Structural behaviour of partially loaded areas and concrete hinges

Author: Tomislav Markić
Language: English
external pageDOI: 10.3929/ethz-b-000592082

Abstract

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.