1,721,058 research outputs found
An anisotropic plastic‐damage model for 3D nonlinear simulation of masonry structures
No full textPredicting the structural response of masonry structures with acceptable accuracy is paramount to safeguard the historical heritage and build new constructions with safety margins adequate to modern standards. However, due to the heterogeneous nature and anisotropic response of masonry, such prediction is still difficult to achieve, where most current masonry representations are based upon homogeneous isotropic material models or even more simplified masonry macro-elements. In this article, a novel anisotropic constitutive model to be used in detailed 3D continuum FE representations is described. This is based upon the application of the transformed-tensor method to an isotropic uncoupled plastic-damage model, which is further enhanced by additional novel features enabling the proper definition of the shear behavior both in terms of yielding surface and damage evolution while increasing local computational robustness. Illustrative examples at different scales are presented, highlighting the characteristics and potential of the developed masonry material model. Focus is placed on the mechanical behavior under uniaxial and biaxial stress states considering pure compression on wallets with varying inclination of the material principal axes and the out-of-plane response of wall components. The numerical results confirm the ability of the proposed constitutive model to predict typical masonry anisotropic response characteristics, which cannot be accurately represented by standard isotropic representations commonly used in professional practice and research
Optimal sensor placement for structural parameter identification
No full textThe identification of model material parameters is often required when assessing existing structures, in damage analysis and structural health monitoring. A typical procedure considers a set of experimental data for a given problem and the use of a numerical or analytical model for the problem description, with the aim of finding the material characteristics which give a model response as close as possible to the experimental outcomes. Since experimental results are usually affected by errors and limited in number, it is important to specify sensor position(s) to obtain the most informative data. This work proposes a novel method for optimal sensor placement based on the definition of the representativeness of the data with respect to the global displacement field. The method employs an optimisation procedure based on Genetic Algorithms and allows for the assessment of any sensor layout independently from the actual inverse problem solution. Two numerical applications are presented, which show that the representativeness of the data is connected to the error in the inverse analysis solution. These also confirm that the proposed approach, where different practical constraints can be added to the optimisation procedure, can be effective in decreasing the instability of the parameter identification process
Identification of mesoscale model parameters for brick-masonry
Full text linkRealistic assessment of existing masonry structures requires the use of detailed nonlinear numerical descriptions with accurate model material parameters. In this work, a novel numerical-experimental strategy for the identification of the main material parameters of a detailed nonlinear brick-masonry mesoscale model is presented. According to the proposed strategy, elastic material parameters are obtained from the results of diagonal compression tests, while a flat-jack test, purposely designed for in-situ investigations, is used to determine the material parameters governing the nonlinear behaviour. The identification procedure involves: a) the definition of a detailed finite element (FE) description for the tests; b) the development and validation of an efficient metamodel; c) the global sensitivity analysis for parameter reduction; and d) the minimisation of a functional representing the discrepancy between experimental and numerical data. The results obtained by applying the proposed strategy in laboratory tests are discussed in the paper. These results confirm the accuracy of the developed approach for material parameter identification, which can be used also in combination with in-situ tests for assessing existing structures. Practical and theoretical aspects related to the proposed flat-jack test, the experimental data to be considered in the process and the post-processing methodology are critically discussed
Multiscale model calibration by inverse analysis for nonlinear simulation of masonry structures under earthquake loading
Full text linkThe prediction of the structural response of masonry structures under extreme loading conditions, including earthquakes,requiresthe use of advanced material descriptionsto represent the nonlinear behaviour of masonry. In general, micro-and mesoscale approaches are very computationally demanding, thus at present they are used mainly for detailed analysis of small masonry components. Conversely macroscale models, where masonry is assumed as a homogeneous material, aremore efficient and suitable for nonlinear analysis of realistic masonry structures. However, the calibration of the material parameters for such models, which is generally basedon physical testing of entire masonry components, remains an open issue. In this paper, a multiscale approach is proposed, in which an accuratemesoscale modelaccounting for the specific masonry bond is utilised invirtual tests for the calibration of a more efficient macroscale representation assumingenergy equivalence between the two scales. Since the calibration is performed offlineat the beginning of the analysis, the method is computationally attractive compared to alternativehomogenisation techniques. The proposed methodologyis applied to a case study consideringthe results obtained in previous experimental testson masonry components subjected to cyclic loading, and on a masonry building under pseudo-dynamic conditions representingearthquake loading.The results confirmthepotential of the proposedapproach and highlight somecritical issues, such asthe importance of selecting appropriatevirtual tests for model calibration,which can significantlyinfluence accuracy and robustness
Pushdown Tests on Masonry Infilled Frames for Assessment of Building Robustness
No full textThe research presented in this paper addresses the influence of nonstructural masonry infill on the resistance of multistory buildings to progressive collapse under sudden column loss scenarios. In particular, the structural response of infilled frames in peripheral bays is investigated within the scope of a design-oriented robustness assessment framework previously developed at Imperial College London. This allows due consideration of structural redundancy, ductility, strength, dynamic effects, and energy absorption capabilities in a unified manner. The realistic contribution of masonry panels toward collapse arrest is examined considering the results from full-scale laboratory tests performed on different two-bay frames with brick-masonry infill subjected to incremental pushdown deformation, capturing the dominant deformation mode found following removal of an edge column. In these physical tests, it is observed that the failure mechanisms and damage patterns displayed by the infill panels under pushdown deformation are similar to those activated by lateral pushover loading. Clear evidence of diagonal cracking and shear sliding, eventually culminating in crushing of the compressed corners, is noted. Different infill configurations are tested, including central openings and an initial gap between masonry and frame elements. Overall, a global stable response is observed even in the presence of severe damage in the masonry panels, delivering a monotonic supply of energy absorption with increasing downward displacement. The outcome from this experimental research provides mechanically sound and quantifiable evidence that nonstructural masonry infill panels in peripheral frames offer a reliable and efficient source of enhanced robustness under column loss events. Because of the widespread application of masonry infill panels, this is believed to be particularly relevant within the context of retrofitting operations for robustness enhancement of existing structures, as a result of the growing demand for upgraded resilience of urban infrastructure. Similarly, due account for masonry infill subject to proper quality control during the construction process is recommended for rational robustness design of new buildings
Advanced segment-to-segment methods for adaptive contact analysis of solid continuum structures
No full textThe current work is motivated by the ongoing need for efficient and robust methods of contact simulation in large displacement structural analysis problems. Among the various methods for contact discretisation in structural systems featuring solid continuum bodies, segment to segment methods are popular for their relative numerical stability and ability to provide a continuous and complete coupling of the displacement fields over the contacting surfaces. The current work improves on the segment-to-segment contact approach based on the classical Lagrange multiplier method, introducing efficiencies and workarounds in multiple aspects, namely: (1) the contact formulation; (2) the contact detection/enforcement scheme; and (3) the spatial search and the adaptive generation of contact elements. The large displacement contact formulation based on the classical Lagrange multiplier method is derived from first principles in a modular manner, where greater formulation clarity is introduced, and several expressions are streamlined for increased computational efficiency. Novel reduced order variants of the contact formulation are then proposed which further reduce the computational cost without a significant compromise on accuracy. Element-, node- and quadrature-based schemes for detecting and enforcing contact are investigated with workarounds proposed for identified weaknesses. The current work also proposes a spatial search method with increased efficiency at the lower levels of the search hierarchy for irregular meshes of smooth surfaces, based on a bottom-up hierarchical clustering algorithm which results in a relatively uniform distribution of cluster sizes and a neater cluster arrangement at the lower levels of the hierarchy. A novel intermediate procedure between the global and local stages of the contact search is proposed which uses the geometry of the external solid elements to generate tight-fitting bounding volumes at a low cost and further shortlists segment pairs sent to the local stage. Several numerical examples are presented which demonstrate the capabilities of the developed methods.Open Acces
Critical local damage scenarios for robustness assessment of irregular structures
Full text linkModern-day architects tend to use singular and non-repeatable shapes for building
structures. Currently, scenarios for robustness assessment of such structures are defined based
on engineering judgement, which can result in designs that are insufficiently robust. This thesis
focuses on development of a rigorous robustness assessment framework capable of reliably
determining critical scenarios for irregular building structures.
To reduce the computational demand associated with analysis of detailed numerical
models of building structures undergoing robustness assessment, a novel shell element capable
of effectively modelling reinforced concrete slabs under extreme loading conditions is
developed. A comparative study conducted in this work demonstrates the computational
efficiency of the new element without compromising the accuracy of the modelled response.
To address the needs of robustness assessment in the early design stage, a gradient based
methodology, founded on the general concepts of topology optimisation, is developed. This
methodology is capable of determining critical zones of the considered structure efficiently and
to obtain a good estimate of its robustness. The developed methodology is based on an
enhancement of the robustness assessment framework developed at Imperial College London,
enabling a rational evaluation of robustness for sudden local damage scenarios. To facilitate the
convergence of the developed procedure, a simple yet reasonably accurate approach for
estimating the sensitivity of the robustness measure to the changes in damage parameters is
developed and verified.
To provide a reliable framework for estimating robustness at the final stage of design, an
efficient global approach based on a meta-modelling technique is developed within the
robustness assessment framework. This approach benefits from the developed variant of multifidelity kriging algorithm. The proposed modification utilises a set of independent low fidelity
models, obtained through the subdivision of the high-fidelity model into a set of a priori
established components. Employing the developed approach these models can be assembled to
construct a composite low fidelity model for the response of the whole structure.
The developed framework is finally applied in a detailed case study to the robustness
assessment of the Agora Garden Tower. This study successfully identified all of the critical
scenarios as well as their respective robustness measures. The results of the robustness
assessment indicated that concepts commonly accepted for the regular structures do not
necessarily hold for specific types of irregular ones, which provides strong support to the
original motivation for the conducted research.Open Acces
Advanced interface modelling for 2D shell & 3D continuum problems
Full text linkThis work is motivated by the need for an efficient yet accurate approach for static and dynamic contact analysis of large-scale structures which can a) capture the optimum con- tact position with a moderate number of contact elements, and b) enable across-partition adaptive contact analysis within a parallel processing environment. In addressing these two issues, a novel adaptive node-to-surface contact approach is proposed to discretise the contact boundaries and to trace the evolution of contact locations.
Contact search is a demanding process that can become quite complicated for certain types of problem. In this work, an efficient and robust contact search method is proposed, which can a) locally track the master facet of a given slave node despite the appearance of highly non-smooth contact surface, including surfaces with concave/convex regions or with distinct boundaries as well as reversible normals, and b) globally reallocate the master-slave contact pairs based on the penetration state without an expensive global search, providing an effective adaptive contact approach.
A dual-interface-based domain decomposition method emphasising across-partition con- tact coupling is proposed. A pair of fully decomposed node-to-surface contact element are proposed to discretise the across-partition contact boundaries. The assumption of small incremental displacements is adopted, which a) avoids the excessive coupling between the decomposed master and slave, b) reduces significantly the communication overhead, and c) facilitates a flexible across-partition adaptive analysis. This strategy is found to provide good results for a sufficiently small time- or load-step, and it also facilitates mix-dimensional contact simulation.
Another interest in current thesis is the inaccuracy in non-smooth plates modelled us- ing 2D displacement-based shell elements. In this work the dominant factor causing the inaccuracy is recognised as the incompatible tangential rotations on the two sides of the in- tersection. A 3-noded coupling element is introduced to impose a continuous constraint to couple the incompatible rotations. The significance of the discontinuity in the shell-based folded structure and the effectiveness of the coupling element is demonstrated through numerical studies comparing shell-based models to high fidelity solid-based models.Open Acces
Energy consistent nonlinear dynamic contact analysis of structures
Full text linkThis work is motivated by the need for a numerically stable dynamic contact algorithm, for use with finite element (FE) analysis including both material and geometric nonlinearities, which imposes the appropriate full kinematic compatibility between the interfaces of impacting boundaries during a persistent dynamic contact.
Several methods were previously developed based on Lagrangian multipliers or penalty functions in an attempt to impose the impenetrability condition of dynamic contact analysis. Some of these existing algorithms suffer from lack of numerical stability, and most of them are incapable of accurately predicting the persistent contact force, hence they would not be suitable for frictional dynamic contact analysis.
The numerical stability and energy conservation characteristics of conventional frictionless dynamic contact algorithms using Lagrangian displacement constraints and penalty functions are investigated in this thesis. Two energy controlling dynamic contact algorithms are proposed in conjunction with the well-known Newmark trapezoidal rule, namely, regularised penalty method and Lagrangian velocity constraint. Although energy consistent, the state of the art for these two methods is somewhat similar to the conventional displacement constraints in the sense that acceleration compatibility is not imposed when simulating problems featuring persistent dynamic contact.
In this work, a novel and superior energy controlling-algorithm is proposed which overcomes the aforementioned shortcomings. The proposed DVA method enforces the displacement, velocity and acceleration compatibilities (referred to as DVA constraint in this work) between the impacting interfaces, which in contrast to existing algorithms can be used for FE analysis of problems exhibiting geometric and material nonlinearities. The advanced DVA method is devised such that the kinematic compatibilities at the interface are consistent with the solution for a continuous system without any special treatment in the time-integration or solution procedure of the penetrating interface boundaries. Furthermore, this can be achieved in conjunction with all of the prevalent implicit time-integration schemes such as the trapezoidal rule, midpoint rule, HHT-α and the most recently developed Energy-Momentum family of Methods.
Finally, utilising the proposed dynamic contact algorithms, a novel multi-constraints node-to-surface dynamic contact element is formulated and programmed within a geometric and material nonlinear dynamic FE analysis software. Several verification examples of frictionless mechanical contact are presented to demonstrate the superiority and performance of the developed node-to-surface contact element in conjunction with the proposed DVA constraint as well as the Lagrangian velocity constraint, providing a robust and accurate solution procedure for highly nonlinear dynamic contact analysis.Open Acces
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