A three-story mixed-use building near Broadway was the first project in Fargo where we saw the design team push full base isolation. The site sat on 40 feet of lacustrine clay, and the spectral acceleration from a 2,500-year event would have pushed a fixed-base frame past drift limits. We ran nonlinear time-history analyses with isolator properties tuned to the site-specific response spectrum. The isolation period shifted the structure well past the peak demand. That project taught us how much the Red River Valley soils amplify long-period motion, and we have carried that lesson into every isolation scheme since. When the soil profile includes thick, soft clay over glacial till, the ground motion at the surface looks nothing like the rock outcrop hazard. A seismic microzonation study helps define the basin edge effects that isolation bearings must accommodate.
The Red River Valley clays amplify long-period motion, so an isolation period of 3 to 4 seconds often yields the best drift reduction.
