Andreas J. Kappos

Cyclic Load Behavior of Low-Slenderness Reinforced Concrete Walls: Design Basis and Test Results

This study addresses the problem of cyclic shear in squat reinforced concrete walls and attempts to assess the validity of current design provisions, both in Europe--Eurocode 8 (EC8)--and in the United States--American Concrete Institute 318 (ACI318). This paper describes a comprehensive experimental program involving 11 wall specimens, six with shear span ratios of 1.5 and five with 1.0, detailed to the provisions of EC8. Problems in applying these provisions are pointed out and comparisons with the corresponding ACI318 provisions are also made. The wall specimens are reinforced against shear, either conventionally (orthogonal grids of web reinforcement) or with cross-inclined bars. The effects of web and edge reinforcement ratio, of axial load level, and of the quality of construction joints are also investigated. Results show that properly designed and reinforced walls can reach their flexural capacities, even when their aspect ratio is as low as 1.0, that sliding shear in this category of walls is not a major problem, and that cross-inclined (bidiagonal) web reinforcement can effectively and economically control sliding and the subsequent pinching of the hysteresis loops, particularly when these bars intersect close to the critical section.

Development of Seismic Risk Scenarios Based on a Hybrid Method of Vulnerability Assessment

A hybrid methodology of vulnerability analysis is presented, involving elements from both empirical and theoretical methods. A model for correlating analytically calculated structural damage indices to loss (in monetary terms) is also proposed and calibrated against available statistical data. Probability damage matrices derived using this methodology are incorporated into a cost-benefit model tailored to the problem of estimating the feasibility of seismically rehabilitating the existing stock of reinforced concrete buildings in Thessaloniki, Greece. Losses calculated using the suggested procedure are found to be in good agreement with losses incurred during the 1978 Thessaloniki earthquake. The results of the present study also indicate that benefit/cost ratios for reinforced concrete buildings are quite low. Hence, it appears that a pre-earthquake strengthening programme is not economically justifiable.

Evaluation of behaviour factors on the basis of ductility and overstrength studies

The paper focuses on the evaluation of behaviour factors for seismic design of structures, with due consideration to both their ductility and overstrength. Firstly, inelastic strength spectra for typical target ductilities are presented for earthquake motions representative of those expected in Southern Europe, in particular Greece, the country with the highest seismicity in Europe. Mean spectra for different soil conditions are derived, using a scaling procedure, which is different from that usually found in previous studies, and reduces the associated scatter. The ductility-dependent component of the behaviour factor is then estimated on the basis of the corresponding inelastic spectra and comparisons with previously suggested values are made. The overstrength-dependent component is subsequently addressed, with particular emphasis on low-rise and medium-rise reinforced concrete structures designed to procedures typical in current European construction; both inelastic time-history and pushover analyses are used to this purpose. A tentative proposal for an appropriate (period-dependent) form of the behaviour factor is made, and it is found that the expression for the behaviour factor adopted in Eurocode 8 is reasonably conservative and can be used if more complex procedures are to be avoided.

ANALYTICAL MODELS FOR BRICK MASONRY INFILLED R/C FRAMES UNDER LATERAL LOADING

Proposed in this paper are two analytical models for predicting the inelastic response of unreinforced brick masonry infills in reinforced concrete frames subjected to monotonic and reversed cyclic loading. The first model is based on the traditional diagonal strut concept, while the second one is a simple isoparametric element with shear deformation only. All the essential characteristics of the hysteretic behaviour of the panel, including strength and stiffness degradation, pinching and slippage, are explicitly taken into account. The models are implemented in a general-purpose program for the inelastic time-history analysis of structures, and are used for studying the seismic behaviour of typical multistorey frames with various arrangements of infill panels, including structures with an open ground storey. The results of the analysis are in agreement with both experimentally observed behaviour and with experience regarding seismically damaged buildings.

Seismic Response of Adjacent Buildings with Similar or Different Dynamic Characteristics

The seismic response of adjacent buildings in series, having similar or different dynamic characteristics, is studied using SDOF systems subjected to a base motion. The phase difference (due to travelling seismic waves) in the starting times of excitation of adjacent structures is taken into account, as well as the changes on response quantities due to pounding. A parametric study is carried out to investigate the influence of the main parameters of the problem. The analysis indicated an increased response of the end structures, as well as of the most rigid structures in the series. The critical problem of the required width of seismic joint to minimise the effect of pounding was studied and found to lie well below that recommended by some modern seismic codes.

Uncertainty analysis of strength and ductility of confined reinforced concrete members

The strength and ductility of reinforced concrete beam and column cross-sections with varying geometries and levels of confinement are investigated. Material properties are modelled as random variables and their effect on section behaviour is assessed through fibre modelling and the Response Surface Methodology. The section ductility is in many cases found to be mainly dependent on the ultimate concrete strain. The uncertainty involved in the estimation of the latter using various models for confined concrete is examined with the aid of existing experimental data. Other failure criteria such as first hoop fracture and buckling of the longitudinal bars are also considered. Monte Carlo simulations show that a significant amount of variability exists in both the strength and ductility of confined concrete sections due to variations in material properties. In the case of ductility, this variability is greatly enhanced if model uncertainty is taken into account, especially for high axial loads. Furthermore, simple expressions are derived for the estimation of the strength parameters for any cross-section subject to varying material properties. Finally, the confined concrete model and curvature ductility provisions of Eurocode 8 are evaluated.

Fragility curves for reinforced concrete buildings in Greece

The latest developments of a methodology developed by the authors and their co-workers for estimating direct losses from earthquakes in reinforced concrete (R/C) buildings are presented; they concern the derivation of capacity curves and vulnerability (fragility) curves in terms of peak ground acceleration (PGA), as well as spectral displacement, for all types of R/C buildings that are common in Greece. The vulnerability assessment methodology is based on the hybrid approach, which combines statistical data with appropriately processed results from nonlinear dynamic or static analyses that permit interpolation and (under certain conditions) extrapolation of statistical data to PGAs and/or spectral displacements for which no data is available. A detailed discussion of the limitations of the hybrid approach is provided, along with a proposal for improving the quality of results by applying a weighting technique to both the analytical and the statistical input data.

Probabilistic evaluation of behaviour factors in EC8-designed R/C frames

A methodology for the probabilistic assessment of reinforced concrete (R/C) frames which takes into account material variability, confinement model uncertainty and the uncertainty in local and global failure criteria is applied for the derivation of vulnerability curves for the serviceability and ultimate limit states of a multi-storey frame designed to Eurocode 8. By combining the uncertainties affecting structural vulnerability and seismic hazard, the seismic reliability is quantified in terms of the probability of failure for any given design life period. It is found that, while adequate safety margins exist for the ULS, the reliability against the SLS strongly depends on the structural criterion adopted for the definition of this state. The variability in the actual behaviour factor of this frame is also estimated, and the appropriateness of the EC8 specified value is assessed.

A re-evaluation of scaling techniques for natural records

Abstract The study focuses on the problem of reducing scatter in the response calculated from time–history analysis using natural records, by proper scaling of these records. Two ground motion data sets from two different seismotectonic environments are used, each one evenly distributed with respect to rock and alluvia sites. The first part of the study focuses on the effect of scaling on elastic and inelastic spectra for strength and displacement. It is found that in the intermediate and long period range any of the three velocity-related parameters studied are appropriate to use. In the case of inelastic spectra, use of SI scaling along the entire period range leads to a reasonably low amount of scatter in both strength and displacement spectra, with COV hardly exceeding 0.5 within any interval of practical interest. The problem is then addressed in the context of response variability in realistic MDOF systems, with focus on multistorey frames. Narrow-band spectrum intensities calculated on the basis of elastic or inelastic pseudovelocity spectra are suggested as an alternative method of scaling in structure-specific studies. Both proposals based on the narrow-band elastic SI concept lead to COV values between 10 and 40% in the calculated drifts and member ductilities, as well as a reasonably uniform distribution of scatter along the building height.

Cyclic Tests on Seismically Damaged Reinforced Concrete Walls Strengthened Using Fiber-Reinforced Polymer Reinforcement

Results from tests on low-slenderness reinforced concrete walls designed to modern code provisions, initially subjected to cyclic loading to failure and subsequently conventionally repaired and then strengthened using fiber-reinforced polymer (FRP) jackets, are presented. Repair involved replacement of damaged concrete by a high-strength mortar and lap-welding of fractured reinforcement in the plastic hinge region, while strengthening involved wrapping of the walls with FRP jackets, as well as addition of FRP strips at the wall edges, to enhance both flexural and shear capacity. A key parameter was the way in which carbon FRP strips added for flexural strengthening were anchored; combinations of glass FRP (GFRP) anchors and anchoring strips, as well as anchoring steel plates, were used. Results show that the addition of steel plates to the GFRP anchors and strips lead to a more effective anchorage, with strength increases of up to 30%.