Tomislav Markic

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Markic

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Tomislav

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0000-0003-1212-6121

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Publications 1 - 10 of 22

Large‐scale experiments on concrete hinges under general loading
Markic, Tomislav; Kaufmann, Walter (2023)
Structural Concrete
The structural behavior of concrete hinges under general loading is far from being properly understood, which is to a large extent due to the lack of pertinent experimental data. This paper contributes to filling this knowledge gap by presenting and discussing the results of an experimental campaign on one-way Freyssinet concrete hinges. Seven large-scale concrete hinges were tested in the Large Universal Shell Element Tester, which allowed the investigation of their behavior under general loading by all six stress resultants. A combination of digital image correlation and distributed fiber optic measurements allowed a deeper insight into the structural behavior of the specimens. The tested hinges could sustain very high axial stresses exceeding 4.5 times the uniaxial concrete compressive strength, large rotations of over 60 mrad, and shear stresses in transverse and longitudinal directions up to 2.24 times the axial compressive stress. The resistance to bending moments about the strong axis and torques also proved to be significant. A moderate amount of reinforcement crossing the hinge throat considerably increased the shear resistance at low axial stresses and produced a ductile shear behavior.
Experimental and analytical investigation of the inelastic behavior of structures isolated using friction pendulum bearings
Tsiavos, Anastasios; Schlatter, David; Markic, Tomislav; et al. (2017)
Procedia Engineering ~ X International Conference on Structural Dynamics, EURODYN 2017
Current American and European code provisions prohibit yielding of base-isolated structures. Therefore, the majority ofexisting base-isolated structures are designed elastically. This study aims at investigating the necessity of the elastic design of thesestructures through the analytical and experimental investigation of their inelastic behavior.This analytical investigation is performed using a two-degree-of-freedom model of a base-isolated structure. The bilinearhysteretic behavior of the structure and the isolator is simulated via a Bouc-Wen model. Numerous simulations of the response tostrong ground motion excitations were performed using Matlab and Opensees models.The experimental investigation performed in this study is based on the response of a reduced-scale base-isolated steel structureto strong recorded ground motion accelerations applied using the shaking table of the IBK Structural Testing laboratory of ETHZurich. The part of the structure designed to develop inelastic behavior is a pair of steel coupons that can be easily replaced aftersuch damage. The structure is base-isolated using four friction pendulum bearings provided by MAGEBA.The experimentally observed inelastic behavior of base-isolated structures is compared to the analytically simulated behavior.A relation between experimentally obtained strength and displacement ductility of these structures is presented. The influence of awide range of response parameters is quantified and presented. The experimentally obtained data is compared to an analyticallyderived strength-ductility-period relation for seismically isolated structures. This comparison serves to validate the proposedanalytical relation and to increase the understanding of the behavior of inelastic seismically isolated structures.
Stress field solution for strip loaded reinforced concrete blocks
Markic, Tomislav; Kaufmann, Walter; Amin, Ali (2018)
Engineering Structures
Betongelenke - Stand der Technik und Entwicklungspotential
Kaufmann, Walter; Markic, Tomislav; Bimschas, Martin (2017)
cemsuisse Forschungsbericht
Direct tension testing of SFRC - Some peculiar effects of the end restraints
Amin, Ali; Markic, Tomislav; Kaufmann, Walter (2019)
Proceedings of the 10th International Conference on Fracture Mechanics of Concrete and Concrete Structures
In principle, a direct tension test is the ideal test that should be used in experimentally determining the softening, or residual, parameters of steel fibre reinforced concrete (SFRC). In the context of design, these material properties are then used to feed into models for shear, flexure etc. This is the approach that has been adopted in the recently released standard for the design of concrete structures in Australia (AS3600-2018). However, there are many parameters which may influence the results of the uniaxial tension test, and the choice of boundary conditions for the test is one of the most relevant ones. Three boundary or end conditions are possible: fixed-fixed, fixed-rotating, and rotating-rotating. In this paper, results of uniaxial tensile tests on twelve identical SFRC direct tension specimens tested with the end conditions listed above are presented. Each condition exhibits behaviours not present in the theoretically ideal tensile softening curve and investigating this is the focus of the present study. It is concluded that the fixed-rotating end conditions serves as a compromise to the issues associated with the other test setups and seems to be more suited for uniaxial tension testing of softening SFRC.
Effect of the boundary conditions on the Australian uniaxial tension test for softening steel fibre reinforced concrete
Amin, Ali; Markic, Tomislav; Gilbert, R. Ian; et al. (2018)
Construction and Building Materials
Partially loaded areas in reinforced concrete: Experimental campaign and model validation
Markic, Tomislav; Morger, Fabian; Kaufmann, Walter (2022)
Engineering Structures
This paper presents the results of a comprehensive experimental campaign investigating the structural behaviour of concrete blocks subjected to partial area loading. A total of 62 concrete blocks were loaded up to failure by applying a compressive load over a limited area. The campaign focused on plane cases of partial area loading, i.e. the load was applied over the (almost) entire block length and thus dispersed mainly in one direction. The test parameters included (i) the specimen geometry; (ii) the size of the loaded area; (iii) the concrete strength; (iv) the conventional reinforcement ratio, layout, and detailing; and (v) the steel fibre content. Modern measuring techniques were applied to gain insight into the crack development, the reinforcement activation, and the contact pressure distribution over the loaded area. The test results, together with experimental data found in the literature, substantiate the fundamental mechanical behaviour identified in previous theoretical studies by the authors. It is shown that the Dual-Wedge and Dual-Cone stress fields presented in the companion paper can accurately reproduce the influence of the investigated test parameters. Moreover, the bearing capacity predictions by these stress fields correlate significantly better with the experimental data than existing design rules implemented in current design codes.
Direction-dependent behaviour and size effect of steel fibre reinforced concrete based on the double punch test
Karrer, Simon; Markic, Tomislav; Lee, Minu; et al. (2022)
Materials and Structures
The post-cracking behaviour of steel fibre reinforced concrete (SFRC) is typically determined following an inverse analysis of flexural prism tests. Although these tests have significant practical merit, it has been argued that the anisotropy of the material due to the dispersion and orientation of the fibres cannot be accounted for in these tests. Multidirectional double punch tests on cubes have been proposed to overcome these issues. These tests are also well-suited to study size effects. However, no generally accepted inverse analysis method for these tests presently exists. This paper presents a simple and mechanically consistent inverse analysis procedure to estimate the residual post-cracking strength of SFRC from the results of double punch tests conducted on cubes. To explore the potential and limitations of this methodology, an experimental investigation was conducted on 30 double punch tests on cubes of varying sizes, varying fibre dosage and loading direction with respect to the concrete casting direction. The results demonstrate that the approach provides useful comparative information on the anisotropy of the material, however further investigation on the input parameters is required to prove its reliability in quantifying the residual tensile stress offered by the fibres.
Structural behaviour of partially loaded areas and concrete hinges
Markic, Tomislav (2023)
Concrete hinges are monolithic connections in structural concrete, shaped to carry high axial compressive forces and undergo significant rotations while developing little moment resistance about at least one hinge axis. This behaviour is particularly useful for articulating linear elements under high axial compressive loads, such as bridge piers and arches. It is achieved by suitably tapering the member section and centring forces and deformations in a small zone, referred to as the hinge throat. Nowadays, concrete hinges are mainly used to reduce restraint stresses in monolithic structures (e.g. integral bridges) as an alternative to conventional bearings. Despite the century-long experience, which confirms the numerous advantages of concrete hinges over conventional bearings (particularly in terms of cost-effectiveness, durability, robustness, sustainability, and appearance), engineers are often still reluctant to opt for concrete hinges. The primary reasons are the limited mechanical understanding of their structural behaviour and the lack of sufficient experimental data, which is reflected in inadequately substantiated design guidelines. A remarkable knowledge gap exists even for the hinge behaviour under the fundamental loading case of axial compression without hinge rotation, which essentially corresponds to that of a partially loaded area. The behaviour under general loading (i.e. bending moments and shear forces) entails even greater uncertainties and open questions. Due to these issues, the potential of concrete hinges is far from being adequately exploited. This thesis aims at alleviating the uncertainties arising when designing and assessing concrete hinges by theoretically and experimentally investigating the underlying mechanisms governing their structural behaviour and proposing mechanically substantiated and experimentally validated models for their analysis and design. To this end, the first part of the thesis is devoted to an in-depth investigation of the structural behaviour of partially loaded areas, i.e. regions where high, concentrated compressive forces are applied over limited contact areas. The accurate understanding and modelling of this fundamental structural engineering problem are of paramount importance to be able to consistently model the more complex behaviour of concrete hinges under general loading. Several discontinuous stress field solutions are proposed to consistently account for the multiaxial strength of concrete and the favourable effect of confinement reinforcement. The results from an extensive experimental campaign conducted on 62 partially loaded concrete blocks show that, compared to the existing, semi-empirical design rules, the proposed stress fields yield very accurate and significantly less conservative predictions of the bearing capacity, thereby enabling a much better exploitation of the structural capacity with reduced uncertainties. The second part of the thesis investigates the structural behaviour of one-way concrete hinges under general loading. In order to extend the very scarce available test data, seven large-scale concrete hinges with light reinforcement crossing their throat were tested in the Large Universal Shell Element Tester at ETH Zurich. The specimens were subjected to multi-stage load histories targeting various failure types (due to axial compression, hinge rotation, shear force parallel or perpendicular to the hinge axis, bending moment about the strong axis, and torque). The experiments (i) confirm the remarkably high compressive strength and deformation capacity of the confined concrete in the hinge throat observed in previous studies, (ii) demonstrate that the hinge can sustain large shear forces and torques if sufficient axial compression is provided, and (iii) reveal that a moderate reinforcement crossing the hinge throat considerably increases the shear resistance at low axial stresses and prevents brittle shear failures. Building on the insights gained from the experiments and previous theoretical studies, the analytical modelling of concrete hinges is revisited using approaches compatible with current design standards. A cross-sectional analysis with confined concrete properties is proposed for the behaviour under axial forces and bending moments, where the strength of the triaxially compressed concrete in the hinge throat and the adjacent blocks is determined with the stress fields developed in the first part of the thesis. The shear strength of concrete hinges is investigated with failure mechanisms inspired by failure modes observed in experiments. Based on the shear strength, a simple approach to estimate the torsional resistance of the throat is also proposed. Overall, the proposed models are mechanically more consistent and agree better with the available experimental data than previously proposed modelling approaches. Moreover, they predict significantly higher resistances, allowing for a more efficient design and potentially fostering the application of concrete hinges in future projects.
Shaking table investigation of inelastic deformation demand for a structure isolated using friction-pendulum sliding bearings
Tsiavos, Anastasios; Markic, Tomislav; Schlatter, David; et al. (2021)
Structures
This study aims at investigating experimentally if seismically isolated structures can manifest inelastic behavior when they are subjected to strong earthquake ground motion excitation. A steel structure is seismically isolated with four friction pendulum bearings and subjected to an ensemble of recorded earthquake ground motion excitations using the shaking table of ETH Zurich Laboratory. The structure is designed to concentrate its inelastic behavior on a mechanical clevis connection comprising two hinges and a pair of replaceable steel coupons. The use of this configuration enables the adjustment of the strength of the structure and thus the parametric investigation of its displacement ductility demand µ for a wide range of earthquake ground motion excitations. The experimentally derived values of the ductility demand µ for the seismically isolated structure are compared with the analytically determined values obtained through the use of Ry-μ-Tn relations for fixed-based structures and the corresponding relations developed for seismically isolated structures.

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