Analytical model for CFRP strengthened circular RC column under elevated temperature

In order to increase the load carrying capacity and/or increase the service life of existing circular reinforced concrete bridge columns, Carbon Fiber Reinforced Polymer (CFRP) composites could be utilized. Transverse wrapping of circular concrete columns with CFRP sheets increases its axial and shear strengths. In addition, it provides good confinement to the concrete column core, which enhances the bending and compressive strength, as well as, ductility. Several experimental and analytical studies have been conducted on CFRP strengthened concrete cylinders/columns. However, there seem to be lack of thorough investigation of the effect of elevated temperatures on the response of CFRP strengthened circular concrete columns. A concrete confinement model that reflects the effects of elevated temperature on the mechanical properties of CFRP composites, and the efficiency of CFRP in strengthened concrete columns is presented. Tensile strength and modulus of CFRP under hot conditions and their effects on the concrete confinement are the primary parameters that were investigated. A modified concrete confinement model is developed and presented

A distributed shear and flexural flexibility model with shear-flexure interaction for R/C members subjected to seismic loading

Α beam-column type finite element for seismic assessment of R/C frame structures is presented. This finite element consists of two interacting, distributed flexibility subelements representing inelastic flexural and shear response. Following this formulation, the proposed model is able to capture spread of flexural yielding, as well as spread of shear cracking, in R/C members. The model accounts for shear strength degradation with inelastic curvature demand, as well as coupling between inelastic flexural and shear deformations after flexural yielding, observed in many experimental studies. An empirical relationship is proposed for evaluating average shear distortion of R/C columns at onset of stirrup yielding. The proposed numerical model is validated against experimental results involving R/C columns subjected to cyclic loading. It is shown that the model can predict well the hysteretic response of R/C columns with different failure modes, i.e. flexure-critical elements, elements failing in shear after flexural yielding, and shear-critical R/C members

Modelling of R/C members accounting for shear failure localisation: Hysteretic shear model

Reinforced concrete (R/C) frame buildings designed according to older seismic codes represent a large part of the existing building stock worldwide. Their structural elements are often vulnerable to shear or flexure-shear failure, which can eventually lead to loss of axial load resistance of vertical elements and initiate vertical progressive collapse of a building. In this study, a hysteretic model capturing the local shear response of shear-deficient R/C elements is described in detail, with emphasis on post-peak behaviour; it differs from existing models in that it considers the localisation of shear strains after the onset of shear failure in a critical length defined by the diagonal failure planes. Additionally, an effort is made to improve the state of the art in post-peak shear response modelling, by compiling the largest database of experimental results for shear and flexure-shear critical R/C columns cycled well beyond the onset of shear failure and/or up to the onset of axial failure, and developing empirical relationships for the key parameters defining the local backbone post-peak shear response of such elements. The implementation of the derived local hysteretic shear model in a computationally efficient beam-column finite element model with distributed shear flexibility, which accounts for all deformation types, will be presented in a forthcoming paper

Seismic retrofit schemes for RC structures and local-global consequences

A review of repair schemes for reinforced concrete frame buildings is presented in this paper, within the context of global objectives of the intervention process. Local as well as global intervention measures are discussed and their technological application details outlined. The effect of the reviewed repair schemes on the member, sub‐assemblage and system performance are qualitatively assessed. The important role of the foundation system in the rehabilitation process is outlined and measures that are consistent with the super‐structure intervention methods are given. The paper concludes with a global assessment of the effect of repair methods on stiffness, strength and ductility, the three most important seismic response parameters, to assist researchers and practitioners in decision‐making to satisfy their respective intervention objectives. The framework for the paper complies with the requirements of consequence‐based Engineering, where the expected damage is addressed only when consequences are higher than acceptable consequences, and a cyclical process of assessment and re‐assessment is undertaken until the community objectives are deemed to be satisfied

Strengthening of short splices in RC beams using Post-Tensioned Metal Straps

This paper investigates the effectiveness of a novel and cost-effective strengthening technique using Post-Tensioned Metal Straps (PTMS) at enhancing the bond behaviour of short lap spliced steel bars in reinforced concrete (RC) beams. Twelve RC beams with a short lap splice length of 10d b (d b = bar diameter) at the midspan zone were tested in flexure to examine the bond splitting failure. The effect of confinement (no confinement, internal steel stirrups or external PTMS), bar diameter and concrete cover were examined. The results show that, whilst unconfined control beams failed prematurely due to cover splitting, the use of PTMS confinement enhanced the bond strength of the spliced bars by up to 58 % and resulted in a less brittle behaviour. Based on the test results, a new analytical model is proposed to predict the additional bond strength provided by PTMS confinement. The model should prove useful in the strengthening design of substandard lap spliced RC elements

Evaluation of code criteria for bridges with unequal pier heights

One of the challenges associated with Eurocode 8 and AASHTO-LRFD is predicting the failure of irregular bridges supported by piers of unequal heights. EC8 currently uses “moment demand-to-moment capacity” ratios to somewhat guarantee simultaneous failure of piers on bridges, while AASHTO-LRFD relies on the relative effective stiffness of the piers. These conditions are not entirely valid, in particular for piers with a relative height of 0.5 or less, where a possible combination of flexure and shear failure mode may occur. In this case, the shorter piers often result in brittle shear failure, while the longer piers are most likely to fail due to flexure, creating a combination of different failure modes experienced by the bridge. To evaluate the adequacy of EC8 design procedures for regular seismic behavior, various irregular bridges are simulated through a non-linear pushover analysis using shear-critical fiber-based beam-column elements. The paper investigates the behavior of irregular monolithic and bearing-type bridges experiencing different failure modes, and proposes different methods for regularizing the bridge performance to balance damage. The ultimate aim is to obtain a simultaneous or near-simultaneous failure of all piers irrespective of the different heights and failure mode experienced

Structural fire safety design of square and rectangular tubed-reinforced-concrete columns

The tubed-reinforced-concrete (TRC) columns have gained increasing use in the high-rise buildings and large-span stadiums in China, whereas the structural fire design method for square and rectangular TRC columns is still missing. Finite element analysis of the fire performance of square and rectangular TRC columns was conducted using a sequentially-coupled thermo-stress model. This model yielded good predictions against the experimental results of square TRC columns. Fire tests on two rectangular TRC columns were also conducted and presented in this paper, to further calibrate the model. Parametric studies were then conducted, through which it was found that the fire resistance of square and rectangular TRC columns decreases with the increase of load ratio or slenderness ratio and increases as the sectional dimension enlarges. Sectional aspect ratio has a minor influence on the fire resistance of rectangular TRC columns. A practical design method was proposed, for the first time, to determine the fire resistance of square and rectangular TRC columns with or without fire protection. The buckling reduction coefficient could be determined by the EC3 buckling curve (c) or the JGJ/T471 buckling curve. The heat transfer equations given in the Japanese AIJ recommendation were modified to determine the temperatures of the steel tube and rebars and equations were proposed to determine the equivalent temperatures of the concrete core