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Target Structural Reliability Analysis for TsunamiHydrodynamic Loads of the ASCE 7 Standard
Gary Chock, Dist.M.ASCE1; Guangren Yu, M.ASCE2; Hong Kie Thio3; Patrick J. Lynett, M.ASCE4
Abstract: Many coastal areas in the United States are subject to tsunami hazard. The public safety risk has been partially mitigated throughwarning and preparedness of evacuation, but community disaster resilience requires that critical and essential facilities provide structuralresistance to collapse. Furthermore, there are coastal communities in the states of Alaska, Washington, Oregon, California, and Hawaii wherethere is insufficient time for evacuation. The Tsunami Loads and Effects Subcommittee of the ASCE/Structural Engineering Institute (ASCE/SEI) 7 Standards Committee has developed a new Chapter 6, titled Tsunami Loads and Effects, for the 2016 edition of the ASCE 7 standardMinimum Design Loads for Buildings and Other Structures (Minimum design loads for buildings and other structures). The new ASCE 7provisions for tsunami loads and effects implements a unified set of analysis and design methodologies that are consistent with probabilistichazard analysis, tsunami physics, and reliability analysis. The purpose of this paper is to provide analysis of the structural reliability basis fortsunami-resilient design of critical and essential facilities, taller building structures, and tsunami vertical evacuation refuge structures.Probabilistic limit-state reliabilities are computed for representative structural components carrying gravity and tsunami loads, utilizingstatistical information on the key hydrodynamic loading parameters and resistance models with specified tsunami load combination factors.Through a parametric analysis performed using Monte Carlo simulation, it is shown that anticipated reliabilities for tsunami hydrodynamicloads meet the general intent of the ASCE 7 standard as stated in its Chapter 1 commentary. Importance factors consistent with the targetreliabilities for extraordinary loads (such as seismic events) are determined for tsunami loads. DOI: 10.1061/(ASCE)ST.1943-541X.0001499. 2016 American Society of Civil Engineers.
Author keywords: Tsunami; Safety; Reliability; ASCE 7 standard; Safety and reliability.
Tsunamis are infrequent events but can be extremely destructive.A tsunami is a series of great waves primarily caused by a majorsubduction zone earthquake with large-scale displacement of thesea floor or by initiation of submarine landslides. Many coastalareas in the United States are subject to tsunami hazard. The publicsafety risk has been partially mitigated through warning and pre-paredness of evacuation, but community disaster resilience requiresthat critical and essential facilities provide structural resistance tocollapse. Furthermore, there are coastal communities in the states ofAlaska, Washington, Oregon, California, and Hawaii where there isinsufficient time for evacuation. With these circumstances, theavailability of taller buildings that are tsunami resistant is a directbenefit to public safety. The recent catastrophic tsunamis in theIndian Ocean (2004), Samoa (2009), Chile (2010), and Japan(2011) indicate that an explicit structural design procedure for risk
mitigation of tsunami damage to major structures is much needed.Multistory buildings, which are necessary for tsunami safety whereevacuation cannot be completely assured, can be economicallydesigned for life safety or better performance for large tsunamiswith local strengthening of relatively few components.
The 2016 edition of the ASCE 7 standard,Minimum design loadsfor buildings and other structures will introduce a new Chapter 6,titled Tsunami Loads and Effects (ASCE 2016). Tsunami risk cat-egories are based on the risk categories defined in Section 1.5 ofASCE 7, with some modifications for specific types of structuresallowed per ASCE 7-16 Section 6.4 for tsunami conditions in thelocal community (the local government would need to identify theircritical facilities versus those facilities that are not uniquely requiredfor posttsunami functionality). Tsunami Risk Category III andTsunami Risk Category IV buildings and other structures located inthe tsunami-prone states of Alaska, Washington, Oregon, California,and Hawaii shall be designed to resist the tsunami loads and effectsdetermined for a maximum considered tsunami. Recent UnitedStates and international research and validation by documented casestudies of tsunami flows and their effects on structures were utilizedin developing this chapter and commentary, along with the develop-ment of new probabilistic tsunami design zone maps (Chock 2016).
Tsunami Vertical Evacuation Refuge Structures
In ASCE 7-16, a tsunami vertical evacuation refuge structure is aspecial type of Risk Category IV structure with additional designrequirements. This structure serves as a designated point of refugeto which a portion of a communitys population can evacuate whenhigher ground is not reachable in time before tsunami arrival. Aparticularly important consideration is the elevation and heightof the refuge, since it must provide very high reliability of structurallife safety for the occupants within a portion of the refuge that is not
1Structural Engineering Institute Fellow, Chair, ASCE 7 Tsunami Loadsand Effects Subcommittee; President, Martin & Chock, Inc., 1132 BishopSt., Suite 1550, Honolulu, HI 96813 (corresponding author). E-mail:firstname.lastname@example.org
2Structural Engineer, Martin & Chock, Inc., 1132 Bishop St., Suite1550, Honolulu, HI 96813. E-mail: email@example.com
3Vice President, Principal Seismologist, AECOM, 915 Wilshire Blvd.,7th Floor, Los Angeles, CA 90017. E-mail: firstname.lastname@example.org
4Professor, Dept. of Civil and Environmental Engineering, Univ. ofSouthern California, KAP 224D, Los Angeles, CA 90089. E-mail: email@example.com
Note. This manuscript was submitted on May 12, 2015; approved onDecember 17, 2015; published online on May 24, 2016. Discussion periodopen until October 24, 2016; separate discussions must be submitted forindividual papers. This paper is part of the Journal of Structural Engineering, ASCE, ISSN 0733-9445.
ASCE 04016092-1 J. Struct. Eng.
J. Struct. Eng., 04016092
inundated. Therefore, additional conservatism is necessary in theestimation of inundation height for the design of these specialoccupancy structures. The minimum elevation for a tsunami verti-cal evacuation refuge structure is the maximum considered tsunamirunup elevation anticipated at the site, increased by 30%. A site-specific tsunami inundation time history analysis is required fora tsunami vertical evacuation refuge, as well as an importancefactor of 1.25.
Tsunami Hazard Analysis
The enduring lesson of recent catastrophic tsunamis is that histori-cal records alone do not provide a sufficient measure of the poten-tial heights of future tsunamis. Engineering design must considerthe occurrence of events greater than scenarios in the historicalrecord, based on the underlying seismicity of subduction zones.To achieve a consistent structural performance for community resil-ience, a probabilistic tsunami inundation geodatabase for designis digitally available with the ASCE tsunami design provisions.The tsunami design zone is the area vulnerable to being inundatedby the maximum considered tsunami (MCT), which is taken ashaving a 2% probability of being exceeded in a 50-year period ora 2,475-year mean recurrence interval. A probabilistic tsunamihazard analysis (PTHA), which is similar to probabilistic seismichazard analysis (PSHA), was performed to produce hazard-consistent offshore tsunami amplitude waveform data and tsunamidesign zone inundation and runup map data. An early example ofthe PTHA procedure was initially performed for California(Thio et al. 2010), which was subsequently refined in accordancewith a peer review study sponsored by the National TsunamiHazard Mitigation Program (California Geological Survey 2015).
Tsunami loads and effects are based on the MCT that is char-acterized by the inundation depth and flow velocities during thestages of inflow and outflow at the site. There are two proceduresfor determining the MCT inundation depth and velocities at a site:(1) the energy grade line analysis (EGLA); and (2) a site-specificinundation analysis (Chock 2015). The EGLA is fundamentally ahydraulic analysis along a topographic transect from the shore lineto the runup point which takes the runup elevation and inundationlimit indicated on the tsunami design zone map as the given sol-ution point. It generally produces values of design flow parametersthat are conservatively greater than numerical modeling of the sametransect, and so it is always performed as the minimum baselineanalysis of the site. (Carden et al. 2015). Site-specific inundationanalysis utilizes the offshore tsunami amplitude, the effective waveperiod that is considered a co