The Los Angeles Times proclaimed the start of a “New Frontier” for earthquake safety: a phenomenon kicked off by the city of Santa Monica, which recently adopted the most comprehensive seismic retrofit ordinance in the nation.
An Owner’s desire to evaluate the seismic performance of an existing building varies. Some national, regional and local Owner’s simply have a genuine concern for knowing the seismic vulnerability of their buildings. Other reasons Owners perform evaluations can include an adopted City Ordinance, a policy trigger for analysis or modification of the building, a requirement for a financial transaction, or buildings with State employee tenants requiring special analysis, just to name a few.
We are currently performing dozens of these evaluations on projects throughout California, efficiently utilizing the ASCE standard ASCE/SEI 41-13. Copyrighted in 2014 by the American Society of Civil Engineers, the standard was developed and written to combine the previously adopted standards ASCE 31 and 41 into a single document for the Seismic Evaluation and Retrofit of Existing Buildings. Whereas past retrofit designs often did not align with evaluations due to having documents with differing criteria for evaluations versus design, this single document coalesces both evaluation and design of existing building retrofits providing one coordinated methodology. Seismic evaluation is defined as an approved process or methodology of evaluating deficiencies in a building that prevent the building from achieving a selected Performance Objective. Seismic retrofit is defined as the measures taken to improve the seismic performance of a building by the correction of deficiencies identified in the evaluation relative to a selected Performance Objective.
The initial step in the process is to assist the client in establishing or selecting a Performance Objective which is a combination of a desired Structural and Non-Structural Performance Levels paired with Seismic Hazard Level(s). The chart below demonstrates the different Performance Level terminology:
Following the establishment of a Performance Level, the Seismic Hazard is established based on the seismicity at the building site determined by historical data, with consideration of proximity to known faults and their activity as well as the specified Seismic Hazard Level(s).
Evaluation procedures based upon the selected Performance Objective, level of seismicity and building type are identified in the following flowchart:
Each Tier of evaluation becomes more detailed and complex. The Tier 1, Screening Procedure, is a quick checklist of structural and non-structural components of the building. A Tier 2, Deficiency-Based Evaluation procedure, utilizes more involved checks of the building to provide a deeper understanding of the building’s design. A Tier 3, Systematic Evaluation Procedure, provides a full building review including linear and non-linear / performance based analysis and design options.
The final step in the review process is to prepare an evaluation report to communicate the results of the evaluation to the owner, local jurisdiction or agency requesting the evaluation. Depending upon the availability of information and the scope of the evaluation, the extent of the report may range from a letter to a detailed document.
If the seismic evaluation suggests that a seismic retrofit is warranted, the next step is to perform the design for the retrofit. A Seismic Retrofit design will utilize the evaluation report to identify the seismic deficiencies relative to the selected Performance Objective. One or more of the following strategies to retrofit the deficiencies may be considered:
Nishkian Chamberlain has extensive experience in seismic retrofit projects and is currently working with building owners to assist in identifying the impact that retrofitting in accordance with the City Ordinances will have on their building assets. We are currently working as part of an advisory council to evaluate over 50 buildings for one client that is proactively looking to understand the overall vulnerability of their portfolio.
Should you need the assistance of a trusted advisor to guide you through the uncertainty of the City Ordinances, evaluate an existing building or make additions/modifications to an existing building, contact us at NCInfo@Nishkian.com or give us a call at 310.853.7180 for cost effective, creative seismic retrofit solutions. You can also go to our Contact page to connect with any one of our offices in your region.
Engineering News-Record recently published a list of the largest new projects started in the Mountain States region last year. The list ranks the 60 largest projects that broke ground and real construction got under way on them between January 1 and December 31, 2016. The projects are located in the following states: Utah, Colorado, Wyoming, Idaho, Montana, and the Dakotas. The 2016 list of top starts is ranked in order by dollar volume, and shows the cost of the top projects in the region totaled over $4.5 billion. The list also enumerates an impressive mix of public- and private-sector work reflective of the growing economic diversity of most states in the region, with projects launched in the health care, hotels and resorts, transportation, educational facilities, office, mixed-use, and multi-family residential sectors.
At the top of this year’s regional list is our Yellowstone Club Core Village project, a 550,000 square feet mixed-use base village in which 48 ultra-luxurious residences, a spa, pool, fitness area, restaurants, and full-skier service facilities are being added to Yellowstone Club, a world-class private resort in Big Sky, Montana. With a $312-million construction cost, the Yellowstone Club Core Village is one of the largest projects in price and size in the history of Montana.
The Nishkian team is incredibly proud to be involved in the Yellowstone Club Core Village project. Credit and kudos also go to our project team: Hart Howerton, GE Johnson Construction/Jackson Contractor Group JV, Stresscon/EnCon United, Cross Harbor Capital Partners LLC, Discovery Land Co., Yellowstone Development, and everybody else involved. We are thrilled for a great start on the largest project in the Rockies!
For the full list of the top 60 projects, please visit ENR 2016 List of Top Project Starts in the Mountain States
By Aerik Carlton
Structural fire consideration has been taking some large steps recently, with several codes and standards having added or altered structural fire sections. At Nishkian Dean, we have examined these structural fire design codes and methods from a structural engineering prospective for our clients and readers below.
A summary of these structural fire design codes includes:
ASCE 7-16 Minimum Design Loads and Associated Criteria for Buildings and Other Structures has added a new appendix to address structural fire considerations.
AISC 360-16 Specification for Structural Steel Buildings has refined and made additions to Appendix 4: Structural Design for Fire Conditions.
The National Institute of Science and Technology (NIST) has drastically expanded the Gaithersburg, Maryland, Fire Testing Lab to include a live-fire hood capable of testing a multi-story building portion with the aim of obtaining empirical large-scale structural system data to further refine and validate their free computational fluid dynamics fire modeling software. Europe, the United Kingdom, New Zealand, and Japan have all adopted performance-based methods for structural fire, and the US industry has been slow to recognize and allow this building design approach. These developments considering structural fire represent a paradigm shift for structures in a fire situation.
Prescriptive methods are the standard for structures in the US, but structural engineering codes and standards are trending toward performance-based methods. Fire Protection Engineers (FPE) have been using performance-based methods for fire for a couple of decades in the US, and Structural Engineers (SE) are beginning to develop similar methods (to be on par with our international colleagues). However, there are some marked differences in focus between FPE and SE. FPE considers smoke ventilation, egress, fire prevention systems, notification systems, and compartmentalization to restrict fire spread, while SE considers the effect fire has on the structures’ ability to remain stable and support service loadings.
Meuller et al. (2014) illustrated this difference in dramatic fashion by testing a reinforced concrete bearing wall under a single-sided heating condition. Prescriptive fire resistance methods consider the tested bearing wall as having a 2-hour rating, due to its thickness. However, the project found upon testing the wall, a complete failure occurred at approximately 42 minutes.
Building fires are rare events—the annual likelihood that a business-occupied building will experience a fire in any given year is on the order of 0.05% (Xin and Huang 2013). But just because the chances are rare does not mean we should not keep improving and refining our fire designs. We have been using prescriptive methods for building fire resistance for nearly 100 years, and yet we still don’t have a good representation of the effectiveness of these provisions.
We could potentially eliminate a lot of conservatism in our fire-resisting elements and still maintain a similar, or possibly improved, building performance at a lesser cost to building owners and developers. Through performance-based design approaches, we could eliminate prescriptively required fire resisting elements (pending jurisdictional fire official approval) such as a reduction of compartment walls thickness through polypropylene fiber addition to the concrete mix to reduce fire response spalling or by refining structural member fire resistance with intumescent paint. There is also a possibility that we could shift our designs toward structures that are more easily reparable after a fire, thus increasing the resilience and life cycle of our buildings.
If your project has structural fire requirements, or you have any questions about the updated codes, please feel free to contact Nishkian Dean.
Aerik Carlton, is an Engineering Designer with Nishkian Dean a structural engineering consulting firm in Portland, Oregon.
Mueller, K. A., Kurama, Y. C., and Mcginnis, M. J. (2014). “Out-of-Plane Behavior of Two Reinforced Concrete Bearing Walls under Fire: Full-Scale Experimental Investigation.” ACI Structural Journal, 111(5).
Xin, J., and Huang, C. (2013). “Fire risk analysis of residential buildings based on scenario clusters and its application in fire risk management.” Fire Safety Journal, 62, 72–78.
IMPORTANT UPDATE – CORRECTION TO THIS BLOG POST:
The earlier version of the blog article “Cadence Apartments project in South San Francisco starts construction soon” posted on April 27, 2017 incorrectly notes Devcon Construction as the architect. This blog post has been corrected. The design architect for Cadence Apartments is TCA Architects. Our sincere apologies for this error.
Construction is scheduled to start in May at an old Ford car dealership in South San Francisco that has been sitting vacant for years. The development, on Cypress Avenue, will offer 260 luxury apartment units and 12 townhouses, two of which will be affordable by the standards of the city’s Below Market Rate Inclusionary Housing Program. The development will be only one quarter mile from the South San Francisco Caltrain station, offering tenants access to downtown San Francisco to the north and Silicon Valley to the south.
The two buildings are each comprised of five levels of wood framing above two levels of concrete parking. Both buildings use wood and concrete shear walls as their primary lateral force resisting system.
Sares Regis is a major developer and property manager in Northern California. The Cadence development will add to their portfolio of over 7 million square feet of developed space. Devcon is the architect and contractor for this project. The design architect for Cadence Apartments is TCA Architects. Devcon is the general contractor for this project.
By Rachel Wong, S.E., CAPM®
With the January 1st implementation of 2016 California Building Code (CBC), there is a new Building Code in town. Much of the 2016 CBC is similar to the previous 2013 CBC with respect to Structural Engineering with minor updates scattered throughout. However, one of the more significant updates was in regards to existing buildings. The 2015 International Existing Building Code® (IEBC) was adopted with the 2016 CBC as the latest and greatest guideline for existing building repair and modifications.
Originally drafted in 2003, the IEBC has been in the International Code Council (ICC) family of codes for over a decade, but faced limited adoption due to the presence of IBC/CBC Chapter 34 for existing buildings. Previously, IBC/CBC Chapter 34 was responsible for minimum requirements in existing building modifications, but had limited content for the variety of projects it covered. In 2014, the code committee decided that Chapter 34 should be eliminated in favor of the more fully-depicted IEBC. The IEBC maintains much of the prior CBC information, while expanding and clarifying specific topics. For example, a path for compliance of relocated buildings is provided in IEBC, and was previously considered to be a design “grey area”. Within IEBC Appendix A, a series of subsections are now provided for masonry, wood, concrete, and steel design, which previously were beyond the scope of Chapter 34.
But a lot of familiar requirements are present in IEBC, too. Previous CBC Sections 3402 to 3411 can now be found incorporated into the contents of IEBC Chapter 4, and Section 3412 has been relocated to IEBC Chapter 14. The CBC retrofit/strengthening threshold of 5% gravity/10% seismic modifications to existing elements without requiring strengthening to these elements is still applicable for Level 2 alterations that impact less than 50% of the building area.
The IEBC provides options for either prescriptive compliance of a structure or performance-based compliance, and permits the use of alternate methods as well. One of the seismic retrofit documents that go hand-in hand with these provisions is the relatively new ASCE 41-13 document, which will be featured in our upcoming May article.
Viscous Damper Brace Frames in an existing Steel Building
(Performance-Based Compliance Upgrade)
Each of the Nishkian offices has extensive experience with providing cost-effective solutions in retrofit, alteration and additions to existing structures. Should you need assistance with understanding how the new code will affect your existing building project, do not hesitate to contact one of our offices to receive expert assistance with any questions you may have. We are here to help!
Construction is wrapping up on a new condominium building tucked off South 19th Avenue in southwest Bozeman. Sitting on 2.0 acres at the intersection of Graf St. and Enterprise Blvd. in Meadow Creek Subdivision, Talbach House is in a prime location close to the Oracle office campus, and within minutes to Montana State University and Downtown Main Street. The brand new three-story condominium building consists of 66 condominium units totaling 64,652 square feet (6,006 square meters). Bitnar Architects served as the master architect leading the design, collaborating with developer and builder Cadius Partners dba CP Haus, and structural engineer of record Nishkian Monks.
The condominiums at the Talbach House range in size from 625-square-foot one-bedroom, one-and-a-half bath options to two-bedroom, two-and-a-half bath options with just over 1,000 square feet featuring high ceilings, open floor plans, 8-foot tall, sliding glass windows with breathtaking views, large entertaining kitchens, and higher-end interior features as well as European-inspired design elements. Talbach’s secure building includes two exercise rooms, 1G internet speeds, outdoor storage, and 112 parking spaces on-site. With Meadow Creek Subdivision’s protective covenants and architectural guidelines, the Talbach House offers spectacular views of the Bridger, Spanish Peaks, Mount Blackmore and Tobacco Mountain ranges, open-space parks, walking/hiking trails, recreation, and other amenities.
The structure is on a gently sloping site, which required some cut and fill to allow a ground level with no interior steps. Above grade, the exterior and interior walls are light-framed wood construction with wood siding at the exterior walls. The roof framing was accomplished with pre-engineered gang-nailed trusses. In addition to the residential building, the project features covered parking with storage rooms. The building is founded on conventional concrete strip and spread footings with a slab-on-grade at the ground level.
With one-bedroom units starting around $173,500, the Talbach House provides home buyers a comparatively affordable option in Bozeman. As the project finally wraps-up, exterior siding panels are being installed giving Talbach House its contemporary look. These past few weeks crews have installed the lobby windows leaving an inviting space for resident mail and the lounge area. Steel staircases on the East end and the North end of the building were also installed. Talbach House also includes two elevators and a staircase in the center of the building. Installation of the steel balconies throughout the building have been completed. The balcony structure will accommodate the 5’x17’ steel and concrete balcony area on the second- and third-floor with glass railings and metal privacy screens. The first-floor suites will have a concrete patio walking right out to the grass area. Interior work is also near its end. The project is expected to be completed before the end of Q2-2017. To learn more about Talbach House Condos, click here.
Nishkian Monks is proud of its growing contribution to Bozeman’s affordable housing inventory and general amenity spread. Do not hesitate to contact any of the Nishkian offices if we can be of service to you on your next residential project.
We were thrilled to hear the news: Chad Norvell, a project engineer at Nishkian Dean in Portland, was named one of ten New Faces of Civil Engineering Professionals in 2017 by the American Society of Civil Engineers (ASCE).
The nationwide recognition program promotes the bold and humanitarian future of civil engineering by highlighting the achievements of the next generation of C.E. leaders. Presented annually, the recipients are chosen based on their contributions to society and their dedication to improving the quality of life for all.
Norvell was officially recognized during ASCE’s annual Outstanding Projects and Leaders (OPAL) Gala on March 16, 2017, in Arlington, Virginia.
Photo 1: 2017 OPAL Awards for New Faces of Civil Engineering
At Nishkian Dean, Norvell specializes in the seismic retrofit of buildings—he has designed efficient seismic retrofits for more than a dozen schools in the greater Portland area, and he works closely with the state government to help school districts fund these projects. He also promotes the profession through his participation with Engineers Without Borders (EWB), and through his ongoing work with the Earthquake Engineering Research Institute (EERI) to help with earthquake relief in Haiti and Nepal.
We took a moment to speak with Chad and to learn more about his dedication to serving the public good through his work. Read on for our Q&A:
Let’s start at the beginning—what inspired you to become an engineer?
I had a fascination with architecture since childhood, and originally studied it in college with the intention of becoming a practicing architect. After two years in architecture school, I realized that while there were some things I was good at, there were more things that I just wasn’t good at. I decided to leverage my strength in math and science and switch to structural engineering, which ended up being a perfect fit.
There are so many different specialties within our field. What prompted you to focus on seismic issues specifically?
As a structural engineer educated and practicing on the West Coast, some study and understanding of seismic phenomena and loads is unavoidable. I find seismic issues interesting for two reasons: for one, it’s just more challenging and requires another level of ingenuity and creative problem-solving beyond typical structural design.
Secondly, seismic issues can have a large impact on society beyond the engineering of structures. For example, the National Earthquake Hazards Reduction Program (NEHRP), the federal government program that funds seismic research, supports the study of social science issues related to earthquakes, in addition to the geologic sciences and structural engineering topics that we would expect. This kind of interdisciplinary field of study is interesting to me.
Could you tell us more about your work with Engineers Without Borders in Nicaragua? What types of projects are you working on with them?
In college, I spent four years as a member of our EWB chapter, serving as chapter president for two of those years. We worked primarily in a region on the West Coast of Nicaragua to provide engineering support for community problems that the government did not have resources to address. This was valuable for the local communities, and was a great learning experience for us as engineering students.
One of the two major projects we worked on was mitigating yearly flooding at an elementary school, and our second major project aimed to help a remote coastal community produce enough potable water to serve the communities’ needs.
I’ve also worked with several other EWB chapters to provide support for their projects. Most recently, I helped establish seismic criteria and perform structural analysis for a school in Ethiopia.
Could you tell us more about your relief work with Earthquake Engineering Research Institute?
After the earthquakes in Nepal and Haiti, I worked with teams in the US that were developing rebuilding guidelines for each country. We developed a guide to earthquake-safe construction for Haiti that was later translated to Haitian Creole, and for Nepal, we provided support for the revision of their building code.
You’re also participating in EERI’s first Learning from Earthquakes field study program—what does that entail?
After major earthquakes occur around the world, EERI and other organizations send reconnaissance teams to investigate building failures. This information is studied, and eventually leads to valuable technical information that advances our seismic design procedures. But this investigation only happens in the weeks immediately after an earthquake.
The idea behind “resilience reconnaissance” is to continue doing field investigation in earthquake-affected regions periodically for years after the event, tracking changes in various critical community services like housing, business, health care, and education. By doing this, we get an understanding of a community’s resilience to earthquakes, not just a building’s, which also serves as valuable information for communities in areas of seismic risk.
Photo 2: EERI Team in Chile 2017
Today, non-profit organization DiscoverE announced that Chad Norvell is one of the winners of the 2017 class of New Faces of Engineering honorees. The announcement coincides with the second annual Global Day of the Engineer, a worldwide day of celebration and volunteerism that shines a spotlight on the work done by engineers and inspires the next generation of engineering and technology professionals. DiscoverE’s New Faces of Engineering recognizes the work of up-and-coming engineering professionals, age 30 or younger, who are making their mark on their industry. These talented individuals are honored for having dedicated themselves to using their skills and education to help engender a better world. These young engineers serve as inspirations both for their colleagues and for the next generations coming up behind them. The highly-coveted award, started in 2003, is recognized nationally by their peers as a top honor for young engineers and continues to grow in prestige. In addition to recognizing young engineering professionals, DiscoverE also honors engineering students through its New Faces of Engineering College program. This year’s class includes young professionals innovating solutions throughout a cross-section of industries, including energy, food security, infrastructure, medicine, aerospace and the environment. Previous honorees have gone on to launch global businesses and NGOs.
All four Nishkian firms join together in congratulating Chad. To learn more about the 2017 New Faces of Engineering Honorees, please visit DiscoverE at http://www.discovere.org/our-programs/awards-and-recognition
2100 University Avenue in East Palo Alto was recently featured in the Silicon Valley Business Journal. The 210,000 square-foot office building features a four-story atrium with impressive skylights – an aesthetic designed by San Francisco-based Korth Sunseri Hagey Architects (KSHA). The expansive open floor plan paired with a warehouse aesthetic and an outdoor terrace has proven attractive to tenants. It has been confirmed that tech-giant Amazon will lease the entire space and fill it with 1,300 employees. This will increase the number of jobs in the city of East Palo Alto two-fold. The project has stirred much discussion about gentrification of the small city on the Peninsula – East Palo Alto’s Mayor commented on the impact the project could have not only on the city’s revenue, but also on the cost of housing.
The four-story concrete office building is connected to a six-story parking structure by a two-story bridge suspended one story above the ground. Both the office building and the parking structure utilizes concrete shear walls.
The general contractor for University Square is Devcon Construction and the developer is The Sobrato Organization.
For more information about the project, please visit: http://www.bizjournals.com/sanjose/news/2017/03/22/amazon-scoops-up-university-square-in-east-palo.html?ana=e_sjo_bn&u=q0hFpWnjiaOOg2k6iyXpZw0cc4406e&t=1490391765&j=77722911
For a video-rendering of the project, please visit: https://vimeo.com/124629180
The demand for affordable housing in urban areas is increasing as rent prices skyrocket. Many developers are now required to include a percentage of affordable housing units to obtain approvals for their new multi-family construction projects. Additionally, affordable housing projects that wish to access alternate funding sources are often required to meet various green building requirements.
Recently, Nishkian Chamberlain began work on a new affordable housing project in Ventura, California. The project is unique in that it is entirely comprised of affordable housing apartment units. In order to receive some state and federal funding, the project is required to meet several green building targets. One such target is the use of “Advanced Framing”.
In short, Advanced Framing is energy and material-efficient wood framing. Conventional wood framing, typically used for many years, includes many structural redundancies (double top plates, three-stud corners, multiple jack studs, double or triple headers, etc.). The goal of Advanced Framing is to eliminate unnecessary redundancies and achieve savings in material usage while also taking advantage of opportunities for increased energy efficiency.
There are various techniques commonly used in Advanced Framing. Which technique(s) are used is determined on a project-to-project basis. One technique applied to a recent project was the use of engineered wood floor joists. Using engineered wood floor joists allowed larger joists spans and wider joist spacing, leading to savings in materials, increased construction efficiency, and reduced construction costs. It is critical for the structural engineer on the project to coordinate joist depths, which may be larger due to the increased spans and spacing, with the architect and MEP early on in the project to minimize conflicts and rework later on.
Additionally, for this project, the increased floor joist spacing affected the design of the floor sheathing. A larger span rating was required, and serviceability characteristics such as floor vibration were investigated. It was recommended to use a thicker floor sheathing than what we use for Conventional Framing to achieve the required span rating, reduce vibrations and increase the perceived stiffness of the floor.
In addition to using engineered wood floor joists, other Advanced Framing techniques include:
These techniques can be mixed and matched to accommodate the unique aspects of each project while still achieving the targeted green building requirements.
In conclusion, the demand for affordable housing is increasing, especially in high density urban areas where housing is in short supply and rents are high. Affordable housing projects often target green building requirements such as the use of Advanced Framing techniques to obtain State and Federal funding. Advanced Framing techniques can be used individually or in combination to achieve the desired green building criteria.
If you are interested in finding out how using Advanced Framing techniques can benefit your next project, Nishkian Chamberlain has significant experience in this technique and would be happy to discuss your construction and development needs. You may contact Craig Chamberlain at CChamberlain@nishkian.com or (310) 853-7180.
By Chad Norvell, PE
Historically, tsunamis have been poorly understood by the public. Films often show tsunamis as towering tidal waves that cast deep shadows over tall buildings on the coast before violently crashing down. Video footage from the 2004 Indian Ocean tsunami showed the world what tsunamis really are—a wall of water that doesn’t necessarily tower over the coast, but that moves through with unstoppable force.
In this article, we will explore important tsunami basics, review the tsunami risk in Oregon, introduce changes to structural loading standards that now include tsunami loads, and discuss essential research findings out of Chile that affect our local understanding of tsunami risk.
Tsunami Causes & Terminology
In the Pacific Northwest, we are increasingly aware of the risk of a devastating Cascadia Subduction Zone earthquake. This potential future earthquake is likely to be associated with a large tsunami that will strike the coasts of Washington, Oregon, and Northern California, as well as echo around the Pacific basin, reaching as far as Japan and Australia. The figure below illustrates how a subduction zone earthquake triggers a tsunami.
When discussing tsunamis, it is important to understand the terminology used. What does it mean to say that in Pulicat, India, during the 2004 Indian Ocean tsunami, the maximum runup was 3.2 m and the inundation limit was 160 m? The figure below illustrates the primary tsunami measurements.
Source: U.S. Geological Survey Tsunami Terms
RUNUP ELEVATION: The difference between the elevation of maximum tsunami inundation and the reference sea level elevation.
INUNDATION DEPTH: The depth of the tsunami relative to grade level at the point of interest (e.g., where the structure is).
INUNDATION DISTANCE or INUNDATION LIMIT: The maximum horizontal distance inland inundated by the tsunami.
Oregon’s Tsunami Risk
According to the U.S. Geological Survey (USGS), Oregon has 25,000 residents, in addition to 55,000 tourists, who could be at direct tsunami risk (defined as being within the Tsunami Design Zone, or TDZ) along 300 miles of coastline subject to inundation. Two ports, a fuel depot hub, and $8.5 billion in essential facilities are located within this risk zone as well. The Oregon Department of Geology and Mineral Industries (DOGAMI) has produced a series of tsunami inundation maps covering the entire Oregon coastline, showing the areas at risk of inundation for both Cascadia Subduction Zone earthquakes [the left figure below] and Alaskan-Aleutian Subduction Zone earthquakes [the right figure below.]
Source: Oregon Department of Geology and Mineral Industries
Analysis by Oregon Public Broadcasting in 2015 showed that “about a third of schools, hospitals, police and fire stations along the Oregon coast are within a potential tsunami zone.” In Seaside, approximately 80% of residents live at elevations of 15 feet above sea level or lower, when DOGAMI estimates that even a small Cascadia Subduction Zone tsunami would have a wave height of over 20 feet. Further development on the Oregon coast will rely on structural designs that are tsunami-resistant.
New Tsunami Structural Load Standards in ASCE 7-16
The American Society of Civil Engineers (ASCE) publishes a document called “Minimum Design Loads for Buildings and Other Structures,” commonly referred to as ASCE 7. This consensus-based standard specifies the minimum required design loads for all the types of load commonly encountered in the structural design of buildings, including dead, live, wind, and seismic. ASCE 7 is incorporated by reference in the International Building Code (IBC), making its provisions law in much of the United States. The recently published 2016 edition of ASCE 7 (ASCE 7-16) includes a new chapter on tsunami loads, and new regulations on when buildings must be designed with consideration of tsunami loads.
In line with current practice for earthquake loads, ASCE 7-16 defines a Maximum Considered Tsunami (MCT) as the tsunami that has a 2% probability of exceedance in 50 years, or an average return period of 2,500 years. The runup elevation associated with the MCT is designated as the Tsunami Design Zone (TDZ), and hazard maps (like Oregon’s tsunami inundation maps) are based on that elevation. TDZ maps for Washington, Oregon, California, Alaska, and Hawaii are included in ASCE 7-16.
The provisions of ASCE 7-16 require consideration of tsunami loads only for Risk Category III and IV structures within the TDZ, generally meaning structures that could pose a great risk to human life if they failed (such as schools) or emergency services buildings (such as police and fire departments.). However, local jurisdictions have the option of designating threshold elevations under which even Risk Category II buildings (typical commercial and residential structures) would need to be designed to resist tsunami loads. In areas characterized by flat coastal planes (for example, Tillamook, Oregon), evacuation from the tsunami zone may be impossible, in which case vertical evacuation into relatively tall public or commercial buildings designed to resist tsunamis could save thousands of lives.
Designing structures to resist tsunamis requires consideration of four types of tsunami load:
ASCE 7-16 includes procedures for determining each of these loads.
Lessons Learned from Tsunami Research in Chile
Chile suffered significant tsunami damage associated with the M8.8 Maule earthquake in February 2010. Since then, observations from the tsunami, along with sophisticated research at the Universidad Técnica Federico Santa María and CIGIDEN (National Center for Investigation of Integrated Management of Natural Distasters) under Dr. Patricio Catalán, have yielded important and surprising lessons about tsunami behavior, two of which are summarized below.
The understanding of risk to our infrastructure from tsunamis is still not as mature as that of seismic and wind risk, but significant advances are being made, particularly via lessons learned from recent tsunamis like the 2004 Indian Ocean Tsunami, the 2010 Maule Tsunami in Chile, and the 2011 Tohoku Tsunami in Japan. These lessons are now being incorporated into the standards that structural engineers use to design buildings, providing us valuable tools for building more resilient communities in areas of tsunami risk. This is particularly important for us in the Pacific Northwest, where we have long coastlines and many communities at risk of inundation in a Cascadia Subduction Zone earthquake and tsunami.
Contact Nishkian Dean for more information or to discuss your project on the Oregon coast.
Chad Norvell, PE is a Project Engineer with Nishkian Dean a structural engineering consulting firm in Portland, Oregon.