A Displacement-Based Design Approach for Seismic Retrofitting of RC FRames with Steel Braces

The introduction in Europe of the new regulation for seismic design will impose stricter performance requirements for building structures. This will result in a significant number of existing reinforced concrete structures having inadequate seismic resistance and therefore requiring intervention. One of the possible retrofitting options is the application of steel braces. However, this approach is not widely used due to the lack of seismic regulation in this field. The main goal of this work is the development of a fast and reliable design method that can promote the retrofitting of reinforced concrete frames with steel braces. In the last years, the scientific community has demonstrated that seismic design based in displacements is more adequate and rigorous that the traditional based in forces. Focused in obtaining the fundamental parameters for the application of this new method, it has been realized static and dynamic non-linear analysis in reinforced concrete structures strengthened with concentric steel braces. The results enable the definition of parameters such as displacement profiles, yielding displacements and target displacements among others. Based on the obtained results, it is proposed a displacement based method that enable, in a simple way, to design the retrofitting system so that the hybrid structure presents adequate seismic behaviour.N/

Development and Verification of Innovative Modelling Approaches for the Analysis of Framed Structures Subjected to Earthquake Action

The last decades have witnessed significant progresses in the development of improved numerical models for structural analysis, as proven by hundreds of dissertations, theses, reports and journal papers dedicated to advanced constitutive models of materials and algorithms to model sectional, element and structural response. Nevertheless, the seismic behaviour of structures involves a number of nonlinear material and geometrical phenomena that, ultimately, are impossible to capture exhaustively in a single model. Furthermore, past studies showed that the most correct modelling options from the scientific viewpoint are sometimes challenged by experimental evidence. This thesis intends to contribute to the ongoing effort of progressively bridging the existing gap between solid theoretical principles adopted in nonlinear modelling and experimental results from shake table or other experimental techniques. Such goal is firstly pursued through the application of a sensitivity analysis to the simulation of the dynamic behaviour of three distinct structures with distributed plasticity beam-column fibre-based elements based on Euler-Bernoulli beam theory. The latter were tested in international blind prediction challenges wherein the author and/or supervisors participated with encouraging results. The goodness-of-fit for each approach is assessed through comparisons between numerical and experimental results in terms of lateral displacements as well as accelerations (when available), following two post-processing strategies: a more conventional one based on the error associated to the peak values measured during each record, and another using the frequency content characteristics of the entire response history. Sensitivity parameters included equivalent viscous damping, element discretization scheme, strain penetration effects, material constitutive models, numerical integration algorithms and analysis time-step size. The conclusions, which are interpreted in the light of state-of-the-practice recommendations and established theoretical frameworks, address fundamental modelling decisions for engineers and researchers. The referred sensitivity analysis identified the simulation of strain penetration effects as particularly relevant. They can significantly impact the seismic response of structures, contributing up to 40% of the overall lateral deformation of RC framed structures. Within this context, the last chapters of the thesis present a novel bond-slip model for RC structures that simulates the member-end deformations associated with strain penetration effects. The model, which in its final form is implemented as a zero-length element, was developed so that it is compatible with any general fibre-based frame analysis software. In a nutshell, the element response is determined from cross-sectional fibre integration, where at each rebar the anchorage mechanism is explicitly modelled through a series of virtual integration points distributed along the anchorage length. The analysis is carried out by an algorithm that enforces both equilibrium and compatibility at every integration point, making use of a state-of-the-art bond stress-slip cyclic constitutive relation applicable to a wide variety of anchorage conditions. Therefore, features such as the expected failure mode (pullout or splitting), or parameters such as the concrete strength, embedment length, cyclic degradation, amplitude of steel strains, rebar type (plain or ribbed), transverse pressure, level of confinement and bond conditions can be explicitly modelled. The element was implemented in a structural analysis software and its performance was assessed against several experimental tests, showing an encouraging accuracy while retaining appreciable computational efficiency.info:eu-repo/semantics/draf

Seismic design of RC frames retrofitted with concentric steel braces

The implementation in Europe of a new seismic design code (Eurocode 8) will impose strict performance requirements to building structures. This will result in a significant number of existing reinforced concrete structures showing inadequate seismic resistance and hence requiring intervention. One of the available retrofitting techniques consists of inserting steel braces in the original RC structure. In spite of the advantages of using this retrofitting approach, the adoption of this technique is still limited due to the lack of seismic design rules. The interaction between the steel braces and the RC members turns the design in an intricate task. The main goal of the research presented in this paper was to develop a simple yet reliable design procedure that can promote the retrofitting of reinforced concrete frames with steel braces. The proposed method is based on displacements and accounts for the interaction of the steel elements on the deformation capacity of the RC members. To this end, nonlinear static and time-history analysis of reinforced concrete structures strengthened with concentric steel braces have been performed with the objective of deriving a number of key parameters (displacement profiles, yield drifts and target displacements) which are essential for the application of the new design procedure. The paper closes with an application of the proposed method to a RC structure with inadequate seismic resistance. The performance assessment of the retrofitted structure, carried out with nonlinear static analysis, confirms the validity of the design procedure and demonstrates the merits of adopting displacement-based seismic design approaches.N/

Review of Strategies for Modelling Beam-to-Column Connections in Existing Precast Industrial Rc Buildings

In recent earthquakes, it has been observed that precast RC structures has shown, in several cases, a poor performance presenting damages on structural and non-structural elements, highlighting the vulnerability of industrial buildings. Beam-to-column connection was pointed as one of the main source of damage. Precast concrete buildings are common in the industrial parks. One-story industrial building constituted by a frame system of beams and columns, with hinged beam-to-column connection are the most common structural configuration. In this way, it is important to characterize this type of buildings to understand its seismic behavior in order to develop new methodologies and solutions for design this type of buildings and improve your performance. The presented work is focused on beam-tocolumn connections that play a determining role on precast structures. The proposed work is the review of the different strategies to model beam-to-column connections in a precast industrial RC building is presented. To perform the analyses, the structural software Opensees was chosen. Nonlinear static analyses were performed. The results are presented and discussed.info:eu-repo/semantics/publishedVersio

Blind Prediction of a Full-Scale RC Bridge Column Tested Under Dynamic Conditions

The definition of appropriate modelling approaches combined with a consistent software framework is a topic of major importance in the present days for structural engineering in general and, particularly, for earthquake engineering. The accuracy of the results obtained in the recent “Concrete Column Blind Prediction Contest 2010” for a full-scale reinforced concrete bridge column tested on the NEES Large High-Performance Outdoor Shake Table, seems to indicate that current modelling strategies are on the right track. The 1.2 m diameter cantilevered column spans 7.2 m from the footing. A massive 230 tonne reinforced concrete block supported by the column generates the inertial forces to mobilize the column capacity. Seismic performance was investigated under 6 ground motions, starting with lowintensity shaking and bringing the column progressively to near-collapse conditions. Based on the obtained results from the pre-contest simulation, as well with the post-contest analysis, it was possible to extract some important conclusions regarding the application of several strategies, namely the use of different type elements, element discretization, constitutive laws for materials.N/

A fibre flexure-shear model for seismic analysis of RC-framed structures

Whilst currently existing modelling approaches of reinforced concrete behaviour allow a reasonably accurate prediction of flexural response, the determination of its shear counterpart needs further developments. There are various modelling strategies in literature able to predict the shear response and the shear-flexure coupling under monotonic loading conditions. However, very few are the reported models that have demonstrated successful results under cyclic loading, as in the seismic load case. These considerations lead to this research work focused on the development of a flexure-shear model for reinforced concrete beam-column elements. A reliable constitutive model for cracked reinforced concrete subjected to cyclic loading was implemented as bi-axial fibre constitutive model into a two-dimensional Timoshenko beam-column element. Aim of this research work is to arrive at the definition of a numerical model sufficiently accurate and, at the same time, computationally efficient, that will enable implementation within a Finite Element package for nonlinear dynamic analysis of existing non seismically designed RC structures that are prone to shear-induced damage and collapse.info:eu-repo/semantics/publishedVersio

Analysis Issues on Seismic Assessment of Existing Structures

Many existing RC structures worldwide were designed for gravity loads only, with inadequate lateral load resistance, lateral stiffness and poor detailing of the reinforcement. Moreover, the concept of regularity both in plan and elevation that characterise a good conceptual design was, in most cases, not taken into account. The introduction in Europe of the regulations for seismic assessment of existing structures imposed stricter performance requirements for building structures. In order to take into account the poor seismic behaviour of such buildings, recent seismic codes – namely EC8 and NTC08, introduce a number of prescriptions regarding issues such as analysis type, load distribution, accidental eccentricity, etc. At the same time, these codes give room for engineering judgment to be used with reference to the definition of structural and non-structural elements such as slabs or infill walls, and obviously leave it up to the analyst decisions regarding Finite Elements typology, meshing, mass modelling, etc. The main goal of this work is thus to provide an extensive and wide evaluation on the influence of each of the abovementioned parameter on the seismic assessment of structures. For this purpose, nonlinear static analysis, as well as nonlinear dynamic analysis are performed on a real building that has also been experimentally tested in the past. The results will hopefully provide indications on the relative importance of each modelling parameter or decision.N/

Assessment of the seismic behavior of a precast reinforced concrete industrial building with the presence of horizontal cladding panels

The latest earthquakes in Europe exposed some critical problems in the connections of cladding panels in industrial precast reinforced concrete (PRC) structures. These connections did not perform as desired, causing the panels to fall, leading to significant nonstructural damage that resulted in the loss of human life and significant socio-economic impacts due to the interruption of business. Furthermore, in addition to the behavior of the cladding system itself, it is still not clear to what extent it can influence the overall seismic performance of the main structure. Making use of a simplified macroelement, the present study assesses the seismic performance of commonly employed cladding-to-structure connections, as well as the interaction of cladding panels with industrial PRC buildings. The analyses were carried out considering a PRC building representative of a Portuguese industrial park, studied with and without cladding panels. The seismic behavior of the structure was assessed considering both nonlinear static and dynamic procedurespublishe

Modelling Strain Penetration Effects in Rc Walls with Smooth Steel Bars

Reinforced concrete (RC) framed and dual wall-frame structures represent an important fraction of the building stock in some European cities. Many of these were constructed without seismic provisions. Therefore, it is important to assess their ability to withstand seismic events and to define strategies to reduce their potential vulnerability. This work focuses on the numerical simulation of the seismic behaviour of RC walls making use of fibre-based nonlinear beam-column elements. Even though it is common to assume a perfect bond between the reinforcing bars and concrete, the relative bond-slip deformations between the two materials can contribute up to 40% of the total lateral deformation of columns when ribbed rebars are used and up to 90% when in the presence of smooth rebars. This paper intends to be a contribution to understand the importance of these effects in old RC walls, namely through the consideration of both ribbed and smooth rebars with different anchorage lengths and provide indications regarding the use of a simplified bond-slip model.info:eu-repo/semantics/publishedVersio

Using Nonlinear Static Procedures for the Seismic Assessment of Irregular RC Buildings

The application of Nonlinear Static Procedures (NSPs) to assess the seismic vulnerability of existing structures has become widely accepted and extensively used in the literature as well as in engineering practice. Nevertheless, their success in predicting the response of irregular buildings is not yet fully verified. The main goal of the present study is to evaluate the capability and accuracy of some of the existing nonlinear static procedures (N2 and ACSM, in this case) to estimate the seismic performance of irregular structures. In order to accomplish this objective, four existing buildings, irregular in plan and elevation, were subjected to an extensive number of nonlinear static and dynamic analyses. The comparisons, focused on both global and local response parameters provide first indications on the reliability of static procedures to estimate the actual response of irregular RC buildings.N/