19 April 2018

Yorkshire Bridges: 21. Knostrop Footbridge, Leeds

This is the newest of all the structures I saw on my trip to Leeds, having only opened to the public in October 2017. It carries a branch of the Trans Pennine Trail cycle/footpath across the River Aire, and it was installed as part of works for the first phase of the Leeds Flood Alleviation Scheme. The bridge was designed by Knight Architects and Mott MacDonald, and built by contractor BAM Nuttall.

Approaching this along the path from the nearby Thwaite Mills, my first thought is what a real oddity this bridge is. It feels a little like a space-age intruder in a semi-urban wilderness: super-sleek architectural design, the sort of thing that would feel right at a visitor "destination" but feels much more of a surprise on this out-of-the-way pathway. I found it disconcerting, at least to begin with.

The 70m long bridge spans across the new Knostrop weir, a series of three inflatable weir structures intended to be adjustable in times of flood. The weirs (and associated fish passes) are staggered in plan and are separated by slender concrete walls, which also form the supports for the bridge.

There seems to be nothing technical published online about the bridge design, but there are some images of construction at the website of steelwork contractor SH Structures, which provide a little illumination.

The YouTube video above is worth a look first, as the early sequences include some overhead views of the bridge layout.

The bridge crosses the river at what looks like roughly a 45-degree skew, and sits on a series of support legs which run parallel to the river (hence at 45 degrees to the bridge), plus a square abutment at one end. The bridge is straight in plan, but widens out with curved standing areas above each support position.

In cross-section, the bridge is a shallow multi-cellular steel box girder, with a smoothly curved soffit plate. The intersection of the plan geometry with the curved underside leads to a distinctive "scalloped" profile along both edges of the bridge.

The bridge legs are formed from simple steel plates, each only 50mm thick, which must be amongst the most slender footbridge supports anywhere in the UK. It's not quite down to Ney and Partners standards, but it's quite remarkable nonetheless. The legs are attractively shaped as well, holding the bridge delicately to one side of the weir so that bridge users get a good view of the site's main attraction.

The construction photos indicate that the pier legs also form diaphragm plates within the box girders, and that each box span is bolted through the diaphragm. I guess a cover plate was then welded across the finished joints.

Presumably the "leaf" piers are thin enough simply to flex under thermal expansion and contraction, although I think the effects must be complex due to the high skew. They must also be short enough to be safe against buckling against vertical load, so a careful balance of stiffness is required.

The parapets comprise stainless steel ribs with timber leaning rails. I like the simplicity of the main parapet section, but the upper rail, presumably to provide standard cycle parapet height, looks a little like an afterthought.

Nonetheless, this is a very high-quality bridge to find in such an unexpected place.

Further information:

1 comment:

GRB said...

This bridge shows the potential beauty of steel and also its strength. The 50mm seem thin but in reality quite strong Here in Denver, Colorado, USA. I see construction crews placing what looks to be approximately 3/4" plates over large holes in the roadway during construction projects. These plates are able to support the traffic which would include some potentially very heavy trucks. The 2" inch steel plates used to support this bridge are significant pieces of change (as in money). Money is often the reason some particular project takes on a certain form. I reckon each of those plates cost a pretty sum, but building concrete forms is not cheap either. All things considered there may not be much difference in construction cost, particularly when you consider the ease and speed of working steel. I'm not an engineer, nor a tradesman, but I have been an avid "do it yourselfer" my entire life. I know almost every trade as I have had to teach myself over many years. Of all the materials I have worked with, I enjoy steel the most. I know how to arc weld, and steel is easy to join and actually very easy to work with the proper tools. I'm not sure of the limitation, but down at the local steel supply house I have seem them cut fairly think steel plates in a large sheer press that slices thick steel plates like butter. They can punch holes the same way. Computer controlled plasma cutters can cut rather intricate designs in steel plate, again I presume effortlessly with the right equipment.

What brought me to your sight was the Figg / FIU concrete abomination. Looking at these elegant steel structures in the UK, one really has to wonder why Figg and FIU were so excited about concrete. I have worked with concrete too, but it's always tricky. It's obviously not very forgiving when things go wrong. Anyway the use of what appears to be stainless steel in the Knostrop Footbridge is also intriguing. Through this sight, I am understanding your belief that bridge engineering designs should be daring and bold. It's obviously an achievable goal, but the designers still have to have a proper understanding of the limitations of the materials they are using, and design accordingly.