Vibroacoustic analysis of the Woodsol timber frame building concept

Abstract

The segment of buildings with four to eight storey is a major share of the building construction market in urban areas. Despite the fact that building with timber has both economical relevance and environmental advantages, the use of timber for urban buildings is still limited to pilot projects. The main reason for this is the limited economical competitiveness of timber buildings due to the risk associated with a lack of knowledge and experience with timber design and construction, compared to the standard concrete and steel buildings. The Woodsol project aims at filling this knowledge gap by developing a timber building concept for urban buildings with a high degree of architectural flexibility. The concept is based on moment resisting timber frames and long-span floor elements. The work presented in this thesis addresses the acoustic aspects and focuses on four main areas of interest: i) the strategy to reliably and accurately determine the acoustic properties of the floor elements, ii) the verification and utilisation of vibration velocity measurement methods to determine the impact sound insulation, iii) the typical properties of the hollow-box floor elements and the influence of the moment resisting connections and iv) possible practical solutions. Low frequency impact sound determines to a large extent occupants’ annoyance in lightweight buildings. At the same time, challenges exist in determining accurately the low frequency sound insulation properties of building elements. Therefore, throughout this work, I put a strong focus on the low frequency range. I adopted a strategy based on experimental activities on prototype floor elements and a full scale system mock-up. I studied the modal behaviour of the floor elements by means of the Experimental Modal Analysis and determined the radiated sound power under impact excitation using the Integral Transform Method (ITM). I performed a parameter study to investigate the effect of the size and of the boundary conditions of the hollow-box floor elements on the modal behaviour and on the sound radiation. I observed a strong dependency of both aspects on the studied parameters at frequencies below 150 Hz. This highlighted some limitations of the standard laboratory measurement procedure according to ISO 10140-2 and ISO 10140-3 in the low frequency range. The ITM is still a rather new method within building acoustics and therefore part of the activities was aimed at verifying the adequacy of the method for our purposes. I compared the ITM with other vibration velocity measurement methods and with the standard laboratory measurement procedures. I demonstrated the reliability of the ITM and showed how the ITM can overcome the highlighted limitations. The airborne and impact sound insulation performance of the Woodsol floor element were included in a comparison of the properties of similar objects. The effect of the additional mass in the cavity or on top of the elements was investigated and we determined the effect of different floating floor solutions on top of the hollowbox. This allowed to identify the parameters governing the acoustic performance of this type of elements and their respective relationships. Within the common flooring solutions in the Norwegian building industry, I identified the practical solutions that can fulfil acoustic requirements according to the Norwegian standard for office, residential and educational buildings, when used with the Woodsol floor elements. More advanced solutions were proposed to meet higher requirements. Overall, within this work I was able i) to highlight the main parameters governing the acoustic behavior of the hollow-box floor elements, ii) to identify and validate a measurement method that allows us to accurately measure the radiated sound power under impact excitation in-situ, iii) to describe the general airborne and impact sound insulation performance of hollow-box floor elements and iv) to finally suggest practical solutions that might fulfil the acoustic requirements according to the Norwegian standards.