NEW YORK CITY – Seventy attendees gathered for the CTBUH New York Chapter‘s seminar on seismic design, hosted at Langan’s New York office. Throughout the evening, participants heard from a number of technical experts from across the architecture, construction and engineering fields.
Earthquakes affect individual buildings in different ways, depending on the building’s height, foundation structure, adjacent buildings, distance from epicenter, flood plain level and other factors. Primary and secondary effects include shaking, ground rupture, liquefaction, lateral spreading, earthquake induced landslip, rockfall, tsunami and fire.
Regardless of a building’s condition, it’s important to be prepared—and even lower-probability areas like NYC need to be prepared because the impact would be quite high.
In NYC, Amy Macdonald, Associate Principal, Head of Resilience, Thornton Tomasetti, emphasized, “we’re not in an area that has no risk—while the seismic hazard is not nearly as high as on the west coast—it still exists and needs to be incorporated into the design and emergency preparations for buildings and infrastructure.”
First, engineers “do investigations of ground conditions” and then based on results, they look at how types of soil(s) involved will impact foundations, Martin explained. Site classes include bedrock (A and B), dense soil (C and D), and soft soils (E and F)—each with different effects and projected interactions with earthquakes.
“Fundamentally, seismic design has a level of uncertainty to it,” as Garlock emphasized in his presentation with Martin.
Along with Martin, Garlock explained how engineers “design for a site-specific maximum credible event” and “to understand risk, you have to multiply hazard by assets and vulnerability.”
In each spot, the confluence of gravity, wind and other natural forces—from the air and on the ground—result in the specific effects of any given earthquake on an individual building. However, often impacts from neighboring structures are even more significant than forces of nature—and as a result, shorter buildings have higher risks than taller buildings. Additionally, Garlock said, “wind controls over seismic for most tall buildings,” but shorter buildings don’t have those same protections.
To sum up, Garlock added, “Anything that we can possibly foresee for our clients, we’ll do it. They’ll make the decision, but we’ll make sure we have the conversation.”