Performance-based Design Framework Research Articles

Use of Crescent Shaped Braces for Controlled Seismic Design of Ductile Structures

It is known that the seismic response of a structural system is highly influenced, in addition to the earthquake input, by the dynamic characteristics of the system itself. This paper presents an approach for the identification of the characteristics of the structural system resisting to horizontal loads which enables to satisfy given seismic performance objectives. This is achieved by considering a total conceptual separation between the structural systems resisting to vertical and horizontal loads. The proposed approach is first briefly developed in general within a Performance-Based Seismic Design (PBSD) framework and then fully applied to the case study of a five-storey steel building structure. It is composed of three basic steps: (1) identification of the fundamental characteristics which should be possessed by the horizontal resisting system to satisfy a multiplicity of performance objectives, (2) development of a peculiar horizontal resisting system composed of ”crescent shaped braces” which are specifically calibrated to satisfy given performance objectives, (3) verification, by means of appropriate time-history analyses, of the seismic performances achieved. In detail, the horizontal resisting system is calibrated to satisfy a multiplicity of performance objectives through the identification of an ”objectives curve”, in the Force-Displacement diagram, of the mechanical characteristics of the structure. The calibration is obtained by methods/tools borrowed either from Direct Displacement-Based Design (DDBD) or Force-Based Design (FBD), depending on the specific performance objective to be imposed. The applicative example has been carried out with reference to three performance objectives and has led to the identification of a horizontal resisting system composed of special bracing elements capable of realizing a sort of properly-calibrated seismic isolation called crescent-shaped braces. The results obtained through non-linear dynamic analyses have shown that the proposed approach leads to the congruity between the imposed and the achieved seismic performances.

Large scale reliability-based design optimization of wind excited tall buildings

Recent trends in the design and analysis of large scale wind excited structures, such as tall buildings, are towards performance-based design strategies. This is partly due to the success of these types of frameworks in such fields as seismic engineering and partly due to the ever growing computational power at the disposal of engineers. While there is tremendous interest in developing appropriate tools for wind engineering, a simple transfer of methods from seismic engineering is not possible due to the inherently complex nature of the exciting mechanism. The successive performance-based optimization of the structural system is a further leap forward that should be tackled simultaneously if problems of practical interest, often characterized by thousands of performance constraints, are to be solved. In this paper a component-wise performance-based design framework is proposed, based on the concept of a directional fragility model that rigorously combines the directional building aerodynamics and climatological information. An efficient reliability-based design optimization scheme is then proposed, based on decoupling the traditionally nested optimization loop from the reliability analysis carried out through the proposed performance-based design framework. The decoupled optimization problem is solved by defining a series of approximate explicit sub-problems in terms of the second order response statistics of the constrained functions.

Focusing on reducing the earthquake damage to facade systems

Earthquake engineering is facing an extraordinarily challenging era. These challenges are driven by the increasing expectations of modern society to provide low-cost, architecturally appealing structures with high seismic performance. Modern structures need to be able to withstand a design level earthquake with limited or negligible damage such that disruption to business be minimised because of the economic consequences of such downtime.
 Technological solutions for seismic resisting structural systems are emerging. However, within the goal of developing a seismic-resisting building, not only the structural skeleton of the building but the entire system must be fully protected from damage. This includes the non-structural components of the building such as the claddings, ceilings and contents. Substantial studies are still required to develop technological solutions and design methods capable of achieving such an earthquake resistance structure.
 This paper presents a review of current technology for facades, including design guidelines for seismic-resistant non-structural components and the steps made towards a performance-based design framework. Alternative conceptual strategies and technical solutions to reduce the damage to non-structural elements will also be introduced.

Neural network based prediction schemes of the non-linear seismic response of 3D buildings

Since early 1980s new families of computational methods, termed as soft computing (SC) methods, have been proposed. SC methods are based on heuristic approaches rather than on rigorous mathematics while their use in various areas of computational mechanics is continuously growing. Artificial neural networks (ANNs), which have been applied in many engineering fields, are among the most popular SC methods. Computational earthquake engineering is a computationally intensive field where ANNs have been used for the simulation of the structural behaviour under static or dynamic loading. Performance-based design (PBD) is the current trend for the seismic design of structural systems where the structural performance is assessed for multiple hazard levels, requiring significant computational effort. In this work a new adaptive scheme is proposed in order to predict the structural non-linear behaviour when earthquake actions of increased severity are considered. The predicted structural response by ANNs can be used in the PBD framework when dynamic analyses are performed, aiming at reducing the excessive computational cost.

Factors Influencing Maximum and Residual Deformations of SDOF Systems Subjected to Large Ground Motions

The maximum and residual deformations of structures subjected to strong ground motions are the most importance indexes, particularly under the performance-based design framework, thus understanding the influencing factors is of great importance to seismic design. In this study, single degree of freedom (SDOF) systems with varying structural properties are analyzed using a series of strong ground motions from FEM/SAC project. The influences of three structural parameters, i.e., yield force, second stiffness after yielding, and stiffness degradation, on the maximum and residual deformations are investigated based on the statistics of the analysis results. The analysis results suggest the follows: (1) larger yield forces lead to smaller residual and maximum deformations for short period structures, and they lead to smaller residual deformations but no necessarily smaller maximum deformation for intermediate and long period structures; (2) larger second stiffness lead to smaller residual and maximum deformations for short period structures, and they lead to smaller residual deformations but no necessarily smaller maximum deformation for intermediate and long period structures; (3) smaller stiffness degradation index leads to smaller maximum deformations but larger residual deformations.

Evaluation of ASCE-41, ATC-40 and N2 static pushover methods based on optimally designed buildings

Alternative static pushover methods for the seismic design of new structures are assessed with the aid of advanced computational tools. The current state-of-practice static pushover methods as suggested in the provisions of European and American regulations are implemented in this comparative study. In particular the static pushover methods are: the displacement coefficient method of ASCE-41, the ATC-40 capacity spectrum method and the N2 method of Eurocode 8. Such analysis methods are typically recommended for the performance assessment of existing structures, and therefore most of the existing comparative studies are focused on the performance of one or more structures. Therefore, contrary to previous research studies, we use static pushover methods to perform design and we then compare the capacity of the outcome designs with reference to the results of nonlinear response history analysis. This alternative approach pinpoints the pros and cons of each method since the discrepancies between static and dynamic analysis are propagated to the properties of the final structure. All methods are implemented in an optimum performance-based design framework to obtain the lower-bound designs for two regular and two irregular reinforced concrete building configurations. The outcome designs are compared with respect to the maximum interstorey drift and maximum roof drift demand obtained with the Incremental Dynamic Analysis method. To allow the comparison, also the life-cycle cost of each design is calculated; i.e. a parameter that is used to measure the damage cost due to future earthquakes that will occur during the design life of the structure. The problem of finding the lower bound designs is handled with an Evolutionary type optimization algorithm.

Some Remarks on Experimental Estimation of Damping for Seismic Design of Civil Constructions

Experimental estimation of damping is currently not a comprehensively solved problem. Although modal frequencies and mode shapes can be measured confidently and quite easily by means of dynamic tests on civil structures, an accurate identification of damping ratios needs further development. Experimental values can be characterized by large error bounds mainly because damping is strongly influenced by the magnitude of the dynamic response of a structure.In this paper, the main issues concerning identification of modal damping ratios are discussed in order to define methods for their reliable estimation, to reduce uncertainties and characterize error bounds. According to this aim, a procedure for optimized modal parameter estimation via Stochastic Subspace Identification is also proposed. Then data collected from an extensive literature review are analyzed depending on the type of structure in order to point out the main factors affecting damping. Moreover, a comparison with provisions about structural damping given in National and International seismic codes for structural design of civil structures is reported. This process demonstrates that the development of a damping database based on homogeneous and reliable estimates is required for calibration of predictive formulations to be used in the framework of performance-based seismic design.

Stiffness-Strength-Ductility-Design Approaches for Crescent Shaped Braces

It is known that the seismic response of a structural system is highly influenced, in addition to the earthquake input, by the dynamic characteristics of the system itself. This paper presents an approach for the identification of the characteristics of the structural system resisting to horizontal loads which enables to satisfy given seismic performance objectives. This is achieved by considering a total conceptual separation between the structural systems resisting to vertical and horizontal loads. The proposed approach is first briefly developed in general within a Performance-Based Seismic Design (PBSD) framework and then fully applied to the case study of a five-storey steel building structure. It is composed of three basic steps: (1) identification of the fundamental characteristics which should be possessed by the horizontal resisting system to satisfy a multiplicity of performance objectives, (2) development of a peculiar horizontal resisting system composed of crescent shaped braces which are specifically calibrated to satisfy given performance objectives, (3) verification, by means of appropriate time-history analyses, of the seismic performances achieved. In detail, the horizontal resisting system is calibrated to satisfy a multiplicity of performance objectives through the identification of an "objectives curve", in the Force-Displacement diagram, of the mechanical characteristics of the structure. The calibration is obtained by methods/tools borrowed either from Direct Displacement-Based Design (DDBD) or Force-Based Design (FBD), depending on the specific performance objective to be imposed. The applicative example has been carried out with reference to three performance objectives and has led to the identification of an horizontal resisting system composed of special bracing elements capable of realizing a sort of properly-calibrated seismic isolation called crescent-shaped braces. The results obtained through non-linear dynamic analyses have shown that the proposed approach leads to the congruity between the imposed and the achieved seismic performances.

Liquefaction-Induced Settlement of Pile Groups in Liquefiable and Laterally Spreading Soils

The results of a series of dynamic centrifuge tests on model pile groups in (level) liquefied and laterally spreading soil profiles are presented. The piles are axially loaded at typical working loads, which has enabled liquefaction-induced settlements of the foundations to be studied. The development of excess pore pressures within the bearing layer (dense sand) was found to lead to a reduction in pile capacity and potentially damagingly large coseismic settlements. As the excess pore pressure increased, these settlements were observed to exceed postshaking downdrag-induced settlements, which occur due to the reconsolidation of liquefied sand around the pile shaft. In resisting settlement, the pile cap was found to play an important role by compensating for the capacity lost by the piles. This was shown to be achieved by the development of dilative excess pore pressures beneath the pile cap within the underlying loose liquefied sand which provide increasing bearing capacity with settlement. The centrifuge test data show good qualitative and quantitative agreement with the limited amount of model and full-scale data currently available in the literature. The implications of settlement for the design of piled foundations to serviceability conditions in both level and sloping ground are discussed, with settlement becoming an increasingly important consideration for laterally stiffer piles. Finally, empirical relationships have been derived from the test data to relate suitable static safety factors to given increases in excess pore pressure in the bearing layer within a performance-based design framework (i.e., based on limiting displacements).

Use of Cumulative Ductility Spectra Within a Deformation-Control Format for Seismic Design of Ductile Structures Subjected to Long Duration Motions

Due to the large economical losses derived from recent seismic events, design methodologies that are based in the explicit control of the dynamic response of structures have been formulated. This article introduces, within the framework of performance-based design, a numerical methodology for the seismic design of structures that are expected to undergo severe cumulative plastic demands. The methodology explicitly considers and integrates: (1) the structural and non-structural performance of the building; (2) the life safety limit state; and (3) the prevention of low cycle fatigue.

Probabilistic Performance-Based Optimal Design of Steel Moment-Resisting Frames. II: Applications

Design codes are migrating from prescriptive procedures intended to preserve life safety to reliability-based design. All stakeholders are given the opportunity to speak the common language of risk and structural designs can be developed to not only reliably preserve life safety after rare ground motions, but minimize damage after more frequent ground motions (minimize life-cycle costs). A companion paper presents a methodology for using an evolutionary (genetic) algorithm with radial fitness and balanced fitness functions to generate solutions to optimal design problems formulated within a probabilistic performance-based design framework. The present paper outlines application of the automated algorithm to design steel frames with fully restrained and a variety of partially restrained connections. Comparison of optimal designs resulting from application of the algorithm proposed with those found in the literature is made. Detailed discussion of algorithm performance and response of the resulting optimized designs during pushover and time-history analysis is provided.

The Role of Inelastic Torsion in the Determination of Residual Deformations

Recent developments in performance-based seismic design and assessment approaches have emphasized the importance of properly assessing and limiting the residual (permanent) deformations typically sustained by a structure after a seismic event, even when designed according to current code provisions. The performance-based design framework for residual deformations, previously developed by the authors for 2-D regular structures, is further extended to the behavior of 3-D irregular (asymmetric in-plan) buildings. The seismic response of a set of single-story systems, comprising of seismic resisting frames, and modeled to represent alternative materials (concrete or steel), is investigated under uni-directional earthquake loading excitations. Different layouts in plan, leading to either torsionally unrestrained or restrained systems, are considered. The influence of varying torsional restraint is investigated to define how residual diaphragm rotations and center-of-mass displacements are affected by changing levels of stiffness and strength, or mass eccentricity. From these findings additions to the previously proposed estimation procedure are made, with a specific example used to validate the suggested changes. Finally, a general example is developed based on currently available methods of evaluating maximum torsion response. It is suggested that such approaches are likely to be insufficient as they do not explicitly define how seismic-resisting elements are influenced by inelastic torsional response.

Low yield metal shear panels as an alternative for the seismic upgrading of concrete structures

The paper addresses the problem of seismic retrofitting of existing concrete structures through an innovative methodology based on low yield metallic shear panels. These panels are introduced in specific places in the concrete structure enhancing its strength and stiffness. However, the most important improvement concerns the energy dissipation capacity of the retrofitted structure. The paper presents a preliminary design methodology within the performance based design framework and details for the realization of the intervention. Elaborated non-linear finite element models are used in order to verify the quality of the results obtained from more simple models used in everyday engineering practice.

Merging energy-based design criteria and reliability-based methods: exploring a new concept

This paper examines the potential development of a probabilistic design methodology, considering hysteretic energy demand, within the framework of performance-based seismic design of buildings. This article does not propose specific energy-based criteria for design guidelines, but explores how such criteria can be treated from a probabilistic design perspective. Uniform hazard spectra for normalized hysteretic energy are constructed to characterize seismic demand at a specific site. These spectra, in combination with an equivalent systems methodology, are used to estimate hysteretic energy demand on real building structures. A design checking equation for a (hypothetical) probabilistic energy-based performance criterion is developed by accounting for the randomness of the earthquake phenomenon, the uncertainties associated with the equivalent system analysis technique, and with the site soil factor. The developed design checking equation itself is deterministic, and requires no probabilistic analysis for use. The application of the proposed equation is demonstrated by applying it to a trial design of a three-storey steel moment frame. The design checking equation represents a first step toward the development of a performance-based seismic design procedure based on energy criterion, and additional works needed to fully implement this are discussed in brief at the end of the paper. Copyright © 2006 John Wiley & Sons, Ltd.

Scaling of spectral displacement ordinates with damping ratios

The next generation of seismic design codes, especially those adopting the framework of performance-based design, will include the option of design based on displacements rather than forces. For direct displacement-based design using the substitute structure approach, the spectral ordinates of displacement need to be specified for a wide range of response periods and for several levels of damping. The code displacement spectra for damping values higher than the nominal value of 5% of critical will generally be obtained, as is the case in Eurocode 8 and other design codes, by applying scaling factors to the 5% damped ordinates. These scaling factors are defined as functions of the damping ratio and, in some cases, the response period, but are independent of the nature of the expected ground shaking. Using both predictive equations for spectral ordinates at several damping levels and stochastic simulations, it is shown that the scaling factors for different damping levels vary with magnitude and distance, reflecting a dependence of the scaling on the duration of shaking that increases with the damping ratio. The options for incorporating the influence of this factor into design code specifications of displacement response spectra are discussed. Copyright © 2004 John Wiley & Sons, Ltd.

基礎の耐震設計用地下震度に関する研究

This paper describes the design seismic coefficient (DSC) of the embedding foundation of building structures, which is required for the design of foundations such as basements and pile foundations. In the current Building Standard Law (BSL) of Japan, DSC is empirically specified as one-half of the base shear coefficient of a superstructure. However, the seismic design requirements in the BSL were revised in June 2000 to a performance-based design framework. A new seismic design method called the "response and limit strength calculation"(RLSC), which incorporates the effect of the soil-structure interaction, was developed. In the BSL revision, however, DSC is not referred to and must be evaluated by the conventional method mentioned above. Applying the numerical procedures employed in the development of RLSC, a practical evaluation method of DSC is investigated. A regressive formula is derived from the DSC and the ratio of the embedded depth to the foundation width.

Framework for Performance-Based Design of Building Structures

Abstract: This article proposes a new framework for performance-based design (PBD) of building structures. This framework was proposed under the 3-year Japanese Government Comprehensive Research and Development Project on “Development of a New Engineering Framework for Building Structures” launched in the fiscal year of 1995. The primary objective of the project is to create a system in which the performance of buildings is clearly stated, and consumers, that is, occupants, are well informed of how their buildings will perform and how much it will cost to maintain their performance. The framework emphasizes the establishment of target performance, the performance evaluation, and the performance statement as the main three elements. It also stresses that an institutional framework and support systems need to be provided to enable PBD to be practiced efficiently. The implementation of the proposed framework is also expected to promote engineering innovation, progress in building engineering, and globalization. The new framework will also bring other benefits, such as improved design techniques, greater design flexibility, and international harmonization. It is also important for building structural performance to become one of the most important indexes for consumers to define a building's value. The Japanese building code was changed to the performance-based code, based in the clear and comprehensive manner proposed in this article. In the United States, “Vision 2000” (SEAOC, 1995) was published, then many research activities were conducted simultaneously.

Elements of a representation framework for performance-based design

We describe selected aspects of a computer-based representation framework aimed at supporting performance analysis during schematic and detailed design. The framework is based on two distinct building representations that include sufficient information for a range of performance analysis tasks. Automated, bi-directional mappings maintain consistency between the two representations. Schematic building configurations are defined in a sheet-type representation, and may be expanded into a solid-type representation, where outlines of building and space enclosures may be decomposed into construction layers or components. Rapid model manipulation is supported by spatial partitioning techniques. A case study illustrates how the framework could be used in performance-based design practice.

Optimal seismic design of steel frame buildings based on life cycle cost considerations

AbstractA multi‐objective optimization procedure is presented for designing steel moment resisting frame buildings within a performance‐based seismic design framework. Life cycle costs are considered by treating the initial material costs and lifetime seismic damage costs as two separate objectives. Practical design/construction complexity, important but difficult to be included in initial cost analysis, is taken into due account by a proposed diversity index as another objective. Structural members are selected from a database of commercially available wide flange steel sections. Current seismic design criteria (AISC‐LRFD seismic provisions and 1997 NEHRP provisions) are used to check the validity of any design alternative. Seismic performance, in terms of the maximum inter‐storey drift ratio, of a code‐verified design is evaluated using an equivalent single‐degree‐of‐freedom system obtained through a static pushover analysis of the original multi‐degree‐of‐freedom frame building. A simple genetic algorithm code is used to find a Pareto optimal design set. A numerical example of designing a five‐storey perimeter steel frame building is provided using the proposed procedure. It is found that a wide range of valid design alternatives exists, from which a decision maker selects the one that balances different objectives in the most preferred way. Copyright © 2003 John Wiley & Sons, Ltd.

Ground motion evaluation procedures for performance-based design

The objective of performance-based earthquake engineering (PBEE) is the analysis of performance objectives with a specified annual probability of exceedance. Increasingly undesirable performance is caused by increasing levels of strong ground motion having decreasing annual probabilities of exceedance. Accordingly, the evaluation of ground motion intensity measures is a vital component of PBEE. This paper provides a brief synthesis of ground motion analysis procedures within a performance-based design framework, and is a summary of a recent report to which the reader is referred for details. The principal topics addressed are probabilistic characterizations of source, path, and site effects, with the discussion of these effects focusing principally on applications in active regions such as California.