Monitoring the axial shortening of principal tower using embedded distributed fibre optic sensors

Abstract

Among the many challenges in the design and construction of high-rise buildings is the effect of differential axial shortening that the vertical load-bearing elements of the structure undergo. Axial shortening is time-dependent and very difficult to predict in advance. Therefore, it is essential for contractors to be able to measure accurately the amount of shortening that takes place in the constructed levels, in order to inform the setting out of new levels and the installation of façade cladding. We report on an innovative method that can be used to measure the axial shortening at any point along the height of individual concrete columns and walls, and at any point in time during construction. The monitoring system makes use of distributed strain and temperature measurements obtained using Brillouin-based techniques, from fibre optic cables that are embedded vertically in the structure during construction. This monitoring system was trialled for the first time at Principal Tower, a 50-storey reinforced concrete high-rise building constructed in London between 2016 and 2018. Two perimeter columns and two core walls were instrumented and monitored during and after construction, over a 20-month period. The unprecedented spatial and temporal resolution of the measured data enabled the project team to gain important insights into the nature of axial shortening. Of notable importance is the effect that variation in ambient temperature had on the shortening of individual members as well as on the rate of change of this shortening.