M. J. N. Priestley

DISPLACEMENT-BASED SEISMIC ASSESSMENT OF REINFORCED CONCRETE BUILDINGS

Seismic assessment of existing reinforced concrete frame and shear wall buildings is discussed. Building on an earlier preliminary assessment procedure incorporating aspects of capacity design into a systems approach for assessment, suggestions are made towards a displacement-based, rather than forced-based, approach to determining available seismic capacity. Based on results from recent experimental programs, procedures are proposed for assessing member strength including column and beam-column joint shear-strength, that result in less conservative estimates of performance than would result from application of existing code rules.

Myths and Fallacies in Earthquake Engineering--Conflicts Between Design and Reality

Current practice in seismic analysis and design is examined, with particular reference to reinforced concrete structures. The attitude of the paper is deliberately iconoclastic, tilting at targets it is hoped will not be seen as windmills. It is suggested that the current emphasis on strength-based design and ductility leads us in directions that are not always rational. A pure displacement-based design approach is advanced as a viable alternative. Improvements resulting from increased sophistication of analyses are seen to be largely illusory. Energy absorption is shown to be a mixed blessing. Finally, accepted practices for flexural design, shear design, development of reinforcement, and the philosophic basis of capacity design are questioned.

VISCOUS DAMPING IN SEISMIC DESIGN AND ANALYSIS

The characterisation of viscous damping in time history analysis is discussed in this paper. Although it has been more common in the past to use a constant damping coefficient for single-degree-of-freedom time history analyses, it is contended that tangent-stiffness proportional damping is a more realistic assumption for inelastic systems. Analyses are reported showing the difference in peak displacement response of single-degree-of-freedom systems with various hysteretic characteristics analysed with 5% initial-stiffness or tangent-stiffness proportional damping. The difference is found to be signiffcant, and dependent on hysteresis rule, ductility level and period. The relationship between the level of elastic viscous damping assumed in time-history analysis, and the value adopted in Direct Displacement-Based Design is investigated. It is shown that the difference in characteristic stiffness between time-history analysis (i.e. the initial stiffness) and displacement based design (the secant stiffness to maximum response) requires a modification to the elastic viscous damping added to the hysteretic damping in Direct Displacement-Based Design.

Stability of Ductile Structural Walls

Based on the observed reponse in tests of rectangular structural walls subject to severe simulated earthquake actions and theoretical considerations of fundamental structural behavior, recommendations are made for the prediction of the onset of out-of-plane buckling. It is postulated that the major sources of instability of the compression zone of the wall section within the plastic hinge region are inelastic tensile steel strains imposed by preceding earthquake-induced displacements, rather than excessive compression strains. Relevant design recommendations are made.

THE LIMITATIONS AND PERFORMANCES OF DIFFERENT DISPLACEMENT BASED DESIGN METHODS

Displacement based design (DBD) methods are emerging as the latest tool for perfor-mance based seismic design. Of the many different DBD procedures proposed in recent years there are few that are developed to a standard suitable for implementation in modern design codes. This paper presents the findings of a study that uses eight different DBD methods to undertake the seismic design of five different case studies. Some significant limitations with the eight methods have been identified through their application to realistic design examples. The study also shows that despite all of the DBD methods using the same set of design parameters, a large variation in design strength is obtained. Finally, through non-linear time history analyses the performance of each method is assessed. The performance assessment indicates that each of the eight DBD methods provide designs that ensure limit states are not exceeded. It is hoped that by presenting the limitations and comparing the required strength and performance of the methods, developments will be made that will enable designers to undertake DBD with ease and confidence.

DYNAMIC BEHAVIOUR OF REINFORCED CONCRETE FRAMES DESIGNED WITH DIRECT DISPLACEMENT-BASED DESIGN

The Direct Displacement-based Design methodology is applied to six reinforced concrete tube-frame structures and tested using inelastic time-history analyses. Using the established design method inter-storey drifts are found to exceed assumed drift limits, and a series of changes to the design displacement profiles and lateral force distribution are proposed to improve agreement. These changes are then applied to the six frames and further time-history analyses carried out at different earthquake intensities. The inter-storey drift behaviour is found to be significantly improved, with code-based drift limits consistently satisfied. Finally a revised form of the Modified Modal Superposition is proposed to account for higher-mode amplification of column shear forces, while a simple intensity-dependent scaling factor to be applied in the capacity design process is developed for column bending moments. The suggested equations are applied to the frame designs, and found to be in acceptable agreement with time history results at a range of earthquake intensities.

Estimating the Higher-Mode Response of Ductile Structures

While the importance of higher-mode actions is appreciated within the engineering community, the affect that ductile nonlinear response has on higher-mode characteristics and the subsequent implications this has for design has received little attention. In this article, the manner in which the higher-mode response of frame-wall structures is affected by inelastic behavior is closely examined and a means of accounting for this in design is proposed. The work focuses firstly on the characteristics of the higher modes present at the development of peak response and then considers how these characteristics would affect the total forces in the building. The study utilizes a series of nonlinear time-history analyses of two different groups of RC frame-wall structures subject to a suite of real records. It is shown that a new modal analysis approach that incorporates transitory inelastic modal characteristics gives significantly improved predictions of peak base shear in frame-wall structures than more traditional modal analysis methods which use elastic higher-mode characteristics. The issues associated with the use of transitory inelastic modal characteristics are discussed and various challenges that would need addressing for the prediction of other response parameters and structural types are identified.

DIRECT DISPLACEMENT-BASED DESIGN OF FRAME-WALL STRUCTURES

A direct displacement-based design (DBD) procedure for structures that comprise both frames and walls is presented in this paper. Within the new procedure, strength proportions between walls and frames are assigned and are used to establish the design displacement profile before any analysis has taken place. Knowledge of the displacement profile and recommendations for the combination of frame and wall damping components enables representation of the structure as an equivalent single-degree of freedom system. The Direct DBD process is then utilised to set the required strength level which is proportioned to the structure in line with the initial strength assignments. To test the design methodology, two sets of 4-, 8,- 12-, 16- and 20-storey reinforced concrete structures are designed. The first set considers frame-wall structures in which the frames are parallel to the walls and the second considers structures in which link-beams connect from the frames directly onto the ends of the walls. A suite of time-history analyses are conducted to validate the methodology, which is seen to perform excellently.

DEVELOPMENT OF AN INNOVATIVE SEISMIC DESIGN PROCEDURE FOR FRAME-WALL STRUCTURES

A displacement-based design (DBD) methodology for structures that are comprised of both frames and walls has been proposed and then tested through examination of several case studies. Strength proportions between walls and frames are decided before any analysis has begun. The various steps required to represent a frame-wall structure as a single-degree of freedom system are identified and a Direct DBD process is then adopted to set the required strength level. To test the design methodology, two sets of 4, 8, 12, 16 and 20-storey reinforced concrete structures are designed, with different proportions of the total strength assigned to the frames. A suite of time-history analyses have then been conducted to validate the methodology, which is seen to perform reasonably well. Finally, improvements to the procedure are outlined and areas for future research are identified.

STRATEGIES FOR REPAIR AND SEISMIC UPGRADING OF BOLU VIADUCT 1, TURKEY

Seismic assessment of a viaduct severely damaged in the Duzce (Turkey) earthquake of 12 November 1999 is described. The purpose of the assessment was to determine whether the bridge could be repaired and retrofitted to a state where it could survive a design earthquake larger than the Duzce event, and more than twice as intense as the original design earthquake, while responding within the serviceability limit state. A number of controversial issues were raised as a consequence of this design brief, and are described in this paper.