Special Topics in Structures

Single Abstract

342938 - Direct Generation of ASCE 7 Site-Specific Risk-Adjusted Spectrum, including Site Amplification Effect

Saturday, April 21
8:00 AM - 9:30 AM
Location: 204AB

ASCE 7 standard switched to risk adjusted spectrum for 2E-4 annual risk target (1% probability of collapse in 50-years) in its 2010 edition. In doing so, it embraced the risk-based design approach that had already been adopted for nuclear facilities in ASCE 43-05. However, unlike ASCE 43, which seeks to meet the target risk for individual structures, systems, and components (SSCs) in a facility for a specified limit state of response), ASCE 7 seeks to meet the risk threshold for the system-wide performance attribute of collapse prevention for the structure as a whole. In any case, ASCE 7 required convolution of the collapse fragility curve with the hazard curve to obtain the risk-adjusted spectral acceleration value. In contrast, ASCE 43 provides empirical equation, which is based on a closed form solution, to directly obtain the risk-adjusted acceleration value if the acceleration value and slope of the hazard curve is known at the annual exceedance frequency equal to the risk target. The empirical equation is based on a closed form solution (based on the assumption that the hazard curve is essentially linear in log-log space in the range of annual frequencies of exceedance). Several studies have shown that the closed form and empirical solutions are accurate within a few percent compared to the solution associated with the full convolution method.

As far as site amplification effect is concerned, ASCE 7 requires application of prescribed amplification coefficients (Fa and Fv) to the risk-adjusted acceleration at Site Class B/C boundary (i.e., the soil horizon below grade corresponding to USGS' ground motion model). For site-specific spectrum, ASCE 7 Chapter 21 requires that the spectrum at grade be obtained by performing site amplification analysis using the soil properties and the spectrum at reference horizon for ground motion model). This is incorrect because it disregards the fact that the motion at reference horizon is represented by an entire hazard curve, and that the associated soil column behavior depends on the intensity of shaking applied at the reference horizon level (i.e., for soft soils, the rarer high intensity shaking has lower amplification compared to the more frequent lower intensity shaking). To reflect the site amplification correctly, it is necessary to include the site amplification characteristic into the convolution method for obtaining risk-adjusted acceleration value. The site amplification effect is often represented as exponential or as a straight line (with varying slope, as in case of ASCE 7 Chapter 11). It turns out that such behavior can be directly accounted for in integration associated with a closed form solution that considers linear hazard curve (in log-log space).

In this paper, a method will be presented to directly obtain the ASCE 7 risk-adjusted spectrum by accounting for hazard curve slope and soil nonlinearities. The proposed method is technically more accurate (in terms of accounting for site amplification effect) and simpler (in terms of avoiding numerical integration (convolution) of the hazard curve with the fragility curve.

Sanjeev R. Malushte

Bechtel Fellow and Technology Manager
Bechtel Corporation

Sanj Malushte is a Bechtel Fellow and Technology Manager at Bechtel NS&E Corporation in Reston, VA. He received his Ph.D. from Virginia Tech in engineering mechanics, where he also obtained master’s degrees in civil engineering and engineering mechanics. Additionally, he has a master’s degree in technical/general management from George Washington University.

Dr. Malushte has worked at Bechtel for over 27 years, and was elected as one of the youngest Bechtel Fellows in 2005. Bechtel Fellow is the highest technical recognition within Bechtel, bestowed for duration of one’s employment on only 15 to 20 engineers/scientists among 55,000-plus worldwide employee population in recognition of their contributions to Bechtel and the profession.

As a Bechtel Fellow and Technology Manager, Dr. Malushte serves as a Senior Technical Advisor by providing wide range of technical and technological oversight and consultation in many areas of structural and earthquake engineering. He has worked on many U.S. and international nuclear, fossil, and renewable power projects in diverse roles as a design/resident engineer, analyst/technical specialist; and in various functional/direct supervision roles. Dr. Malushte has also provided technical consultation to several Bechtel projects for Department of Energy (DOE) nuclear facilities, chemical demilitarization facilities, mining and metals facilities, civil infrastructure, and oil/gas/chemical facilities.

From 1999 to 2014, Dr. Malushte taught as an adjunct faculty member at Johns Hopkins University, where he taught several graduate level structural engineering courses. He has also taught American Society of Civil Engineers’ (ASCE) two-day ASCE 7 Seismic seminar, and has taught many in-house seismic/structural engineering seminars within Bechtel. He has led, participated in, supervised, sponsored, or peer-reviewed several funded research projects for Bechtel, EPRI, DOE, Purdue University, Virginia Tech, and University at Buffalo in the areas of earthquake engineering, composite design, and computational fluid mechanics, and has presented and/or co-authored over 70 journal or conference papers/presentations. Dr. Malushte has also peer-reviewed manuscripts for many journals, served on many conference organizing committees, and is a past associate editor of ASCE’s Journal of Structural Engineering. He has also actively participated in the organizing committees for many ASCE and SMiRT conferences, and served as session organizer/chair for many technical sessions.

Dr. Malushte is a fellow of ASCE and ICE (UK), and is licensed as a civil, structural, and mechanical engineer in several U.S. states (including California) and the UK. He has been an active member of the DOE-1020 standard committee as well as several ASCE and AISC standard committees, and was the founding chair of AISC standard committee for Modular Steel-Plate Composite (SC) Structures for Nuclear Facilities. Specifically, Dr. Malushte has been a longtime member of ASCE 7 Main and Seismic Load committees, and ASCE 4 and ASCE 43 committees. He has served on several peer-review panels for the National Science Foundation, the National Institute of Standards and Technology, AISC, DOE, and is a past member of the ASCE Policy Committee for Energy, Environment, and Water. He is also a past Associate Editor of ASCE’s Journal of Structural Engineering. Dr. Malushte is a member of Virginia Tech’s Advisory Board for College of Engineering.

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