The National Building Code of Canada (NBCC 2020) sets clear performance objectives for seismic design, and in Burlington the site class often plays a defining role. Much of the city sits on glacial till and lacustrine deposits overlying the Queenston Shale, producing site class C and D profiles that amplify ground motion differently than the rock reference. Base isolation seismic design cuts the cord between ground shaking and structural response. Rather than strengthening the superstructure to resist forces, we insert horizontally flexible isolation units at the foundation level. These units elongate the fundamental period and concentrate deformation where it can be managed. For Burlington projects on softer lakeplain sediments near the Burlington Bay shoreline, the period shift must be tuned carefully to avoid resonance with the soil column. Our experience shows that pairing isolation with a site-specific seismic microzonation helps refine the spectral demand before finalizing bearing parameters. The approach works for new steel and concrete frames, and increasingly for retrofit of essential facilities that must remain operational after a design-level event.
An isolation system shifts the fundamental period past 2.0 seconds, cutting spectral acceleration by a factor of three or more compared to a fixed-base structure on Burlington's site class C soils.
