The integrity of historic fibrous plaster ceilings under dynamic loading

The collapse of the Apollo theatre ceiling, London, 2013, emphasised the importance of researching historic fibrous plaster ceilings. Ceilings are subject to dynamic forces, and this study identifies and quantifies dynamic loads in historic theatre ceilings by in-situ monitoring using accelerometers. Flexural, tensile and compressive tests were conducted on new and historic fibrous plaster to establish properties of ceiling panels and supporting fibrous plaster wads. Forces required to initiate microcracks and ultimate failure were compared with the magnitude of recorded accelerations. In-situ work made two important discoveries: technical activity in-between shows induces higher accelerations than performance sound pressure levels, and steel wire rope (supporting light/sound systems) making contact with ‘top hats’ (tubular channels in cylindrical drilled ceiling holes) is the primary dynamic loading in routine operation, with acceleration peaks typically ≈ 15 g (≈35 N force). Simulating accelerations in ceiling-panel laboratory tests resulted in displacements up to 0.1 mm and 200 microstrain. Results in new fibrous plaster established forces and displacements when initial microcracks occur in flexure (≈700 microstrain), tension (≈7000 microstrain) and compression (≈10000 microstrain). Results demonstrated that accelerations recorded in-situ would be insufficient to cause cracks in new, or well-maintained historic, theatre ceilings and wads. Ceilings in different buildings may vary in condition, therefore minimising steel wire rope-top hat contact would reduce the probability of ceilings sustaining damage. This study provides the first quantitative evidence of the dynamic behaviour and material performance of original wads and theatre ceilings, transforming understanding of their properties. Results will directly guide conservation policy and practice, enabling heritage engineers, conservation professionals, custodians and building owners, to make evidence-based decisions that enhance the long-term safety and preservation of historic interiors. • Fibrous plaster ceilings and wads are common in historic buildings e.g. theatres. • Historic ceilings are subjected to dynamic loading during routine operation. • Dynamic loading is identified and quantified through monitoring a theatre ceiling. • Tests on new and historic fibrous plaster demonstrate forces causing microcracks. • Tests enhance understanding of properties and behaviour of fibrous plaster ceilings.