Ermüdungsverhalten von Stahlgussknoten in Brücken aus Stahlhohlprofilen

Due to their aesthetic and structural advantages, tubular space truss structures are enjoying increasing popularity in modern bridge construction. The use of cast steel nodes for the joints between the circular hollow section members is also becoming increasingly popular. The fatigue design of such joints however requires additional knowledge with respect to their fatigue resistance. The present work deals with the global fatigue behaviour of cast steel nodes used in longitudinal truss girders of steel-concrete composite bridges. The global fatigue behaviour of cast steel nodes in a truss girder is quantified on the basis of experimental investigations. The relative influence of the resistance of the cast steel node and the resistance of the welds is analysed as a function of various parameters. An economically optimal fatigue design consists of adapting the fatigue resistance of the cast node to that of the welds. The experimental results show very clearly that the fatigue behaviour is governed by the welds in all tested configurations. Their fatigue behaviour is therefore investigated in a research program conducted simultaneously. The present work concentrates on the adaptation of the resistance of the cast steel nodes to that of the welds. Consequently, allowable initial casting defect sizes must be defined as a function of the required fatigue resistance of the welds. Using a numerical boundary element model, they are calculated for cast steel nodes in a typical steel-concrete composite bridge. The results show that brittle failure of a node containing cracks does not occur. A crack depth equal to 90 % of the wall thickness at the crack location is therefore chosen as the failure criterion, assuming that a through-thickness crack is unacceptable. Varying between 28 and 88 % of the wall thickness, the resulting allowable initial casting defect sizes are very large. Aiming at a general application for steel-concrete composite bridges, the results of the numerical investigations are represented by an approximate formula for the stress intensity factor based on a constant correction factor. This approximation results in a considerable simplification of the procedure for the fatigue design of cast steel nodes established for the aforementioned typical composite bridge. The approximate formula is used to perform a parametric study. The influences of the utilisation ratio under traffic and fatigue loads, the cast steel fracture toughness and yield strength and the node dimensions on the defect size are described. Assuming a fracture toughness likely to be encountered in practice and a mean utilisation ratio of the node as well as a range of node dimensions, the allowable initial casting defect sizes are quantified. The present work shows that the fatigue resistance of the welded joints needs to be improved substantially in order to benefit from the high fatigue resistance of the cast nodes. Weld details including a backing ring offer a higher fatigue resistance than those without. Using the proposed design concept, allowable initial defect sizes in cast steel nodes can be estimated as a function of the required fatigue resistance

Fatigue of bridge joints using welded tubes or cast steel node solutions

In the design of recently constructed steel-concrete composite bridges using hollow section trusses for the main load carrying structure, the fatigue verification of the tubular truss joints has been a main issue. Recent research on the fatigue behaviour of such joints has focussed on circular hollow section (CHS) K-joints with low diameter-to-thickness ratios - a geometric characteristic typical to tubular bridge trusses. Analytical and experimental research was carried out and joints with both directly welded tubes and cast steel nodes were studied. This paper presents the main results of these studies and shows comparisons between welded and cast steel solutions. The key issues for the design and fabrication of both types of nodes are reviewed and recommendations for the design and fabrication of tubular bridge structures are made

Fatigue safety of riveted bridges - Part 2: Verification based on the monitoring data of the project "Railway Bridge at Eglisau"

Fatigue safety of riveted bridges Part 2: Verification based on the monitoring data of the project "Railway Bridge at Eglisau". Long term monitoring over one year has been conducted on the riveted Railway Bridge over the Rhine at Eglisau. Measured values were exploited by rainflow analysis and served as the basis for the verification of fatigue safety. As the locations of measurements are generally not identical with the cross sections of verification, measured strains respectively stresses, were extrapolated to the relevant verification cross section by means of factors that were obtained by structural analysis. Using these values, all fatigue relevant structural details were first verified with respect to the fatigue limit. Then, damage accumulation calculation according to the Palmgren-Miner rule and based on Wohler curves for riveted details was performed for those structural details where the fatigue limit check was not fulfilled. Sufficient fatigue safety could finally be verified for the whole riveted structure and an additional service life of at least 50 years for the most fatigue relevant structural element

Fatigue safety examination of a riveted railway bridge using data from long term monitoring

Long term monitoring of structural elements of a 115 years old riveted railway bridge structure of high value as cultural heritage has been conducted. Monitored values were exploited by Rainflow analysis and served as the basis for the fatigue safety verification. As the locations of measurements are generally not identical with the cross sections of verification, measured strains were translated to the relevant verification cross section by means of factors that were determined by structural analysis. Using these values, all fatigue relevant structural details were first verified with respect to the fatigue limit. Then, damage accumulation calculation according to the Palmgren-Miner Rule was performed for those elements where the fatigue limit check was not fulfilled. Sufficient fatigue safety could finally be verified for the entire riveted structure and additional service duration of at least 50 years for this riveted structure could be validated