Collisions and fractures: a predictive theory

We investigate collisions of solids which can fracture. Equations of motion and constitutive laws provide a predictive theory. Assuming the collision as instantaneous, the equations of motion are derived from the principle of virtual work introducing new interior forces which describe the very large stresses and the very large contact forces resulting from the kinematic incompatibilities. They are interior volume percussion stresses and interior surface percussions both on the unknown fractures and on the colliding surface. In order to approximate these equations, we assume solids are damageable. In this point of view, it results that velocity is continuous with respect to space but its strain rate is very large in a thin region where the material is completely damaged, so approximating a fracture. When the velocity before collision is very large, the damaged zone may be large accounting for parts of the solid completely transformed into powder. The constitutive laws result from dissipative functions satisfying the second law of thermodynamics and able to model the fracturation phenomenon at the macroscopic engineering level. Representative numerical examples confirm that the model accounts for the fracturation qualitative properties

Metamodeling choices for seismic vulnerability assessment of BRB-retrofitted low-ductility RC frames

Damage incurred in low-ductility reinforced concrete (RC) buildings during recent earthquakes continues to underline their structural vulnerability under seismic shaking. Among the viable seismic retrofitting procedures, passive control systems such as buckling-restrained braces (BRBs) have emerged as an efficient strategy for structural damage mitigation through stable energy dissipation while providing additional strength and stiffness to low-ductility buildings. Although quantifying the beneficial effects of BRBs for vulnerability reduction through seismic fragility curves has been suitably investigated in literature, almost all such studies consider a deterministic description of the BRB device. This study illustrates a metamodeling framework rooted in statistical learning techniques for efficient seismic vulnerability assessment of BRB-retrofitted low-ductility RC frames. The framework develops multidimensional probabilistic seismic demand models for response prediction of a case study retrofitted frames as a function of ground motion characteristics as well as the design parameters of the BRB device. These demand models when compared against damage states capacity estimates subsequently yields vector-based seismic fragility functions that provide notable advantages over unidimensional fragility curves in terms of efficiency as well as generality. Additionally, uncertainties stemming from a multitude of sources can also be conveniently captured and propagated through the different stages of statistical model development. The proposed study aims to help researchers, stakeholders, and even device manufactures by providing a convenient tool for vulnerability evaluation of retrofitted structures with reasonable accuracy and enhanced efficiency of computation

Phase-field modelling of failure in hybrid laminates

In this paper, the complex failure process of unidirectional hybrid laminates under uniaxial loading condition is reproduced and investigated by a one-dimensional phase-field model. The key ingredients of the approach, describing the mechanical response of a hybrid composite made of two different layers, are: (i) a phase-field method, based on a variational formulation of brittle fracture with regularised approximation of discontinuities for the two layers, (ii) cohesive law for the adhesive interface that connects the layers and (iii) robust and consolidated numerical strategy for the solution of the non-linear discretised problem. Explicit and well detailed simulations are shown for four peculiar failure mechanisms and the outcomes validated against experimental results available in literature. The model is able to discriminate among these different failure mechanisms according to the geometrical and mechanical properties of the hybrid composite. Both delamination of the adhesive interface is followed and crack patterns within the materials are fully determined. Finally, the proposed approach opens new perspective studies in higher dimension settings

Assessment and retrofitting of a RC building through a multi-hazard approach: Seismic resistance and robustness

Most of the existing buildings in seismic prone regions have been built before the publication of modern design provisions against seismic events and progressive collapse. Nonetheless, some studies have highlighted the possible interaction between earthquake resistance and structural robustness, the latter being of interest to either individual extreme hazards (e.g., blast, impact, fire) or interacting hazards (e.g., landslides produced by seismic events). While retrofit strategies to improve the seismic performance of reinforced concrete (RC) structures have been widely investigated since many years, the topic of mitigation strategies against progressive collapse received very little attention. Progressive collapse can be described as a special type of structural collapse that involves several components of the structure as consequence of an initial localised damage. The present study aims at investigating whether and how much seismic retrofitting may improve not only the earthquake resistance but also robustness. A four-storey, five-bay, RC frame building designed according to Eurocode 2 is considered as a case study. The frame was assessed by evaluating: 1) the capacity of the structure to redistribute loads after a local damaging event; 2) the seismic capacity of the structure. Non-linear static analyses, i.e., PushDown and PushOver analyses, were carried out in OpenSees to evaluate the robustness and seismic resistance of the structure, respectively. The progressive collapse capacity was evaluated under two relevant column-removal scenarios, i.e., the sudden loss of an internal and an external column, while the seismic resistance was assessed under two load distributions, i.e., proportional to the first vibration mode and to the inertia masses. Subsequently, the impact of retrofitting with carbon fibrereinforced polymers on both structural robustness and seismic resistance was evaluated. The use of the retrofit measure allowed, on the one hand, the removal of all the shear failures due to horizontal seismic actions and, on the other hand, to increase the robustness of the structure

FROM "MODELS" TO "REALITY", AND RETURN.SOME REFLECTIONS ON THE INTERACTION BETWEEN SURVEYAND INTERPRETATIVE METHODS FOR BUILT HERITAGE CONSERVATION

It's well known that more and more accurate methodologies and automatic tools are now available in the field of geometric survey and image processing and they constitute a fundamental instrument for cultural heritage knowledge and preservation; on the other side, very smart and precise numerical models are continuously improved and used in order to simulate the mechanical behaviour of masonry structures: both instruments and technologies are important part of a global process of knowledge which is at the base of any conservation project of cultural heritage. Despite the high accuracy and automation level reached by both technologies and programs, the transfer of data between them is not an easy task and defining the most reliable way to translate and exchange information without data loosing is still an open issue. The goal of the present paper is to analyse the complex process of translation from the very precise (and sometimes redundant) information obtainable by the modern survey methodologies for historic buildings (as laser scanner), into the very simplified (may be too much) schemes used to understand their real structural behaviour, with the final aim to contribute to the discussion on reliable methods for cultural heritage knowledge improvement, through empiricism

Seismic performance of Steel MRFs retrofitted with BRBs: Influence of the design decisions for the devices

Buckling Restrained Braces (BRBs) represent an effective strategy for the seismic retrofit of existing steel Moment Resisting Frames (MRFs), as they contribute to increase the strength, stiffness and energy dissipation capacity of the frame. Nonetheless, the design choices made during the retrofit process have a significant impact on the performance of the structure. For example, the inclusion of ‘large’ BRBs (i.e., high yielding strength and stiffness) may contribute to limit the deformation demands in the MRF; nonetheless, it may also induce large forces in the beams and columns of the existing structure. On the other hand, the inclusion of ‘smaller’ BRBs (i.e., low yielding force and stiffness), while allowing reaching the required safety requirements, may not be able to protect the MRF from damage. Additionally, the sizing of the BRB elements has an influence on the seismic demand parameters affecting the global performance of structural and non-structural components (i.e., peak and residual drifts, as well as storey accelerations). The present study investigates the impact of the design choices in the seismic performance of a retrofitted three-storey case-study frame by considering three retrofit options. The case-study MRF for the bare frame and the three retrofit configurations are modelled and numerically investigated in Opensees by monitoring local damage states (e.g., damage in BRBs, beams, columns, panel zones). First, a comparison is made in terms of non-linear static analyses to identify the deficiencies of the structures. Then, a fragility analysis is carried out through Incremental Dynamic Analyses (IDAs) accounting for the influence of the recordto-record variability. Finally, a comparison is made in terms of local and global Engineering Demand Parameters, by developing fragility curves for the components, for storey drifts and accelerations

Rocking damage-free steel column base with friction devices: development of advanced 3D finite element models in ABAQUS

Column bases are critical components of a seismic-resistant steel building as they transfer gravity and lateral forces to the foundations. This paper focuses on the finite element (FE) modelling of a rocking damage-free steel column base, which uses post-tensioned high strength steel bars to achieve self-centering behavior and friction devices to provide energy dissipation capacity. Contrary to conventional steel column bases, the monotonic and cyclic moment-rotation behavior of the column base can be easily described using simple analytical equations. An advanced three-dimensional FE model for the column base is developed in ABAQUS. The techniques used to overcome convergence issues in numerical simulations as well as the constitutive laws for the nonlinear behavior of materials and friction interfaces are described in detail. The FE model provides results that are in very good agreement with the results from the analytical moment-rotation equations. The FE model results are also used to validate a simplified numerical model in OpenSees. Moreover, the FE model provides results that help to assess the level of stress concentration in critical areas of the column base and to evaluate a step-by-step design procedure that ensures damage-free behavior, self-centering capability, and adequate energy dissipation capacity

Rocking damage-free steel column base with friction devices

Earthquake resilient steel frames, such as frames with self-centering connections or frames with passive energy dissipation devices, have been extensively studied during the past decade but little attention has been paid to their column bases. The paper presents a rocking damage-free steel column base, which uses post-tensioned (PT) high strength steel bars to control rocking behavior and friction devices to dissipate seismic energy. Contrary to conventional steel column bases, the rocking column base exhibits monotonic and cyclic moment-rotation behaviors that are easily described using simple analytical equations. Analytical equations are provided for different cases including yielding or loss of posttensioning in the PT bars and their efficiency is compared with numerical results from a three-dimensional non-linear finite element (FE) model in ABAQUS. Moreover, a simplified model is developed in OpenSees to evaluate how the use of the rocking column base affects the global behavior of a self-centering moment-resisting frame. Nonlinear dynamic analyses show that the rocking column base fully protects the first story columns from yielding and eliminate the first story residual story drift

Experimental Evaluation of a Rocking Damage-Free Steel Column Base with Friction Devices

This paper presents the experimental evaluation of an earthquake-resilient rocking damage-free steel column base, previously proposed and numerically investigated by the authors. The column base uses post-tensioned high-strength steel bars to control its rocking behavior, and friction devices to dissipate seismic energy. It is equipped with a circular steel plate with rounded edges, which is used as a rocking base. The rounded edges prevent stress concentration and damage of the contact surfaces, whereas the circular shape allows rocking toward all plan directions. In contrast to conventional steel column bases, the proposed column base exhibits monotonic and cyclic moment–rotation behaviors that are easily described by analytical equations. This allows the definition of a step-by-step design procedure which ensures damage-free behavior, self-centering capability, and energy dissipation capacity for a target design base rotation. The experimental tests, presented in this study, were conducted under monotonic and cyclic loads demonstrating the damage-free behavior even under large rotations. The experimental results were used to validate the design procedure and to calibrate refined three-dimensional (3D) nonlinear finite-element models that will allow further investigations

Probabilistic seismic demand modeling of local level response parameters of an RC frame

Probabilistic methods to evaluate the seismic vulnerability of reinforced concrete (RC) frames are largely used in the context of performance based design and assessment, often describing the structural response using global engineering demand parameters (EDPs) such as the maximum interstory drift. While such EDPs are able to synthetically describe the structural behavior, the use of local EDPs is necessary to provide a more realistic and thorough description of failure mechanisms of low-ductility frames lacking seismic details. The objective of this paper is to investigate viable probabilistic seismic demand models of local EDPs, which may be used in developing fragility curves for the assessment of the low-ductility RC frames. The present work explores adequate regression models, probability distributions and uncertainty variation of the demand models. In addition, the adequacy of several ground motion intensity measures (IMs) to be used for predictive modeling of local EDPs is investigated. A realistic benchmark three-story RC frame representative of non-ductile buildings is used as a case study to identify key considerations