- Title
- Finite element modelling of chain‐link drapery for rockfall protection
- Creator
- Tahmasbi, Soheil
- Relation
- University of Newcastle Research Higher Degree Thesis
- Resource Type
- thesis
- Date
- 2020
- Description
- Research Doctorate - Doctor of Philosophy (PhD)
- Description
- Rockfall represents a significant threat to human life and to infrastructures along many road and railway transport networks. Rockfall drapery systems are effective low maintenance protection systems that can be used to mitigate rockfall risk. Drapery systems have been around for more than fifty years but limited research has been conducted on their performance and dynamic response upon impact. The experimental testing of drapery systems typically incurs very high cost and significant technical constraints, all of which can be reduced by resorting to numerical simulations. However, the intricate modelling techniques required to capture the realistic responses of such systems leads to computationally expensive numerical models. Such high computational costs make it impractical to utilise complex modelling techniques to simulate the response of large systems, such as draperies. When this research project started, there was no published comprehensive dataset, either experimental or numerical, on the dynamic response of drapery systems made of chain-link meshes, and their ability to control the trajectory of falling rocks. The two objectives of the research presented in this thesis were to develop a computationally efficient chain-link mesh model and to acquire new knowledge on the dynamic response of the drapery systems made of chain-link meshes. The first part of this research focused on developing a computationally efficient three-dimensional FE model. This model is based on a hybrid approach where the drapery mesh is modelled using two different finite element formulations. In the zone of impact, where plastic deformations occur, the exact architecture of chain-link wires is modelled using beam elements, while homogenised shell surfaces are used far from the impact where the mesh mostly experiences elastic deformation (peripheral zones). Elastic constitutive relationships are implemented for all shell elements, while plasticity and ductile damage constitutive behaviour is assigned to all wires in the impact zone. Such a hybrid approach leads to considerable computational saving by reducing the number of degrees of freedom of the model and thereby the size of its corresponding global equation (dynamic equilibrium equation). The hybrid model was calibrated and validated against the results of experimental drop tests on 4.2 m per 4.2 m chain-link meshes. The hybrid model resulted in an almost 50% and 70% reduction in the CPU time for small (4.2 m by 4.2 m mesh panel) and large mesh sizes (12.3 m by 4.2 m mesh panel), compared to a model without shell elements. The second part of this PhD research deals with the implementation of the hybrid mesh model into a full-scale model of a drapery system with the objective being to investigate the dynamic response of draperies. Two different models were used for this study: a simplified one and a realistic one. The simplified model consisted of a vertically hanging drapery mesh impacted by EOTA blocks coming at different impact velocities and angles. The idea of this model was to provide preliminary information on the possible interaction mechanisms between the mesh and the block. This model was also used to assess whether it is possible to predict the occurrence of mesh failure for a given block size and set of impact conditions without accounting for the slope. The realistic model included a drapery mesh resting on a rock slope with an irregular surface geometry where the block, positioned at the top of the slope, was released and fell between the mesh and the rock slope. The model was applied to a real case slope scenario to assess its capability to capture a rockfall behind a drapery mesh, after which the performance of the system was investigated by a comprehensive simulation program. Two block shapes, four block sizes and five chain-link meshes were used in the simulations. The analyses were conducted in terms of block trajectory, block residual energy, dissipation mechanisms and anchor load. The simplified model showed that the mesh can reverse the rotation of blocks upon impact and that the weight of the mesh relative to the weight of the block is an important parameter of the mesh/block interaction. The heavier the mesh, the less it displaces upon impact, for a given set of boundary conditions. Although the simplified model revealed some fundamental interaction mechanisms, it showed its limits when it comes to predicting the occurrence of mesh failure. Indeed, the occurrence of mesh failure is not only dependant on mesh properties and boundary conditions, but also on mesh/slope interaction and slope geometry (because it affects block trajectory and hence mesh impact). The simulations were successfully run on the full-scale drapery systems using the hybrid approach for a specific slope, producing the first comprehensive database of block trajectory, block energy evolution, drapery performance and development of anchor loads upon rockfall behind a drapery mesh. Using a rough slope surface in the model showed that a small change in block trajectory induced by the mesh could result in a large change in trajectory after a rebound off the slope, depending on where the block impacts the slope. It is best to run simulations to try to capture all possible changes in trajectory and, hence, loading to the mesh. Small blocks were found to bounce back and forth between the block and the mesh, while large blocks tended to slide against the mesh with a lesser number of impacts on the slope. Increasing the relative weight of mesh against block was found to result in more impacts of the block upon the slope, which can lead to more energy dissipation but a higher chance of orthogonal impact with the mesh, which increases loading. The simulations reveal the ability of meshes to reverse the rotation of blocks, especially for small blocks, which can contribute to reducing the block energy. It was observed that the occurrence of mesh failure depends greatly on the boundary conditions (including the top anchors, bottom rope, slope geometry, and mesh weight relative to the block weight) and impact conditions (which is a function of block trajectory, and is highly variable). In addition, rotational energy can be problematic for mesh failure when blocks have sharp edges. Finally, there seems to be an optimal mesh size for a given block size, whereby the mesh is light enough to displace upon impact in order to partly absorb the impact energy, but also strong enough to resist the impact.
- Subject
- rockfall; chain-link drapery; finite element method; hybrid approach; numerical modelling; ABAQUS; thesis by publication
- Identifier
- http://hdl.handle.net/1959.13/1417011
- Identifier
- uon:37147
- Rights
- Copyright 2020 Soheil Tahmasbi
- Language
- eng
- Full Text
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