to the others. It can be seen looking down to the river from the adjacent A66 highway bridge.

For anyone who would like to visit these bridges, there are details and directions in the Bowstrings over the Greta leaflet (linked below) and also on the Lake District Miles Without Stiles website. Free parking is available next to Keswick leisure centre, and there's a splendid pub in Threlkeld where weary, hungry or thirsty pontists can rest before returning along the route.

Further information:

04 September 2013

Cumbria Bridges: 8. CKPR Bridge 74

I'm nearing the end of this set of bridges which formerly comprised part of Thomas Bouch's Cockermouth Keswick and Penrith Railway (CKPR) line.

This penultimate bridge is another upright bowstring truss. Unlike the previous examples, Bridges 69 and 71, this one has overhead bracing between the trusses, although so little as to be barely worth having.

The bridge lies a short distance to the east of a lovely, short little railway tunnel. It spans 122 feet and has a pronounced skew. As with most of the other bridges on the line, it was strengthened during its lifetime by the addition of new cross-girders, additional diagonal bracing struts to the top chord of the trusses, and additional vertical stiffening on the face of the arches. You can see clearly in the photographs how much heavier the strengthening members are compared to the original metalwork.

Although the railway line was closed in 1972, and converted to a foot/cycle trail quite recently, there are proposals to reopen the entire line as a railway (not just the Keswick to Threlkeld section forming the trail). These plans are being promoted by an independent group, not by any government body, and I have to say I think the likelihood of it ever happening is somewhere very close to nil.

They argue that the retention of the original railway bridges would make it easier to reinstate a railway line, but from what I saw when I visited, the present cycle and foot trail is extremely popular, and diverting it would be an expensive and possibly unpopular option.

Although the bridges may broadly have the strength to carry local passenger services, they are not in marvellous condition and would require close examination, repair and repainting to be of any use for a new railway line. There are difficult obstacles elsewhere on the route, where the original track formation has not been protected against encroachment, and it seems unlikely to me that there is a business case of sufficient strength to attract the necessary private investment.

Further information:

03 September 2013

Cumbria Bridges: 7. CKPR Bridge 73, Rowsome

This is the last of the "upside-down" bridges as you head east along the Keswick to Threlkeld trail. This one is the shortest span at 80 feet.

Unlike the other two similar structures I've already covered, this bridge has retained small X-frame parapets, giving it quite a different appearance. It's still difficult to imagine the bridge as it must have been when it carried rail traffic however. I assume it had a timber deck with the rails supported on large longitudinal timbers, width the crossbeam outriggers supporting a maintenance footpath.

I was able to get a better view from directly underneath this bridge, showing the plate girder which was added when the bridge was strengthened in 1924-8. I wonder how the load was shared between the new girder and the original bowstring trusses, and suspect the girder did the lion's share of the work after it was introduced.

It's interesting to compare the two side views of the bridge in these photographs. The north face is considerably "greener" than the south, as you might expect. I wonder what the original engineers, Thomas Bouch and colleagues, would think of their structures being left in such a visibly dilapidated state. Perhaps they would just be pleased to see the bridge still in use, nearly 150 years after the railway line was originally built.

Further information