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This article presents two case studies for a full-probabilistic assessment of existing concrete bridges combined with the use of the finite element method for the evaluation of the internal forces. Since the structures already exist, additional information about material parameters and loading conditions can be collected. With a sensitivity analysis, the contribution of each parameter to the total uncertainty is examined and the relevance of their actual value and variation is shown. The first case study is a single-span reinforced concrete frame bridge on the German highway A92 constructed in the year 1985. With a span of around 5 m, but a width of 27 m and a skewness of 70 gon, the bridge is modelled as a three-dimensional finite element model. The second case study is a two-span reinforced concrete frame bridge from 1986 with span lengths of about 7,50 m each. The superstructure is a 45 cm thick slab and the base plate has a thickness of 80 cm. The assessment is performed based on the semi-probabilistic safety concept according to the codes first and secondly with a full-probabilistic calculation. In this article the selection of parameters for the relevant input data are presented and discussed. The results of the sensitivity analysis are used for the setup of measurements to acquire important data for the modelling in the critical limit states.
In reliability assessment of existing bridges, collected information about the structure can be utilized to modify partial safety factors and to refine characteristic values individually. In this way, uncertainties covered in initial calculation models and arising from assumptions can be reduced as well as biases identified and corrected. This allows for more realistic assessments and more appropriate decisions on actions to be taken. However, the on-site observation of information is generally associated with certain costs. For this reason, an efficient data-supported reassessment requires that only the individually relevant testing tasks are solved. Additional sensitivity analyses can further be used to specify the requirements on certain inspections, such as the testing result quality to be achieved, in order to avoid too imprecise or too expensive measurements on-site. In this contribution, the methodology for the demand-actuated definition of measurement objectives is demonstrated using the example of non-destructive concrete testing methods and findings from reassessments of two prestressed concrete bridges. Each case study includes the semi-probabilistic identification of structural weak points in ultimate and serviceability limit states, a comparison with respective probabilistic sensitivity analyses, and the subsequent derivation of parameters that are a) measurable and b) highly relevant in the individual reliability analyses.
AbstractIm zweiten Teil dieses Beitrags werden die Grenzzustandsfunktionen für die Nachweise im Grenzzustand der Tragfähigkeit für Biegung mit Normalkraft, Querkraft mit Querkraftbewehrung und Torsion vorgestellt. Die Eingangsgrößen der mechanischen Widerstandsmodelle werden durch zahlreiche Abbildungen illustriert. Auf Grundlage der probabilistischen Modelle aus dem ersten Teil dieses Aufsatzes und der mechanischen Modelle aus diesem zweiten Teil werden die probabilistischen Nachweise geführt. Die Ergebnisse aus der Zuverlässigkeitsanalyse werden mit den semiprobabilistischen Berechnungsergebnissen verglichen. Mittels Parameterstudien wird gezeigt, welche Eingangsgrößen im Standsicherheitsnachweis die größten Einflüsse auf die Bauwerkszuverlässigkeit haben. Die Messung dieser Parameter ist entsprechend besonders nützlich. Auf dieser Basis wird abschließend gezeigt, welche Chancen sich aus der Durchführung von zerstörungsfreien Bauwerksprüfungen für die Nachrechnung ergeben.
AbstractDer Erhalt der bestehenden Verkehrsinfrastruktur ist eine große Herausforderung, da die Bauwerke fortwährend altern, die Verkehrsbeanspruchung zunehmend steigt und deren umfassender Ersatz mit Blick auf begrenzte Budgets und Ressourcen nicht möglich ist. An Bestandsbauten können tatsächliche Materialkennwerte, Geometrien oder Einwirkungen objektspezifisch erfasst werden. Die Nachrechnung von Bestandsbrücken erfolgt in Deutschland gemäß Nachrechnungsrichtlinie nach Stufen 1 und 2 mit dem semiprobabilistischen Teilsicherheitskonzept. Nur im Ausnahmefall wird eine messbasierte Bewertung oder eine wissenschaftliche Betrachtung gemäß Stufen 3 und 4 durchgeführt. Die Einbeziehung zusätzlicher Informationen aus der Zustandserfassung bietet jedoch viele Vorteile für eine realitätsnahe Bewertung. In probabilistischen Berechnungsmethoden können solche Kenntnisse direkt in der Berechnung berücksichtigt werden. Ziel dieses zweiteiligen Beitrags ist die Vorstellung der Potenziale und Vorteile bei der direkten Berücksichtigung der Ergebnisse von zerstörungsfreien Prüfverfahren bei der Zustandsbewertung von Brückenbauwerken. Das Konzept wird an einer Spannbetonbrücke vorgestellt. Der erste Teil befasst sich mit den Möglichkeiten zur Überführung der Eingangswerte der üblichen semiprobabilistischen Berechnung in stochastische Modelle. Ausgehend von der semiprobabilistischen Nachrechnung nach derzeitigem Regelwerk wird die probabilistische Modellierung aller relevanten Basisvariablen für die Grenzzustände der Tragfähigkeit Biegung, Querkraft und Torsion ausgeführt.
Moisture testing and mapping
(2024)
Most building materials are porous and hydrophilic. Thus, for condition assessment and damage detection a determination of the material moisture is required. In this lecture, a short overview is given regarding moisture transport in porous media in the hygroscopic range. Then, several non-destructive measurement techniques are introduced. Here, the focus lies on their practical application and limitations.
Durch das alkalische Porenwassermilieu ist Stahl im Beton vor Korrosion geschützt. Unter ungünstigen Umgebungsbedingungen (Karbonatisierung, Chlorideintrag) kann die passive Deckschicht auf der Stahloberfläche zerstört werden. Im weiteren Verlauf können sich korrosionsbedingte Folgeschäden, wie Risse und Abplatzungen am Bauwerk ergeben. Um notwendige Sanierungsmaßnahmen sind frühzeitige und weitgehend zerstörungsfrei ermittelte Informationen über das aktuelle Korrosionsverhalten der Stahlbewehrung von großer Bedeutung. Die Potentialfeldmessung ist ein etabliertes und weit verbreitetes Verfahren zur Beurteilung des Korrosionszustandes der Bewehrung in Stahlbetonbauwerken. Mit Hilfe dieses Verfahrens können Bereiche aktiv korrodierender Bewehrung zerstörungsfrei lokalisiert werden. In der Regel kommt diese Messmethode bei der Detektion chloridinduzierter Korrosion zum Einsatz. Wie und unter welchen Randbedingungen dieses Messverfahren zielführend eingesetzt werden kann, ist im Merkblatt B 03 der dgzfp erörtert.
Electrochemical potential mapping is a recognized technique for the localization of corroding reinforcing steels. Areas of corroding reinforcement steel can be located in a non-destructive manner by means of this procedure. Half‐cell potential measurements are used in order to detect chloride‐induced corrosion. However, it is not recommended in order to assess the risk of carbonation‐induced corrosion. For this purpose, determining the carbonation depth and the concrete cover appears more appropriate. The content of this specification exclusively refers to the application of mobile, local variable reference electrodes, which are only placed on the concrete surface while measuring. The technique distinguishes itself thereby from the range of corrosion monitoring systems with stationary installed reference electrodes and sensor systems, respectively, whereby it is possible to continuously track measurements within the area of the installed electrodes. However, these methods are not dealt with in this specification.
DIN 4871
(2024)
The Bundesanstalt für Materialforschung und -prüfung (BAM) collaborates with TNO to develop a software framework for automated calibration of structural models based on monitoring data. The ultimate goal is to include such models in the asset management process of engineering structures. As a basis for developing the framework, a multi-span road bridge consisting of ten simply supported steel box girders was selected as a test case. Our group measured output-only vibration data from one box girder under ambient conditions. From the data, we determined eigenfrequencies and mode shapes. In parallel, we developed a preliminary structural model of the box girder for the purpose of predicting its modal properties. In this contribution, we provide an overview of the measurement campaign, the operational modal analysis, the structural modeling and qualitatively compare the identified with the predicted modes. As an outlook, we discuss the further steps in the calibration process and future applications of the calibrated model.
Managing aging engineering structures requires damage identification, capacity reassessment, and prediction of remaining service life. Data from structural health monitoring (SHM) systems can be utilized to detect and characterize potential damage. However, environmental and operational variations impair the identification of damages from SHM data. Motivated by this, we introduce a Bayesian probabilistic framework for building models and identifying damage in monitored structures subject to environmental variability. The novelty of our work lies (a) in explicitly considering the effect of environmental influences and potential structural damages in the modeling to enable more accurate damage identification and (b) in proposing a methodological workflow for model‐based structural health monitoring that leverages model class selection for model building and damage identification. The framework is applied to a progressively damaged reinforced concrete beam subject to temperature variations in a climate chamber. Based on deflections and inclinations measured during diagnostic load tests of the undamaged structure, the most appropriate modeling approach for describing the temperature‐dependent behavior of the undamaged beam is identified. In the damaged state, damage is characterized based on the identified model parameters. The location and extent of the identified damage are consistent with the cracks observed in the laboratory. A numerical study with synthetic data is used to validate the parameter identification. The known true parameters lie within the 90% highest density intervals of the posterior distributions of the model parameters, suggesting that this approach is reliable for parameter identification. Our results indicate that the proposed framework can answer the question of damage identification under environmental variations. These findings show a way forward in integrating SHM data into the management of infrastructures.
Nowadays geotechnical engineering firms have powerful software tools to extent their consult-ing business also into dynamic soil-structure interaction, which before has been restricted to a rather small community of specialized experts in this field, and they certainly do. This is par-ticularly true with respect to non-seismic sources, that is all kinds of human induced vibrations. Hence, there is a demand from clients as well as from contractors to have guidance on the re-quirements as well as the limits of numerical modelling of soil-structure interaction. From the literature as well as from relevant standards, recommendations for the numerical modelling of soil-structure interaction problems involving seismic actions are well known, e. g. ASCE/SEI 4-16. There are, however, some particularities when dealing with human-induced vibrations, which are absent in seismic analyses. For human-induced excitations very little specific guid-ance has been published in the past. A machine foundation on a homogeneous half space ex-cited by harmonic loads with excitation frequency between 4 Hz and 64 Hz has been ana-lysed by means of several commercially available software packages. Parametric studies have been performed to verify if recommendations for seismic soil-structure analyses are valid for non-seismic analyses as well. This paper provides details on the benchmark example and the most important conclusions from the undertaken parametric studies.
ISO 14837-32:2015 and DIN EN 1998-1/NA:2021 as well as prEN 1997-2:2022 allow for us-ing correlations between the results of in-situ soil penetration tests and shear wave velocity (or shear modulus) to determine soil properties to be used in dynamic analyses. While the ISO and prEN standards even provide some recommendations on specific correlations to be used, the DIN standard does not. Due to the statistical nature of such correlations their general applica-bility has to be verified. We collected data sets from test sites from Germany as well as New Zealand at which cone penetration tests (CPT) as well as seismic site investigation methods were conducted. These sites comprise sandy soils as well as clayey soils, mixed soils as well as glacial soils. We compare the results of several correlations between CPT results and shear wave velocity. The accuracy of such correlations is assessed with respect to the accuracy of seismic in-situ tests. It turns out that for clean sands such correlations between CPT and Vs have a similar order of variability as seismic in-situ tests conducted at the same site. The higher the fines portion of the soil, the higher the variability of the statistical correlations, and conse-quently the less the general applicability. For glacial soils and other special soil types usage of statistical correlations to determine dynamic soil properties is not recommended.
The design of bridges often overlooks the vertical component of earthquakes or considers it of secondary importance, despite compelling evidence indicating specific structural damage caused by primary earthquake waves. Conversely, during the operational phase, the combined influence of ground motion and moving loads from vehicles can significantly impact the structural health monitoring (SHM) of bridges. This study aims to evaluate the simultaneous effect of vertical earthquake vibrations and moving vehicle loads on simply supported bridges. The research employs a practical methodology based on the eigenfunction expansion method to analyze change of deflection due to the effect of these concurrent forces under seven different earthquake records. It is shown that within a realistic range of vehicle mass and velocity, the average of changing the maximum deflection at the mid-span of the main beam (denoted as M_n) reaches up to 163% under various scenarios. Subsequently, the seismic parameters influencing this phenomenon are identified through a statistical analysis of set of 100 different earthquake records with unique features. A linear regression equation is presented to predict the M_n based on the earthquake specific properties. Additionally, to control the vertical vibration of bridge systems, a novel vibration suppression system utilizing steel pipe dampers is introduced, and its reliability is examined across a broad spectrum of bridge flexural rigidity. The results indicate that the system's efficiency depends on M_n and the soil type of the bridge construction, enabling a reduction in structural sections (up to 27%) while achieving the same maximum target deflection in the initial state. This efficiency leads to a more economical design solution, emphasizing the potential benefits of the proposed system for practical application.
Ausgehend von den Grundlagen der Korrosion stellt der Vortrag die Möglichkeiten des Korrosionsschutzes mit Zink vor. Dabei wird sowohl auf den kathodische Korrosionsschutz mit aktiven Zink-Anoden, als auch auf den Korrosionsschutz durch Deckschichtbildung an Zink und Zinküberzügen eingegangen. Veränderte Umweltbedingungen in Europa in den letzten Jahrzehnten, ermöglichen mit Schichtdicken um 80 µm einen Korrosionsschutz von über 100 Jahren, wenn konstruktive Randbedingungen beachtet werden.
NDT Data-Supported Structural Reassessments based on Regression Relations in Measuring Data Analysis
(2024)
Non-destructive Testing (NDT) provides valuable data about structural elements, supporting the assessment of existing infrastructures without incurring additional structural damage from inspections. Simultaneously, the uncertainty in measurement, which quantifies the quality of measurement results, plays a crucial role in decisions aimed at, e.g., optimizing maintenance strategies, rehabilitation works and Structural Health Monitoring (SHM) implementations. The Guide to the Expression of Uncertainty in Measurement (GUM) framework has already been considered for non-destructive concrete testing. Regarding that, extensive measurements with sophisticated scopes need to be conducted by experts as an obligatory process.
This study illustrates how NDT results describing the inner structure of a concrete element can effectively support the reassessment of bridges in operation. To achieve this, the study considers the various measurable positions of the resisting longitudinal tendons of a bridge structure to investigate the displacement change under dynamic service loads.
Furthermore, this study aims to simplify and optimize existing NDT data analysis procedures by employing regression analysis, enabling the detection of structural features. This regression analysis yields a modifier for determining the correct depth of an object within the structural element. The method is validated through laboratory experiments, including the use of an ultrasonic measurement system. As a result, it provides unbiased and accurately measured results, while ensuring that the measurement data can remain uncorrelated. Major advantages include efficient computation, a wider scope, and avoiding redundant information about the measuring process.
The findings demonstrate that employing the proposed NDT analysis method, with its enhanced practicability, can significantly augment the efficiency of NDT data-supported structural reassessments across various scenarios.
Ultra-High-Performance Fiber-Reinforced Cementitious Composite (UHPFRC) offers several advantages compared to concrete, notably due to the strain hardening behavior under tensile actions. Structures made of this composite material are lightweight and highly durable, thanks to the UHPFRC waterproofing quality. Nonetheless, the tensile behavior leads to a different cracking pattern than conventional concrete and is not fully understood yet. This paper presents a combined approach using both passive ultrasonic (US) stress wave (or acoustic emission) and active US stress wave monitoring to localize and quantify damage progression in a full-scale UHPFRC beam during experimental load testing. The proposed monitoring approach involves 24 US transducers that are embedded randomly throughout a 4.2- meter-long laboratory UHPFRC T-beam. Continuous monitoring enabled accurate localization of US stress sources caused by loading-induced cracking as well as from pulses generated by the embedded US transducers. This study shows that it is possible to predict the location and shape of the macro-crack that is linked to structural failure early on, i.e., just after the end of the elastic domain. This combined approach opens new possibilities to monitor the structural behavior and detect damage on UHPFRC structures before they affect the structural behavior in terms of deflection and strain.
Combined passive ultrasonic (US) stress wave [better known as acoustic emission (AE)] and active US stress wave monitoring has been shown to provide a more holistic picture of ongoing fracture processes, damage progression, as well as slowly occurring aging and degradation mechanisms in concrete structures. Traditionally, different data analysis techniques have been used to analyze the data generated from these two monitoring techniques. For passive US stress wave monitoring, waveform amplitudes, hit rates, source localization, and b-value analysis, among others, have been used to detect and locate cracking. On the other hand, amplitude tracking, magnitude squared coherence (MSC), and coda wave interferometry (CWI) are examples of analyses that have been employed for active US stress wave monitoring. In this paper, we explore some of these data analysis techniques and show where their respective applications and limitations might be. After providing an overview of the monitoring approach and the different data analysis techniques, results and observations from selected laboratory experiments are discussed. Finally, suggestions for further work are proposed.