322 research outputs found
Non linear analysis of R/C shear walls subjected to seismic loadings
No full textA robust model is proposed for the nonlinear analysis of R/C shear walls under cyclic seismic loadings. The attention is focused on developing the R/C membrane element for two-dimensional problems and the R/C plate element for three-dimensional ones. The reinforcing bars are modelled as multiple smeared steel layers with uniaxial stress-strain response. At this stage of the research the bond-slip between concrete and rebars is not taken into account. An isotropic plastic-damage constitutive law developed by some of the authors is used to simulate the concrete behaviour. The two damage parameters are profitably employed for detecting the damage state of the structures and for interpreting their inelastic response.
The model is validated by comparison with several experimental results on R/C panels and shear walls from the literature. The chosen examples demonstrate the ability of the model in reproducing both flexural and shear failures. The tests are carried out in quasi-static conditions. The accuracy of the model in reproducing the cyclic behaviour of R/C members is notable
Seismic Analysis of Reinforced Concrete Frames with a Scalar Damage Model
No full textThe analysis of the dynamic response of reinforced concrete frames subjected to earthquake still represents an open issue in the field of the civil engineering. Experimental tests are of difficult realisation and expensive in terms of cost and time because of the large geometrical dimension of the frames necessary for the result be effectively representative. For their flexibility and their cheapness numerical methods could override these drawbacks but they present even greater difficulties for the non linearity of the mechanical behaviour of the reinforced concrete and for the numerical effort that step-by-step dynamic analyses imply.
A constitutive law for the material, to be used in the dynamic non-linear analysis of structures, must join the capability of well describe the material behaviour with the requirements of low computational weight and few number of material parameters. In literature (e.g. RILEM [1988]) a wide variety of constitutive models for concrete can be found, mainly based of the fracture and plasticity theories, but their application to real problem rarely have been done.
In this work a two-parameter scalar damage model, developed by Faria and Oliver [1993], for the concrete and an elastoplastic uniaxial law for the reinforcing steel have been adopted to describe the global behaviour of the reinforced concrete. The application of the model to the simulation of the seismic response of a frame is presented (Scotta [1997]) and the effect of mansory infills on the global behaviour of the frame has been investigated. The reliability of the proposed numerical model is demonstrated by good agreement of the numerical results with the corresponding ones experimentally obtained by Negro and Verzelletti [1994,1996].
Reference
• RILEM, International Union of Testing and Research Laboratories for Materials and Structures, “Fracture mechanics of concrete. From theory to applications. Parts A and B.”, RILEM Technical Committee 90, FMA, 1988.
• Faria R., Oliver X., “A rate dependent plastic-damage constitutive model for large scale computation in concrete structures”, Monografia CIMNE, n. 17, Barcelona (Spain), 1993
• Negro P., Verzelletti G., Magonette G. E., Pinto A. V., “Tests on a four storeys full-scale R/C frames designed according to Eurocodes 8 and 2: preliminary report”, Report EUR 15879, European Commission, Joint Research Centre, Ispra, Italy, 1994.
• Negro P., Verzelletti G., “Effect of infills on the global behaviour of r/c frames: energy considerations from pseudodynaic tests”, Earth. Eng. and Struct. Dynam., Vol. 25, pp. 753-773, 1996.
• Scotta R., “Analisi meccanica di strutture in calcestruzzo mediante modelli di danno”, Phd. Thesis, University of Padova, Italy, 1997
Non-Linear Behaviour Modelling of RC Panels Subjected to In-Plane Loads
No full textReinforced concrete panels find widespread use in many engineering structures and accurate prediction of their structural behaviour is important in achieving a safe structural design. The shear strengths of these panels depend strongly on the softening of concrete struts in the principal compression direction due to the principal tension in the orthogonal direction. Intensive investigations of the nonlinear structural behaviour of RC panels and shear walls by finite elements method have been reported in the last decades. Despite commendable progress made in developing new computational methods, accurate and efficient prediction of both the overall load-deflection and the local stress-strain cyclic responses of RC panels is still challenging because of the complicated nonlinear behaviour of these structures, especially in the case of coupled in-plane membrane-shear nonlinear behaviours. The main issues are the development of proper finite element models and the enhancement of effective constitutive laws for concrete, for reinforcement and for their interactions.
Many finite element models have been developed for the nonlinear analysis of RC elements and generally there are three types of models: the discrete model, the smeared-crack model, and the layered model. In the discrete approach [1]-[2], the concrete and steel reinforcement are modelled separately by two different types of finite elements. The creation of discrete models can be quite difficult especially for complex structures. Since a large number of degrees of freedom are generated in the discrete model, it is significantly less efficient, which is of particular concerns in the nonlinear analysis of these structures [2]. In the smeared-crack model [3]-[4], the cracking of concrete and the degradation of its material properties are considered by using averaged stress-strain relationships, that are established directly from full-scale biaxial tests. The resulting models turn out to have low computational efficiency or even to cause numerical instability. The layered approach has been widely used for FE analysis of RC structures, and it has been demonstrated to be effective, particularly in predicting the cracking and the ultimate behaviour of RC panels and slabs in bending and shear [5]-[6]. In this model, the element is formulated by assembling a finite number of concrete layers and equivalent smeared steel layers. Each layer may have different material properties corresponding to its particular material states, and the material properties of each layer are usually assumed to be constant throughout the thickness of the layer. In this case the material constitutive laws for general stress states can follow analytical approaches as the theory of fracture and the theory of continuum damage mechanic. For out-of-plane loaded slabs, cracking and crushing of concrete and yielding of reinforcement through the thickness of the cross-section can be monitored progressively using the layered model, thereby providing an accurate and realistic representation of the structural behaviour [6].
The aim of the work herein is the investigation of the nonlinear modelling of reinforced concrete panels by means of a concrete constitutive law based on damage mechanics applied to a layered quadrilateral element. The concrete constitutive law, that took its bases on the works of Faria et al. [7], Lee et al. [8], Berto et al. [9], is presented in its general formulation having the possibility to represent softening isotropic and orthotropic material behaviour. The tensile branch takes into account the concrete energy of fracture and the tension-stiffening effects. A particular effort has been made to improve the convergence speed through the definition of an adequate secant material stiffness matrix. For what concerns the reinforcing steel, in sake of simplicity, a simple elastic-plastic law has been used with both kinematic and isotropic hardening. The material models have been implemented in the finite open source code Opensees of the University of California, Berkeley [10]. The already implemented quadrilateral layered element has been enhanced with the possibility of taking into account more than one nonlinear material.
The validation of the proposed model has been made by comparison with entire experimental sets such as Bhide and Collins [11] and Mansour and Hsu [12]. These test campaigns have been chosen for representing a wide range of coupled membrane-shear nonlinear behaviours. In particular Bhide and Collins [11] carried out 32 tests on square panels applying combined tension, compression and shear stressed on their edge whereas Mansour and Hsu [12] presented 12 full-size reinforced concrete panel tests investigating the behaviour of reinforced concrete membrane elements under reversed cyclic shear stresses. These last set outlined the effects of the variation of angle of steel bar orientation with respect to the applied principal vertical stress and different percentages of reinforcing steel in the panels. The results of the numerical simulations are presented critically with the aim of showing the achievements and the model drawbacks in order to clearly delineate the future developments. The model showed its ability to interpret the experimental evidences especially in uniaxial stress states, biaxial compression and biaxial tension both locally and discretely, but it demonstrated the need of improvements on biaxial tension-compression due to its simplified definition of the damage limit surface in these stress regions.
References
[1] Nonlinear analysis of reinforced concrete slabs by a discrete finite element approach, J. Jiang, F.A. Mirza, Comput. Struct. 65 (4), 585–592, 1997.
[2] Nonlinear finite element for reinforced concrete slabs, K. Phuvoravan, E.D. Sotelino, J. Struct. Eng., ASCE 13 (4), 643–649, 2005.
[3] The modified compression field theory for reinforced concrete elements subjected to shear, F.J. Vecchio, and M.P. Collins, ACI Journal, 83 (2), 219-231, 1986.
[4] Multiscale modeling of reinforced/prestressed concrete thin-walled structures, A. Laskar, J. Zhong, Y.L. Mo, T.T.C. Hsu, Interaction and Multiscale Mechanics, 2 (1), 69-89, 2009.
[5] Cracking and punching shear failure analysis of RC flat plates, Y.C. Loo, H. Guan, J. Struct. Eng., ASCE 123 (10), 1321–1330, 1997.
[6] A layered shear-flexural plate/shell element using Timoshenko beam functions for nonlinear analysis of reinforced concrete plates, Y.X. Zhang, M.A. Bradford, R.I. Gilbert, Finite Elements Analysis and Design, Elsevier, 43, 888-900, 2007.
[7] A strain-based plastic viscous-damage model for massive concrete structures, R. Faria, J. Oliver, M. Cervera, International Journal of Solid and Structures, 35, 1533-1558, 1998.
[8] Plastic-Damage Model for Cyclic Loading of Concrete Structures, J. Lee, L. Fenves, J. Eng. Mech., ASCE, 124 (8), 892-900, 1998.
[9] An orthotropic damage model for masonry structures, L. Berto, A. Saetta, R. Scotta, R. Vitaliani, International Journal for Numerical Methods in Engineering, 55 (2), 127-157, 2002.
[10] Annual workshop on Open System for Earthquake Engineering Simulation, L. Fenves, Pacific Earthquake Engineering Research Center, UC Berkeley, 2005.
[11] Reinforced concrete elements in shear and tension, S.B. Bhide, M.P. Collins, University of Toronto, Publication n. 87-02, 1987.
[12] Behavior of Reinforced Concrete Elements under Cyclic Shear. I: Experiments, M. Mansour and T.T.C. Hsu, J. Structural Engineering, ASCE, 131 (1), 44-53, 2005
La modellazione numerica delle strutture della basilica e le verifiche di stabilità e di vulnerabilità sismica
No full text
Reinforced concrete structural elements cast into wood-chip cement formworks subjected to compression and out-of-plane bending
No full textThe mechanical response of reinforced concrete walls cast into permanent woodchip-cement formworks and subjected to eccentric compression has been assessed with experimental tests of full-scale panels and an analytical approach has been proposed. The load-bearing capacity of the wall panel is assured by the inner reinforced concrete grid structure, composed of vertical columns and horizontal ribs, reinforced with vertical and horizontal steel bars. The experimental specimens were realized varying the concrete cross-section and tested varying the eccentricity of the applied vertical load. Both reinforced and non-reinforced panels were tested. An analytical model is proposed to evaluate the load-bearing capacity, based on dimensionless curves, which provide reduction coefficients of strength based on eccentricity and slenderness, considering second-order effects. The analytical model demonstrates a reliable prediction of the experimental peak loads
Influence of wall assembly on behaviour of cross-laminated timber buildings
No full textThis paper describes a new wood-joint numerical model which applies commercial software and illustrates how it can be used for reliable estimation of appropriate behaviour factor (q) in multi-storey, cross-laminated timber buildings. The model is based on a macro-element approach and can reproduce the load–displacement hysteretic response of the steel–wood and wood–wood joints typically used in such structures. The panels are regarded as elastic and all non-linearities are assumed to take place in the connections, which are modelled by appropriate macro-elements. The model was calibrated and validated according to the results of cyclic shear wall tests and fullscale shake table tests. After validation, the model was used to carry out a parametric study to assess q values of three-storey buildings. Lastly, the influence of the two different wall assemblies on the building’s seismic response and q values was examined
Valutazione numerica del comportamento sismico e del fattore di struttura “q” di edifici in legno con pareti tipo XLam
No full textIn the X-Lam buildings, the energy dissipation capacity through inelastic behavior is concentrated in the connections between the wall panels and the footings. It has been possible to reproduce the behavior of wood-to-wood and wood-to-footings connections by properly combining several springs with simple work-hardening elasto-plastic constituent models in a complex macro-element. By using such nonlinear multi-springs connections in a finite element model it has been possible to reproduce the experimental results of monotonic and cyclic tests on single panels and plane walls as well as of a three-storey cross-laminated wooden building tested on a shaking table.
The Xlam buildings modeling rests on the hypothesis that the nonlinear behavior of the wall is exclusively due to the (angular and hold-down) connections, whereas the XLam panels are always in the elastic field. The numerical simulations that have been performed have accurately reproduced the results of the experimental tests both in terms of shape of the force-displacement hysteresis curve and assessment of dissipated energy. The experimental tests carried out on a three-storey building tested on the shaking table have been numerically reproduced. The results of the nonlinear analysis in the time domain have confirmed the effectiveness of the numerical model in reproducing the behavior of the whole building. On the basis of these preliminary validations, the model has also been used to predict the displacements and forces on the structure subjected to seismic excitations and, therefore, their most appropriate ‘q ductility factor’. Two different approaches have been used. The first one calculates ‘q’ as the ratio between the acceleration leading the structure to a near-collapse condition and the acceleration leading the structure to the elastic limit. The second one defines the ‘ductility factor’ as the ratio between the base shear calculated for near-collapse seismic intensity in case of elastic response and in case of dissipative response of the connections. The results from such analyses confirm that the adoption of a ‘q’ factor equal to 3 is appropriate for the design of the examinated structures
Seismic Behavior of Precast Buildings With Dissipative Connections
No full textRecent earthquakes in southern Europe highlighted that the connections of cladding panels to R.C. frames in precast buildings had a major role in the structural collapse. For this reason, there is an urgent need for a review of the design methods for these connections as well as for an improvement in the manufacturing technology. This article aimed to assess the efficiency of dissipative panel-to-structure and roof connections in R.C. precast buildings. A parametric study consisting of linear and non-linear analyses on one case-study building is performed. Different sensitivity analyses are performed varying their mechanical properties (i.e., stiffness, strength, and ductility) to analyze the behavior of the CP/frame connections. The study focuses on dissipative connections with an elastic–plastic behavior, placed between cladding panels (CPs) and frames in precast buildings with stacked horizontal cladding panels. The introduction of dissipative CP/frame connections implies the inclusion of panels in the global seismic resisting system. The “panels + frame” system highlights a high stiffness until the yield strength of the CP/frame connections is reached. The results, obtained from non-linear dynamic analyses (NLDAs), clearly show how the proposed connection improves the structural seismic performance. By contrast, this is no longer true for R.C. precast structures with flexible diaphragms, especially for intermediate columns, far from panels aligned to seismic action. In this case, significant and unexpected axial forces arise on out-of-plane connections between panels and columns. The integration of an efficient diaphragm is essential to prevent these critical issues both on intermediate columns and CP/column connections; it enables the dissipative capacity of the “panels + frame” system, and it significantly limits the forces and displacements of intermediate alignments. Unfortunately, the achievement of a rigid diaphragm is not always feasible in precast buildings. A possible alternative to activate dissipative capacities of the roof diaphragm with limited in-plane stiffness is the use of dissipative connections linking roof beams and main beams. The solutions described in this article can be applied both in the design of new buildings and for the seismic upgrading of existing ones with easy-to-install and low-impact applications
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