06 December 2016

London Bridges: 45. Greenwich Reach Swing Bridge


This is the last of four bridges that I visited on a recent trip to London, and the most recent, having only been completed in January 2015.

This swing bridge spans the mouth of the Deptford Creek, where it empties into the River Thames, and it forms part of the Thames Path on the river’s south bank. The Creek here is navigable, so this is an opening bridge. It was designed by Flint and Neill with Moxon Architects, with mechanical engineering by Eadon Consulting, and built by Raymond Brown Construction with SH Structures.

I had seen this bridge design presented at a conference, and came to the site expecting a level of disappointment. Elevations and photographs of the bridge indicated it to be somewhat cramped and truncated in its appearance. The mast seemed too short, cut off just above the cable stays rather than allowed to reach a polite conclusion higher in the air, and the 8m backspan seemed far too short to properly balance the 44m main span, whether structurally or visually. Awkward rectangular casings surrounding the bottom of each cable were also a real distraction.

It's worth noting that this was a design-and-build project, a process often noted for its ability to compromise quality in the pursuit of cheapness. The project had previously endured a stop-start history, with a number of designs proposed without ever getting anywhere.

In reality, the bridge was not only less awful than I had feared, but quite impressive in many ways. I got the feeling that here was a design team doing their best to eke what quality they could out of a tight budget and highly constrained site.

For the most part the bridge design is attractively simple, and I particularly like the parapets, which have a very clear and uncomplicated design, making good use of stainless steel handrails and infill mesh.

The front span of the deck is a plated box girder with outriggers supporting the main deck plate, while the back span is a plated box formed from a simple folded geometry. It is over-large compared to the front span, because it is so short that it needs to be deep enough to carry sufficient ballast to maintain even loads on the bridge's support. I'm guessing that land ownership restrictions are what led to it being so short.

The parapet mesh is also featured below deck level, presumably to prevent any attempts at climbing the bridge from underneath.

The mast is also in plated steel, although in a different manner. The masts of cable-stayed footbridges are normally hollow-sections, to provide sufficient strength and stiffness against twisting effects. Here, the mast comprises two steel plates arranged in the form of an open-tipped V, held apart by flat plates in a "ladder" arrangement. The bracing plates are inclined diagonally, which provides a modicum of torsional stiffness and prevents the main plates from buckling.

Flat plates exposed to wind often generate vortex shedding behaviour, which can cause vibration and even damage. The mast therefore incorporates a tuned mass damper half way up its height, hidden behind a cover plate, to prevent excessive vibration.

An interesting feature of the design is the use of colour. Most of the bridge is painted a very pale grey. Yellow is used to highlight the main elements of the bridge opening mechanism. A much darker grey is used for the bridges two approach ramps and western staircase, presumably to visually demarcate the main structure from ancillary elements.

The darker grey is also used on the box girder below the main deck span, including the various struts which support the deck plate. The effect is to put the structure below floor level in permanent "shadow", emphasising the thin fascia plate along the edge of the deck and making it look very slender. This is fine, but I don't think it works very well where it meets the back span: it emphasises the mis-match between the two, and leaves the front span looking far too spindly to be quite right.

I also can't admit to being a particular fan of the cable stay protection sleeves, which I assume are there to prevent vandalism. This is a difficult detail to treat successfully, and I've certainly seen far worse elsewhere, but if they are necessary, they are a necessary evil. Compare the similar detail on the Media City Footbridge, which has no anti-vandal shrouds but which elevates the level at which the cables connect to elements of the deck steelwork.

I do, however, love the timber benches which are situated below each of the cable shrouds. These are beautifully shaped and very well made. Each one is a different size, reflecting the different inclination of each cable that they sit below. They serve the dual function of providing a resting place and also of preventing people walking or cycling across the bridge from accidentally hitting their head on the cables.

I also very much like the various gates which are used to keep people off the deck while the bridge is being opened. There are four in total, each one detailed carefully according to its location. At the rear end of the bridge, there is a curved gate which slides into place below the approach steps, and a simpler swing gate to close off the ramp (also where the bridge operator stands to control the bridge movement).

At the front end of the bridge there are two swing gates which close off the steps and ramp, and when not in use, these are detailed to dovetail against each other, minimising the space they take up. It's a neat, very well-considered detail.

Who would have thought there would be so much to discuss on what is ultimately a relatively modest bridge? To be honest, there's quite a bit more which could be said, but I think this is enough. I've included a few more photographs to illustrate some of the other features of the bridge.

Perhaps my favourite has little do with the bridge's detailed design, but more to do with its situation at the mouth of the Deptford Creek. All along the south bank of the Thames in this area there are views across towards London's Docklands district, but there's something about the bridge which makes you stop and look afresh. It's a crossing point and also a vantage point.









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05 December 2016

Footbridge 2017 reminder


We're getting close to the end of the year now, which means this is probably a good time to remind any interested readers about deadlines for the Footbridge 2017 conference.

The conference will take place in Berlin next year from 6th to 8th September. The organisers are hoping to take the event in different directions to previous Footbridge conferences. As well as papers on innovation and dynamics, they are particularly looking for submissions which "tell a story" about a design or perhaps a new material or construction idea. They want contributors not just to share the facts, but explain their learning, to discuss not only their own work but enter into debate with others. It has the potential to be much more interesting and inspiring than a conventional conference.

They're also running an open request for design submissions, having identified six sites in Berlin where no footbridge is currently planned, but where one could be considered. It's not a competition as such, but a way to inspire creative contributions, which may be featured in a special book to commemorate the exercise, as well as discussed at the conference itself.

I'm particularly pleased to see that they plan to publish the conference papers freely online in April 2018, which I think is an excellent initiative.

The deadline to submit abstracts or to register to submit a design is 1st January 2017.

01 December 2016

London Bridges: 44. London & Blackwall Railway Viaduct


This is a much more attractive bridge than its next door neighbour, the footbridge discussed in the last post.

The London and Blackwall Railway was one of London’s first railway lines, completed in 1840 to connect London’s Docklands to the City of London. The engineer was Robert Stephenson, although the route was planned by John Rennie.

Adjacent to the Limehouse Basin, the railway is carried on a particularly splendid brick arch viaduct. According to a plaque nearby, this was designed by Robert Stephenson and George BidderHistoric England’s website states (I'm pretty sure incorrectly) that it was designed by Robert’s father, George Stephenson.

The viaduct is interesting for several reasons. The typical spans have semi-elliptical arch barrels each comprising four brick rings, but they vary in span, with span increasing towards the canal. There are then three main segmental spans, much longer at 87 ft span (26.5m), with arch barrels of bonded brickwork effectively the equivalent of ten brick rings thick.

A contemporary account from 1840 states that the more typical arches are of 30 ft span, "constructed of five rings, but shew only three on the face, which gives the entire structure a light appearance". This is obviously wrong, with four rings being visible, but it would be no surprise to find that the arch barrel below the tracks is thicker.

The 1840 account also explains why the bridge has cast iron rather than solid brick balustrades: "iron standards of the railing, very properly introduced instead of solid parapets, the ill effects of which are daily experienced on the Greenwich Railway: the noise to passengers is of a stunning description".

The main spans have stone piers and springing stones, as well as stone cornices and keystones, all topped with the attractive cast iron balustrade. However, the main thing that impresses me about this viaduct is the shear boldness and depth of the main arches. I’m no expert, but these look to have been very ambitious in brickwork at a time when many large span arches would still have been in stone masonry.

The bridge has lasted well, and following the closure of the railway in the 1960s, it was revived as part of the Docklands Light Railway. As can be seen in the photographs, some of the piers have been festooned with small pipework, which I assume to be an ad-hoc attempt to direct water seepage in the piers to a suitable drainage system.




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29 November 2016

London Bridges: 43. Limehouse Basin Footbridge

While visiting the Rotherhithe Tunnel footbridge, I spotted this span through a gap between some buildings, and decided to take a closer look.


I don't know who designed or built this bridge, or when, and indeed that's probably for the best.

The bridge spans the canal right next to Limehouse Basin, just below the Commercial Road Locks. It provides a key link in the footway around the canal basin (now part of the Jubilee Greenway), spanning Regent's Canal.

It's a steel cable-stayed bridge, with a portal frame tower supporting the deck via an asymmetric fan-type cable layout. There are three pairs of main span cables, and one pair of back-stay cables. The bridge deck consists of steel edge girders supporting curved steel parapets and a timber plank deck.

At first sight, it's a slightly clunky but unobjectionable bridge, noticeable mainly for being awkward in height relative to the adjacent railway viaduct. There are two landings on the span which would have been better eliminated as they put ungainly kinks into the otherwise smoothly curving alignment of the main bridge steelwork.

Approaching more closely, two other features stood out for me.

The first is that the support arrangements are odd, with the deck bolted directly to steel supports at both the pylon and at the "tip" of the cable-stayed deck. With an asymmetric cable-stayed bridge, the normal arrangement is to fix the span at the position of the back-stays, and have an expansion joint and sliding bearings at the tip, so that the deck is loaded only in compression, and thermal expansion and contraction is catered for.

The second thing of note is that not only are all the support cables pretty slack, one has a pronounced kink in it, showing that it has failed inside a turnbuckle connection. Slack cables are not unusual on poorly designed short-span cable-stayed footbridges: it's often difficult to get sufficient weight in the bridge deck to ensure that all the cables remain tight. But it's unusual to find that none of the cables are tight, as this indicates that the entire deck is gaining no support from the cables.

On closer inspection, I spotted a half-joint in the main deck girders towards the tip end of the deck. This explained the fixed support at this end of the bridge, as any movement can take place in the half-joint instead. However, I was completely unprepared to then find a second half-joint on the other end of the span, closer to the mast, between the first and second pairs of stay cables.

This is a truly bizarre design. If the bridge deck has a genuine half-joint at both ends, then it presumably cannot transmit axial loads in either direction. However, a cable stayed bridge cannot function without axial loads in the deck, as these are necessary to balance the horizontal component of forces in the inclined cables.

The conclusion is therefore simple: none of the cables are in fact carrying any load from the deck, nor are they designed to do so. This accounts for their slackness. All four pairs of cables, and their supporting mast, are structurally redundant, just an affectation.

One positive thing about the slack cables is that this allows them to be threaded through the parapet railings in order to be connected directly to the deck stringers. Truly, this has to be one of the oddest arrangements I've seen on a bridge like this.

This is not the first time I've encountered "fake" cable-stay bridges (see, for example, this bridge in Salford). However, it is probably the most gratuitously stupid example that I've seen. There is, I think, sometimes a place for deceit in structural design – sometimes it may be necessary to give the visual impression of something rather than the reality because otherwise a structure may simply look wrong.

Here however, there is already a fine Victorian railway viaduct that can be admired – there was never any need for a short-span footbridge to try and compete for attention in this deeply ridiculous manner.






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27 November 2016

London Bridges: 42. Rotherhithe Tunnel Footbridge


This bridge has been in place since long before I started writing this blog, so it's perhaps a surprise that I've managed to cover 41 other bridges in London before finally getting round to this one.

This footbridge spans across the northern approach road to Rotherhithe Tunnel in east London, providing a direct route from St James Gardens at its north end to various streets and housing at its south end. The south end spills onto a piazza behind the horrible post-modern ventilation building which sits over the west portal of another tunnel, the Limehouse Link, completed in 1993.

The bridge connection is useful but not vital. There’s an alternative route a short distance to the east, but that involves crossing the Rotherhith Tunnel roadway at grade, so the bridge is a welcome alternative.

The footbridge was built for £550,000 in 1998, to a design by Nicholas Lacey and Partners with Whitby Bird and Engineers and WS Atkins. It was reportedly nicknamed the "butterfly bridge" at the time. Obviously something was in the air as a similar twin inclined arch bridge also nicknamed the "butterfly bridge" was built in Bedford the year before, with a different design team.

Whitby Bird and Lacey also collaborated on the St Saviour's Dock swing bridge around the same time, and there are definitely some similarities.

The Rotherhithe span comprises two tubular steel arches from which a steel bridge deck is suspended by stainless steel cables. Each arch rib has twin sets of cables, arranged to form a triangular arrangement when viewed along the axis of the bridge. This has two benefits: by doubling the number of cables, each cable can be smaller in size, which may be considered by some to be visually preferable; and in addition, the angle of the cables ensures that the arch rib is stable against out-of-plane buckling, allowing the arch to be more slender. Nonetheless, it's not an arrangement normally seen on bridges of this type. For a relatively small span such as this, it's unlikely that the arch will be greatly reduced in size, and the effect of having more cables can be visually confusing, counteracting any benefit from their slenderness.

The bridge deck itself consists of stiff frames spanning transversely between the points of cable attachment and carrying the main loads from the bridge deck. These frames project laterally beyond the area of the deck, because of the triangulated cable arrangement. The deck is then stiffened longitudinally by means of a cable truss, additional cables running below the deck connecting the lateral frames. The frames themselves incorporate steel tie bars as their lower element, a pointlessly expensive indulgence.

It's difficult not to conclude that the longitudinal cable below the deck provides little or no structural benefit, and that the profusion of wires and metal is more confusing than elegant. This proliferation of metalwork is the point of similarity with the St Saviour's Dock footbridge, and it seems regrettable now.

Other than the structure, the most notable feature of the bridge is the chainmail barrier on each side, which is there to stop objects being blown, knocked or dropped onto traffic below. Considering their size, these are reasonably unobtrusive. My photos make them look more transparent than they feel when seen in real life, at least when standing on the bridge.

Generally, the bridge seems to be in good condition, although the paving slabs are coming loose and one section of handrail has gone missing.

Overall, this is an interesting, but flawed, little bridge. I have the feeling it is not very well known at all, and would be interested to see how many readers were already familiar with it.


 
 

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