Understanding the issues
In order to understand what is causing the unexpected movement in the bridge it has been necessary to carry out a number of tests. A thorough review of the analytical model used to test the design has been conducted by the project engineers and independent checking engineers in parallel with this testing, in order to gain the fullest possible understanding of the issues.
The tests carried out on the structure have endeavoured to recreate the conditions experienced on Saturday 10th June in two key ways:
- Groups walking across the bridge attempting to recreate the phenomena;
from the Building Research Establishment (a 'grandstand shaker' - pictured below - designed, as the name suggests, to test the impact of crowd movements on stadia) is being used to replicate the forces.
- Specialist shaking equipment
These tests were filmed from four locations. Transport Research Laboratories (TRL) monitored vertical and horizontal loading at key points. We are now starting to get factual information back from the analysis of this test data, and are continuing to analyse the footage.
Review of the mathematical model
As explained in the engineering section, the designers used a complex mathematical model to test their design and predict how the bridge would behave. As the behaviour of the bridge witnessed on June 10th was outside expectations, we have revisited the model to compare the predictions with the measured responses of the bridge as built.
Following best practice principles, our original design and analysis was carried out twice in Arup's by two totally independent teams. In addition to this an independent firm, Mott Macdonald, conducted their own separate analysis. The process of checking and double checking the design has now been repeated.
The results of this review and comparison show that, apart from the unexpected movement, the bridge is reacting as predicted. There are some marginal - and explicable - differences in the figures but none represent a significant departure from the model, and none could explain the unexpected movement.
Are Arup working on this alone?
No. Together with the other members of the design team - Foster & Partners and Sir Anthony Caro - we are working with a number of international experts in order to bring the best world thinking to the issue. The Building Research Establishment (BRE) and the Transport Research Laboratory (TRL) are assisting with the testing of the bridge. Mott Macdonald continue in their role as Category 3 checking engineers, and Flint & Neill have been retained as our advisors to review the solution.
In addition, we are in discussions with a number of academic experts and institutions, and have commissioned research from Imperial College and from Southampton University.so what DID happen?
All suspension bridges move, sometimes imperceptibly. The problem in this instance arises when the vibration caused by groups of people moving across the bridge tie in with the horizontal frequencies of the bridge, setting up a perceptible lateral movement.
As the motion becomes more noticeable, people instinctively adjust their pace in time with the movement of the bridge in order to feel more stable. When large groups of people do this at the same time it reinforces - and emphasises - the movement in the bridge which becomes uncomfortable.Understanding why it happened
The structure of the bridge was designed in accordance with best practice, and all applicable codes and standards were not only met but exceeded. However these codes and standards do not address the movement that occurred at the opening of the bridge.
As a result of the work we have done, and are continuing to do, the knowledge base of the industry will be expanded and future projects will be able to take account of these new measures and understandings in their designs.Designing a solution
So what's the solution? There are two principal options to resolve the movement. The first is to stiffen the bridge in order to change the frequency at which it oscillates so that people can no longer lock in with the rate at which it moves. The second is to utilise damping, dissipating the energy in the system using the same principle as shock absorbers in a car.
At present, the most likely option is that we will install some form of damper. The exact form that the solution will take is dependent on determining the amount of damping required. This can only be defined once we understand fully the input force from the pedestrians. Very little data exists on this subject at present. We have therefore commissioned a series of tests to quantify the characteristics of the force exerted by different numbers of people, and of different populations (the elderly, young children, young adults etc.).
Once the results of these tests are known we will be able to make a final decision on the most appropriate course of action.
Will the bridge still move when it reopens?
Movement is an inherent characteristic of suspension bridges so yes, the bridge will still move. However, the excessive and uncomfortable movement experienced on June 10th will be corrected.
So, when will the bridge reopen?
Until we have completed our investigations, we can give no firm indication of a reopening date. However, it will be a matter of months rather than weeks.