A New Bridge Proposal - Road Bridge with a Cross-Laminated Timber Slab

Abstract: In this thesis, a new bridge type is proposed. It is a steel bridge with a slab of cross-laminated timber elements. The bridge is intended for road traffic, and it has a span length of 26 m. It aims to be competitive when a short construction time is important. Long construction time could be a critical issue when launching a bridge over a railway where disruption of traffic flow is associated with high costs. The design of the new bridge type was based on an existing steel-concrete composite bridge built in Ulricehamn in Sweden. The design was limited to the main structural elements of its superstructure. The biggest difference to the reference bridge is that the new bridge type do not have composite action between its girders and slab. Since steel and timber have very different moduli of elasticities, composite action is not believed to be effective. For the design of the cross-laminated timber slab, rolling shear in cross-layers due to large concentrated loads was decisive. Whereas for the design of the steel structure, deflections in the serviceability limit state was governing. The cross-laminated timber slab did not provide sufficient torsional stiffness to the main girders. Thus, the deflections of the bridge became too large. Therefore, planar and cross-bracing spaced every 3.25 m were required. The bracing created a virtual box-section with large torsional stiffness. The construction cost of the superstructure was estimated for the proposed bridge and the reference bridge. The cost of the cross-laminated timber slab and the concrete slab were comparable. However, the steel structure of the new bridge type was more expensive. Therefore, the cost of the superstructure of the new bridge type was higher. To construct the proposed bridge, it was estimated that it would take two weeks to launch the main girders and to mount the cross-laminated timber elements. Furthermore, it could take up to three months to erect the superstructure of the reference bridge. Consequently, the higher cost of the superstructure of the new bridge type could be compensated by the fact that it is faster to build. Therefore, it may be competitive for projects where short construction time is necessary.