Estimating the (remaining) service life of timber bridges with the use of factor methods

Abstract

Timber bridges are much applied structures in the Dutch landscape. In contrast to the other two most common building materials, concrete and steel, the use of timber is however almost exclusively limited to bridges for lighter traffic, e.g. pedestrians and bicycles. Examples of timber traffic bridges in Germany, Scandinavia and even in the Netherlands, show however that, at least structuraly, timber can be perfectly used in the construction of bridges for heavier traffic. Although structuraly feasible, there are still a number of doubts and uncertainties that go hand in hand with timber bridges. One of them is the maximum reachable service life. This research focusses on the ability to estimate this service life with the help of so called factor methods. The service life of a timber element in the Netherlands, with respect to biological decay, is in the basis determined by two main factors: the natural durability of the timber specie, and the moisture and temperature levels of the timber during its service life. In the case of timber bridges, where the timber elements are subjected to the weather, this can be broken down in three main influencing factors, namely:
1. Natural durability of the used timber specie 2. Local climate (Temperature, rainfall and humidity) 3. Detailing of the bridge (Protection against water accumulation) With these three main influencing factors in mind, factor methods can be used to estimate the service life of bridge elements. These factor methods estimate the service life based on reference situations, and use modification factors to modify these reference situations into the actual situation of the element. In this research two different factor methods have been reviewed in depth: the DuraTB method, developed in Sweden, and the TimberLife method, developed in Australia. The two factor methods have in this research been compared with each other and after that their usability and accuracy has been tested with the use of two reality checks on existing bridges in Amsterdam. For the reality checks, first an expected bridge condition was determined with the help of the two factor methods. After this the actual condition of the bridge was determined by an in-field visual inspection. Then the expected and the actual conditions were compared in order to obtain insight in the correctness of the service life estimation of the two factor methods. In addition, the role of factor methods in the processes of service life planning and especially during the calculation of the total costs of ownership (TCO) has been discussed. An example of a TCO calculation has been performed for two bridge designs, in which the focus was put on the incorporation of both the DuraTB and the TimberLife factor methods. Even though this TCO example is not representative from a total cost point of view (since the values of the different costs were guessed), it does show how factor methods can be used in the calculation of these total costs.