Arzhang Alimoradi

Selection and Scaling of Ground Motion Time Histories for Structural Design Using Genetic Algorithms

This paper presents a new approach to selection of a set of recorded earthquake ground motions that in combination match a given site-specific design spectrum with minimum alteration. The scaling factors applied to selected ground motions are scalar values within the range specified by the user. As a result, the phase and shape of the response spectra of earthquake ground motions are not tampered with. Contrary to the prevailing scaling methods where a preset number of earthquake records (usually between a single component to seven pairs) are selected first and scaled to match the design spectrum next, the proposed method is capable of searching a set consisting of thousands of earthquake records and recommending a desired subset of records that match the target design spectrum. This task is achieved by using a genetic algorithm (GA), which treats the union of 7 records and corresponding scaling factors as a single “individual.” The first generation of individuals may include a population of, for example, 200 records. Then, through processes that mimic mating, natural selection, and mutation, new generations of individuals are produced and the process continues until an optimum individual (seven pairs and scaling factors) is obtained. The procedure is fast and reliable and results in records that match the target spectrum with minimal tampering and the least mean square of deviation from the target spectrum.

Benchmark Problems in Structural Design and Performance Optimization: Past, Present, and Future - Part I

In 1981, the Optimal Structural Design Committee of ASCE identified impediments preventing structural engineers from adopting widespread application of optimization techniques in design. Almost 30 years later, many of the structural optimization algorithms that have been developed are still being tested on simplistic models proposed in the 70s and 80s. There is a need for new realistic system benchmarks that could serve as testbed application and verification of novel methods of structural optimization. We discuss the advancements of the past and present a venue for development of future benchmark problems. This paper serves three purposes: it challenges developers of optimization techniques to tackle larger scale problems by posing new yet practical problems; it encourages practitioners to realize the strength of structural optimization techniques in developing safe and cost effective designs; and it seeks to persuade design professionals and academicians to efficiently utilize available computational power and paradigms in solving today’s engineering design and optimization problems. In an upcoming paper, review of performance of emerging design optimization techniques in handling the newly proposed optimization benchmarks will be studies. Future research and development needs will be discussed.

FUZZY PATTERN CLASSIFICATION OF STRONG GROUND MOTION RECORDS

Classification of earthquake strong ground motion (SGM) records is performed using fuzzy pattern recognition to exploit knowledge in the data that is utilised in a genetic algorithm (GA) search and scaling program. SGM records are historically treated as “fingerprints” of certain event magnitude and mechanism of faulting systems recorded at different distances on different soil types. Therefore, databases of SGM records of today present data of complex nature in high dimensions (many of the dimensions—or SGM parameters in time and frequency domain—are presently available from different archives). In this study, simple ground motion parameters were used but were combined and scaled nonlinearly such that the physical properties of the data could be preserved while reducing its dimensionality. The processed data was then analysed using fuzzy c-means (FCM) clustering method to explore the possibility of meaningfully representing earthquake SGM data in lower dimensions through finding subsets of mathematically similar vectors in a benchmark database. This representation can be used in practical applications and has a direct influence on the processes of synthesising ground motion records, identifying unknown ground motion parameters (e.g. soil type in this study), improving the quality of matching SGM records to design target spectra, and in rule generalisation for response. The results showed that the stochastic behaviour of earthquake ground motion records can be accurately simplified by having only a few of motion parameters. The very same parameters may also be utilised to derive unknown characteristics of the motion when the classification task on “training” records is performed carefully. The clusters are valid and stable in time and frequency domain and are meaningful even with respect to seismological features that were not included in the classification task.

Sustainable Structural Design: Outlook and Potentials

Sustainable structural design is enjoying a new vogue with vague practices, little formal definitions, and almost no rigorous treatment. This is partly because structural engineering services are influenced and convoluted with construction and architectural methods, many of which a design engineer has a little influence on. I refer to sustainable structural design technologies collectively as the methods and techniques that are either cognizant of their effects on the ecological and natural resources (design for sustainable resources) or to technologies that result in structures with extended lifetime and usability (sustainable structures), although the two are not equivalent and should not be confused. An early reference to a need for sustainability in the United States was due to the President’s Commission on Critical Infrastructure Protection that was formed in 1996 under an executive order by President Clinton to “recommend a comprehensive national policy and implementation strategy for protecting critical infrastructures : : : and assuring their continued operation” (Haimes 1998). A significant body of academic work only started appearing in the literature approximately five years after (e.g., Micic 2003). The construction industry is among the top three greenhouse gas contributors in key industrial sectors in the United States [U.S. Environmental Protection Agency (USEPA) 2009]. With the human population expected to reach 9.6 billion by 2050 (Sullivan 2013), there will be significant pressure on suppling resources for infrastructure construction and maintenance with ramifications for the market price of goods and irreversible effects on the living and natural environment. Although structural design may have less influence than construction practices, to the greenhouse gas emissions (one may argue that structural design follows the laws of mechanics and mathematics that are inherently sustainable by nature), its effects is most noticeable through extension of life, operability, and comfort while minimizing the use of natural and synthetic resources. This is in no way a minute undertaking. Structural engineers are directly and solely responsible for the choice of structural systems and materials that resist the loads and meet the safety and serviceability requirements for a constructed facility. These choices directly affect the considerations of sustainability. Responsible structural engineers should add to the mix of design limit-states, provisions of life extension, simplicity in design, and minimal use of resources. Can this be explicitly and quantifiably formulated into design? A sustainable design objective may be defined as design of a facility with extended lifetime and exceeding performance compared with current practices with no added resources. Such a design objective needs quantifiable and probabilistic measures of lifetime, performance, and resource utilization. This used to be pragmatic within the realm of structural optimization (for which structural engineers were trained particularly for their role in early aircraft design), but current sustainable design methods often ignore or poorly implement the procedures of mathematical optimization in design. As of this writing only a few engineering programs in the United States regularly offer structural optimization courses in their curriculum—a probable cause for this shortcoming. There are some important questions about sustainable design to consider: • A given design may use green materials and construction practices but result in a shorter life span of a constructed facility. Would such a practice be considered sustainable? • Besides CO2 emission and equivalents, other byproducts are at play during the planning and design processes. Take, for example, a framing system made of high-performance polymers whose production leads to higher CO2 and equivalent emissions compared with the production of steel but offers significantly enhanced lifespan for a project. Should use of such framing system be encouraged or abandoned? How would this decision change by considering that nonbiodegradable polymers do not easily disintegrate and require energy for recycling therefore adding to the waste production? • How are current practices quantifiable in terms of lifecycle performance? In the context of performance-based design, where performance objectives are communicated with stakeholders, can structural engineers provide probabilities of annual energy consumption of a facility along with the expected lifespan of their designs quantitatively? • Are older structures and designs more sustainable than the new designs in terms of their effective lifespan? Consider, for example, a historic and a modern building at a local university campus. What are the characteristics of sustainable structural systems? • Do local architectural practices result in more sustainable structures than modern architecture? Construction of adobe and rural buildings in many parts of the world, for example, has little effect on the natural environment and offers positive experience for inhabitants (they have evolved through many generations considering comfort, local costumes, and culture), but they have an extremely poor record of safety particularly during natural hazards. Could safety and sustainability become competing objectives under certain design conditions? • Economy is a major part of design. Sustainable design technologies won’t fly unless businesses start competing over their proven advantages of safety, affordability, aesthetic, reliability, and functionality. What is the role of structural engineering in ethically and scientifically promoting the proven advantages of sustainable technologies? • Nontechnical issues deserve attention as well. Sustainable design technologies will not flourish when the experience of inhabitants is not positive. Discomfort with fluctuating temperatures, locked windows, insufficient natural light, and frequently clogged waterless urinals are nonstructural issues, but they affect the business of sustainable structural design.

Evolutionary System Identification of Coupled Shear-Flexural Response for Seismic Damage Detection

SUMMARY We present a comprehensive set of computational tools, hereinafter called JAMAADA, recently developed for the analysis of strong motion data of instrumented buildings and damage detection implications. By using the system, structural engineers have a legion of advanced analytical tools at their disposal to (1) identify the possibility of damage, (2) to recognize the extend of damage, and (3) to some degree find the location of damage within a structure virtually minutes in the aftermath of an earthquake. The methods have been applied to more than 80 instrumented buildings which have recordings from more than one earthquake. The results indicate that the proposed methods, when used in combination, can provide very useful information regarding the status of a building immediately after an earthquake by simple and rapid analysis of sensor data and prior to any building inspections. JAMA-ADA utilizes a single-input multiple-output identification procedure of timevariant and time-invariant systems for estimation of modal properties of the instrumented buildings as they respond to ground shaking. The system identification procedure uses a novel solution of dynamic response of multistory buildings in an evolutionary algorithm. Most structural identification procedures proposed to date only provide some estimates of the modal quantities from which any inference to the commencement of damage may not be trivially available. However in our proposed method, besides modal properties such as mode shapes and periods, an overall estimate of the participation of shear to flexural deformations in the lateral response of a building structure can be obtained. As a result, any changes in the mode of response to and from flexural and shear dominant can be monitored in a time-varying fashion. The method is fully automated such that it can be used virtually without any delay in the aftermath of an urban earthquake.

Identification of Input Ground Motion Records for Seismic Design Using Neuro-fuzzy Pattern Recognition and Genetic Algorithms

A data classification system is designed by pattern recognition to preprocess data that can be utilized in a g enetic algorithm (GA) that performs search and scaling task of finding strong ground motion (SGM) records for seismic design. Tectonic settings, regional consideration of seismology and site characteristics are taken into account as well as nonlinear stru ctural response and performance-based design requirements for selection of input motion records. The objective of this study is twofold. First, a better understanding of SGM characteristics is attained by applying statistical pattern recognition techniqu es. Second, the classification of records makes it possible for the GA -based search and scaling methodology, developed in an earlier study by the authors, to present more appropriate input motion records in the design bin for the site under consideration. Better seismic hazard representation would result in reduced uncertainty in demand estimation in the probabilistic performance -based context, which in turn will enhance structural performance and cost efficiency of design.