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Performance-Based Seismic Design for Safer High-Rises

F5 Tower

The City of Seattle knows that building codes for downtown Seattle are not safe for tall buildings in a strong earthquake. That’s why it now requires performance-based seismic design for all buildings over 240 feet tall.

What is Performance-Based Seismic Design?

Seismic design usually follows a prescriptive code, sort of like following a cookbook. Performance-based seismic design is a more rigorous seismic analysis, performed by a team of experienced geotechnical and structural engineers. Because the design doesn’t follow the cookbook code, this alternate design procedure must be done by top engineers, so that it meets the intent of the code while also going beyond the code in certain respects. It must also be peer-reviewed by experienced engineers—often the people who participated in developing the code in the first place.

Doug Lindquist, a principal geotechnical engineer with Hart Crowser, describes it this way: “Performance-based design is a design method where the geotechnical and structural engineers proactively evaluate the performance of a structure in terms of displacements, forces, moments, and damage level. Performance-based design often results in a more resilient, constructible, and valuable structure compared to prescriptive/reactive methods.”

In the early 2000s, Hart Crowser was the first local geotechnical firm to use modern performance-based seismic design methods in the Pacific Northwest. Our engineers have incrementally improved on our proprietary methods and procedures over the last 18 years.

When and Where is Performance-Based Seismic Design Used?

Performance-based seismic design is used for buildings taller than 240 feet—around twenty-four stories or higher. It is used in areas zoned for high-rises, and only when allowed by the local permitting jurisdiction (e.g., Seattle and Bellevue).

Examples of our 20+ performance-based seismic design projects include:

  • Rainier Square Tower, Seattle (850 feet tall)
  • F5 Tower, Seattle (660 feet tall)
  • Russell Investments Center, Seattle (598 feet tall)
  • Lincoln Square Expansion, Bellevue (two towers, 450 feet tall)
  • Cirrus, Seattle (440 feet tall)
  • Midtown 21, Seattle (322 feet tall)

Major western United States cities allowing performance-based seismic design include Seattle, Bellevue, Portland, San Francisco, San Jose, Oakland, Los Angeles, and San Diego.

Advantages

Safer Design

Typical building design following the International Building Code (IBC) is based on the Design Earthquake (DE), which is defined as two-thirds of hazard level of the Risk-Adjusted Maximum Considered Earthquake (MCER). Using performance-based seismic design, the geotechnical engineer works closely with the structural engineers to analyze the building under both the DE and the MCER hazard levels. Because the building is analyzed under the higher MCER loading, the engineers have a better understanding of how the building will behave when subjected to strong ground motions. After review of many performance-based design projects, the City of Seattle identified deficiencies in the typical building design methods and now requires performance-based seismic design for all buildings taller than 240 feet.

Faster Construction and Lower Development Costs

When a building is so tall, the building code requires a dual seismic restraint system. This is like wearing both a belt and suspenders. If it’s a good belt, you don’t need the suspenders, and vice versa. Using performance-based seismic design allows you to build using one or the other. Just as it’s faster and more economical to dress donning only one fashion accessory, it’s faster and more economical to build only one structural system. This is allowed when the design engineers perform detailed analyses showing that the single system achieves the desired performance goals of the structure.

Improved Views and Higher Building Value

Eliminating cross-bracing or other exterior seismic restraint systems improves the building’s views, allowing floor-to-ceiling windows, which make the building more desirable to tenants.

Recent Advances

ASCE 7-16

Although it will not be required for use until 2020, improved methods in ASCE 7-16 have been used by Hart Crowser engineers since 2015. Certain provisions of this new code document allow for the removal of some of the extra conservatism built into the current building code. Hart Crowser was the first to use these methods in the Pacific Northwest, which result in reduced construction costs compared to older methods.

Ground Motions

Horizontal pairs of ground motions are provided by the geotechnical engineer to the structural engineer, who simulates the seismic response of the building subjected to these motions using a building model in the PERFORM 3D. There are thousands of ground motions in multiple public databases for geotechnical engineers to choose from to give to the structural engineer for design. Over the last 18 years, Hart Crowser has developed tools and techniques to identify, select, and scale the optimum ground motions that meet the source characteristics (e.g., magnitude, mechanism, spectral shape, site conditions, and source-to-site distance) and reduce the error between the target spectrum and ground motion spectra. This eliminates unnecessary conservatism and reduces construction cost compared to using less ideal ground motions.

Seattle Basin Amplification

The Seattle Basin amplifies ground motions compared to motions outside of a basin. Hart Crowser has been at the forefront of the practical implementation of research on the Seattle Basin into building design. Doug Lindquist has presented at both the 2013 and 2018 workshops on the subject organized by USGS and the City of Seattle.

Future Improvements

Future improvements will include enhanced scenario modeling to determine the strength of shaking at a building site (e.g., the M9 project) and additional advancements on incorporating basin amplification into design.

Lincoln Square Expansion

Lincoln Square Expansion in Bellevue, Washington.

Geotechnical Lessons from the Tohoku Earthquake

Japan landslide area

Rockslide (background) and flood protection (foreground) in Ishinomaki City, Japan (Photo: Dave Swanson, Reid Middleton)

The magnitude 7.3 earthquake that struck Japan six days ago is a reminder of the more devastating magnitude 9.0 earthquake that struck March 11, 2011. In an earlier post we mentioned a reconnaissance team that traveled to Miyagi Prefecture in Japan in May 2011 after the earthquake and tsunami.

In the landslide area photo above from 2011, the light colored rock slope failed even with reinforcement that protected the slope to the left. The entire land area settled, which allowed Tsunami and high tide water access to the shoreline. Fortunately, in this area the Tsunami water was not as high as other areas, so the buildings weren’t washed away. Blue tarp temporarily protects the river bank from overtopping at high tide.

Doug Lindquist of Hart Crowser had these observations about the geotechnical damage:
Damage generally happened in known geologic hazard areas (tsunami zones, areas near past landslides, liquefiable areas, and reclaimed land).
• Liquefaction damage was extensive even 150 kilometers away from the fault rupture. (Seattle is about 100 kilometers from the Cascadia Subduction Zone.)
• Ground improvement measures are effective.
• Engineering methods can reasonably estimate the liquefaction hazard.
• Newer structures performed well when designed considering known geologic hazards.

As the reconnaissance team report reminds us, a similar earthquake will happen along the Cascadia Subduction Zone, off the coastline from northern California to British Columbia. The impacts of this event on our communities and industry will depend on the actions we take now to prepare for it. The lessons learned from Japan can be applied in our own communities.

For more details on the reconnaissance team’s findings, along with some fascinating photographs, see the report here.

Driving on Styrofoam, Building on Pillows

Geofoam at SR 519

Geofoam at SR 519

You may have seen this recent blog headline: In New York, Buildings ‘Sleep’ on These Giant Red Pillows. Since that headline was called out in an engineering-related notice, you might have assumed it had something to do with seismic stability or that it was related to geotechnical engineering. After all, a recent Washington State Department of Transportation project (SR 519) used giant blocks of styrofoam in the foundation for access ramps and pedestrian areas.

To be more specific, SR 519 had the first application of geofoam approved by the Seattle Department of Transportation. Geofoam, or lightweight expanded polystyrene, is essentially a type of Styrofoam, and is used as lightweight fill in areas where heavier materials would be problematic. For the SR 519 project, using Geofoam helped protect hundred-year-old utilities. Meanwhile, highrises now can have huge rubber or fluid-filled shock absorbers, or Teflon-coated pegs.

But if you clicked on that blog headline about pillows expecting to see an earthquake engineering technology, you would have been delightfully wrong. The blog entry is about a stunning art installation, not about engineering. Although you might wonder whether there is an underlying truth to the art.

Take a look.