Sheep’s Green bridge collapses

Well it hasn’t actually collapsed but we might have been better off it had! The headline on the front page of Newsletter 59 was ‘New Sheep’s Green Bridge promised.’

Last year, Cambridgeshire County Council was given a large grant (Growth Area Delivery Grant) of money for cycling infrastructure improvements in ‘South Cambridge’. The list of proposed projects was passed by the County’s Cabinet and then the ‘Area Joint’ Committee following somewhat brief consultations with the public and stakeholders. One of these was to replace the Sheep’s Green bridge, which is only 1.2 m (4 ft) wide, and to provide ramps rather than steps up to it. The Campaign strongly supported the bridge improvements, but had concerns that other money was to be spent ‘upgrading’ pavement cycle paths, with nothing spent to improve conditions of cyclists on the road. See:

In July we had sight of the proposed drawings for the new bridge and approaches, as did other local groups. The bridge was wider, but it was also higher, bigger, and longer, with new ‘approach’ spans of tens of metres with high balustrades. Several local councillors and local groups took strong objections to the design and the intrusion into the area. The escalating cost was also giving concern to council officers who could see the grant not even being big enough for just the bridge.

Why does a bridge here cost so much?

The bridge proposed by the County Council’s consultants, W S Atkins, has a span of 25 m (82 feet) rather than 64 feet of the original. As a rule of thumb the cost of just the span is proportional to the square of the length, so a 25 m span would cost nearly twice the cost of a direct replacement, and of course a 25 m span won’t fit on the old foundations. Such bridges are also now required to take heavy loads, more in fact than double decker buses parked end to end! The requirements are for a ‘crush’ crowd of people with a safety factor of 1.7.

This steep ramp would be replaced
Image as described adjacent

Then there is the ‘approach’. Currently that has steps, and for cyclists and the disabled a slope of not greater than 1:15 is required. Any change here requires this, but because the proposed bridge was about a metre higher, the approach slopes would be some 30 m longer. If these were ‘earth ramps’ they would act as a ‘dam’ in times of flood flow, and the Environment Agency was not happy, nor would those in Newnham whose houses flooded. So approach ‘spans’ are proposed, but ‘Highway’ standards require a balustrade 1.4 m high where cyclists are permitted, and gaps must not permit a small child to get through, even if the fall is only one metre onto a grass surface. So we have approaches that are more intrusive than the existing bridge.

But there is still a problem of access. Heavy lifting gear and huge lorries would be needed to lift out the old span (we were told), and later to lift in this new one.

This requires a temporary road, possibly 500 m long, over a sensitive area.

As you can see it all adds up!

So what is happening now?

The money has to be spent this financial year, and a new proposal is on the table just to improve the approaches to the existing span. Even this is costed at £250 000. We are concerned that the approaches are less than the recommended width and will cause yet more congestion adjacent to the existing narrow span, and that when the bridge is eventually replaced the approaches will need reconstructing!

The bridge itself would stay for the present
Image as described adjacent

We know that other interest groups are not happy with the new proposal, and the Campaign is very unhappy with a proposal using £250 000 of cycleway money that does not improve facilities for cyclists.

We are hoping that a meeting will be arranged to bring interested parties together, and that we may get approaches suitable for cycling, that are not intrusive, and do not prejudice a new bridge at a later date.

Would there have been an alternative?

We asked the Bridge Engineer at Sustrans for his advice, and also asked others. The existing bridge is bolted together in sections with the largest weighing little more than two tons. A simple ‘off-road’ crane could be used for removal and transport to the road. A simple ‘bowstring’ arch of the same span as the existing bridge but wide enough for both pedestrians and cyclists would cost about £25 000 to manufacture.

If constructed in sections (two sides, bracing, and deck) a similar crane and method could be used for assembly. Such a bridge would carry a load of 50 tons! No doubt the foundations would require widening and strengthening, but modern ‘mini-pilers’ such as are used for ‘underpinning’ work on houses, could easily access both sides.

Ironically when I first looked at this bridge a number of years ago I thought a major obstacle to upgrading was the duct of telephone cables secured to one side. Moving such services without disruption can be very expensive. I now find that these cables are redundant and present no problem at all!

Jim Chisholm