Roberto Scotta

Analysis of Chloride Diffusion into Partially Saturated Concrete

This paper describes the governing equations of moisture, heat, and chloride ion flows through concrete within the framework of a distributed parameter model. The coupling terms and the nonlinearity of the problems are taken into account and a numerical procedure based on the finite element method is developed to solve the set of equations. This study examined chloride diffusion even in partially saturated concrete by considering the variability of ion diffusion coefficients with concrete parameters. Chloride intrusion in various environmental conditions was investigated and the effect of moisture flux in transporting dissolved ions through the porous media is considered. Comparisons with experimental tests are also carried out to show the reliability and the effectiveness of the proposed numerical model.

A fully coupled dynamic model for two-phase fluid flow in deformable porous media

Abstract A fully coupled dynamic model is presented for the analysis of water and air flow in deforming porous media, in fully or partially saturated conditions. The solid displacements and the pressures of fluids are taken as primary unknowns of the model. The finite element method is used for the discrete approximation of the partial differential equations governing the problem. The mathematical framework and the numerical implementation of the model are given in detail and the adopted approximations are put into evidence. First the model is validated with respect to documented experiments on partially saturated soil behaviour in quasi-static condition. Then the results of a full dynamic analysis are shown and discussed. In this paper, merits and drawbacks of the proposed model are highlighted.

A scalar damage model with a shear retention factor for the analysis of reinforced concrete structures: theory and validation

Abstract A local isotropic single parameter scalar model that can simulate the mechanical behaviour of quasi-brittle materials, such as concrete, is described. The constitutive law needs the mechanical characteristics and the fracture energy of concrete to be completely defined. The damage parameter is obtained directly from the value of an equivalent effective stress in order to reduce the computing effort. Due to the unique damage parameter, this model is suitable for the study of quasi-static problems involving monotonically increasing loads. The problem of localisation and mesh dependency have been partially overcome by using an enhanced local method in which a characteristic internal length related to the mesh dimension is employed instead of the characteristic fracture length. In this work, the model was enriched further with the introduction of a shear retention factor that accounts for the friction between the two surfaces of a crack. These new features assure a real improvement of the damage model, maintaining nevertheless its simplicity and low computing cost and making it suitable for the practical solution of large scale problems. Several numerical simulations of experimental tests, concerning fracture tests on concrete specimens and beams failing in shear, have been performed for the validation of the model. The main results from the numerical analyses are described and compared with the experimental ones.

Behaviour factor for innovative massive timber shear walls

Four massive wooden shear walls were analysed via experimental tests and numerical simulations. The specimens differ mainly in the method used to assemble the layers of timber boards: two of them are the well-known Cross-Laminated-Timber panels with glued interfaces, the other two are innovative massive timber panels adopting steel staples or wooden dovetail inserts to connect the layers. Quasi-static cyclic-loading tests were performed for each wall and main results are presented and analysed. A non-linear numerical model was calibrated on experimental results and used to perform non-linear dynamic analyses on specifically designed three-storey shear wall. The methods ensuring a reliable estimation of the intrinsic behaviour factor are presented and the definition of yielding and failure condition is discussed. The intrinsic behaviour factor values were calculated using results from non-linear dynamic analyses. Three limits of failure condition were analysed to estimate the correlated Peak Ground Acceleration and therefore the behaviour factor. A final interpretation of the obtained results is presented and some instructions about the choice of the suitable behaviour factor are given.

Passive control of precast building response using cladding panels as dissipative shear walls

Current design approaches consider cladding panels (CPs) in precast RC buildings as non-structural elements not interacting with the frame. Frame/CP connections are typically designed only for wind or seismic actions perpendicular to the panel plane; unrestrained displacements in the panel plane are presumed. Past seismic events have clearly demonstrated the inadequacy of this design approach. CPs do in fact interact with the frame, modifying the response of buildings subjected to seismic action. This means that substantial and unexpected forces act on the connections and may lead to connection failure. Evidence of such out-of-plane forces in joints arising in buildings with deformable roof diaphragms is given here. This work proposes using CPs as structural elements to enhance the seismic performance of framed industrial buildings. If the friction acting between the panels is taken into account, CP walls become part of the resisting structure, leading to a dual frame/wall system. This concept can be used both in the design of new buildings and the retrofitting of existing ones. The case study reported here was conducted with CPs arranged horizontally and with pinned connections. We show that the results are highly sensitive to the assumed friction coefficient, and also that they differ substantially from analytical design values obtained disregarding friction effects. Friction between panels is known to be unpredictable, especially when structures are subjected to seismic movement with a strong vertical component. The consequence is that forces and displacements in framed buildings are random in a way which cannot be accepted in structural design. This work therefore proposes control of friction magnitude with suitable devices between panels: CPs can act as resistant and dissipative shear walls in industrial buildings, and are effective and relatively low-cost.

Concrete-Plated Wooden Shear Walls: Structural Details, Testing, and Seismic Characterization

AbstractThis paper discusses the structural characterization of a novel hybrid shear-wall system formed by coupling standard platform-frame panels with an external reinforced concrete shelter formed of precast slabs screwed to the wooden frames. The external RC skin is intended as a supplementary bracing system, increasing strength and dissipative capacity of the bare timber frame. The structural performance of such hybrid shear wall under monotonic and cyclic loading was first theorized analytically on the basis of code provisions and then confirmed via experimental tests. The novel shear walls demonstrated to fulfill the requirements prescribed by Eurocode 8. In particular, the analyzed system belongs to high ductility class (HDC). Finally the seismic response of a reference building realized with the innovative hybrid shear walls was simulated by means of a numerical model validated on experimental tests; the suitable behavior factor for the building was estimated.

Light Steel-Timber Frame with Composite and Plaster Bracing Panels

The proposed light-frame structure comprises steel columns for vertical loads and an innovative bracing system to efficiently resist seismic actions. This seismic force resisting system consists of a light timber frame braced with an Oriented Strand Board (OSB) sheet and an external technoprene plaster-infilled slab. Steel brackets are used as foundation and floor connections. Experimental cyclic-loading tests were conduced to study the seismic response of two shear-wall specimens. A numerical model was calibrated on experimental results and the dynamic non-linear behavior of a case-study building was assessed. Numerical results were then used to estimate the proper behavior factor value, according to European seismic codes. Obtained results demonstrate that this innovative system is suitable for the use in seismic-prone areas thanks to the high ductility and dissipative capacity achieved by the bracing system. This favorable behavior is mainly due to the fasteners and materials used and to the correct application of the capacity design approach.

A Dissipative Connector for CLT Buildings: Concept, Design and Testing

This paper deals with the conception and characterization of an innovative connection for cross-laminated timber (CLT) panels. The connection is designed to provide an adequate level of dissipative capacity to CLT structures also when realized with large horizontal panels and therefore prone to fragile shear sliding failure. The connector, named X-bracket, has been theorized and designed by means of numerical parametric analyses. Furthermore, its cyclic behavior has been verified with experimental tests and compared to that of traditional connectors. Numerical simulations of cyclic tests of different CLT walls anchored to the foundation with X-brackets were also performed to assess their improved seismic performances. Finally, the analysis of the response of a 6 m × 3 m squat wall demonstrates that the developed connection provides good ductility and dissipation capacities also to shear walls realized with a single CLT panel.

Optimal design of seismic retrofitting of RC frames with eccentric steel bracing

This paper focuses on optimal design for dissipative steel bracing in seismic retrofitting of gravity load designed RC frame buildings. An optimized iterative force/strength-based design procedure is presented, for simultaneous yielding of bracing of all storeys, in order to induce an global damage mechanism in the building which maximizes the hysteretic damping effect. The procedure complies with the capacity design rule, whereby the bracing system enters the plastic range before the existing RC frame. The iterative design procedure of the system is applied here to a regular RC shear frame, with various design behavior factors.

NUMERICAL SIMULATION OF THE COUPLED TENSION-SHEAR RESPONSE OF AN INNOVATIVE DISSIPATIVE CONNECTION FOR CLT BUILDINGS

This work presents a numerical macro-element model able to simulate the dynamic response of an innovative ductile and highly dissipative bracket for assembling of crosslaminated timber structures. This bracket resists to both tensile and shear forces and has been conceived to realize all the seismic-resistant joints of the building with a unique type of connection able to maximize the seismic capacity of the entire structure. The main issue of these kinds of connection is the reliability of numerical models in reproducing the coupled tensionshear behaviour and dissipative capacity with reduced computational effort, so as to simulate the non-linear response of complex buildings. With this aim, a numerical macro-element model was developed within the finite-element framework OpenSees using an assembly of linear beams and plastic hinges capable of simulating the complete tension-shear strength domain of the connection. The macro-element model was calibrated referring to the results from quasi-static cyclic-loading tests of the connector performed in pure shear and pure tension. The coupled tension-shear behaviour of the macro-element model was then validated on the results from independent numerical simulations performed using detailed 3D models with solid finite elements, including material and geometric non-linearity. Obtained results demonstrate that the developed macro-element model is able to describe accurately the hysteretic behaviour of the bracket with a very low computational effort. Therefore, it can be conveniently adopted to simulate the seismic response of complex structures. 247 Available online at www.eccomasproceedia.org Eccomas Proceedia COMPDYN (2017) 247-254