Repairing a Bridge
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|Repairing an old bridge is not
a good way of achieving fame. But if you can do it without too much
annoyance to the people who go over it and under it, you have done a great
The diagram below shows (not to scale) the junction of the A40 trunk road and the M5 motorway.
The junction has five bridges. After many years of use, the A40 over the M5, the bridge in the middle of the diagram, was weakened by ageing of the concrete. Heavy vehicles on the A40 were diverted around the roundabout to keep them off the bridge. Eventually it became necessary to repair the bridge.
Because the bridge had been cast as a single huge slab, it could not have been removed without closing both roads entirely. They would have remained closed until a new bridge could have been installed. The process could have been speeded up by using prefabricated bridge sections. During this period, traffic on the A40 would have had to use the roundabout to bypass the missing section, and one or more lanes, and possibly all, of the M5 would have been closed at various times.
Here is a diagram of the main span of the bridge as it was constructed, with some vertical exaggeration. You can see that it was a fairly typical overbridge with haunched beams.
The adopted solution to the problem retained the existing bridge span, but replaced the concrete piers by new ones made from steel girders. By using splayed legs, tied together at the top, the effective main span was significantly reduced, with a resulting reduction in stresses.
The deepest parts of the beam were no longer at the positions of greatest bending moment, but with the reduction in overall stress, this did not matter. Tying the legs together prevented longitudinal forces being applied to the deck, which had not been designed for a strutted construction.
The pictures below should give some idea of the result, which even succeeds in looking reasonably neat. The whole repair was performed without having to close either of the roads, and was a triumph of ingenuity. Remember that at all times, the deck had to be adequately supported, and at the same time, the new legs had to be installed among whatever supports were currently in use, whether they were the original piers or temporary piles. In the first picture you can see people standing around on the motorway, wondering what has stopped the traffic. This was nothing to do with the bridge: it was caused by a problem further along the motorway, but it illustrates very clearly the huge consequences of even a minor problem on busy roads. Once the queues have built up, the problem propagates to other roads, as people start looking for alternative routes, or are diverted by the police.
Although the finished support system looks reasonably simple, applying it is not trivial. It is imperative to share the stresses correctly among all the supports, and to avoid introducing unwanted stresses into the deck. In fact, if at any stage of the repair, the span had been allowed to move more than the specified amount, cracks would have started.
The next pictures show the two bridges taking the A40 over the roundabout.
The repair at this junction was a slow business, but it was achieved with no more inconvenience to motorists than occasional lane closures and speed restrictions. The next picture is an aerial view, showing glimpses of four of the five bridges.
Repairing an Approach
|Here is a cantilever bridge of a type which is found in
several places in Gloucestershire and Wiltshire. This one crosses
the Barnwood bypass, east of Gloucester. There is another one about
a kilometre to the east. The narrow roads leading to these exhibit
cracks typical of slumping on embankments. The local ground is
largely clay. Some pictures below show this. The last picture
shows cracking due to drying out of the soil in a nearby field, showing
that the problem is not limited to the roads.
The diagram below shows how slumping can occur on a hillside or an embankment. The ground breaks along a roughly cylindrical surface.
These lines were made using pseudo random numbers to simulate random deviations from a line. There is a clear resemblance to the cracks, which roughly follow the stress lines, but are deviated by random variations in the ground.
These pictures show that the approaches have not only slumped locally, they have also slumped as a whole, relative to the bridge. At the date of taking the photographs, one end had been marked for repair, and one end had actually been repaired, by adding some black-top. The traffic volume on the road is very low, as it serves only a farm and two houses. So it is cheaper to repair the road by adding black-top from time to time than by shoring it up or rebuilding it.
Between the bridge and the farm, the road surface has been re-covered with black-top to hide great variations in its surface. A few months later, the edges of the road are already cracking, and grass has begun to grow in the cracks.
A Bridge Needing Repair
These pictures show an arch bridge after some voussoirs had fallen off. If even one voussoir falls out, the compressive force on all the others in its row are to some extent relieved, and those nearest to the gap are held in place only by the adhesion of the mortar. The release of stress means a release of strain, and that implies that the relieved row of voussoirs tends to become slightly longer than the neighbouring row. There will thus be shear stress between the broken row and the unbroken row. This shear stress will in fact tend to share the compressive stress between the two rows, especially far from the gap.
There are implications for a repair. If any structure contains more than the minimal set of parts that will keep it standing, loss of a part may not be disastrous: the stresses are simply shared among other parts. But when a repair is attempted, simply fitting a new part is not enough: in theory, the gap should be jacked until the stresses are similar to their values before the break. Sometimes the loss of stress is irreparable. When the Britannia bridge over the Menai Straits caught fire, the internal stressing put in by Stephenson was lost, and the spans now have arches to support them. In the example of the skew bridge, any one row of voussoirs is so lightly stressed that simple cementing new bricks is probably an adequate repair. An example where repair is difficult is a large piece of pottery that breaks, and the pieces are found not to match when held together. This happens when internal stresses and strains were locked in during the cooling process after firing. They are released by the breaking, and the parts take slightly different shapes.
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