Analytical and experimental response of timber joists strengthened with an external underslung post-tensioned system

Abstract

Floors made of timber load-bearing joists often do not meet modern load demands and strict code requirements. In pursuit of retrofitting such floors, this paper investigates the feasibility of strengthening timber joists with an external underslung post-tensioned system composed of a V-shaped tension chord and a midspan strut. An analytical approach is employed to assess the system’s response under live loads. Based on these findings, a short parametric study is used to identify the key parameters influencing the structural behaviour of the strengthened joists. To validate the theoretical results, an experimental programme is conducted on 9 full-scale specimens obtained from an existing XI-century building. The specimens were divided into three series based on a variable span-to-rise ratio of the underslung post-tensioned system. The experimental tests confirm the correct functioning of the system when applied to timber joists. Furthermore, the comparison between the analytical and experimental outcomes demonstrates consistent agreement across the various cases investigated, confirming the efficiency of the proposed system in increasing the load-carrying capacity of the timber joists. Specifically, the underslung post-tensioned system significantly enhances the load-carrying capacity of timber joists, increasing the flexural strength and stiffness up to 2.0-2.5 times and contributing to support up to 35% of the live loads.