ASCE 7-16 TSUNAMI LOADS
Tsunami Loads and Effects: Guide to the Tsunami Design Provisions of ASCE 7-16
|Publication Date:||1 January 2020|
Geographic Extent of Applicability
The 2016 edition of the ASCE 7 "Tsunami Loads and Effects" chapter is applicable only to the five US states bordering the Pacific Ocean-Alaska, Washington, Oregon, California, and Hawai'i. Coastal regions in these five states are exposed to a quantifiable probabilistic tsunami hazard resulting from subduction zone earthquakes. The MCT is defined as having a 2% probability of being exceeded in a 50-year period. This represents a 1:2,475 annual probability of exceedance, often referred to as the 2,500-year mean return interval (MRI) tsunami. The 2,500-year MRI was selected to be consistent with the ASCE seismic design provisions, which are based on a maximum considered earthquake (MCE) with the same 2,500-year MRI.
In ASCE 7-16, Chapter 6, the 2,500-year MCT is represented by the offshore tsunami amplitude and characteristic period at 100 m bathymetric depth with respect to mean sea level along the entire coastline of Washington, Oregon, California, and Hawai'i, and most of the southern coastline of Alaska. These data are available for download from the ASCE Tsunami Hazard Tool. Figure G3-3 shows an example of the MCT wave data offshore from Seaside, Oregon. The extent of coverage of the ASCE 7-16 offshore tsunami maps is indicated in ASCE 7-16, Figure 6.1-1. A portion of the southern coastline of Alaska was not included because of the lack of adequate resolution bathymetry and topography. In addition, the northern and western Alaska coastlines were not included because they are not directly exposed to tsunamis generated by subduction zone earthquakes and presently lack adequate data to perform a probabilistic tsunami hazard analysis. For similar reasons of data availability, other US territories such as Puerto Rico, US Virgin Islands, Guam, Northern Mariana Islands, and American Samoa were also not included in this first version of the MCT mapping. The commentary to Chapter 6 recommends that for Tsunami Risk Category III and IV buildings in these locations, consideration be given to performing probabilistic site-specific tsunami hazard analysis for use in implementing the tsunami design provisions.
As mentioned in Chapter 1, subduction zone earthquakes are not the only possible source for generating tsunamis. The ASCE 7-16 tsunami design zone maps include the effects of three potential local faults in Puget Sound-the Seattle Fault, Tacoma Fault, and Rosedale Fault (Wei et al. 2017). Local tsunamis can also be generated by large landslides at or near the coast and by underwater volcanic eruptions. In particular, Alaska has a history of co-seismic landslides generating local tsunamis in fjords. Because of a lack of information on likely return periods for these landslide-induced tsunamis, they were not included in the ASCE 7-16 Tsunami Design Zone maps.
Applicability to Buildings and Other Structures
The ASCE 7-16 "Tsunami Loads and Effects" chapter requires that tsunami design be applied to all TRC IV structures located within the tsunami design zone (TDZ) and to all TRC III structures with inundation depth exceeding 3 ft at any point within the building footprint. In the commentary, the local jurisdiction is encouraged to require tsunami design for TRC II buildings with sufficient height to provide emergency refuge for people stranded within the TDZ. This will also enhance the resilience of the community since these buildings are likely to be structurally sound after the tsunami, requiring only replacement of nonstructural elements below the maximum tsunami flow elevation for them to resume their pre-tsunami function.
Short TRC II buildings such as light-framed single- and multiple-family residences and low-rise office buildings and those that are only a single story (like warehouses and airplane hangars) are not required to be designed for tsunami loading. In general, the economic cost of hardening these more vulnerable buildings for tsunami loads is not justified considering the inability of these low-rise or single-story structures to provide safe refuge for people stranded in the tsunami zone or any postinundation functionality.