Vibrational performance of timber floors constructed with metal web joists

Abstract

Metal web engineered timber joists are increasingly being used nowadays to replace traditional solid timber joists for constructing intermediate-span timber floors in low-rise houses and long-span floors in commercial buildings. Vibrational performance criteria become crucial for serviceability design of timber floors and often control the design. This paper presents an experimental study carried out on the dynamic performance of a series of full-scale floors constructed with metal-web joists with focus on modal frequencies, modal shapes and damping. The effects of a range of floor component configurations including spacing of joists, type, size, number and location of strongback, ceiling, etc., were examined. In general, joist spacing, strongback bracings and ceiling did not significantly influence the fundamental frequency and the corresponding damping ratio, required for the design of timber floors, but they did influence higher modal frequencies. The measured damping ratio for the fundamental mode was 0.9% on average, which is below the recommended value of 1% in Eurocode 5 Part 1–1 and much smaller than the recommended value of 2% in the UK National Annex to the code. The use of strongbacks, however, could considerably reduce the number of first-order modes below 40Hz, used to determine the unit impulse velocity response, and in turn it could result in easier fulfilment of velocity design criterion.