Feasibility of Medium Span Steel-Timber Hybrid Bridges

Abstract

Concrete and Steel are the materials with the largest market share in the construction industry and have been for a long time.
With environmental awareness increasing, timber is regaining popularity due to the potential for carbon neutral and even carbon negative construction.
Use of structural wood elements in bridges, however, is often limited to foot- and cycle bridges.
In this thesis, key aspects with regards to the design of heavy traffic bridges incorporating timber members are identified.
A fully steel bridge and an equivalent bridge, combining timber members with steel are designed within boundaries set by a case study. These designs are developed to a level sufficient for an adequate comparison of the bridges.
The basis on which the bridges are compared are laid out, followed by the conditions the bridges are subjected to. These are based on typical conditions found in an urban Dutch environment.
Analytic equations are automated, by way of python scripts, for the analysis and optimization of the steel bridge longitudinal dimensions under simplified ULS loading. After the optimization is complete, these initial bridge dimensions are verified with a 2D plate element model in SCIA engineer. The full loading for the bridge during utilization, save for accidental loading, is then modelled and the bridge dimensions are adapted in order to meet ULS, SLS, and fatigue conditions.
Several potential versions of a bridge with timber members are considered. Following this, a bridge with a mostly timber superstructure, supported by a self-anchored cable system is further worked out. For this, a SCIA model, with 1D elements and subjected to the same loads as its steel counterpart, is produced. The incompatible combination of 1D elements, thick cross sections, and surface loads is addressed by the use of connector elements (“dummy members”) and individual load panels per member.
After the global optimization of the bridge dimensions with regards to ULS and SLS, the connections are designed with a combination of detail 2D element FE models and analytic equations. The forces and support conditions of the connections follow from the global bridge design. A fatigue check is then run on the timber members of the bridge.
Subsequently, the durability and eco- costs of the bridges are computed. The data for the durability estimation of the steel bridge is based on experience within IV- Infra and the durability of the timber bridge is estimated using the RISE factor method. The eco- costs of the bridges are computed using the IDEMAT database.
The results from the analyses are discussed based on this.
Finally, aspects of relevance in the design of timber bridges are synthesized and recommendations for the application of bridges and further research are given.