Thomas Paulay

The design of ductile reinforced concrete structural walls for earthquake resistance

In the design of reinforced concrete multistorey buildings, in which lateral load resistance has been assigned to structural walls, the emphasis should be on a rational strategy in the positioning of walls and the establishment of a hierarchy in the development of strengths to ensure that in the event of a very large earthquake brittle failure will not occur. The preferred mode of energy dissipation should be flexure in a predictable region. Therefore failures due to diagonal tension or compression, crushing of concrete in compression, sliding along construction joints, instability of wall elements or reinforcing bars and breakdown of anchorages should be suppressed. These aims may be achieved with the application of a deterministic design philosophy and they necessitate special detailing and dimensioning of potentially plastic regions of walls. In several areas differences exist between code provisions and practices in the United States and New Zealand.

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.

EQUILIBRIUM CRITERIA FOR REINFORCED CONCRETE BEAM-COLUMN JOINTS

To contribute to a better understanding of the behavior of beam-column joints in ductile reinforced concrete frames, the laws of statics are used to demonstrate the disposition of internal forces. It is shown that due to joint shear forces, which result in extensive diagonal cracking of the core concrete, significant orthogonal tensile forces are generated. It is postulated that as in the case of linear elements, joint shear reinforcement is necessary to sustain a diagonal compression field rather than to provide confinement to compressed concrete in a joint core.

Torsional mechanisms in ductile building systems

It is postulated that in order to estimate torsional effects on the seismic response of ductile building structures, the associated plastic mechanism to be developed in the three-dimensional system should be identified. The proposed approach is very different from that embodied in building codes. Inelastic structures are classified as either torsionally unrestrained or restrained. It is shown that clearly defined mechanisms that are to be mobilized, enable the acceptable system ductility demand to be estimated. This should ensure that the corresponding demands imposed on critical translatory elements of the system do not exceed their established displacement ductility capacity. To this end familiar quantities, such as element yield displacement and stiffness, are redefined. Comparisons are made of the intents of existing codified design approaches and those emphasising the role of imposed inelastic displacements. A simple treatment of the consequences of earthquake-induced inelastic skew displacements is also addressed. The primary aim of the paper is to offer very simple concepts, based on easily identifiable plastic mechanisms, to be utilized in structural design rather than advancement in analyses. Detailed design applications of these concepts are described elsewhere. The approach is an extension of the deterministic philosophy of capacity design, now used in some countries.

The displacement capacity of reinforced concrete coupled walls

With the identification of criteria of performance-based seismic design, the need to focus on estimations of displacement capacities of ductile system emerges. This involves redefinitions of some properties of reinforced concrete structures. A system comprising components with very different characteristics, a coupled wall structure, is used to demonstrate how displacement and ductility capacities, satisfying specific performance criteria, can be predicted simply, even before the required seismic strength of the system is established. An attractive feature of this approach is that the strengths of components, which contribute to the required seismic strength of the system, may be freely chosen. The astute designer may advantageously exploit this freedom.

A Displacement-Focused Seismic Design of Mixed Building Systems

A postulated prediction of displacements in ductile reinforced concrete building systems is based on a redefinition of basic structural properties. Contrary to the ability of traditional techniques, the proposed approach permits displacement limits, relevant to ductile mixed systems, i.e., those with markedly differing components, to be established before details, such as strengths, are addressed. This should lead to significant benefits at the stage of preliminary design. Acceptable displacement limits, associated with currently introduced direct displacement-based seismic design strategies, can be readily and simply established. Similarly, limits of displacement ductility demands on components and the system, associated with current force-based approaches, can be estimated already as part of the preliminary design.

Displacement-based design approach to earthquake-induced torsion in ductile buildings

An unconventional, yet very simple, seismic design strategy for asymmetric-plan reinforced concrete ductile building systems is postulated. The proposal considers the need to address primarily inelastic displacement demands and available capacities, particularly those of lateral force-resisting elements remote from the centre of twist, rather than torsional strength. A brief review of existing and widely codified design procedures, applicable to elastic systems, is presented in order to demonstrate their irrelevance to needs arising during ductile seismic response. The emphasis of the proposed design approach is on the determination of a reduction of the anticipated seismic displacement demand on the system, necessary to ensure that the available inelastic displacement capacity of appropriately detailed critical elements is not exceeded. The concept of torsional restraint is used as a basis for a response classification. A few examples illustrate applications. Definitive design recommendations, which are very simple to use, are offered.

A Re-definition of the Stiffness of Reinforced Concrete Elements and its Implications in Seismic Design

It is postulated that for the purposes of seismic design the ductile behaviour of lateral force-resisting components, elements and indeed the entire building system, can be satisfactorily simulated by simple bi-linear force-displacement relationships. This enables the displacement relationships between the system and its lateral force-resisting elements at a particular limit state to be readily evaluated. To this end some widely used fallacies, relevant to the transition from elastic to inelastic behaviour, are exposed. A re-definition of yield displacements and consequently stiffness, allows much more realistic predictions of the most important feature of seismic response, element displacements, to be made. The concepts introduced are rational yet very simple. Their applications are closely interwoven with the designer’s intensions. The strategy provides the designers with unexpected freedom in the assignment of strengths to lateral force resisting elements, such as frames or structural walls. Contrary to current design practice, whereby a specific global displacement ductility capacity is prescribed for a particular class, the designer can determine the acceptable displacement demand to be imposed on the system. This should protect critical elements against excessive displacement demands.

Deterministic seismic design procedures for reinforced concrete buildings

Abstract A brief review is given of a deterministic design philosophy with respect to earthquake resisting ductile structures for reinforced concrete buildings. This was developed recently mainly in New Zealand. In this approach a preferred hierarchy in the development of energy dissipating mechanisms is postulated. Some applications of ‘capacity design procedures’ relevant to beams, columns and shear walls, are outlined. The paramount importance of quantifiable good detailing is emphasized and the relevance of this with respect to shear effects in plastic hinges, the confinement of compressed concrete, and bond between reinforcement and concrete are examined. These aspects of the specific seismic environment are also utilized to show fundamental differences in structural behaviour when effects of gravity loads or seismic displacements are compared.

Are Existing Seismic Torsion Provisions Achieving the Design Aims

A simple approach to the consideration of torsional effects on the ductile seismic response of buildings is suggested. Instead of increasing torsional strength, the control of twist, which may amplify local inelastic translational deformations, is emphasised. This may be achieved when assuring in the design that some residual stiffness in ductile systems is available. To this end a classification in terms of torsional restraint is suggested. It is postulated that traditional codified techniques, based on the evaluation of torsional effects on elastic systems, are largely irrelevant to ductile structural response. The primary consideration of inelastic deformation demands rather than strength is advocated. The presentation addresses foremost concepts of torsional behaviour and their relevance to routine seismic design, rather than advancement in analytical techniques.