Long-term behaviour of prestressed timber decks in road bridge design

Abstract

Conventional building methods are still based on the reinforced concrete industry. In the last decades, timber has become more popular because it could be a more sustainable alternative. However, pure timber is not always an option, especially when slender design is required by the client. Because of its low elastic modulus, deflections often require an increased deck height. Therefore, this research focuses on the strengthening of timber bridge decks with reinforcement and prestress in order to increase their slenderness (= the ratio of the span to the height of the cross-section). This might make timber decks more competitive to reinforced concrete designs regarding slenderness. The problem is that prestressing timber decks will lead to creep deformations that induce losses of prestress force. This research is focused on modelling the creep deformations and the resulting resistance losses of prestressed timber decks.

First, a cross-sectional model is developed to be able to find the initial resistance of a reinforced- and prestressed timber deck. This model is based on an incrementally increasing curvature so that the deck behaviour can be quantified from zero load to the failure load. Second, a time dependent model is developed to find the displacements and resistance through time. The timber bridge deck is modelled with ODE systems. The ODE's are used to find the (1) displacements and (2) strains of the deck. To obtain the time dependent behaviour of the deck, a viscoelastic E-modulus is substituted into the displacement- and strain equations. This viscoelastic E-modulus decreases with time, which causes an increase in displacements (= creep displacement). In the same way, the strains are modelled over time. The time dependent creep stains are implemented in the cross-sectional model to find the reduced resistance of the timber deck.

The outcomes of the model suggest that large prestress forces lead to negative creep deflections (= creep in upwards direction). Meaning that for the right value of the prestress force, also zero creep deflections can be obtained. Besides creep, the instantaneous deflections are a large part of the total deflections. According to the results of the model, the instantaneous deflections can be decreased by up to 70%. Regarding the resistance, the final increase of bending moment resistance can reach up to 30% by incorporating prestress (at t = 50 years, including losses due to creep). Due to creep, prestress force is lost over time, resulting in a decreased deck resistance. This research shows that the creep losses result in a bending moment resistance decrease of up to 12%. Taking this into account, a bridge deck with a slenderness of 31 to 33 will be able fulfil its requirements after 50 years of service life. Depending on the client requirements, a slenderness of over 34 can be reached.

Using Eurocode, creep deformations are calculated with a simplistic and conservative method. The model that is built in this research gives a more advanced way of determining the creep deformations of a timber deck. This leads to more realistic quantification of creep behaviour. However, Several factors still cause uncertainty in the model. Therefore, experiments with timber decks should be done to obtain more accurate data for the creep behaviour. The model from this research can be calibrated according to data from experiments, which will increase the reliability of the results.