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SEAOC Webinar: Beyond Site Class - Where Your Building is Sited Greatly Influences Seismic Vulnerabi

Date and Time

Wednesday, December 18, 2024, 12:00 PM until 1:00 PM

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Webinars

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Beyond Site Class - Where Your Building is Sited Greatly Influences Seismic Vulnerability


Ashwani Dhalwala, MS, (UC Berkeley, 1967-68) SE, FSEAOC, Hon. SEAOSC1,
Martin B. Hudson, PhD, PE, GE2

1Principal, AEC Solutions, Santa Monica, CA, Chair, SEAOSC Steel Committee
2University of California, Los Angeles, Principal, Hudson Geotechnics, El Segundo, CA Member, SEAOSC Steel Committee
12 noon - 1:00 p.m. PST, Wed., Dec. 18

Influence of soil conditions on seismic response of structures was originally researched by Prof. H.B.Seed at UC Berkeley back in 1967-68 when he founded the discipline of Geotechnical Engineering. Considerable progress in understanding this phenomenon has been made since then culminating in the Site Specific Ground motion provisions and definition of Site Classes including provisions of Soil Structure Interaction (SSI) as required. However a very critical part of site response and structural evaluation is understanding the resonance that occurs when the building periods are in close proximity to the ground motion periods which are dictated largely by the time history of earthquake motions as these motions transmit thru soil strata and the natural site period, the root cause of site resonance, if it exists. Site resonance among other mechanisms, forms a second peak or sometimes multiple peaks in the site response spectra. This was observed during several earthquakes such as the Superstition Hills, Landers, Northridge, Christ Church and more recently the M7.8 Turkey Earthquake. The duration of these peaks is far longer than the duration of the peak period of underlying seismic motions and can potentially cause significantly higher probability of collapse and greeter damage.

The presentation addresses the effects of site resonance and site conditions under which it can exist. In addition, per the ASCE 7-22 provisions, with a multi-period spectrum, a small change in the period of earthquake motion can modify the spectral acceleration by a very large amount, this itself presents another scenario of building performance as the peak in the response spectrum shifts. Procedures to assess site resonance period using available drill-boring log data or by on-site measurement of ambient vibrations by measuring horizontal-to-vertical spectral ratio (HVSR) are described. The presentation also discusses an example of using site specific drill-boring log data for reliably estimating the site resonance period and demonstrates how the soil resonance site period differs from the site-specific peak response period per ASCE 7 16 and ASCE 7 22. Finally, specific structural systems and examples of construction details which are affected by site response are discussed.

 





Ashi Dhalwala

Ashwani (Ashi) Dhalwala, M.S., P.E. Principal, AEC Solutions, specializes is Earthquake Engineering and Earthquake Dynamics of structures. He received his M.S.C.E. in 1969 from the Structural Engineering and Structural Mechanics Division of UC Berkeley with fellowships including a fellowship offer for PhD work. He started his seismic structural engineering career at T.Y. Lin & Associates. During his professional practice of more than 55 years, he has performed seismic design, seismic risk evaluations, vibration monitoring and retrofitting/updating/repair of several existing and new building structural systems in Reinforced/Post-tensioned
concrete (including tilt-up and lift slab buildings), Steel, Masonry and Wood varying in height from single story to 73 storys located in Southern California, Western US, Alaska and Puerto Rico. After the Northridge Earthquake, he was appointed to the SEAOSC/L.A. City Task force advisory committees on Non-Ductile Reinforced Concrete Structures, Type V Construction and Non Structural Building Elements. His focus thru-out his career has been to promote concepts which enhance the seismic performance of both existing and new buildings.

For the past ten years he has been actively researching both the effects of Soil Period and advanced concepts of Soil Structure Interction (SSI) and develop practical solutions to address these very important issues in order to enhance the resiliency of Southern California considering the unique seismicity of the region.

He is a member of the SEAOC College of Fellows, an Honorary
Member of SEAOSC, and currently chairs the SEAOSC Steel
Committee.


Martin (Marty) Hudson, has over 30 years of industry experience covering a variety of building types, sizes, and market sectors. Dr. Hudson is a licensed Civil and Geotechnical Engineer. He obtained his B.S. and M.S. degrees from the University of California, Los Angeles and Ph.D. in Civil Engineering from the University of California, Davis.

Dr.  Hudson has worked as a geotechnical engineer of record on some of the largest projects in California history, including the Wilshire Grand Tower, Ronald Reagan UCLA Medical Center, the New Century twin tower development, and the LADWP Headworks Reservoir. He also has voluminous experience with commercial building and stadium development of all sizes throughout the United States. He has worked as one of the plan check reviewers on behalf of the City of Inglewood for the geotechnical aspects of the SoFi Rams Stadium. For several major transit projects, including the Los Angeles Metro Purple Line Westside Extension, he has served as Project Manager for the geotechnical and environmental investigations. He has extensive experience with deep excavations, shoring, soil nailing, MSE walls, ground improvement, shallow and deep foundations including driven piles, drilled shafts, caissons, and micropiles. He also has extensive experience with ground motion studies on many high-rise buildings, reservoirs, subway stations and tunnels, and hospital buildings, including base-isolated and damped structures. He is a member of the Los Angeles Tall Buildings Structural Design Council, the Hospital Building Safety Board for the State of California, an Honorary Member of the Structural Engineers Association of Southern California  (SEAOSC), and serves on the seismology committee of SEAOSC. He has served on the Board of the American Society of Civil Engineers Los Angeles Branch Geo-Institute and on the Board of SEAOSC. He also is an Adjunct Professor in  geotechnical engineering at the UCLA Henry Samueli School of Engineering and Applied Science. He has dozens of publications on seismic and geotechnical engineering topics.
Martin Hudson



Structural Engineers Association of California | SEAOC Foundation
1201 J St., Ste. 200
Sacramento, CA 95814
seaoc@seaoc.org | (916) 447-1198

Structural Engineers Association
of Central California (SEAOCC)

Structural Engineers Association
of Northern California (SEAONC)

Structural Engineers Association
of Southern California (SEAOSC)

Structural Engineers Association
of San Diego (SEAOSD)

Members of the National Council
of Structural Engineerings Associations (NCSEA)