Strengthening monumental timber floors with a reversible intervention to accommodate acoustic measures

Abstract

For centuries timber has been the most commonly used material for constructions. All over the world examples of timber structures that date back from a few centuries are still standing today. The Netherlands feature about 60,000 national monuments with internally timber structures. These timber structures often need strengthening due to increasing loads that are applied. The strengthening techniques are narrowed down to a few options whenever historical ceiling designs, such as ceiling paintings, plaster works or timber beams need to remain intact.

Timber floors encounter two major challenges: low stiffness which leads to limited load-bearing capacity and low surface mass which leads to poor acoustic sound insulation. This research seeks to provide structural engineers to make a factual choice in an early design stage for using a reversible strengthening technique on a monumental timber floor preserving original appearance. This strengthening technique is verified for strength, stiffness and acoustics, both airborne and structure-borne sound transmission. To this end, the following research question was formulated: "What is the influence on the strength, stiffness and acoustic properties of monumental timber floors by strengthening them with multiple layers of plates fastened on top of the existing floor?"

To answer the research question, a case study was performed on two monumental floors of the Prinsenhof Museum in Delft. These floors were investigated for their current strength, stiffness and acoustic properties. Then, equations were derived which considered the separate timber plates in the reinforcement technique as an equivalent layer. This equivalent layer is used to determine the effective stiffness for mechanically connected girders by the gamma-method. These equations were validated using software for 2D frameworks. Subsequently, a parameter study was used to determine the influence of the parameters on the strength, stiffness and acoustic properties. Finally, by applying the reinforcement technique to the case study based on the results of the parameter study, the strengthening technique was assessed more in detail by taking into account the influence of the non-cooperating intermediate layer.

The results of the parameter study and the case study demonstrated that the strengthening technique increase the strength and stiffness of the floor considerably. However, by increasing the stiffness, the connections between the additional timber plates and the reinforced component becomes governing. Furthermore, the increase in stiffness does not significantly improve the acoustic sound insulation, as this is mainly governed by the surface mass of the timber floor.

It is therefore concluded that strengthening of monumental timber floors, by means of several layers of separate timber plates fastened on top of the floor, is an effective way to achieve the desired strength and stiffness. Thick plates, small spacing between fasteners and inclined fasteners are a requirement to achieve higher strength and stiffness. However, additional measures must be taken to meet the sound transmission requirements. For the original perseverance of the monumental timber floor and reversibility, these measures would be dry floating floors. A significant fact is that dry floating floors only add mass and do not increase stiffness, which lowers the maximum allowable load on the floor.

A major limitation of this thesis is the consideration of the non-cooperating intermediate layer between the additional timber layers and the reinforced component. This intermediate layer results in multiple shear planes between the reinforced component and the additional timber plates. This thesis suggested to use a factor, determined from Roensmaens et al. (2020) research, to convert the multiple shear planes into a single shear plane. It is therefore important that further research investigates the influence of these multiple shear planes that do not contribute to the bending stiffness of the reinforced component.