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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.

Towering Hills for Beauty and Strength

Governors Island

Photo: Timothy Schenk

A dozen years ago an American port representative was asked how his port was preparing for rising sea levels. “Well…we aren’t,” he answered, somewhat sheepishly, because he knew they should be. Back then, the public was skeptical of the controversial topic, and frankly many ports had too many other priorities. But now public officials see the situation in a new light. They are taking advantage of waterfront development projects to make property not only more resilient to climate change, but also more beautiful and beneficial to the public.

A perfect example is the 40-acre Governors Island Park and Public Space in New York. West 8, an urban design and landscape architecture firm, transformed the abandoned former military island into a green oasis with an extraordinary 360-degree experience of water and sky that has won numerous awards. Part of the makeover involved creating four tall, dramatic hills from twenty-five to seventy feet high. This meant overcoming a major challenge involving Governors Island history.

Governors Island Park and Public Space

Pumice, or lightweight fill (the light colored material) is placed on the water side of the tallest hill. Image courtesy of West 8

From Subway Dirt to Island

Back in 1637, when a Dutch man bought Governors Island for two ax heads, a string of beads, and some nails, the island was only about 72 acres. In 1901, somebody needed a place to discard the dirt from the excavation of New York’s Lexington Avenue subway line. What better place to put it than Governors Island? The dirt widened the island by 100 acres.

Fast forward to the twenty-first century. Now that the island had been sold back to the people of New York for one dollar, it was possible to take advantage of the island’s potential views, which meant building upwards. To create the new hills, West 8 needed to add 300,000 cubic yards of new fill—enough to fill 40 Goodyear blimps. The challenge was to keep that massive amount of dirt from pushing the island built on subway fill out into the harbor.

Hart Crowser worked with the lead civil engineer to make the hills strong yet light. Twenty-five percent of the new fill is from the demolition of structures and parking lots. This made it sustainable and strong. Pumice lightened the load. Some of the fill was wrapped in geotechnical matting, and the steepest slopes used wire baskets. This allowed hills as high as seventy to be built within twenty feet of the shoreline, and allowed for varying slopes and walkways, where the public can safety enjoy the park.

Governors Island reopened to the public on May 28.

How Many Soil Borings Do Development Sites Need?

One of the challenges that developers – both public and private – face from a geotechnical and environmental standpoint is the inherent uncertainty in what’s underground at the development site. Generally, we’d like to know the geologic layers, soil types, groundwater levels and potential environmental contaminants across a site. But trying to characterize a fairly large volume of soil with just a few pieces of information inevitably leaves knowledge gaps.

An illustration for this challenge comes from an unexpected source – a children’s book. “Sam & Dave Dig a Hole,” written by Mac Barnett and illustrated by Jon Klassen, is a funny, deadpan story about two boys (and a dog) who dig a hole, hoping to find “something spectacular” (website here). 

Sam and Dave Dig a Hole

Photos courtesy of Mac Barnett and Jon Klassen

As the boys dig through the ground, they come close to, but never discover, several spectacular gems.

Sam and Dave miss the gem

In fact, they seem to navigate around everything spectacular.

Sam and Dave digging around the gem

While the book is an admittedly whimsical analogy to geotechnical and environmental subsurface exploration, it actually serves to illustrate an important point – there may be more beneath the surface of a site than a couple of borings will indicate. Skimping on borings increases the chances that zones of contamination or soft soils, may be missed, only to be discovered during or after construction. More borings can help fill in gaps and increase confidence that the site has been well-characterized. In many cases, spending bit more money on site exploration may reduce overall project costs by reducing uncertainty about the site and what may be encountered during construction. And depending on project needs and site conditions, the use of less conventional site investigation methods (Cone Penetration Test, strataprobe) may be appropriate. These can often provide better spatial coverage at similar costs to traditional Standard Penetration Test borings, because they’re cheaper. 

Of course, there’s no one-size-fits-all approach to subsurface exploration. The best exploration program for a project will balance project needs, budget, and local experience with geologic conditions. But in order to minimize the chances of pulling a Sam and Dave, maximizing spatial coverage in the explorations program should be a consideration.

Increase Your Points Toward LEED Certification

Federal Center South

Federal Center South, the most energy-efficient office building in the Pacific Northwest, may achieve LEED Platinum. Energy Piles, recycled wood and construction debris, and stormwater infiltration galleries contributed LEED points.

Most LEED points come from efficiency in design and construction areas such as energy, water, materials, and indoor environmental quality. That’s why it may not be obvious how geotechnical engineers and environmental scientists contribute to LEED certification. Since LEED Silver is a requirement for most new public buildings, with LEED Gold the new normal, owners need every possible point. Here are several avenues to gain more:

Sustainable Sites – Several credits are available, including Brownfield Redevelopment (Credit 3); Protect and Restore Habitat, including green roofs (Credit 5.1); and Stormwater Design, including infiltration, reuse, pervious paving, swales, and other LID solutions (Credits 6.1 and 6.2).

Water Efficiency – Credits are typically based on the percentage of reduction in the use of potable water for the new development. Water-efficient landscaping, reuse of rainwater, and capture and reuse of groundwater in the irrigation or building systems can cut water use by 20% or more. Designing efficient filters for graywater recycling can lead to additional points.

Energy and Atmosphere – Credit 2 (On-Site Renewable Energy) allows as many as 3 credits for generating up to 7.5% of the building’s power usage on site. Properly designed ground source heat pump geothermal systems will achieve this goal and these points.

Materials and Resources – Again, several credits are available: Credit 2.1 Construction Waste Management (diverting demolition debris from landfills or incinerators), Credit 3.2 Materials Reuse (reusing salvaged building materials like foundation piles), Credit 4 Recycled Content (using materials such as ground down tires or recycled glass for backfill).

Innovation & Design Process – New or innovative energy saving solutions that have been applied to a site development can be described, justified, and submitted for potential extra points.

Geotechnical and environmental professionals can work with design and construction teams to gain as many as 5 or 6 additional points – and that might be the difference between Silver and Gold, or Gold and Platinum.