On June 9^th, ULI San Francisco hosted a sold-out seminar on high-rise building in the Bay Area entitled “Sink, Swim… or Tilt?” The talk was given by respected structural engineer Carl J. Wilford, P.E., S.E., Vice President of Thornton Tomasetti. Wilford’s 21 years of experience in structural and civil engineering were readily apparent as he broke down the different engineering techniques used to stabilize high-rise buildings in the event of an earthquake.

Wilford began by listing the strengths and weaknesses of different materials that can be used in the construction of a building and the way these materials hold load paths, a concept that is elementary in structural engineering. Concrete offers unlimited forms: it’s durable and keeps material costs low. Steel is useful because it has the best strength-to-area ratio, and can be used to forge strong and stable connections. Compared to steel and concrete, timber is a much greener and more environmentally friendly material to produce. While masonry is by far the most common material used in the construction of buildings worldwide, the Bay Area is not a friendly area in the world for masonry due to high labor costs and seismic activity.

Wilford then highlighted and explained two kinds of structural systems which are integral to understanding the science and physics behind creating safe and durable buildings: lateral and vertical support systems. Lateral systems are more resistant to wind and seismic loads. Some lateral systems have shear walls and/or brace frames made of steel and/or concrete that distribute loads horizontally (laterally), equipping buildings for disaster. Base isolation is another effective method to prevent seismic damage to a building: the building’s columns are set down on a rubber bearing isolator, keeping the building in place while the ground moves.

One of the more unique methods to prevent earthquake damage to a building is to install a mass tuned damper—a massive, solid steel ball that hangs in the heart of the building. In the event of an earthquake, the ball goes out of phase with the shaking building and absorbs energy, stabilizing the structure in which it hangs. This method is used in Taipei 101, a 101-story tower in Taipei, Taiwan.

Soil quality is another factor that is important to consider when constructing a building: when building on poor (loose) soil conditions, it’s often necessary to drive massive piles into the earth in different ways to ensure stability. Another way to address loose soil is a mat or raft foundation, where a building is supported by a giant, 30-foot deep slab of concrete. This was the method used when constructing the 1,100-foot tall Wilshire Grand Center in Los Angeles—the tallest building west of the Mississippi.

The vertical support system, described as an alternative structural system, resists dead (building weight) and live (people) loads. There are a number of ways to create a structural support system for vertical forces, including creating a steel deck with beams, wood sheathing over joists, steel girders, waffle slabs, concrete floor slabs, wood glue lams, and other similar means.

For those of us who never took an engineering course, this was an extremely informative session that “laid the foundation” for a basic understanding of the many challenges that structural engineers face in the seismically unstable Bay Area.

By: Eric Podolsky