Hans De Backer

Fatigue strength application of fracture mechanics to orthotropic steel decks

Modern fatigue design in civil constructions is mainly limited to the use of S-N curves and the hypothesis of Palmgren-Miner, as described in design standards and Eurocode. While using the latter, the fatigue evaluation may be conservative, since the outdated S-N curves are compared to current construction technology and weld properties. This shortcoming has a direct influence on the current design of orthotropic steel decks. To increase the understanding of the fatigue behaviour, an improved analysing tool using linear elastic fracture mechanics and extended finite element model is proposed. As a result, thickness effects are evaluated for both the longitudinal stiffener and the deck plate. These calculations indicated that increasing the thickness of the deck plate and the longitudinal stiffener increases the fatigue life of the structure. However, the thickness should be limited to maintain the advantage of a light-weighted construction.

Determining geometric out-of-plane imperfections in steel tied-arch bridges using strain measurements

The structural behavior of steel tied-arch bridges is determined by the presence of a large compressive force. As a consequence, slender steel arches are highly sensitive to in-plane as well as out-of-plane buckling. At present, no specific buckling curves for out-of-plane buckling exist for nonlinear or curved elements in the international codes and calculation methods. This research is mainly concerned with quantifying the geometric imperfections of real arch bridges based on the results of on-site strain measurements. The arches of six bridges have been equipped with strain gauges along their entire length. For the actual determination of the imperfections, a detailed finite-element model has been developed. It is assumed that the actual geometric imperfections are a linear combination of a number of theoretical imperfections. These imperfections are characterized by a theoretical lateral displacement in a single cross section of the arch. The influence of these theoretical imperfections can be calculated using the described finite-element models. The actual imperfections are determined based on the comparison of measured strain values with the strain values based on a linear combination of theoretical imperfections. This method is verified using the results of topographical measurements of one of the studied bridges. Both sets of results are in agreement when it comes to size and shape of the imperfections. The shape of the resulting geometric imperfection is a half-sine wave for most of the studied bridges. However, the most important result is that for all considered bridges, the size of the imperfections is substantially smaller than the one predicted by design codes.

Efficiency of Closed Stiffener Orthotropic Deck Panels for Railway Bridges

Closed stiffener orthotropic decks can be advantageous for ballasted or non-ballasted railway bridges. This has been achieved successfully on a number of tied arch railway bridges for the high-speed railway network in Belgium. In these bridges, the orthotropic plate contributes to both the deck plate action and the main load carrying system as the horizontal lower chord or arch tie. Although the basic structural action is comparable to that of road bridges, specific differences exist. These differences, which originate from a substantial discrepancy of the load transfer, are addressed in this paper. Railway loads considerably exceed applicable loads for road bridges, which result in significantly larger rib and floor beam dimensions. This has a significant impact on the transverse load spreading in the plate and on the moment redistribution due to floor beam flexibility. Load spreading describes the phenomenon in which a concentrated load is spread throughout a continuous medium. For typical railway bridg...

Application limits for continuously welded rails on temporary bridge decks

The possibility of omitting rail expansion devices from the track configuration, when continuously welded rail is continued over temporary bridge decks, is investigated in detail. More specifically, the related rail track to temporary bridge interaction phenomena are analysed using finite element modelling. A first parametric analysis assesses the additional rail stresses due to moving trainloads and temperature variations, based on stipulations provided in the unit identification code 774-3R. In addition the model is expanded to a more complex structure that is able to simulate the buckling behaviour of the rail track using non-linear methods. Using this model, a second parametric study is performed in which only thermal loading is considered. This allows for determining the parameters, which are predominant in determining the critical buckling temperature of the rails, and for assessing the magnitude of the safety margin necessary, when it comes to thermal buckling of the rails and the temporary bridges. It can be concluded that, depending on the magnitude of two main factors, the lateral ballast resistance and the amplitude of the initial track misalignment, a considerable reduction of the track stability might arise. Therefore, a minimal characteristic lateral ballast resistance of 4 kN is recommended along with a maximal allowable misalignment amplitude of 7 mm has to be prescribed when thermal track buckling has to be considered in the design.

Verification of the performance of the precast concrete lining in the Diabolo Tunnel and Liefkenshoek rail tunnel

During construction of the Diabolo tunnel and the Liefkenshoek railway tunnel in Belgium, strains were monitored in several cross-sections of the precast concrete segmental lining. The obtained measurement data allow evaluating the in situ behaviour of the concrete lining under numerous loading conditions during and after tunnel drive works. The gradual development of strains in the initial stage after ring erection is discussed in relation to the grout loading, hydraulic jack forces and transferred soil and water pressures. Measurement sections below the River Scheldt show that the water level variation, linked to the tides of the North Sea, is clearly depicted in the strain results as a half-daily fluctuation. Results show that despite the rough site conditions, strain monitoring proves a useful verification of tunnel lining performance and an important addition to tunnel design practice.

Weld Penetration in Orthotropic Steel Bridges

This paper reports the results of experimental research concerning the connection between the deck plate and the web of the longitudinal stiffeners in an orthotropic plated bridge deck on a microscopic scale. An important number of test specimens of a weld are studied with the help of a video microscope, to detect the efficiency of the root of the weld. The second part of the paper is concerned with parametric analysis of the lack of weld penetration by using accurate finite element modeling. The results demonstrate that the weld quality often required cannot always be assured, which surely has important consequence on the stresses in the weld and the fatigue resistance.

Efficiency assessment of CFRP laminate reinforcement of a partly deficient post-tensioned fly-over

Immediately after construction, five spans of a newly built concrete fly-over showed insufficient concrete resistance. Carbon fibre reinforced polymer (CFRP) laminates with little compression stiffness were glued to the lower surface of the superstructure to strengthen the concrete slab decks. After installing, assessment of this reinforcement was made by a first loading test, during which concrete strain measurements with wireless gauges were carried out. In addition, the fundamental frequencies have been determined by accelerometers and impact excitation of the viaduct. Comparing the results from these tests on acceptable and deficient decks confirms the insufficient concrete quality of the suspect spans. The time-dependent evolution of creep changes this situation and the CFRP laminates actively influence the characteristics of the five strengthened decks. Hence, strains were monitored constantly during a period of 5 months. Finally, loading tests with trains as well as acceleration measurements were carried out to confirm the effect of the reinforcement.

Impact of Different Conditions on Resin Coated Mortar

In order to increase the structural performance, various techniques are being evaluated, tested and adopted. Structures exposed to adverse environmental conditions, are facing issues i.e. lifespan reduction and pre-mature failure. World-wide polyester resin is considered to be applied for re-novation, repair, especially for structural elements which are facing environmental degradation. This manuscript evaluates the quantitative performance of resin as a coating agent on mortar, under different controlled conditions. The experiment involved casting of specimens, coating 1–3 layers polyester resin on samples, exposure to environmental conditions and water. Compressive strength provide evidence that resin coats increased compressive strength of cubes by 69%. Whilst significant water absorption reduction was also noted which is proportional to number of coats. Univariate general linear model were evaluated by with respect to various parameters on the two conditions i.e. ambient environment and exposure to water. This study concludes that polyester resin may be used in primary structural element members, which are exposed to different environmental scenarios.

Macro-crack initiation life for orthotropic steel decks considering weld heterogeneity and random traffic loading

Abstract This paper presents a multi-scale approach to predict the macro-crack initiation life (MCIL) of welded joints. This fully adaptive multi-scale technique is designed to take into account micro-scale material heterogeneity due to the existence of defects and to consider the uncertainties regarding defect occurrence and characteristics and traffic loading. The procedure of the proposed approach was introduced along with an orthotropic steel deck. Firstly, a homogenisation method was used to link defects in the heat-affected zone with the macro-scale structure. This allows for estimating the effective Paris constant, required for a Paris Law-based fatigue damage analysis. Secondly, a traffic flow, based on weigh-in-motion measurements, was used to simulate the load effects on the examined joint. Afterwards, the extended finite element method was adopted to calculate the stress intensity factors with respect to the weld geometries. Finally, the damage before macro-crack initiation was obtained using the Paris Law. In addition, Monte Carlo simulation was integrated within this multi-scale crack growth procedure to consider the randomness in pore and inclusion occurrence and characteristics. Hence, the MCIL is expressed in the form of a probability distribution. Results show that the proposed approach enables to provide a rational maintenance and inspection intervention time.

Active crack control in continuously reinforced concrete pavements (CRCP)

Continuously reinforced concrete pavements (CRCP) are frequently used in Belgium on roads having heavy traffic because they have a better long-term performance and require low maintenance. Free cracking is allowed in CRCP but the cracking pattern is controlled by the continuously longitudinal reinforcement. The problem is that in reality the crack pattern is not as desired. Distances between cracks are sometimes smaller, creating clusters of cracks. These clustered cracks can evolve over time, and, after many heavy axle load passages, lead to destructive punch-outs.