What the New Seismic Code Means for Your Building

Pacific Northwest design and construction firms are well acquainted with seismic requirements related to structures. But, as we described earlier this summer, the upcoming 2018 International Building Code has implications that are unprecedented when compared to previous code updates.

Our coverage of the upcoming changes, specifically the seismic code updates, has spurred many conversations with our clients and partners. These conversations and common questions inspired us to design and deliver a series of seismic presentations, and the popularity of the presentations drove us to build a follow-up piece. Here, we apply the seismic code to three project types that are common to the region: a residential podium structure, a mid-rise tower and a high-rise tower.

Back Up… What’s Driving These Changes?

The seismic code updates are triggered by recent geotechnical discoveries associated with the Seattle Basin. The bowl-shaped geological depression, which is softer than bedrock and stretches from Everett to Tacoma, focuses and amplifies seismic waves. This basin effect means that earthquakes are magnified and shaking lasts longer. Geotechnical studies of the disastrous Jiji and Tohoku earthquakes, also located in deep basins, helped inform the new code. The data set gathered (from some of the most extensive networks of sensors and monitoring stations in the world at that time) gave scientists and geotechnical engineers new indicators related to the path and depth of ground motions across the basin. Evaluation of this information, when combined with data from more sophisticated equipment and other seismic events, contributes to the improved life safety codes.

Minimizing Impacts of the New Seismic Code

Collaborating with the region’s designers and contractors, we’re exploring techniques and layout options that will maximize performance without sacrificing an owner’s intended goals, budget, or delivery to market. While we acknowledge each site and program is unique, these examples reveal how we’re minimizing impacts for these project types.

Residential Podium

Construction of multi-unit residential projects continues at a fast pace in the greater Seattle area. Many of these are low-rise (6- to 8-stories), wood-framed structures over a concrete podium.

Code changes are unlikely to significantly affect the lateral layout or geometry of this project type, great news for a market dependent on flexible, efficient programming. There will likely be an increase of five to twenty percent in seismic forces, which could represent an increase in the cost of seismic systems. Applying our experience and advanced analytical expertise, we can minimize these code-related costs. Traditional and non-traditional systems are explored when we collaborate with our design and construction partners to create the most efficient layouts possible.

Mid-Rise Tower

Unlike a residential podium project, a 160-foot tower project will be impacted by large increases in seismic forces; likely from 25 to 75 percent.

Vetting options early in design and using advanced analytical studies to more precisely determine a building’s response to earthquakes can significantly offset cost increases. Specifically, this means taking a harder look at alternative seismic systems and strategizing more efficient layouts. We continue to explore ways to optimize structural systems that will lessen the impact on usable space and offset structural cost increases via additional lateral options, higher strength materials, and other techniques.

Additionally, it may now make sense to implement techniques traditionally reserved for taller buildings. For example, we have used soil-structure interaction to model the nonlinear damping response of soils surrounding below-grade structures for towers to significantly reduce concrete and rebar quantities.

High-Rise Tower

While high-rise towers are also impacted by basin effects, these requirements are already implemented on buildings over 240 feet through code-mandated performance-based design (PBD) methodologies. For these projects, state of the art lateral solutions are utilized during design of PBD structures, and we provide options that balance safety, efficient space layout, and constructability.

Ongoing in-house and institutional research, as well as continual participation in code advisory committees, provide the basis to explore solutions that can mitigate potential impacts of the new seismic codes on your project. Give us a call!