Initial details of the extensive testing being carried out by Mott MacDonald on the Hammersmith Bridge in west London have been revealed in an exclusive interview with New Civil Engineer.
Owner of the grade 2* listed bridge, Hammersmith and Fulham Council (LBHF), said the consultant had been appointed to carry out the testing on the bridge to establish the extent of strengthening work required to allow current weight restrictions to be lifted.
Currently the 130 year old, cast iron chain link suspension bridge, has a severe 7.5t weight restriction in place, meaning only one, single decker bus is allowed on the structure at a time. Wardens employed by Transport for London (TfL) are in place at either end of the bridge to enforce this.
Much of the structure of the bridge is original, with the cross girders, the majority of the hangers and suspension chains all dating back to 1887. The foundations for the bridge were built for a previous bridge on the same site and date back to 1827.
Mott MacDonald project director Tim Abbott said many of the component parts now need to be strengthened for the modern day demands on the structure.
“The structure is obviously in need of repair and strengthening, with many components approaching the end of their useful life,” he said. “Some form of re-decking currently looks inevitable, but it’s too early to discuss capacity and where strengthening is required.”
The testing is being carried out during a number of closures of the bridge due to the sensitivity and access needed to carry out the investigation.
The tests are divided into four categories to find out more about the condition and integrity of the bridge, the materials used and their properties, the stresses in each of the members without any traffic on it and to validate and calibrate the structural model of the bridge being built.
The integrity testing will focus on the metallic components which are prone to fatigue and impact damage. To do this, ACFM (alternating current field measurement) testing will be used to find surface cracks in the members without removing the paint. Ultrasonic testing will then be used to find the size of the defects if necessary.
To confirm the material used in the chain suspension links and pins, positive material identification (PMI) and hardness testing is being used.
“This has been challenging as it has necessitated the removal of cast iron cap nuts (using a very large spanner) to expose the pin for testing,” said Abbott.
Destructive testing of the hangers is also able to be carried out on retained sections of original hangers, replaced in a previous repair. From this, the tensile strength and modulus of elasticity can be determined and their load carrying capacity found.
To work out the stresses in the suspension chains under the bridge’s self weight, specialist testing company Veqter is carrying out incremental centre-hole drilling on the members. The method, which involves drilling shallow holes into components, looks at their surface strains and using this the structural assessment can be validated.
To find out the same stresses in the hangers, temporary works will be used to support the deck while four of the hangers are destressed so they can be monitored with strain gauges to determine the dead load in each. Additionally all hangers will have accelerometers fitted to them to establish their natural frequencies under permanent loads. This would highlight any unusual behaviours and allow the force in the remaining hangers to be determined, said Abbott.
The team will also carry out a static live load test, with vehicles of known weights placed on the bridge in 16 different arrangements. From this the difference between measured and calculated values for different quantities can be compared. Again, this will be used to validate the structural model.
To monitor deflections of the bridge while the tests are taking place, geometrical surveying techniques potentially including videogrametry, a technique which takes a video of an object and creates a 3D virtual mesh of it. Key structural components have also been captured in a point cloud survey to build a “true form” structural model.
LBHF cabinet member for environment, transport and residents’ services Wesley Harcourt said: “Hammersmith Bridge is one of London’s most beautiful and iconic structures and holds a place in the hearts of everyone who lives or works in the borough.
“Unfortunately it just wasn’t built for these times and today’s traffic volumes cause continual damage. Although patchwork repairs have kept it in use over the years, the time has come for a full overhaul.
“This refurbishment is a huge task, complicated by the age, complexity and rarity of our bridge and while a time-consuming process, it’s unavoidable if we want to keep traffic flowing into the future.”
A feasibility study and preliminary design for strengthening works is due to be completed by the summer this year. A detailed design is then expected to be carried out as part of a design and build contract with work starting on site at the end of the year.
The problem originally surfaced in 2015 when Transport for London (TfL) said it wanted to route double decker buses over the structure.
The bridge currently had strict weight restrictions of 7.5t and a report by Hyder said some strengthening works would be needed for the 18t load bearing necessary for double decker buses. The report said there was some current over stressing, but this was a first stage assessment and it recommended further assessments.