Development of Tsunami Design Provisions for the ASCE/SEI ... · This code will likely be adopted in the 5 western s\൴ates by 2020, at which time tsunami design will required for

$Page 1: Development of Tsunami Design Provisions for the ASCE/SEI ... · This code will likely be adopted in the 5 western s\൴ates by 2020, at which time tsunami design will required for$
Ian N. Robertson, S.E., [email protected] Professor of Structural Engineering, UH Manoa

Michael Mahoney, [email protected] Senior Geophysicist, Federal Emergency Management Agency

Development of Tsunami Design Provisions for the ASCE/SEI 7-16 Standard

Tohoku Tsunami photograph at Minami Soma by Sadatsugu Tomizawa

Presenter

Presentation Notes

The current status is presented at the ISOPE-PACOMS Conference in Brisbane, Australia, as of Oct. 6 th, 2016, of the effort in the US to develop tsunami load and effects provisions for inclusion in a national standard of loads.

• Minimum Design Loads for Buildings and Other Structures by American Society of Civil Engineers’ (ASCE) Structural Engineering Institute (SEI).

• Consensus design standard updated every 5 years using ANSI-approved process.

• ASCE/SEI 7-10 is adopted by reference by the 2012 and 2015 International Building Codes (IBC), and therefore most US jurisdictions.

ASCE/SEI 7-10

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Presentation Notes

ASCE/SEI 7-10 is the current standard for all loads used in the design of new buildings and other structures in the US. It is referenced by the International Building Code, IBC-2012, which is the official building code for most jurisdictions in the US.

ASCE/SEI 7-10 Minimum Design Loads for Buildings and Other

Structures • Chap 1 & 2 – General and load combinations • Chap 3 - Dead, soil and hydrostatic loads • Chap 4 - Live loads • Chap 5 - Flood loads (riverine and storm surge) • Chap 6 - Vacant • Chap 7 - Snow loads • Chap 8 - Rain loads • Chap 10 - Ice loads • Chap 11 – 23 - Seismic Design • Chap 26 – 31 - Wind Loads

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The ASCE/SEI 7-10 standard provides minimum design loads for dead and live loads, flood loads (which relates to riverine flooding and storm surge coastal flooding, but not tsunamis), snow, rain, ice, wind and seismic loads and effects. However, there is no consideration of tsunami loads and effects.


Structures • Chap 1 & 2 – General and load combinations • Chap 3 - Dead, soil and hydrostatic loads • Chap 4 - Live loads • Chap 5 - Flood loads (riverine and storm surge) • Chap 6 - Vacant • Chap 7 - Snow loads • Chap 8 - Rain loads • Chap 10 - Ice loads • Chap 11 – 23 - Seismic Design • Chap 26 – 31 - Wind Loads

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Presentation Notes

Chapter 6 of ASCE/SEI 7-10 used to be the Wind chapter, till Wind was moved to Chapters 26 through 31.


Structures • Chap 1 & 2 – General and load combinations • Chap 3 - Dead, soil and hydrostatic loads • Chap 4 - Live loads • Chap 5 - Flood loads (riverine and storm surge) • Chap 6 – Tsunami Loads and Effects • Chap 7 - Snow loads • Chap 8 - Rain loads • Chap 10 - Ice loads • Chap 11 – 23 - Seismic Design • Chap 26 – 31 - Wind Loads

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Presentation Notes

In the “soon to be released” ASCE/SEI 7-16 standard, Chapter 6 will now contain a new chapter on Tsunami Loads and Effects.

ASCE/SEI 7 Tsunami Loads and Effects • Subcommittee of 16 members and 14 associate

members formed in February 2011. – One month before Tohoku tsunami.

• Subcommittee met 4-5 times per year for three years. • Processed 8 consensus ballots through ASCE/SEI 7-16

Main Committee, addressing over 1500 comments. • Final version issued for public comment in Fall 2015. • Officially approved for ASCE/SEI 7-16 March 11, 2016. • ASCE/SEI 7-16 accepted by International Code Council

for inclusion in IBC 2018 in December 2017. • ASCE/SEI will also be publishing a companion design

guide in late 2017 with numerous design examples.

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Presentation Notes

The ASCE/SEI 7 subcommittee on Tsunami Loads and effects was officially commissioned in February 2011, just one month before the Tohoku Tsunami. It is chaired by Gary Chock, a structural engineer and president of Martin & Chock, Inc. in Honolulu, Hawaii. The committee has 16 voting and 14 associate members, all of whom contributed to the development of the new Chapter 6. The committee met 4 to 5 times per year over a 3 year period to develop the draft provisions. FEMA P-646 Edition 2 served as a valuable reference for this committee’s work. Past laboratory research and lessons learned from the Tohoku Tsunami and other past events were fundamental to the development of this draft. The draft was submitted to the ASCE 7 Main Committee, comprised of 50 voting members from the other ASCE/SEI 7 loading subcommittees. It took 8 ballots and responding to over 1500 comments from the main committee until consensus was reached on the final Chapter 6, Tsunami Loads and Effects. This final draft was released for public comment in Fall 2015 and all comments were addressed in early 2016. The ASCE/SEI main committee officially approved the new tsunami chapter on March 11, 2016, exactly 5 years after the Tohoku Tsunami. Since then the full ASCE/SEI 7-16 has been adopted by the International Code Council as a reference for the IBC 2018. This code will likely be adopted in the 5 western states by 2020, at which time tsunami design will required for certain essential and critical buildings and structures. Ian Robertson is currently developing a companion design guide that will assist practitioners in application of the new tsunami design provisions.

Fluid-Structure Interaction

Structural Loading

Structural Response

Scour and Erosion

Consequences (Life and economic losses) Warning and Evacuation Capability

Tsunami Inundation Modeling to Define Tsunami Design Zones

Sources and Frequency Tsunami Generation Distant and Local Subduction Zones Open Ocean Propagation Offshore Tsunami Amplitude

Loads and Effects incorporating Coastal, Hydraulic, Structural, and Geotechnical Engineering

Maps based on Probabilistic Tsunami Hazard Analysis (PTHA)

Structural Reliability Validated

Societal Impact Assessment for the Five Western States by USGS

Tsunami-Resilient Engineering Subject Matter Incorporated in ASCE 7

Coastal Inundation and Flow Velocities

Performance by Risk Category

Consensus on Seismic Source Assessment by USGS

Scope of ASCE/SEI 7 Chapter 6

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Presentation Notes

The ASCE/SEI 7-16 Chapter 6 on Tsunami Loads and Effects encompasses the scope identified by the dashed rectangle in this figure. Apart from the seismic source mechanisms, which are well defined by USGS, and the consequences (both life and economic losses) and warning and evacuation systems, all other aspects of tsunami generation, propagation, inundation, structural loading, scour and the design performance requirements of coastal structures are covered by this chapter.

ASCE 7 Chapter 6- Tsunami Loads and Effects • 6.1 General Requirements • 6.2-6.3 Definitions, Symbols and Notation • 6.4 Tsunami Risk Categories • 6.5 Analysis of Design Inundation Depth and Velocity • 6.6 Inundation Depth and Flow Velocity Based on Runup • 6.7 Inundation Depth and Flow Velocity Based on Site-Specific

Probabilistic Tsunami Hazard Analysis • 6.8 Structural Design Procedures for Tsunami Effects • 6.9 Hydrostatic Loads • 6.10 Hydrodynamic Loads • 6.11 Debris Impact Loads • 6.12 Foundation Design • 6.13 Structural Countermeasures for Tsunami Loading • 6.14 Tsunami Vertical Evacuation Refuge Structures • 6.15 Designated Nonstructural Systems • 6.16 Non-Building Structures

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Presentation Notes

This slide shows the subsections of the new chapter, and gives an overview of the topics covered by the provisions. I will briefly present some of these sections.



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The first four sections cover basic scope, general requirements, definitions and notation, and tsunami risk categories.

MCT and Tsunami Design Zone

• ASCE 7 Tsunami Loads and Effects Chapter is applicable only to the states of Alaska, Washington, Oregon, California, and Hawaii, which have quantifiable tsunami hazards.

• The Maximum Considered Tsunami (MCT) has a 2% probability of being exceeded in a 50-year period, or a ~2500 year average return period.

• The MCT is the design basis event, characterized by the inundation depths and flow velocities at the stages of in-flow and out-flow most critical to the structure.

• The Tsunami Design Zone is the area vulnerable to being flooded or inundated by the Maximum Considered Tsunami.

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Presentation Notes

Important aspects of the tsunami provisions are that they only apply to the 5 western states with a quantifiable probabilistic tsunami hazard, namely Alaska, Washington, Oregon, California and Hawaii. Other tsunami hazard areas such as Puerto Rico, Guam, American Samoa, etc. will hopefully be included in future versions of the standard, but there was insufficient funding to perform all of the probabilistic tsunami hazard analysis for these locations in this version. The design tsunami has a 2% probability of exceedance in 50 years, representing a 2500 year average return period. This is the same return period as is used by ASCE/SEI 7 for the Maximum Considered Earthquake. Since much of the NW US and Alaska are governed by near-source tsunamis, the earthquake that initiates the tsunami will clearly have the same return period as the subsequent tsunami, so it is only logical that the two hazards be treated the same. This Maximum Considered Tsunami is then used to develop probabilistic off-shore wave heights and coastal inundation resulting in the Tsunami Design Zone maps used to determine the flow properties at any site in the TDZ.

Consequence Guidance on Risk Categories of Buildings Per ASCE/SEI 7

Risk Category I Up to 2 persons affected (e.g., agricultural and minor storage facilities, etc.)

Risk Category II (Tsunami Design Optional)

Approximately 3 to 300 persons affected (e.g., Office buildings, condominiums, hotels, etc.)

Risk Category III (Tsunami Design Required)

Approximately 300 to 5,000+ affected (e.g., Public assembly halls, arenas, high occupancy educational facilities, public utility facilities, etc.)

Risk Category IV (Tsunami Design Required)

Over 5,000 persons affected (e.g., hospitals and emergency shelters, emergency operations centers, first responder facilities, air traffic control, toxic material storage, etc.)

Visual 20.11

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The ASCE 7-16 code will only require tsunami design for Risk Category III and IV structures. These are critical and essential facilities such as hospitals, emergency operations centers, etc. and large assembly halls, arenas, etc. where large casualties would be expected if the building were to collapse. The code does not require tsunami design for Risk Catergory II buildings, which make up the vast majority of the building inventory. For low rise RC II buildings, such as light framed residential buildings and strip malls, etc. it does not make economic sense to design for tsunami loads. In addition, the tsunami may well overtop these buildings, so they cannot serve as effective refuges for folk trapped in the tsunami inundation zone. Finally, it might give a home owner undue confidence that their home was designed for tsunamis, so they feel safe staying in place, but are then trapped in flow that exceeds the roof of their building. However, for multi-story reinforced concrete and structural steel buildings, such as hotels, office buildings, etc. it would be advisable to design for tsunami loads. This would make these a refuge of last resort for folk trapped in the inundation zone, even though they are not designated as vertical evacuation structures. Tens of thousands of people in Japan survived the Tohoku Tsunami because they sought refuge in buildings that were tall enough and strong enough to survive the tsunami. The code commentary therefore recommends that communities adopting the code consider adding a requirement for tsunami design of Tsunami Risk Category II buildings that exceed some threshold height (probably around 60 feet or higher depending on the location). This would also increase the resilience of the community since these structures would not have to be demolished after the event but could be refurbished and returned to use more rapidly than building an entire new structure.



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The next section of Chapter 6 provides the requirements for the Probabilistic Tsunami Hazard Analysis, PTHA, required to determine the flow conditions at the site of interest.

PTHA Derived MCT • Probabilistic hazard identification required in order to be in the model code. • The ASCE PTHA procedure was peer reviewed by NTHMP stakeholder group. • Subduction Zone Earthquake Sources are consistent with USGS Probabilistic

Seismic Hazard model.

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Presentation Notes

Development of the 2500 year probabilistic Tsunami Offshore wave heights required integration over all seismic sources around the Pacific Ocean varying in size & recurrence rate. This required the inclusion of uncertainties through logic trees & distribution functions. The probabilistic tsunami hazard is primary expressed in the Offshore Tsunami Amplitude at 100m bathymetry offshore. These data will be available digitally on an ASCE website. In addition, source disaggregation of hazard for every site around the coastline will also be available from the same database. This allows for the generation of tsunami scenarios from the dominant sources to perform inundation modeling to generate the Tsunami Design Zone bounded by the inundation limit and runup elevations. These tsunami scenarios can also be used to perform full site-specific time history inundation studies. Source model characterization for probabilistic tsunami events include: Recurrence of the subduction zone sources Mmax of the subduction source regions Coupling ratio (what proportion of long-term plate convergence is released) Scaling Relationships of Mw to average slip and area of rupture Slip Distribution (asperity rather than uniformity) Extent of Rupture – need to consider greater extent for tsunami events than for earthquake ground shaking, in capturing great tsunami events Near-field regional subsidence – general use of Okada model geometry



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The next two sections of the code specify how the site flow conditions can be determined using either the Energy Grade Line Analysis, or the Site-Specific PTHA approach.

Tsunami Flow Characteristics

Two approaches to determine flow depth and velocity

• Energy Grade Line Analysis method based on pre-calculated runup from the Tsunami Design Zone maps – Required for all TRC III and IV buildings

• Site-Specific Probabilistic Hazard Analysis

– Required for TRC IV buildings – Optional for other TRCs – Velocity lower limit of 75-90% EGL method

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Self-explanatory



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The next section shows how the flow properties at the site are used to design the structure.



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The next three sections of Chapter 6 provide the formulas for determining various hydrostatic, hydrodynamic and debris impact loading conditions.

Tsunami Load Conditions • Hydrostatic Forces (equations of the form ksρswgh)

– Unbalanced Lateral Forces at initial flooding – Buoyant Uplift based on displaced volume – Residual Water Surcharge Loads on Elevated Floors

• Hydrodynamic Forces (equations of the form ½ ksρsw(hu2) – Drag Forces – per drag coefficient Cd based on size and element

– Lateral Impulsive Forces of Tsunami Bores on Broad Walls: Factor of 1.5 – Hydrodynamic Pressurization by Stagnated Flow – per Benoulli – Shock pressure effect of entrapped bore

• Waterborne Debris Impact Forces (flow speed and √k m) – Poles, passenger vehicles, medium boulders always applied – Shipping containers, boats if structure is in proximity to hazard zone – Extraordinary impacts of ships only where in proximity to Risk Category III

& IV structures

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(Animated Slide) There are three different hydrostatic load conditions, including buoyancy mentioned earlier. There are four hydrodynamic load conditions that must be considered. And there are three waterborne debris impact conditions for which the structure and its components must be designed.



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Scour and sediment transport are covered in two sections relating to the design of foundations for buildings in the tsunami design zone, and the design of structural countermeasures such as berms, etc. to protect structures from tsunami loading.



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A special section in Chapter 6 applies specifically to the additional requirements for the design of Tsunami Vertical Evacuation Refuge Structures. These are structures designated by the local authority as safe locations for evacuees to use for vertical evacuation from the tsunami. As such, it is paramount that they be tall enough and strong enough to ensure the safety of those at the evacuation levels.

Tsunami Vertical Evacuation Refuge Structures • Tsunami Vertical Evacuation Refuge Structures - ASCE 7 Chapter

6 is intended to supersede both FEMA P646 structural guidelines and IBC Appendix M

• Additional reliability (99%) is achieved through site-specific inundation analysis and an increase in the design inundation elevation

Figure 6.14-1. Minimum Refuge Elevation 21

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Presentation Notes

(Animated Slide). The provisions require that a site-specific tsunami inundation study be performed for all vertical evacuation structures. This study must follow the PTHA procedures outlined in the chapter, and must also comply with the minimum threshold values relative to the EGLA results. Once the resulting tsunami elevation has been determined (red dashed line), a 30% margin of error must be added to this elevation (red line). This is based on the estimated uncertainty in current inundation model results. This increase elevation is then used to design the structure, hence making it stronger, and to set the refuge elevation. A minimum of 10 feet or one story is then added to this elevation and any floor above the resulting elevation is safe for use as a refuge. Reliability analysis has shown the a structure designed as a vertical evacuation building per this approach has greater than a 99% probability of surviving intact during the maximum considered tsunami.



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Finally there are two sections dealing with nonstructural systems that are required to function during and immediately after the tsunami, and non-building structures such as industrial facilities, etc.

Summary • ASCE/SEI 7 Chapter 6 provides a comprehensive method for

reliable tsunami structural resilience, making tsunamis a required consideration in planning, siting, and design of coastal structures in the five western states.

• Probabilistic Tsunami Hazard Analysis is the basis for the development of 2475-yr MRI Tsunami Design Zone maps.

• Specified design procedures are provided for all possible loading conditions.

• Coastal communities are encouraged to require tsunami design for taller Risk Category II buildings to provide a greater number of buildings that will be disaster-resilient.

• ASCE/SEI 7-16 (and Chapter 6) still need to be adopted by the 5 western States into their State building codes.

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The Summary is self-explanatory.

IBC Appendix M • IBC Appendix M, Tsunami Generated Flood Hazard, was

originally submitted using the analysis and structural design aspects of FEMA P-646, Guidelines for Design of Structures for Vertical Evacuation from Tsunamis.

• However, this much of the technical provisions have been superseded by Chapter 6 of ASCE/SEI 7-2016.

• A proposal to update Appendix M for the 2018 IBC was submitted by FEMA and ASCE so it now only refers to the tsunami evacuation and site planning criteria of FEMA P-646.

• It will now defer to the tsunami hazard mapping and structural design guidelines of ASCE/SEI 7-16 Chapter 6.

• The title of Appendix M was also revised to focus only on tsunami vertical evacuation refuge structures.

APPENDIX M TSUNAMI-GENERATED FLOOD HAZARD SECTION M101 REFUGE STRUCTURES FOR VERTICAL EVACUATION FROM TSUNAMI-GENERATED FLOOD HAZARD M101.1 General. The purpose of this appendix is to provide tsunami vertical evacuation planning criteria for those coastal communities that have a tsunami hazard as shown in a Tsunami Design Zone Map. M101.2 Definitions. The following words and terms shall, for the purposes of this appendix, have the meanings shown herein. Refer to Chapter 2 of this code for general definitions.

TSUNAMI HAZARD DESIGN ZONE MAP. A map that designates the extent of inundation by a Maximum Considered Tsunami as defined by ASCE 7-16 Standard.

M101.3 Establishment of tsunami design zone. Where applicable, the Tsunami Design Zone Map shall meet or exceed the inundation limit given by the ASCE 7 Tsunami Design Geodatabase. M101.4 Planning of tsunami vertical evacuation refuge structures within the tsunami design zone. Tsunami Vertical Evacuation Refuge Structures located in a tsunami design zone shall be planned, sited and developed in general accordance with the planning criteria of FEMA P646-12.

Exception: These criteria shall not be considered mandatory for evaluation of existing buildings for evacuation planning purposes.

FEMA P-646 Vertical Refuge Guide • The April 2012 edition of FEMA P646

contains information now in conflict with ASCE/SEI 7-16 Chapter 6 and must be updated to remove those conflicts.

• Funding for a full update was not available from either NOAA/NTHMP or FEMA/NEHRP.

• A “low cost” update has been added to an existing task order and is currently being performed by Ian Robertson.

• The final draft due this spring and will be sent out for review; including NTHMP MES State and territorial representatives.

Guidelines for Design of Structures for Vertical Evacuation From Tsunamis

Thank-You

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Development of Tsunami Design Provisions for the ASCE/SEI ... · This code will likely be adopted in the 5 western s\൴ates by 2020, at which time tsunami design will required for