On Monday, 28. April 2025 at 1PM, the doctoral defense of Stefanie Schoen will take place. The title of her dissertation is: Reliability analysis and design of RC and SFRC structures considering Polymorphic Uncertainties.

Abstract:
Ensuring structural safety requires a precise understanding of uncertainties in material properties, environmental conditions, and modeling assumptions. This dissertation advances the design of reinforced concrete (RC) and steel fiber-reinforced concrete (SFRC) structures by integrating polymorphic uncertainties – combining aleatory and epistemic uncertainties – into a reliability-based framework. While aleatory uncertainties account for inherent randomness, such as material heterogeneity, epistemic uncertainties stem from limited knowledge, including variations in fiber orientation.

Building on semi-probabilistic safety concepts, this study employs probabilistic methods to quantify failure probabilities across ultimate and serviceability limit states. However, accurately estimating low failure probabilities requires a large number of realizations, making finite element simulations computationally prohibitive. To overcome this challenge, reliability analysis is combined with a Transformer-based model for uncertainty quantification.

Finally, reliability-based design optimization reveals that while steel fibers can partially replace conventional reinforcement, their greatest value lies in complementing its – substantially reducing crack width sensitivity to stochastic loads and enhancing durability. Hence, this thesis underscores the importance of integrating uncertainties and the strategic use of steel fibers in the sustainable and durable design of concrete structures.

Interested parties are cordially invited to attend this academic occasion.

For further information please check:

We have published a new work titled "Mechanics of longitudinal joints in segmental tunnel linings: Role of connecting bolts". It is part of the Tunnelling and Underground Space Technology, Volume 161, July 2025, 106601.

The publication is written by: Zhen Liu, Ba Trung Cao, Chen Xu, Xian Liu, Yong Yuan, Günther Meschke

Abstract:
In order to accurately evaluate the moment-rotation relationship for longitudinal joints and to thoroughly elucidate the role of connecting bolts on the mechanism of longitudinal joints subjected to compression-bending scenarios, a nonlinear semi-analytical joint model is developed. This model incorporates the nonlinear behavior of the concrete within the joint influence zone, as well as the contribution of connecting bolts, gaskets, and the contact deformation. The proposed semi-analytical model can simultaneously predict the distribution of the contact pressure, and the stress and deformation field in the vicinity of the joint as well as moment-rotation relationships for both bolted and boltless joints. According to the reference joint test and the proposed semi-analytical model, it is concluded that, under positive bending moment conditions, the load-bearing process of bolted joints can generally be divided into six stages separated by five characteristic points. A comparison of the response of bolted joints versus boltless joints reveals that connecting bolts can effectively delay the expansion of the detached surface, leading to a smoother distribution of the contact pressure, and lower peak contact pressure. The introduction of connecting bolts substantially contributes to maintaining the joint bending stiffness and enhancing the joint bearing capacity, providing bolted joints with greater resilience under environmental disturbances. The influence of the water-proofing gasket on joint behavior is also discussed. Furthermore, parametric analyses are conducted to explore the influence of bolt prestressing, bolt position, and bolt dimension on the mechanical behavior of bolted joints. Based on these findings, a joint design model, based on analytical expressions, is proposed to predict the M-θ relationships for both boltless and bolted joints considering multiple parameters, including axial force levels, bolt contribution, joint configuration, and contact deformation. The joint design model exhibits a high prediction accuracy compared with the semi-analytical joint model, while requiring only simple calculations by analytical equations. To facilitate real-world applications, a design tool based on the proposed joint design model has been developed, which enables efficient and reliable prediction of mechanical behavior of longitudinal joints in segmental tunnel linings.

The publication is now available on Elsevier and can be freely accessed within 50 days (until May 19, 2025) here:
https://authors.elsevier.com/c/1krjH39eM4TfUN

Our latest publication, "New analytical laws and applications of interaction potentials with a focus on van der Waals attraction", is now available as part of the Applied Mathematical Modelling, Volume 145, September 2025, 116100 by Elsevier.

The authors are: A. Borković, M.H. Gfrerer, R.A. Sauer

Abstract:
The paper aims to improve the efficiency of modeling interactions between slender deformable bodies that resemble the shape of fibers. Interaction potentials are modeled as inverse-power laws with respect to the point-pair distance, and the complete body-body potential is obtained by pairwise summation (integration). To speed-up integration, we consider the analytical pre-integration of potentials between specific geometries such as disks, cylinders, rectangles, and rectangular prisms. Several exact new interaction laws are obtained, such as disk-infinite half-space and (in-plane) rectangle-rectangle for an arbitrary exponent, and disk-disk and rectangle-rectangle for van der Waals attraction. To balance efficiency and accuracy, approximate laws are proposed for disk-disk, point-cylinder, and disk-cylinder interactions. Additionally, we have developed a novel formulation for the interaction between a spatial beam and an infinite half-space. The application of the pre-integrated interaction potentials within the finite element method is illustrated via two examples.

The full text can be accessed here:

The recent open access publication "A virtual lab for damage identification in concrete using coda wave interferometry Authors" is written by G. Vu, J.J. Timothy, E.H. Saenger, C. Gehlen, and G. Meschke.

Abstract:
Early identification and prevention of damage in concrete structures can significantly reduce maintenance and repair costs. Weak material degradation, such as load-induced microcracking, generally is a precursor of localized damage in concrete structures can be detected by means of ultrasonic signals. To reliably identify and quantify damage, a systematic method that translates ultrasonic coda signals into the damage state is required. To this end, the effect of material degradation on the coda variations at the specimen level is systematically investigated using a combination of multiscale computational modeling, wave propagation simulations, and Coda Wave Interferometry. The study reveals a strong correlation between relative velocity variation and stiffness variations under stress, confirming the method’s sensitivity to microstructural changes. Simulations of mesoscale concrete models in a virtual lab reveal that relative velocity change increases linearly with stress during initial deformation (up to 1.23%) and decreases significantly during the microcracking stage (−3%), correlating reasonably with experimental data. Additionally, the computational framework enables testing across a robust sample set to estimate the probability of failure, supporting more informed decision-making in structural health monitoring. Finally, a strategy for using specimen scale information to predict the state of damage at the structural scale is presented.

The open access publication is part of the journal Structure and Infrastructure Engineering by Taylor & Francis Online and available here:

The lecture dates for the summer term 2025 are online here: