# 7.3: Earthquake Design of Large Structures

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• 7.3.1: Introduction
It’s impossible to earthquake-proof a building. A look at the intensity scale shows that for intensities of IX and worse, even well-designed and well-constructed buildings can fail. However, most earthquakes have maximum intensities of VIII or less, and well-constructed buildings should survive these intensities. The highest intensity recorded in a Pacific Northwest earthquake was VIII in the 1949 Puget Sound Earthquake and locally on Harbor Island in Seattle in the 2001 Nisqually Earthquake.
• 7.3.2: Seismic Retrofitting
The Starbucks Center occupies a nine-story building that was formerly a Sears catalog store constructed in 1912 on tidal fill next to Elliott Bay. Before Starbucks moved in, the City of Seattle required an earthquake upgrade costing \$8.5 million. Nearly two thousand people were in the building when the Nisqually Earthquake struck. People dove under desks and tables. Some of the walls cracked and a four-foot brick parapet on top of the building crashed to the ground.
• 7.3.3: Base Isolation
The normal approach to providing seismic resistance is to attach the structure firmly to the ground. All ground movements are transferred to the structure, which is designed to survive the inertial forces of the ground motion. This is the reason why your house is bolted to its foundation and your cripple wall is reinforced. In large buildings, these inertial forces can exceed the strength of any structure that has been reinforced within reasonable economic limits.
• 7.3.4: Special Problems
Each large building presents its own set of design problems in surviving earthquake forces, which means that architects must consider earthquake shaking in designing a large structure in a seismically hazardous region such as the Pacific Northwest. I consider the problem of a soft ground floor and the issue of the tuning fork. In a building with a soft ground floor, the ground floor is weaker than the higher floors.
• 7.3.5: Bridges and Overpasses
Freeways and bridges are lifelines, and their failure can disrupt the economy and kill people on or beneath them during an earthquake. The television images of people sandwiched in their cars in the collapse of the double-decker in Oakland, California, the collapsed span of the Oakland-San Francisco Bay Bridge, and the pancaked freeway interchanges in Los Angeles after the Sylmar and Northridge earthquakes were dramatic reminders of the vulnerability to earthquakes of highways and railroads.
• 7.3.6: Engineering Against Ground Displacement
Up to this point, the main hazard discussed has been ground shaking. The Alquist-Priolo Act in California seeks to avoid construction on active fault traces. A large displacement of several feet, particularly vertical displacement, will probably destroy a building constructed across the fault, but building foundations can be designed to survive displacements of a foot or less. It makes no difference whether the displacement is caused by faulting, ground subsidence, or incipient landslides.
• 7.3.7: Decisions, Decisions, and Triage
The astronomical cost of retrofitting bridges brings up a major problem faced by society. As you look at the building inventory in your town or the bridge inventory in your state, you soon recognize that in this era of budget cutbacks in government, the money is not available to retrofit even a sizeable percentage of the inventory. Decades will pass before dangerous buildings are retrofitted, with the retrofit decision commonly based on criteria other than earthquake shaking.

This page titled 7.3: Earthquake Design of Large Structures is shared under a CC BY-NC-SA 4.0 license and was authored, remixed, and/or curated by Robert S. Yeats (Open Oregon State) via source content that was edited to the style and standards of the LibreTexts platform; a detailed edit history is available upon request.