Traditional and operational modal testing of the Washington National Cathedral

This paper investigates differences between traditional and operational modal analysis as applied to a masonry-vaulted structure. For this purpose, we applied the techniques of both traditional and operational modal analyses to the choir fan vaults of the Washington National Cathedral (D.C.), a monumental church of Indiana limestone, vaulted in the 1930's in the Gothic style. Impact hammer excitation was used during traditional modal analyses tests, while five different excitation sources: (1.) peal bells, (2.) carillon bells, (3.) orchestra and chorus, (4.) organ, (5.) ambient, were used during operational modal analyses tests. The first four types of excitations were measured by a microphone. The acoustic input of these excitations, especially the carillon and the orchestra/chorus, was found to approximate a uniform distribution across the bandwidth of interest. System identification was completed by the least squares complex exponential (LSCE) method. In general, the most pronounced of the identified natural frequencies were found to be in agreement for all excitation sources. Certain natural frequencies found by traditional modal analysis, however, were omitted from the sequence of frequencies determined by operational modal analysis.