A model for the nonlinear, time-dependent and strengthening analysis of shear critical frame concrete structures
- Autores: Denise Carina Santos Ferreira
- Directores de la Tesis: Jesús Miguel Bairán García (dir. tes.), Rui Manuel Carvalho Marques de Faria (dir. tes.), Antonio R. Marí Bernat (dir. tes.)
- Lectura: En la Universitat Politècnica de Catalunya (UPC) ( España ) en 2013
- Idioma: inglés
- Tribunal Calificador de la Tesis: Gonzalo Ramos Schneider (presid.), Antoni Cladera Bohigas (secret.), Fabio Biondini (voc.), Mário Pimentel (voc.), Jan G. Rots (voc.)
- Materias:
- Enlaces
- Tesis en acceso abierto en: TDX
- Resumen
Nonlinear fibre beam models, due to its intrinsic simplicity and computational efficiency, are often an adequate alternative to the complex nonlinear plane and solid FE models for the assessment of entire frame structures. Nevertheless, simulations of structural concrete members undergoing relevant shear stresses cannot be performed by these models, as nonlinear shear effects and shear-bending interaction are neglected. In turn, the presence of shear stresses in cracked reinforced concrete (RC) elements leads to a rather complex resistant mechanism which numerical modelling is neither straightforward nor clearly established. Within this problematic, the formulation proposed in this thesis is an upgrade version of an existent flexural fibre beam model for the time-dependent analysis of segmentally constructed RC frames by taking into account the shear effects. The model is devised for the analysis of 2D RC and prestressed frame elements under combined axial, bending and shear forces. Shear-bending interaction is taken into account by means of a hybrid kinematic/force-based sectional approach. The key characteristics of the proposed model are: (i) at the material level RC is simulated through a smeared cracked approach with rotating cracks; (ii) at the fibre level an iterative procedure guarantees equilibrium between concrete and transversal reinforcement, allowing to compute the biaxial stress-strain state of each fibre; (iii) at the section level a uniform shear stress flow is assumed in order to estimate the internal shear stress-strain distribution and (iv) at the element level, the Timoshenko beam theory takes into account the deformation due to shear. As a result, the relevant attributes of the proposed formulation can be resumed as: (i) its capability for considering shear effects in both service and ultimate levels; (ii) the time step-by-step solution procedure enables taking into account the time-dependent response due to creep and shrinkage of concrete, temperature variations and relaxation of prestressing steel considering the multiaxial stress-strain state of the fibres and; (iii) the sequential type of analysis allows capturing the strengthening effects, accounting for the state of the structure prior to the intervention. The model is validated through experimental tests available in the literature, as well as through an experimental campaign carried out by the author. Accordingly, the capacity of the model to efficiently reproduce the behaviour of shear critical beams is demonstrated. The importance of including shear-bending interaction in the numerical analysis is underlined by comparing the results with the ones provided by the pure flexural basis model. The influence of transversal stresses on the time-dependent response of shear and bending dominant beams is also studied with the proposed model. Considering shear effects in modelling the time-dependent response of diagonally cracked RC and prestressed beams is found to be relevant. The proposed model is successfully used to predict the experimental results of a shear damaged and subsequently strengthened RC beam, available in the literature. An alternative strengthening solution for the damaged beam based on post-tensioned stirrups is numerically analysed. This technique showed to be effective to avoid brittle shear failure allowing for the development of all the flexural capacity of the repaired beam. The importance of considering previous damage in the numerical assessment of strengthened RC beams is revealed. Finally, the response of a dismantled prestressed concrete bridge, with deficient shear resistance, submitted to full-scale tests is successfully simulated with the proposed model. In addition, different strengthening proposals based on post-tensioning measures are studied for this bridge. In this manner, the capacity of the model to determine the safety of existent structures and to analyse the performance of strengthening measures is demonstrated.
