Traditional and operational modal testing of masonry vaults

Abstract

This paper discusses the practical and technical differences between the usual form of experimental modal analysis (EMA) and operational modal analysis (OMA), as applied to Gothic-style masonry vaulted structures. In situ vibration measurements were collected from the masonry fan vaults of the National Cathedral in Washington, DC, USA. Instrumented impact hammer excitation was used during the experimental traditional modal analysis (TMA) tests, while excitation by peal bells, carillon bells, organ, an orchestra and chorus, and ambient noise were exploited for OMA. The acoustic inputs of the first four excitation types were measured using a microphone and were found to approximate a uniform acoustic excitation across the frequency bandwidth of interest during the carillon and the orchestra and chorus excitations. For both TMA and OMA, system identification was completed by the least-squares complex exponential method. The natural frequencies were found to be consistent for all excitation sources and modal test techniques. Certain natural frequencies found by TMA, however, were omitted from the sequence of frequencies as determined in the OMA. This paper illustrates that OMA is a suitable alternative to global vibration analysis of a masonry building, especially when there are concerns about the use of a hammer or shaker for this purpose.