Assessing the Dynamic Response of Historical Masonry Pinnacles

The dynamic evaluation of historical masonry structures is a crucial aspect of conservation engineering, particularly when slender architectural components experience vibration-induced distress. The Church of Our Lady of The Assumption and The English Martyrs in Cambridge, UK, is a Gothic Revival church from the late 19th century, including a 65-m-high spire on its western bell tower. The construction material is Combe Down stone, a fine-grained oolitic limestone favoured in Victorian construction for its workability and aesthetic appeal. However, this workability comes alongside reduced strength, relative to other stones used in construction. A monitoring campaign was undertaken to evaluate the dynamic behaviour of pinnacles on the bell tower, following observations of cracking damage affecting both the pinnacles and their connecting stone tie-beams to the tower. Specifically, accelerometers were used to measure the response of a subset of the pinnacles, under both hammer-generated impulse excitation and forced vibration during bell ringing. A non-contact video-based approach utilizing Motion Magnification and WaveCam was concurrently employed, in which high-resolution video re-cordings were processed to enhance the visibility of small-magnitude motions and analysed in comparison with the accelerometric data. Motion Magnification and WaveCam are effective techniques for analysing the structural dynamics of heritage buildings, including at the subpixel level. In combination with accelerometer analysis, this campaign enabled assessment of the liveliness of the pinnacles during both ambient conditions and bell ringing, as well as their typi-cal dynamic response, natural frequency and sensitivity to past damage and recent restoration.