An innovative distributed fibre optic sensor (DFOS) monitoring system that can be embedded within the structure of a high-rise building to continuously measure the shortening of columns and walls along the entire building height, as the building is being constructed, is featured in the monthly journal of the Chartered Institution of Civil Engineering Surveyors.
The article, ‘On the Rise: Monitoring Principal Tower’ presents CSIC’s DFOS monitoring system which was successfully trialled for the first time at Principal Tower, London. CSIC Research Associate Dr Nicholas de Battista led the project in collaboration with the overall project contractor, Multiplex, the frame contractor, Careys and the structural designers, WSP. Principal Tower is a 163m-tall, 50-storey residential building in central London, designed by Foster + Partners and is scheduled for completion in late 2018.
The ability of an engineer to predict the axial shortening of vertical load-bearing elements in tall buildings is fundamental to safe and efficient design and construction. Dr de Battista writes: “These predictions are used as a basis for the contractor to pre-set column and core levels in order to ensure that the as-built floor levels are within acceptable tolerance when the building is handed over to the client. Contractors need to allow for the expected final shortening, particularly when setting slab levels and installing finishes, façade cladding and partitions in lower floors, while the building has not yet been completed. However, the behaviour of concrete under strain over time is notoriously difficult to predict because it depends on multiple variables, including environmental factors, concrete mix properties, construction sequence and load history.”
Principal Tower was constructed using an automated jumpform that incorporated the entire building footprint, including the columns. Fibre optic strain and temperature sensing cables were continuously embedded inside two columns and two locations in the core walls of the tower as construction progressed. CSIC trained the frame contractor’s operatives to install the FO cables and CSIC researchers analysed the data to provide the progressive axial shortening information to the contractor and design engineers. The monitoring data were used to adjust the pre-set levels for the core, columns and unitised façade panels, which was necessary due to the amount of column differential shortening which occurred.
“Until now, it has been almost impossible to measure the axial shortening of high-rise buildings to a sufficient degree of accuracy,” said Dr de Battista. “This monitoring system provides continuous data throughout construction, enabling engineers and contractors to verify predictions and adjust assumptions with unprecedented precision and reliability. We have now developed a robust tool that can be applied to the construction of other tall buildings of any size and height.”
CSIC’s fibre optic monitoring system was also noted by judges of the Concrete Society's Awards for Excellence, where Principal Tower won a Highly Commended award. Judges wrote: “The project also used monitoring for axial shortening of the concrete structure using a fibre-optic system developed by Cambridge University. A complex process, advancing construction in concrete.”