Certain uncertainties: Modelling unusual structures to control vibrations in sensitive areas

Abstract

Mechanical equipment and occupant footfalls are often the most critical sources of floor vibration on the elevated floors of buildings. Achieving stringent vibration criteria on these floors requires sufficiently stiff and massive floor structures to effectively resist the forces exerted by mechanical equipment and user traffic. The difficulty for engineers in modelling these buildings can be exacerbated in structures that are very old and/or of unusual construction. In this paper, two case studies are presented of modelling such structures in order to predict vibrations in sensitive areas. The first structure dates to 1925, and is of massive concrete construction. A new 1 megawatt emergency generator is being installed directly above a floor containing vibration sensitive computer equipment. The challenges associated with the uncertainty of computer modelling of the dynamic properties of the historic structure are explored. The second structure is a large private residence, which has long span floors constructed of concrete, engineered wood joists and steel beams. Excessive vibrations due to footfall activity resulted in cracking of the concrete topping. The challenges associated with modelling this unique structure are explored. Field testing of both structures to examine and verify the accuracies of the assumed model parameters is presented.