Timber and Steel Railway Bridge

Abstract

Timber usage in construction works has increased over the past years due to its better environmental performance. Applying timber elements instead of steel or concrete decreases the emissions impact of the built environment. Architect firm ZJA and Iv-consult are interested in investigating a timber railway bridge as part of the Norwegian infrastructure plan. A kilometre-long double-track railway bridge with 50 m spans is used as a case study to investigate the possibilities of timber application. The objective is to design and assess a timber beam bridge with steel sections at the support.

The aim of this research is to design the sections and their connection. The design requirements and calculation methods used are based upon the Eurocodes. The design is considered to be part of the preliminary design phase of a project. Unity checks of the cross-section and deflection are calculated by use of linear mechanics. An FE model with beam elements is created to determine the force distribution of the bridge and calculate deflection. The strength and stiffness of the connection are calculated using empirical and plastic mechanic methods given in the standards. The resulting stiffness is used in the model to determine the final deflection when accounting for the semi-rigid joint.

The design consists of timber and steel box girders that only carry a single track. On top of the support, a slender stiffened steel box girder of 10 m length is situated, 5 m to both sides. Between the steel sections, there are timber box girder sections of 40 m that have very thick flanges and webs. On top of the structure lies a slab track that is only considered as weight. A dowel-type connection is used with double slotted-in steel plates in all flanges and webs of the timber section. The slotted-in plates have a slightly smaller width than the timber elements and have a depth of around 2.7 m. The steel fasteners are simple dowels.

This thesis has found that a timber and steel beam bridge with box girder cross-sections is governed by the creep deflection, shear strength of the timber, fatigue strength of the steel, and the dimensional stability of the connection. Except for the last issue, each of these limits can be satisfied by increasing the dimensions of each cross-section. Dimensional stability is the timber size change due to moisture content variation. The perpendicular-to-grain stresses become too large due to the configuration of the steel in the connection. The use of modified timber, which decreases the dimensional stability effect drastically, is only suggested as a solution.

An architecturally pleasing structure is designed by adding very limited visual elements but mainly practical and structural elements that have two functions, shown in the figure below. The desired continuity and bridge rhythm is accomplished via a line concept. The abrupt material cross-section changes are partly covered by continuous timber elements. From the practical and architectural adjustments, new cross-section dimensions were determined. The governing unity checks of the new design were calculated and showed the possibility of the design under the assumption the connection shrinkage issue is resolved.

Thus a timber and steel railway bridge has potential. It should be stressed that methods and factors from the Eurocodes are assumed to be applicable to the large timber box girder sections. This assumption is recommended to be checked by means of experimental research on large timber specimens. Additional research on the semi-rigid is advised, as introducing slotted-in steel plates in the web and flange plane can significantly improve the moment capacity.

Finally, two alternative suggestions are given to avoid solving the dimensional stability issue. The first is to design a bridge with lower loading demands, like a road bridge. Secondly, the bridge can be made of only timber sections. Alternating spans would have two hinged connections to form a continuous cantilevered bridge. While the first alternative creates easier circumstances to solve for dimensional stability, the second alternative completely removes the issue by having a connection with lower demands. It is recommended to incorporate connections early on in timber design.