Design of an instant vibration-based warning system and Its operation during relocation works of historic facades

Abstract

Preserved listed building facades may require large-scale and highly technical work when the supporting building structure is at serious risk of collapse. Such is the case described in this paper, where vast facades must be cut into large panels up to 200 m2 and 150 t in weight and carefully laid on the ground. Various engineering works must be carried out to ensure the structural integrity of the panels to be safeguarded. Each panel must be reinforced by a temporary lattice steel structure prior to the disengagement from the supporting building frame. The operations require the use of cutting tools, hitting demolition machines and heavy cranes, which can induce potentially damaging vibrations that should be monitored and processed so that workers can be alerted in real time if certain thresholds are exceeded so that they can proceed more carefully. The paper describes the specifically designed monitoring system, its electronic parts, how they operate and how the data are processed and displayed. The monitoring system, once verified in laboratory tests, is applied to the detachment and overturning activities of a representative full-scale panel, tracking vibration levels and tilting rates. After days of operation and visual observation, it is possible to correlate vibration levels with incipient damage, establishing that peaks below 0.5 m/s2 or RMS values of 0.05 m/s2 are permissible, but that above 1.0 m/s2 or 0.3 m/s2, respectively, activities should be halted. The proposed system has proven to be useful for the intended purposes, making it possible to know the acceptable thresholds and trigger the necessary alarms in real time for the successful course of the work.