Simulation of the in-plane structural behavior of unreinforced masonry walls and buildings using DEM

In this study, a novel computational modeling strategy is proposed to estimate the lateral load capacity and behavior of unreinforced masonry (URM) structures. All commonly noted failure mechanisms are captured via the proposed modeling strategy using the discrete element method (DEM) in three-dimensions (3D). Masonry walls are represented as a system of elastic discrete blocks, where the nodal velocities are evaluated by integrating the equations of motion using the central difference method. Then, the mechanical interactions among adjacent blocks are examined utilizing the relative contact displacements and employed in the contact stress calculation. Through this research, a new stress-displacement contact constitutive model is considered and implemented in the commercial software 3DEC, which includes softening stress-displacement behavior for tension, shear, and compression along with the fracture energy concept. The results of the discontinuum models are validated on small- and large-scale experimental studies available in the literature with good agreement. Furthermore, important inferences are made regarding the effect of block size, the number of contact points, and contact stiffness values for robust and accurate simulations of masonry walls.- (undefined

Enhancing efficacy, performance, and applicability of additive food manufacturing

The advent of three-dimensional (3D) printers has led to many industrial innovations by providing alternatives to conventional manufacturing in various disciplines, including food science and engineering. As an emerging digital fabrication technique, 3D printing relies on computer-aided design and a set of commands for physical production in a layer-by-layer fashion. AM technology has attracted much attention in the food industry due to advantages such as on-demand production, personalization in nutrition, better food textures, printing complex and appealing structures without using molds or fixtures. More recently, with the advances in engineering and digital technologies, four-dimensional (4D) printing has emerged, which is defined as property changes (e.g., shape morphing, color change) of a 3D-printed product after exposure to a stimulus (e.g., pH change, water, heat). Successful adoption of this novel AM technology needs substantial work in selecting appropriate materials, creating geometrical designs, and improving the stability of 3D printed products after processing. The applications of 3D and 4D printing using edible materials should consider the complexity of food systems and the availability of appropriate materials to enable desirable functions of foods. This study focused on using AM technology to create food products that are structurally stable, suitable for post-processing, and anisotropically-actuated via extrusion-based 3D printers. Cookie dough, a popular food containing multiple ingredients, was selected as a model system to investigate the effects of food components, pre-heating, different geometrical properties, and the baking conditions on the printability of food ink. A novel 3D printing methodology was established to create structurally stable cookies by modifying the dough recipe without using gums or stabilizers. The results indicate that the reduction of sugar is an effective way to develop structurally stable cookies by 3D printing. On the other hand, pre-heating improves the printing performance and shape stability of the cookie dough systems by yielding a dense network and affecting conformational changes. The baking conditions and geometrical properties affected the quality of 3D-printed shapes, such as layer cracking due to high-temperature cooking. In addition, this study explored the osmotically driven and anisotropically actuated 4D printing concepts using edible composite films and elucidated two different mechanisms of 4D printing: shape-changing and color-transforming. Edible composites containing ethyl cellulose and gelatin were formed and submerged into water, resulting in osmotically driven structural changes of the composites. The results demonstrate that mismatching in the swelling behaviors of film components and geometrical properties of designed shapes are critical to controlling the shape-morphing behaviors of the composite structures. This study proves the great potential of 3D and 4D printing in the food industry.Includes bibliographical references

Discrete-continuum approach to assess 3D failure modes of masonry arch bridges

There are two main objectives of this research. First, a full masonry arch bridge with all structural components are considered. The failure mechanism of spandrel wall and backfill-masonry interaction are successfully simulated using a 3D discrete-continuum model as validated by previously published experimental data. Moreover, the influence of the frictional resistance between soil and masonry components is discussed. Second, two different skew arches, with different bond patterns, are analysed to understand the influence of construction method (helicoidal and false) on the damage pattern and capacity. The results of the analysis demonstrated that discrete and mixed discrete-continuum approaches can predict complex 3D collapse mechanisms of masonry arch bridges and provides detailed information about their damage progression.(undefined

Discrete element modeling of masonry structures: Validation and application

The failure mechanism and maximum collapse load of masonry structures may change significantly under static and dynamic excitations depending on their internal arrangement and material properties. Hence, it is important to understand correctly the nonlinear behavior of masonry structures in order to adequately assess their safety and propose efficient strengthening measures, especially for historical constructions. The discrete element method (DEM) can play an important role in these studies. This paper discusses possible collapse mechanisms and provides a set of parametric analyses by considering the influence of material properties and cross section morphologies on the out of plane strength of masonry walls. Detailed modeling of masonry structures may affect their mechanical strength and displacement capacity. In particular, the structural behavior of stacked and rubble masonry walls, portal frames, simple combinations of masonry piers and arches, and a real structure is discussed using DEM. It is further demonstrated that this structural analysis tool allows obtaining excellent results in the description of the nonlinear behavior of masonry structures

Simulation of uniaxial tensile behavior of quasi-brittle materials using softening contact models in DEM

This study proposes new contact models to be incorporated into discrete element method (DEM) to more accurately simulate the tensile softening in quasi-brittle materials, such as plain concrete and masonry with emphasis on fracture mechanism and post-peak response. For this purpose, a plain concrete specimen (double notched) and stack bonded masonry prism under direct tensile test are modeled. Furthermore, mixed mode crack propagation is investigated in concrete and brickwork assemblages. Two modeling approaches are proposed, the simplified and detailed meso modeling, both based on DEM. In the simplified meso-model, a smooth contact surface is considered between two separate blocks, whereas the internal structure of the material is explicitly represented as a tessellation into random polyhedral blocks in the detailed meso-model. Furthermore, recently developed tensile softening contact constitutive models implemented into a commercial discrete element code (3DEC) are used to simulate the softening behavior of concrete and masonry. As an important novel contribution, it is indicated that the proposed computational models successfully capture the complete (pre- and post-peak) material behavior and realistically replicate the cracking mechanism. Additionally, a sensitivity analysis demonstrates the influence of the various micro-contact parameters on the overall response of the examined materials.Authors would like to express their gratitude to Itasca Educational Partnership Program (IEP) for their kind support and providing 3DEC softwar

Static and impact response of a single-span stone masonry arch

Unreinforced masonry structures are susceptible to man-made hazards such as impact and blast loading. However, the literature on this subject mainly focuses on masonry wall behavior, and there is a knowledge gap about the behavior of masonry arches under high-strain loading. In this context, this research aims to investigate both quasistatic and impact response of a dry-joint stone masonry arch using the discrete element method. Rigid blocks with noncohesive joint models are adopted to simulate dry-joint assemblages. First, the employed modeling strategy is validated utilizing the available experimental findings, and then sensitivity analyses are performed for both static and impact loading, considering the effect of joint friction angle, contact stiffness, and damping parameters. The outcomes of this research strengthen the existing knowledge in the literature regarding the computational modeling of masonry structures that are subjected to usual and extreme loading conditions. The results highlight that applied discontinuum-based numerical models are more sensitive to stiffness parameters in high-strain loading than static analysis

Discrete Rigid Block Analysis to Assess Settlement Induced Damage in Unreinforced Masonry Façades

In this study, a system of discontinuous rigid blocks is employed to simulate the possible damage mechanisms in unreinforced masonry (URM) façades and load-bearing frame systems subjected to settlement using the discrete element method (DEM). First, the employed modeling strategy is validated utilizing the available experimental results presented in the literature. Once there is a good agreement between the computational models and experimental findings, a sensitivity analysis is performed to quantify the influence of the input parameters defined in the DEM-based numerical model. Finally, the proposed modeling strategy is further utilized to assess the damage pattern that may develop in a URM façade due to uniform and non-uniform settlement profiles. The results of this study clearly show that the discrete rigid block analysis (D-RBA) provides robust numerical solutions that can be employed to visualize and assess the possible damage patterns and related collapse mechanisms of URM masonry systems as an alternative modeling strategy to standard continuum-based solutions

Damping in masonry arch railway bridges under service loads: An experimental and numerical investigation

This article investigates the damping behavior of masonry arch bridges under service loads extracted from experimental data and provides guidelines on how to emulate this behavior in numerical analysis, particularly in discrete element model applications. First, an experimental campaign is undertaken and vibrations on three masonry arch railway bridges under train loads were monitored. The modal damping ratios from several sensors on each bridge were extracted by isolating the modal component of free decay vibrations which commence immediately after the train leaves the bridge. The modal damping ratios identified under service loads were compared with their counterparts identified under ambient vibrations. The suitability of mass-proportional, stiffness-proportional and Rayleigh damping models in emulating damping in masonry arch bridges was evaluated. In the numerical phase of the study, a single-arch masonry bridge was modeled using mixed discrete continuum approach and a moving load analysis was conducted without applying any additional viscous damping. The results of the numerical analysis indicate that the inherent damping in discrete element models provided by their nonlinear nature can be sufficient to emulate the damping behavior of masonry arch bridges under service loads. The research provided in this article is unique in the sense that it combines an experimental study and a numerical study on damping of masonry arch bridges under service loads. Unlike its counterparts in literature, which use either ambient vibrations or seismic action, damping values are computed under appropriate levels of vibration amplitudes for service loads, which is critical to estimate the modal damping ratios accurately under these loads.Peer ReviewedPostprint (published version

In-plane static response of dry-joint masonry arch-pier structures

The majority of historical masonry structures include arches and vaults, constructed with or without (dry-joint) any mortar. This paper focuses on dry-joint masonry, because it is common all around the world among architectural heritage. Furthermore, even if there was a mortar in the original construction, it typically suffers from deterioration over its lifetime, often causing total loss of mortar in many of the joints. Due to large horizontal thrust that can be produced, depending on their geometry, arches are typically supported by heavy buttresses. These structures tend to be difficult to model due to their nonlinear nature and inherent discontinuity, which makes it challenging to evaluate their stability. In that context, it is necessary to have realistic numerical models to better diagnose their structural behaviour in a seismic event and, ultimately, to perform only necessary and beneficial interventions. The main goal of this paper is to assess the seismic performance of various dry-joint arch forms with different masonry pier types (i.e. monolithic and regularly coursed) subjected to incrementally increasing lateral loads proportional to the mass (pushover). To achieve this goal, a parametric study is performed on arch curvature and pier morphology. Moreover, the influence of steel tie-rod reinforcement is also examined on the proposed masonry models. These complex masonry arch systems can be simulated with discrete element modeling (DEM) approach. In this research, a commercial three-dimensional discrete element code, 3DEC, is used; in which masonry units are modeled as distinct blocks with zero tensile strength at their joints. The results reveal that pointed arches provide better seismic resistance than the circular arch form. Furthermore, implemented steel tie-rods yield significant increase in stability for the arch-pier structures, which is quantified on different arch curvatures.- (undefined

Comparison of in-plane and out-of-plane failure modes of masonry arch bridges using discontinuum analysis

This research aims to provide a better understanding of the structural behavior of masonry arch bridges using advanced modeling strategies. Two main contributions are achieved in this article; first, triggering mechanisms for the out of plane failure of spandrel walls are established; second, the influence of soil backfill on the behavior and strength of the bridges is presented through a comprehensive parametric study. Here, masonry arch bridges are modeled using a discontinuum approach, composed of discrete blocks, including also a continuum mesh to replicate infill material, adopting a framework of discrete element modeling. The equations of motion for each block are solved by an explicit finite-difference method, using the commercial software 3DEC. The results of the preliminary analyses are compared with analytical solutions and limit state analysis for validation purposes. Different arch bridge models, representing common geometrical properties in the northwest Iberian Peninsula are analyzed. Transverse effects, damage patterns and collapse mechanisms are discussed under different types of loading. The analysis demonstrated the severe capacity reduction due to spandrel wall failures and the importance of soil backfill in results, only possible by taking advantage of the performed numerical modeling strategy