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Load sharing between stringers in gravel decked log bridges Lansdowne, Matthew Wayne

Abstract

In British Columbia bridge designers have shifted to using steel or concrete for more permanent structures in forest roads. For temporary structures gravel decked log stringer bridges can still be a cost effective alternative. However, as companies move into smaller second growth timber, load sharing between the stringers becomes an important consideration. There are many ways that load sharing between log stringers is achieved. Examples of methods used to distribute the live loads between the log stringers are gravel surfaces, cable lashing wrapped around the stringers, and cross members including cedar cross puncheon and needle beams. In this paper a Finite Element Model (FEM) of a gravel decked log stringer bridge was developed. This model represented the stringers as beam elements and the cable lashing as elements that only transfer vertical loads. The load spread effect of the gravel surface was accounted for by using equations that predict the stress at the base of the gravel deck due to live and dead loads. For simple configurations where hand calculations were possible the results from the FEM were compared to the hand calculations. The comparison of the FEM to the hand calculations indicates the FEM is calculating values correctly. The FEM was then calibrated based on data available from the Forest Engineering Research Institute of Canada for an in service bridge. The FEM model was calibrated by varying the stiffness of the lashing element stiffness, a parameter affecting the load sharing between the stringers. When varying the stiffness of this parameter it was found through comparison of the FEM to the in-situ data that setting the stiffness of the lashing elements to very low values underestimated the load sharing between the stringers, and setting the stiffness to high values overestimated the load sharing. An error minimization analysis was run to determine the optimum lashing stiffness to calibrate the model to the in-situ data. As the calibrated FEM deflections approximated the deflected pattern of the in-service stringers the forces, moments, and reactions of the FEM should approximate the internal conditions experienced by the stringers of the in-service bridge. When considering the calibrated FEM results for the in-situ data it was found that full load sharing between all stringers of the superstructure was not occurring. This indicates it is inappropriate if design assumes full load sharing between all stringers when designing gravel decked log stringer bridges with lashing. Additionally, it was found when lashing the bridge at thirds, the sections of the stringers in the span between the lashing points were acting independently. This action produced elevated stress levels in the stringers due to bending. The elevated stress levels between the lashing points could be of particular concern to designers using smaller second growth stringers.

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