A Seismic Shift in the International Building Code

Partially collapsed building

Earthquake-damaged building in Chile

Changes associated with the 2018 International Building Code (2018 IBC) are coming soon to Washington State. This will affect all buildings that are submitted for permit after June 30, 2020.

No Three-Month Grace Period in Seattle

Typically there has been a three-month grace period in Seattle between the code change date and enforcement. This allowed projects that were designed using an older code to still be permitted under that code if there is a short delay. We have heard from multiple City of Seattle representatives that there will be no grace period this time. Projects submitted on July 1, 2020, must use the new code.

Changes to Seismic Design

There are significant changes to the way the seismic hazard will be considered in ASCE 7-16, which is referenced by the 2018 IBC. Up to now, the process to determine code-based seismic hazard has been generally straightforward. As long as a site wasn’t on liquefiable ground that would turn to Jell-O in an earthquake, an engineer could produce seismic code parameters for a site based on latitude, longitude, and assessment of soil stiffness, called the Site Class.

In ASCE 7-16 the process for determining seismic hazard is more complex. Site-specific seismic studies will be encouraged for a larger range of site conditions (i.e., Site Classes D, E, and F), which includes sites with generally soft or loose soils even if a liquefaction hazard is not present. Such a study involves complex calculations and computer modeling. A developer may choose to take an “exception” and not perform a site-specific study for an affected site, which the code allows in certain situations. However, they would incur a penalty by taking this exception that could require the structural engineer to use higher seismic values for building design than the current code permits. A cost-benefit analysis will provide important information.

For example, a seven-story building on a soft, non-liquefiable site in Lynnwood might be affected by these new code changes. Taking a code exception for this structure would likely result in a significant increase to the seismic loads used in design, compared to the previous code. However, a relatively simple site-specific seismic hazard analysis could potentially reduce these seismic loads with a minor cost to the project. In Seattle and Bellevue, the benefit of this additional analysis is reduced because of Seattle Basin effects that amplify ground motions.

Key Takeaway

It won’t be straightforward to determine whether a site-specific seismic study or use of a code-based design will save money. Finding the best approach for each project will take more up-front communication between the developer, the structural engineer, and the geotechnical engineer. We suggest engaging the geotechnical engineer early in the design process.