Dionysis Biskinis

Strength, Stiffness, and Cyclic Deformation Capacity of Concrete Jacketed Members

This paper highlights an experimental program on rectangular reinforced concrete-jacketed columns, focused on the impact of measures taken to enhance the shear transfer at the interface of the old and new concrete. Columns with plain vertical bars and no detailing were rehabilitated for earthquake resistance with shotcrete jackets connected to the old column through various means and subjected to cyclic uniaxial lateral loading up to ultimate conditions. The effect of different means of connection at the interface on the effectiveness of the jacket was investigated. These test results are supplemented with data from the literature for different means of connecting the jacket to the old column to develop simple rules and expressions for the calculation of the yield moment, the drift at yielding, the secant-to-yield stiffness, and the ultimate drift in cyclic loading of jacketed columns with or without lap splicing at the base of the old column. These rules give the key properties of jacketed columns as ratios of the corresponding quantities of an equivalent monolithic member. These ratios do not systematically depend on the type of measures taken to enhance the shear transfer at the interface of the old and new concrete.

Stiffness and cyclic deformation capacity of circular RC columns with or without lap-splices and FRP wrapping

Mechanics-based models are developed for the moment, the curvature and the chord rotation at yielding of circular concrete columns or piers, their secant stiffness to the yield point and the ultimate curvature and flexure-controlled ultimate chord rotation in cyclic loading. The strain criteria for yielding or ultimate are calibrated on the basis of over four hundred test results. Besides the model for the secant-to-yield-point stiffness which is in terms of the yield moment and chord rotation, an empirical one, independent of the vertical reinforcement, is fitted to the data. The ultimate chord rotation is obtained from a plastic hinge model employing a plastic hinge length, the yield and the ultimate curvatures of the end section and the fixed-end rotation due to slippage of bars from their anchorage zone beyond the column length. All models are extended to columns the vertical bars of which are lap-spliced within the plastic hinge and to columns with FRP wrapping and continuous or lap-spliced vertical bars. The comprehensive portfolio of expressions proposed for the deformation properties of circular columns is fully consistent across the various situations of continuous or lap-spliced bars, with or without FRP wrapping, and with models developed by the authors from much larger databases of rectangular columns in similar situations; the aspects specific to circular sections are limited to the mechanics-based section analysis for moment and curvature, a purely empirical coefficient for the secant-to-yield-point stiffness and the empirical plastic hinge length.

Strength, deformation capacity and failure modes of RC walls under cyclic loading

Past models for the cyclic strength and deformation capacity of reinforced concrete walls are evaluated and new ones are developed using results from 866 wall tests. On the basis of the observed damage, the failure mode is classified into the following categories: in flexure, in diagonal tension, in diagonal compression, or in sliding shear. Among the past models evaluated are those proposed by two of the authors: (a) for the cyclic strength in diagonal tension after flexural yielding or in diagonal compression and (b) for the ultimate chord rotation capacity under cyclic flexure, which were adopted in Eurocode 8-Part 3 and in fib Model Code 2010. Walls with height-to-length ratio <1.2 (“squat”) are considered separately: past models are evaluated and two new ones are proposed on the basis of 321 cyclic tests. Past models for sliding shear strength are modified, using the results from 55 cyclic tests. The predictions for the failure mode and the cyclic strength and/or deformation capacity agree well with the tests.

Models of the flexure-controlled strength, stiffness and cyclic deformation capacity of rectangular RC columns with smooth bars, including lap-splicing and FRP jackets

Cyclic tests of single concrete columns with smooth (plain) bars are not representative of building columns with lap splices at floor levels and story-long starter bars. Column specimens with fixity at top and bottom resemble building columns best, but few of those tested so far had smooth bars and even then without bar lap-splicing at floor level or FRP jackets at column ends. Empirical models based on single-column tests, especially the numerous ones with cantilever-type specimens, cannot be readily extended to columns with smooth bars in real-life buildings. Physical models of the Strut-and-Tie type are developed and are validated or calibrated through comparisons with laboratory tests. Their scope includes anchorage and splicing of bars with either 180° hooks or straight ends. Once validated, they are adapted to real-life multistory rectangular RC columns with smooth bars, in order to obtain the column properties of interest: the chord rotation at yielding and the cyclic ultimate chord rotation, with or without FRP jacketing. Different expressions apply to the top and bottom end of a column in a story, but a single one is used to estimate the column’s effective stiffness. Empirical alternatives fitted to the single-element test results have slightly less scatter than physical models, but caution is needed for their application to columns of real buildings. Simulations of the 3D seismic response of a plan wise asymmetric full size building, tested pseudo-dynamically before or after retrofitting all columns with FRPs or just two of them with RC jackets, provide certain confidence in the extension of the physical models for the estimation of the stiffness and ultimate deformation of columns with smooth bars in real-life buildings.

Upgrading of Resistance and Cyclic Deformation Capacity of Deficient Concrete Columns

Rules and expressions are presented for the calculation of the flexural and shear strength, the secant stiffness to yield point and the cyclic deformation capacity of concrete members retrofitted with fiber reinforced polymer (FRP) or concrete jackets, including the case of lap splicing of the longitudinal bars of the original member within the plastic hinge region. They are developed/calibrated on the basis of test results for retrofitted members, including pre-damaged ones. They represent an advancement over rules proposed earlier by the same authors and adopted in Annex A of Part 3 of Eurocode 8 for the retrofitting of concrete members.

SEISMIC ASSESSMENT OF EXISTING RC BUILDINGS

Two procedures are presented for seismic assessment of individual RC buildings on the basis of member seismic chord rotation demands. In the (simpler) “preliminary” evaluation procedure, inelastic chord rotation demands are estimated from linear analysis, while forces for brittle failure modes are computed in a capacity-design fashion. In the “final” or “detailed” evaluation, inelastic chord rotation demands and member shears are determined via nonlinear static (pushover) analysis. This procedure also requires more information on as-built materials and reinforcement. Verification criteria at three performance levels are proposed, tuned to be on the conservative side for “preliminary” evaluation.

CONCRETE OR FRP JACKETING OF CONCRETE COLUMNS FOR SEISMIC RETROFITTING

Experimental and analytical work is reported for jacketed concrete columns, including short lap splices at the base. 45 rectangular columns with smooth or ribbed vertical bars are subjected to cyclic flexure with constant axial load. Test parameters are the presence and length of lap splices and the type of retrofitting: concrete jackets over the whole column, or CFRP wrapping of the plastic hinge region at varying number of layers and height of application from the base. A 0.7:1 scale two-storey space frame was also pseudodynamically tested before and after retrofitting. Test results are supplemented with past data, to develop expressions for the yield moment, deflection at yielding and ultimate deflection in cyclic loading, for columns with concrete or FRP jackets, as a function of jacket parameters and the length of lap splice.

Strength, stiffness and cyclic deformation capacity of RC frames converted into walls by infilling with RC

In seismic retrofitting of concrete buildings, frame bays are converted into reinforced concrete (RC) walls by infilling the space between the frame members with RC of a thickness of not more than their width. The cyclic behavior of the resulting wall depends on the connection between the RC infill and the surrounding RC members. The paper uses the results from 56 cyclic tests on such composite walls to express their properties in terms of the geometry, the reinforcement and the connection. Properties addressed are: (a) the yield moment at the story base; (b) the secant-to-yield-point stiffness over the shear span of the wall in a story; (c) the deflection at flexural failure in cyclic loading; (d) the cyclic shear resistance, including a sliding shear failure mode. Separate models are given for squat walls failing in shear and for those where the top of the column shears-off. The proposals are modifications of models developed in the past for monolithic RC walls from several hundred cyclic tests; blind application of these latter models as though the walls were monolithic gives, in general, unsafe predictions. By contrast, the diagonal compression strut approach in ASCE41-06 is safe-sided, but gives unacceptably large prediction scatter.