11 March 2014

Tyneside Bridges: 3. Byker Metro Bridge

The third and most recent high-level bridge to cross the Ouseburn Valley is the Byker Metro Viaduct, completed in 1982 for the Tyne and Wear Metro.

It is 815m long and curves in between the Ouseburn Viaduct (1839) and Byker Bridge (1878).

I only had time to visit its western end, which spans the valley. Much of the viaduct, at its eastern end, runs at about 6-7m above ground level, and is visually less imposing.

The bridge was the first glued segmental precast prestressed bridge to be built in Britain. The main spans are 69m long, with haunched concrete box girders supported on twin-stem piers. The bridge's appearance was considered significant enough for the design to be presented to the Royal Fine Art Commission in 1975.

The form of construction lends itself to a site where there was highly restricted access for plant. The concrete segments were precast off site, and winched down the valley slopes on temporary tracks. The length of each segment was just narrow enough to allow it to pass through an opening in the bridge piers, allowing the segments to be manoeuvred along a very narrow construction strip. They were then lifted and erected using the balanced cantilever method. Each segment was bonded using epoxy glue, and stressed in place with short length bars. Further prestressing was added in later stages.

The segments were "match-cast" i.e. each one was cast against its preceding segment, to ensure a perfect fit. Sawteeth keys in the webs, still visible in the completed structure, help align the units during erection. In practice, this method requires very careful measurement and control, as errors have the potential to accumulate over multiple segments. This is exactly what happened with the Byker Metro Viaduct, with a significant twist detected as more segments were added. Special segments had to be cast to correct the twist.

The finished bridge is aesthetically striking, and I think very successful. The plain finishes to the concrete deck contrast with a vertically ribbed finish on the piers and precast parapets. The ribs are nicely detailed, splaying out as the piers widen out towards their base. Horizontal feature grooves have been included on the piers to disguise construction joints.

The splayed piers both give an impression of considerable stability (required to resist centrifugal loads from the metro vehicles), but also make what are quite substantial concrete sections look quite elegant.

I think this is easily one of the most impressive and well thought out bridges on this scale to have been built in Britain.

Further information:

07 March 2014

Tyneside Bridges: 2. Byker Bridge

The second bridge to be built linking the crests of the Ouseburn Valley was the Byker Bridge, built in 1878 to a design by Robert Hodgson. Hodgson had been Robert Stephenson's Resident Engineer for the construction of the Newcastle High Level Bridge, and the original Byker Bridge very much resembled a railway structure, with its tall brick arches.

When first built, users of the bridge had to pay a half-penny toll, although this was withdrawn in 1895.

According to the ICE's Civil Engineering Heritage book, the bridge was widened in 1902, to cantilever the footpaths beyond the edge of the brickwork; and then the widening was "improved" in 1985, adding prestressed concrete beams on cantilevers to support both footways and new crash barriers.

Clearly, "improved" is used only in a technical sense, as the widening has utterly spoiled the appearance of the bridge. The original bridge appears not to have been a great beauty - its tall piers taper slightly, which seems attractive, but the semi-elliptical arches look awkward.

Now, the stubby little concrete cantilevers look quite horrible, and the effect of the deck widening is to hide the crowns of the arches, "squashing" them unpleasantly.

Further information:

05 March 2014

Tyneside Bridges: 1. Ouseburn Viaduct

I'm continuing my series of posts on the bridges of north-east England which I visited as part of an IABSE study tour last year. The second day of the tour had begun with a visit to the delightful Cragside Iron Bridge. From there we went to Newcastle-upon-Tyne.

Three tall bridges span the Ouseburn Valley in Newcastle-upon-Tyne. The Ouseburn Viaduct was the first of the trio to be built, in 1839, to a design by John and Benjamin Green. To place it in context, it was built ten years before Stephenson's High Level Bridge in the same city, and was completed in the same year as Brunel's Maidenhead Bridge for the Great Western Railway. At this time, anything other than a brick or masonry bridge remained relatively unusual on the railway.

Brunel's famous timber railway viaducts in Cornwall would not be completed for another two decades, but timber structures were springing up in various other places. In the same year as the Ouseburn Viaduct, the timber Scotswood Bridge was completed in Newcastle. Within the next decade, timber arch viaducts would be completed at Etherow and Dinting Vale, both near Manchester.

The Ouseburn Viaduct, along with the Willington Viaduct built at the same time on the same railway line, was originally a timber arch structure, built using the Wiebeking system. This involved the use of laminated timber sections, made from Baltic softwood, and preserved by Kyanising.

The timber bridge lasted for three decades, and was replaced in 1869 by the wrought iron bridge which remains there today. The pattern of the ironwork closely followed the geometry of the original timber structure.

In 1885, the bridge was widened, doubling the number of arch ribs, to carry additional railway tracks. Since then much of the valley floor has been infilled, hiding the original stone piers, which appear to have been much taller in old images and drawings of the bridge.

This wasn't an easy bridge to photograph, half hidden by trees and by adjacent structures.

There's a lovely quote in Benjamin Green's ICE paper concerning this bridge, from another consulting engineer:
"It appeared to me that to cross the Ouse Burn and Willington valleys, would be a work of so much labour and cost, that I would not conscientiously recommend the Committee to prosecute their plan. These difficulties, however, are likely to be removed, and most effectually surmounted by the great scientific and practical knowledge of Mr Green, who proposes to pass over them by bridges of peculiar construction, and at a comparatively light expense. To him I would most willingly transfer that responsibility, having no pretensions to skill in bridge building myself."
Further information:

05 February 2014

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:

23 January 2014

Salford Meadows bridge design contest winner revealed

Arup and Tonkin Liu have been announced as the winners of the RIBA Salford Meadows bridge design competition. I last mentioned this scheme back in December when the shortlisted designs were released for public consultation.

Jury member Renato Benedetti describes the design as "poetic, innovative and elegantly engineered". Designer Mike Tonkin highlights its "biomimetic design". Local mayor Ian Stewart noted: "We will now be working with our partners to find the funding to create this stunning new bridge in the heart of Salford that will add to the city’s global reputation."

Sometimes you really just have to sit back in your chair and wonder quite what was going through the minds of all those involved.

The "poetic" nature of the bridge presumably relates to the designer's concept that it was some kind of silvery tendril, sprouting across the river from a seed, an idea which informs not only the bridge's S-shaped plan but also the layout of landscaping at one end. This landscaping is vital to providing a DDA-compliant ramp gradient at a site which, from the contest submissions, appears to have a very large level difference. The Arup / Tonkin Liu design is made to work by eliminating large areas of car parking in front of a pub and adjacent university building.

The structural engineering is interesting. The arch is a triangular steel box section which curves in plan and also twists. Arup's analysis indicates 10mm steel plate to be sufficient, and deformation and local buckling is prevented by the insertion of steel tubes through the box, creating the bridge's distinctive Swiss-cheese appearance. They state that these tubes eliminate the need for the normal box stiffeners and diaphragm plates, and also that the geometry is arranged in such a way that each face of the box can be bent from a flat plate. I find the latter point very hard to believe, given the twisting geometry shown on the visuals, and would reserve comment on the ability of the tubes to eliminate other stiffening. The FEA diagram included in the contest submission is certainly not very convincing.

The bridge deck is of similar geometric complexity, consisting of two paired box girders, again in triangular form, which curve in plan, vary in depth, and intersect and diverge at different places along their length. Although lacking in cheeseholes, the geometry of their surfaces is probably even more difficult to fabricate, and if 10mm plate is used throughout, likely to be subject to all sorts of out-of-tolerance imperfection. It will be a shame if it doesn't look good close up, because the perforated arches provide a pretty handy climbing frame from which it can be examined.

The parapets are a much more conventional system, arranged vertically with steel posts, a timber top rail, and a steel mesh infill. The designer seems to have made the entirely reasonable decision for them not to compete visually with the hyper-real brushstrokes of the main structural elements, which is not unreasonable in the circumstances.
This is an extremely bold design, and one which will be challenging for any designer (let alone the poor builder) to progress. There's little in the contest submission to provide confidence that what is depicted can be delivered at a reasonable level of quality, and I think there's absolutely no chance of it being done for the £2m figure that the designer states.
As with the ill-fated River Wear Crossing, which also emerged from a RIBA competition jury, this is a bridge which is technically demanding to the extent that the risks involved in choosing it are very high indeed. There must be a strong possibility it will also end up as unbuilt, not that this would an unusual outcome for RIBA, with only 2 out of 7 of their bridge competitions that I looked at in 2009 resulting in construction.
Of course, none of these issues are reasons to reject the design. If the customer wants flash and whizz, and can tolerate the likely budget, then this does seem the flashiest and whizziest of the four shortlisted entries. The other three contenders appeared more pragmatic and certainly in some cases less expensive, but I admire the desire to reject conventionality and pursue a flight of fancy. Perhaps that is what was in the jury's heads.
For me, the technical issues are secondary to the aesthetic in this instance, and to me the winning design looks both alien and ugly. Alien, in its twisted melange of retro-futuristic detritus, like the disposable plastic parts from an Airfix model of some end-of-70s science-fiction TV show. And ugly in its carefree trashing of such old-time conventions as innate elegance or harmonious relationship to context.

09 December 2013

Bridges news roundup

Here are a few news links that you have probably already seen. I'm travelling over the Christmas period, so it will probably be January when I finish off my reports on the IABSE study tour of north-east England.

New Thames footbridge gets planning
I really dislike this so-called Golden Jubilee bridge, which is proposed to cross the Thames between Battersea and Chelsea Harbour in London. The proposed span arrangement, with a short middle span flanked by two larger spans, with correspondingly larger support arches, just seems to have a really awkward rhythm to it. Although the bridge has now won planning consent, it still appears to have no funding. Incidentally, if you don't have a BDonline subscription, you can read their news stories by simply copying-and-pasting their headline into Google, then following the link from there: this bypasses the subscription protection.

£60m boost for Thomas Heatherwick's garden bridge
The Golden Jubilee Bridge is a ridiculous enough proposal, but the Garden Bridge is an absolute disgrace, an absolute eyesore where money is no object if it serves to boost the vanity of all involved. The total project budget is £150m, which is absurd for a bridge which is so utterly unnecessary.

Striking but useless… just like Boris
It seems I am not alone in this opinion. "It has just the right combination of whimsicality and instant-wow theme-park tackiness to make it a likely Boris project."

Shortlist announced for Hisingsbron in Gothenburg
So far as I can tell using Google Translate, five proposals out of 24 submissions have been shortlisted in this competition to design a new moveable bridge. The finalists are Snøhetta / WSP, Zaha Hadid, Tyréns / Beam / Schlaich Bergermann, Dissing + Weitling / Lenonhardt Andrä, and Wilkinson Eyre / Ramboll. That's a mighty impressive roll call of bridge architecture names, along with one or two less so. All five shortlisted entries are available online, where you can play guess-the-architect. All I can say is that three out of the five submissions are quite horrible, and you really have to wonder what the participants were thinking.

Beautiful bridge planned for China's Hunan province evokes knots, Möbius strips
This depressingly awful bridge proposal has been all over the architecture blogs, but few have managed such an inappropriate title. Beautiful? In the eye of the beholder, perhaps. If they are blind.

I told you so, St. Patrick’s Island Bridge
Following flood damage during construction earlier this year, parts of the RFR / Halsall designed footbridge had to be disassembled, setting back the date for the bridge's opening. Tallbridgeguy takes the opportunity to say "I told you so".

04 December 2013

Shortlist published for Salford Meadows bridge competition

The four shortlisted entries to the RIBA Salford Meadows footbridge design competition have been made available online, and also as part of a public exhibition. Comments are invited from the public, with an online deadline of 6th December.

I previously showed some of the shortlisted entries here, as well as a selection of the many unsuccessful entries. I gave some more detailed comments on the contest back in June.

I'm a bit pushed for time right now, but if I get a moment, I'll post some more images and thoughts here.