13 April 2010

Kent Bridges: 1. Lockmeadow Footbridge


A recent trip to Kent, in southern England, gave me the opportunity for a short side-trip to the town of Maidstone. This is the home of no less than three modern landmark footbridges, all built around the turn of the millennium. They were promoted by Maidstone Borough Council, with external support from funders such as the Rochester Bridge Trust.

It wasn't the brightest of days, so none of the bridges were shown to their best effect. The first is Lockmeadow Footbridge, spanning the River Medway close to the 14th century Archbishop's Palace. This is a historically sensitive site, and it's apparent that the bridge designers, Flint and Neill with Chris Wilkinson Architects (now Wilkinson Eyre), sought to create as lightweight a structure as possible in an attempt to minimise its visual intrusion.

They won the bridge design contract following a design competition in 1997, and the bridge was opened in October 1999. It was built by Christiani & Nielsen for £650,000 (£3.9k per square metre of deck), with steelwork by D&B Darke Ltd.

The bridge is 80m long, with a 45m main span, but the deck is only 300mm deep. That gives a remarkably slender span-to-depth ratio of 150. The span arrangements are driven by the need not only to bridge the river, but also to clear the adjacent floodplain. The choice of a large central support was driven by construction constraints, with a weak river bank wall and limited access available on the east river bank. The resulting bridge deck is a little awkward, as in dry conditions it has a very low clearance underneath (see main photo at the top of this post). However, the overall structural form, with twin pylons each carrying twin cables, is very striking, particularly in elevation.

To achieve the desired deck slenderness while minimising weight, a cellular aluminium deck has been adopted. A series of extruded profiles, supplied by Dutch firm Nedal, are placed side by side, and locked together by transverse stainless steel prestressing bars at 1.6m intervals. A non-slip texture is obtained by sawcutting the upper surface of the extrusions. Ian Firth's paper in SEI (see link below) gives much more detail on how the deck was conceptualised, designed, and built. The aluminium segments still look in very good condition a decade after the bridge opened in October 1999. The main question that strikes me is why, if this system is so economic and durable, it hasn't received much wider use?

The use of cellular aluminium extrusions in bridge decks isn't unique, with patented systems such as those from Alumadeck and Sapa Front (see paper by Tomasz Siwowski [PDF] for details). But the arrangement used at Lockmeadow was highly innovative, and only one of several aspects of the bridge which are unusual.

The skeletal steel masts are a design which Chris Wilkinson Architects had already used for the entrance canopies to the Dyson Headquarters at Malmesbury (completed just a few months before the Lockmeadow bridge). They remind me a little of the Skylon, which was similarly cigar-shaped, but also evoke the stylings of high-tech architecture. The use of twin plates for each diaphragm is a nice feature.

The parapet posts are resin-filled, curved FRP forms, supporting stainless steel handrails and stainless steel mesh infill. These are surprisingly transparent, but still add to the visual depth of the structure.

The bridge seems to me to be most successful visually when viewed in elevation, where the unusual cable-stay arrangement is most effective. However, it's almost impossible to see the whole bridge from any viewpoint to the north or south. It therefore seems unnecessarily gigantic - the field of view struggles to contain its scale and it's therefore perhaps more domineering than was intended.

Viewed along the deck, the visual complexity of the pylons and cable arrangements again give the impression that bridge is larger than it really is. It seems to be trying a little too hard, grabbing attention rather than relinquishing it.

That was all something of a surprise to me, as having only seen photographs of the bridge before, I'd been very much looking forward to visiting it. Perhaps my expectations were too high; perhaps the overcast day would have made even a structure of filigree glass seem heavy and awkward rather than graceful.

Given the bridge's unusual slenderness, I did my best to get it moving under an oscillating pedestrian load, but without success. Tie-downs secure both ends of the bridge, and I guess these are balanced with the tension in the locked-coil cable stays to limit vertical movement of the deck.

As a piece of structural engineering, it's a tour de force, with a distinctive visual form, several innovative elements, and having been well-engineered to prevent its slenderness leading to vibration problems. Architecturally, I left it with mixed feelings. It's unexpectedly ostentatious, giving the distinct impression that it might work better in a different visual context. But I'd like to visit it again on a sunny summer day, as I suspect that would lead to a more positive appraisal.

Further information:

1 comment:

Furnival said...

HP,

Thanks for this series - it's very interesting to see someone else's opinions on these bridges, having done a similar trip myself.

However, I have to disagree on the topic of dynamic excitation. My experience was that there was a noticeable response to a walking pedestrian. This was obviously known to the local population, as shown by this video of some impromptu "vandal" loading! (http://www.youtube.com/watch?v=tInlW-NcbzI) I was attempting to video the cable wobble, and definitely wasn't expecting then to jump up and down... I thought it was only bridge engineers that did that!

I would hasten to add that it wasn't a problematic vibration at all (relatively long period, so not likely to become resonant). I also much prefer some response when walking over a bridge!

Interestingly, the Kent Messenger Bridge downstream, which I'd actually come to visit, was a complete surprise. I was expecting a far greater response given the lack of bending stiffness, but the period was huge (3secs?) and the acceleration tiny. You could notice it, but it took concentration! It gave me much more confidence in my own dynamic calculations for a stressed ribbon bridge.

Anyway, looking forward to your response to the Downstream Bridge - it's an extraordinary collection of footbridges in a very small space of river.