Closure of Hammersmith Bridge to traffic – what was the ‘final straw?”
I was down in Bristol over the weekend, in the hope that I might catch sight of the mass ascent, when more than 100 balloons were expected to rise into the skies at around the same time. It would have been spectacular, except for the weather. Result: no balloons at all over the weekend.So, plenty of time to browse the internet. One item in particular caught my eye: why was the nearby Clifton Suspension Bridge [not unlike Hammersmith Bridge in concept] to be closed at key moments? The answer is to be found at https://www.cliftonbridge.org.uk/planned-closures/annual-balloon-fiesta-closuresThe text runs as follows:When the Clifton Suspension Bridge was completed in 1864, 500 tons of stone was spread evenly across the structure to test its load bearing capability. However, the stone was laid carefully and did not move, meaning that although the bridge can support this weight, the test only showed that it was able to do so if the load did not move in any way. Today, the bridge is over 150 years old and consists of many original components which are able to safely deal with the demands of modern traffic 24 hours a day, 7 days a week.The bridge is equipped with sensors which monitor its movement and check for any problems. A weighbridge senses vehicles over 4 tons and prevents them from crossing as large weights like this this concentrated in one area would cause local damage to the bridge deck and cause it to flex and bend more than it does under the weight of light passenger vehicles such as cars.However, the most damaging form of loading for this bridge is foot traffic. People can be packed much more densely than vehicles, particularly when large crowds are all travelling in the same direction. Normally foot traffic does not cause us an issue because pedestrians are limited to the footways and people are making their way back and forth individually or in small groups and this is a small weight, spread evenly. However, in 2003 and 2004, when people were allowed to walk across the bridge during the Balloon Fiesta, we found that this was putting a severe level of stress and strain on the structure because large numbers of people clumping into heavy groups to avoid obstacles (such as slower moving pedestrians) create heavy loads which are unevenly dispersed. An even bigger problem is the energy created by so many people walking across the bridge at once which could cause a swaying motion and amplify the natural movement of the bridge. This is called resonance and could lead to a very powerful vibration or wobble, similar to that which closed the Millennium Bridge in London. This is very dangerous as it can cause panic and a stampede as people try to get off the bridge.It is almost impossible for us to regulate pedestrian traffic when such large numbers are involved. To get everyone across safely during this event, we could only allow a maximum of 200 people on the bridge at once. We would have to erect turnstile barriers and employ a large number of stewards at both sides of the bridge to control the flow of people, leading to a large bottleneck in Ashton Court as people would have a very long wait to be allowed on to the bridge.For these reasons, we have had to take the decision to close the Bridge to everyone between 6pm and midnight on Thursdays and Saturdays during the Fiesta. The timing on these two evenings is due to the departure of many thousands of people from the site in a very short period once the night glow and fireworks have finished. We will use this time to carry out essential maintenance and annual inspections, which means that we will be hard at work making the bridge better and safer for you to use for the rest of the year.The bridge will be open as normal throughout the rest of the Fiesta. Please follow the yellow signs if travelling by car, or use Festival Way to get to and from Hotwells and the city centre on foot. We apologise for the inconvenience and hope that you enjoy your visit: we hope you understand our reasons for closing the bridge and that we have to make your safety our priority.==In summary, management of the Clifton Bridge seems to be much more ‘switched on’ than was ever the case with Hammersmith Bridge, or am I doing a disservice to the London council?But then we come to the timing of Hammersmith Bridge earlier this year. Was it not immediately after the Oxford and Cambridge Boat Race, when a routine inspection revealed unusual cracks? Was the bridge closed to pedestrians for that event? Not that I can recall from looking at the television.
James Dixon ● 2648d7 Comments