For details of our one day CPD course on vibration serviceability, which takes place in London on 8th December, please visit www.istructe.org/events/hq/2016/cpd-course-applied-vibration-serviceability
Following a successful modal test at Les Hanois lighthouse, off the coast of Guernsey, the Vibration Engineering Section embarked on a data gathering exercise at Wolf Rock lighthouse 15km southwest of Land’s End, Cornwall. This was the second in a planned sequence of modal tests of offshore rock mounted lighthouses around the British Isles, as part of the EPSRC-funded project STORMLAMP (STructural behaviour Of Rock Mounted Lighthouses At the Mercy of imPulsive waves).
After fears that the testing would be delayed, due to a dense blanket of fog over the sea at Sennen Cove, the team for Wolf Rock, comprising James Bassitt (photo 5), VES Experimental Officer, Research Fellow Emma Hudson (photo 7), Laboratory Technician Ian Moon (photo 4) and Alessandro Antonini, Research Fellow in the School of Marine Science and Engineering at the University of Plymouth (photo 8) set off for the lighthouse from St Just Airport by helicopter.
Due to the bulk of the 350kg equipment, including a horizontal shaker, amplifiers, laptops, cabling, cameras and accelerometers, it took two helicopter journeys, courtesy of Trinity House, to transport the Wolf Rock team to the lighthouse where, on arrival, they decanted the equipment into the service room at the top of the lighthouse and installed the computers in the battery room, lowering equipment throughout the levels by rope.
As well as carrying out modal tests in real time, the team installed accelerometers throughout the levels of the lighthouse, to capture data and measure the response of the structure during stormy weather.
Despite high winds coming off the land across the sea, the modal tests were successful, and the team left the lighthouse after seven and a half hours.
James Bassitt, who managed the data acquisition and on-site modal analysis, said: “The whole exercise was logistically harder than at Les Hanois, because the lighthouse was narrower, taller and access to the helideck was restricted.
“It was difficult to carry out the modal testing, because the background noise from the wind was far outweighing the forces we were generating. But we persevered and completed the testing successfully.”
The next data gathering exercise will take place at Longships Lighthouse, off the coast of Lands End, in August 2016.
Experts in vibration serviceability of civil engineering structures from across the globe met at the University of Exeter over two days to discuss the growing challenge of vibrating structures. The best known example of this problem was the ‘wobbly’ Millennium Bridge in London, a case that brought the field of vibration serviceability from obscurity into the limelight 16 years ago.
To read more, see bit.ly/2arOZWW
The Exe North Bridge (Figure 1) is one of the two almost identical adjacent bridges crossing River Exe and forming a big roundabout in Exeter, UK. It is 60m long and consists of three spans, resting on two wall type pier structures in the river and abutments at the ends. It was constructed in 1969, so it is very close to its 50 years of designed service life.
The Vibration Engineering Section supported The University of Exeter undergraduate engineering student project “Analysis of transverse load distribution of Exe North Bridge superstructure”.
The north span of the test structure was instrumented with 12 strain transducers, which made it possible to study the load shedding characteristics of the deck structure under moving load. As a test vehicle, a four-axle, 32 tonne lorry was used to obtain a quasi-static strain response. The load test was performed overnight to avoid disturbing traffic. The truck made several passes in each lane, stopping every time for 30-45 seconds to record static strain (Figure 2.).
The load test revealed that, although the structure is nearing its 50 years of designed life, it still retains significant strength reserves.
As part of EPSRC-funded project STORMLAMP (STructural behaviour Of Rock Mounted Lighthouses At the Mercy of imPulsive waves), the Vibration Engineering Section carried out a modal test of the Les Hanois lighthouse (photo1) off the coast of Guernsey. This was the first of a planned sequence of modal tests of offshore lighthouses around the British Isles.
Modal tests are used in the aerospace and automotive industries to validate designs through direct measurements on prototypes. They are also used (particularly by VES) to evaluate the dynamic performance and structural condition of bridges, buildings and their components, as well as ‘special structures’ – and this is believed to be the first such exercise ever carried on an offshore lighthouse. Test equipment and procedures were carefully organised to fit within airlift capacity (photo 2) and manhandling constraints onsite, and to maximise chance of success with uncertain time constraints and on-site conditions.
The VES team for Les Hanois, comprising James Brownjohn (photo 3a), Professor of Structural Dynamics, Research Fellow Emma Hudson (photo 3b) and Laboratory Technician Ian Moon (photo 3c) were flown to the lighthouse courtesy of Trinity House to join a maintenance crew already on board. Due to flight limitations, time on board was only four hours, which included two hours for unloading and loading the helicopter and for setting up and packing up the test equipment. However, through meticulous pre-planning and intense on-site activity, the VES team carried out the entire exercise successfully.
During the test, orthogonal pairs of accelerometers were placed in sequences at the ten levels of the lighthouse, from entrance (for boat landing), through engine room, bedroom, lantern level (photo 4) and helideck (photo 5). As well as using the sensors to measure the minute wind-induced sway of the lighthouse, structure vibrations were also forced, using an electro-dynamic shaker located outside the lantern room (photo 6). On-site analysis of the data was used to refine the test procedure in real time, and the results show the test to have captured all the structural information needed for use by other members of the STORMLAMP consortium at Plymouth University and University College London.
Emma Hudson who managed the data acquisition and on-site modal analysis said: “The testing provides an indication of how much movement we can expect from the lighthouses. Based on these modal tests, we can identify which of the lighthouses is especially problematic, then focus on that lighthouse in particular for the rest of the project.”
“We will then leave a set of accelerometers running to log data, so we can measure the response of the structure during stormy weather, when the sea waves apply large forces to the lighthouse. When these measurements are combined with the dynamic model derived from the initial modal test, we can then infer what sort of forces these waves are generating.”
She added: “It was a useful exercise because, in VES, we often deal with footbridges, offices, road bridges and stadia. Lighthouses are such narrow, enclosed spaces, with limited access. So, it was a challenge in terms of placing the equipment, and interesting to see the different dynamics of the structure.”
The next data gathering exercise will take place at Wolf Rock lighthouse, off the coast of Cornwall, in July 2016.
Featured news – Engineering students showcase their wares to industry experts – University of Exeter
Check out the coverage in the Plymouth Herald here
We are delighted to announce that our MSc in Structural Engineering has been accredited by the Joint Board of Moderators on behalf of the Engineering Council. For info, see http://bit.ly/1WyQpxk
Figure 1: Prof. Reynolds explains the operation of tuned mass dampers to MSc students Elmar Talibli and Ed Court
The Mill-on-Exe footbridge (Figure 1) was opened in 2003. The span is supported by two pairs of stay cables, whose tensions are balanced by a massive counterweight, and is equipped with six tuned mass dampers (TMDs), which are designed to suppress pedestrian-induced vibrations.
A team from the Vibration Engineering Section (Figure 2) supported two University of Exeter undergraduate engineering student projects – one about performance of the tuned mass dampers and one about flow of energy from the pedestrian generating dynamic response to damping sinks, including TMDs.
Measurements used a set of 21 Honeywell servo-accelerometers (Figure 3) and a set of 11 APDM Opal inertial measurement units (IMUs). Opals are intended for sports biomechanics applications, but they are well suited for research into human-induced structural vibrations e.g. of footbridges, and this is the first time they have been used to study the flow of energy.
Energy was supplied by undergraduate students, either jumping or running (Figure 4).
The testing provided a set of experimental modal properties, which validated the modeling by the consultant (Flint & Neill) and confirmed the effectiveness of the TMDs.
With 95 per cent of the UK’s international trade transported by sea, rock-mounted lighthouses have a crucial role to play in safe navigation – but the longevity of these historical structures is threatened by extreme weather.
So, the Vibration Engineering Section (VES) have teamed up with researchers at Plymouth University and University College London to start a project assessing six of the most vulnerable lighthouses in the UK and Ireland.
VES Experimental Officer James Bassitt joined Julian Seipp, Senior Technician at Plymouth University on Longships Lighthouse at St Just near Lands End, to begin STructural behaviour Of Rock Mounted Lighthouses At the Mercy of imPulsive waves (STORMLAMP). An EPSRC funded project, STORMLAMP will combine field, laboratory and mathematical/computer modelling methods.
James and Julian were given access to the lighthouse by General Lighthouse Authority, Trinity House and were flown there by helicopter, along with a maintenance crew. There, they retrieved data gathering geophones previously installed on the lighthouse by the University of Plymouth as part of a pilot project leading into STORMLAMP, and replaced them with triaxial accelerometer data logging equipment. In addition, they carried out a full survey of the lighthouse to help them to come up with a long term monitoring solution and testing plan.
James Bassitt said: “I would like to thank Trinity House, including Ian Gorvin and Malcolm Johns for their support on the day and for their help with future planning. We have taken the geophones back to Exeter to benchmark them against highly sensitive accelerometer data logging equipment.
He added: “The next stage will be to carry out a data gathering exercise at a lighthouse off the coast of Guernsey in June.”
As part of STORMLAMP, field instrumentation and procedures will be developed by the University of Exeter (James Brownjohn and James Bassitt) for the highly challenging constraints. Meanwhile, UCL (Dina D’Ayala will create test data guiding of multi-scale numerical simulations for lighthouses that can be used with the full-scale data to diagnose observed performance in the long term monitoring.
These models will link with advanced physical and CFD simulations by Plymouth University’s COAST Laboratory (Alison Raby-who leads the project- and Deborah Greaves).
STORMLAMP will move to a focus of one lighthouse for the following two winters. Based on these modelling exercises, the long term monitoring of a single structure will used to characterise the wave loading in-situ at full scale. The project will support performance management of such structures worldwide.