Sub-project 8: Displacement-Based Design Methodologies 

Sub-Project 8 is divided in two parts. The first part is devoted to Displacement-Based Design (DBD) of buildings, falls under Research Area 2 “Urban Areas” and includes three Tasks. The second one refers to Displacement-Based Design (DBD) of bridges and equipment in industrial facilities, falls under Research Area 3 “Infrastructures” and includes 5 Tasks. The Sub-project is coordinated by UPAT, featuring also the participation of CEA, DENCO, INPG, INSAL, JRC and UPAV.

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A summary of each of the tasks involved in this Sub-Project is provided below:

Task 2.3a.1: Development of member acceptance and design criteria in terms of deformations (including the effect of bi-directionality) at different performance levels

There is a large amount of experimental information on the cyclic deformation capacity of members and connections, as affected by member or connection geometry, by the amount and detailing of the reinforcement and by the (unidirectional) loading. This information will be utilised to develop proposals for acceptable deformations at the different performance levels of interest in seismic design of buildings (Operational/Immediate Occupancy, Life Safety, Collapse Prevention), along with values of the associated safety elements (overall partial safety factors on member deformation capacity) which are consistent with the uncertainty (scatter) of deformation capacity and the target reliability level for that performance level.

Unlike the abundance of unidirectional cyclic test results, the available experimental information on member deformation capacity under bi-directional seismic demands is very limited. This information will be gathered and processed in conjunction with the unidirectional data, in order to extend the deformation-based acceptance and design criteria from the case of unidirectional to that of bi-directional loading. 

Task 2.3a.2: Calculation of member inelastic deformation demands in buildings irregular in plan within a DBD framework

The problem of torsional response due to irregularity of multi-storey reinforced concrete buildings will be analytically/numerically treated for typical structural configurations of the seismic-prone South of Europe. Non-linear dynamic analysis tools will be employed, taking into account the coupling between member stiffness and strength interaction, which is, in general, ignored today. The magnitude of absolute member deformations will be employed as a response criterion, instead of the normally used local ductility factor.

The irregularity in plan will be parameterised and general conclusions will be drawn. Attention will focus on the question of applicability of the “Equal Displacement” rule at the level of member deformations. (In those cases that this rule is applicable, member deformations may be estimated without recourse to pushover analysis to establish the correspondence between the global displacement demand Dt and local deformation demands (plastic hinge or chord rotations). This will be achieved through extensive comparisons of results of nonlinear dynamic and linear analyses - static or modal (“dynamic”) - for a representative sample of irregular buildings.

Attention will also focus on the development of simple rules and procedures for the estimation of the effective elastic stiffness of RC members (for use in a linear analysis emulating a nonlinear one, as well as for the estimation of global displacement demands), before their full dimensioning and detailing.

Task 2.3a.3: Advancement of nonlinear analysis, static (Pushover) or dynamic, as a means for direct design of new buildings

The tendency in Eurocode 8 is for nonlinear analysis, static (Pushover) or dynamic, to become the reference method for the direct design of new buildings. In the long run nonlinear dynamic analysis, with its many advantages over static (Pushover) analysis, will become the reference method for such direct design of new buildings, in the way that linear dynamic (modal) analysis is currently in Eurocode 8 the reference method for buildings over linear static analysis, and the nonlinear dynamic analysis is the reference procedure for base-isolated buildings. The primary obstacle to such a development, namely the lack of detailed specifications and guidelines for member (nonlinear) models, will be removed in this part of the Sub-Project 8 work, through parametric non-linear analyses in our study.

This will be accomplished through comparative evaluation/validation of models of various levels of sophistication, using cyclic test results of members, subassemblies, or full structures (at full- or reduced scale) as the benchmark. In order to represent realistically the structural behaviour under multiaxial and cyclic loading conditions, in-house developed constitutive modelling of varying degree of sophistication will be used. 2-D and 3-D representations of the structural behaviour, as well as different theory approaches (plasticity, coupling between damage and plasticity theory) will be investigated. Comparisons between numerical and experimental results will be carried out, to provide a set of design tools, which will enable the fine-tuning of member response characteristics in accordance with displacement-based design philosophy. The main model parameters affecting the reliability of the predictions of nonlinear analysis for local (member) deformations will be identified (at first sight the primary ones seem to be the effective elastic member stiffness up to yielding and the member ultimate deformation, to be quantified under Task 2.3a.1 above, followed by member strength and energy dissipation through hysteresis). Finally, simple rules will be devised for the estimation of primary model parameters (such as the effective elastic stiffness to yielding) from member geometry and preliminary estimates of the reinforcement, before the building is fully designed. 

Task 2.3a.4: Yield and ultimate deformations of piers (including prestressed ones), as controlled by flexure and/or shear

Relatively simple, yet reliable and accurate tools will be developed for the estimation of (i) the secant-to-yield stiffness of piers (for the analysis) and (ii) pier deformation and shear force capacity (for the verification of piers).

The available test results on circular or hollow piers will be collected and utilised to develop engineering models for: a) the characteristic deformations – e.g. chord rotation, curvature, etc. – at yielding, and therefore for the effective stiffness of the pier; b) the pier shear strength, as this is affected by the magnitude of inelastic cyclic deformations; and c) the ultimate value of the pier characteristic deformations – chord rotations, curvatures, etc. It is noteworthy that the quantitative expressions adopted for some of these pier properties by current codes for the seismic design of bridges (including EC8-Part 2, as well as the 1999 Caltrans Seismic Design Criteria), have been based on limited early data and do not seem to be in agreement with the majority of recent test results.

Part of this work will be devoted to post-tensioned piers. Conventional wisdom in seismic design of bridges is against prestressing piers expected to develop plastic hinging. Nonetheless, recent tests in Japan have demonstrated the beneficial effect of prestressing on the cyclic behaviour of piers: ultimate deformation increases and residual displacements decrease with prestressing. As a result prestressing of bridge piers have received considerable attention recent (April 2002) code drafts in Japan for seismic design of prestressed concrete structures. It is, therefore, worthwhile reconsidering the present attitude in Europe against prestressing of bridge piers. To this end, recent (mainly Japanese) test results will be utilized to derive tools and rules for large cyclic deformation capacity of prestressed piers, for use in their displacement-based design.  

Task 2.3a.5: Development and elaboration of DBD methodologies

The development of simple procedures will be pursued for the estimation of pier inelastic deformation demands (chord or plastic hinge rotations), e.g. through linear analysis, static or modal (“dynamic”), extending therefore the applicability of the “Equal Displacement” rule to the level of member deformations, without recourse to pushover analysis for the correspondence between the global displacement demand Dt and local deformation demands (plastic hinge or chord rotations). This will be achieved through extensive comparisons of results of nonlinear dynamic and linear analyses for a representative sample of bridge configurations.

Moreover, the development of procedures will be pursued, to facilitate direct design of piers on the basis of displacements and deformations, without an unduly large number of iterations between analysis and member verifications.

Task 2.3a.6: Effect on the design of uncertainties regarding displacement capacity of isolators and the associated overstrength

The displacement capacity of certain isolator types will be determined. The possibility of benefiting from the overstrength of each isolator type close to its displacement capacity will be investigated. Construction measures that may enhance the behaviour of the isolators beyond their design displacement capacity will be investigated and proposed. Finally, means will be proposed for accounting in the design of the bridge system for the effects of the uncertainties in the behaviour of isolators once they exceed their design displacement capacity.

Task 2.3a.7: Re-centering (zero-residual displacement) capability of isolation systems

The evaluation of code requirements for the zero-residual displacement (re-centering) capability of seismic isolation systems will be carried out through parametric nonlinear dynamic analyses for the commonly used isolation systems in Europe. From this evaluation, proposals will be developed for any necessary amendments (with the aim of relaxing current code requirements where they are found to be over-conservative, or make them more stringent where they do not provide sufficient safety).

Moreover, displacement-based guidelines will be developed for tentative code provisions for bridges with energy dissipation devices and no discrete seismic isolation interface. In order to achieve this task, parametric studies with nonlinear dynamic analysis will be carried-out for typical bridge systems.