Framework for the Design of Seismically Isolated Part 9 Structures

Despite the recent advances in structural engineering made over the last century, earthquakes continue to pose a major risk to the lives and livelihoods of many communities. British Columbia is well known as a region at risk from seismic hazards, however, significant seismic risk extends to Canadians across the country as 70% of Quebec’s population lives in active seismic regions. Most single-family residential structures in Canada are built using Part 9, Housing and Small Buildings, of the National Building Code of Canada (NBCC). Part 9 provides conservative simplified design and analysis methods that facilitate design without engineering involvement. It is generally believed that Part 9 single family wood frame residential structures are resistant to seismic effects, however, the aftermath of several earthquakes have demonstrated that, while life safety resilience is fairly good, the economic losses are unacceptably high. To protect against this danger to life and livelihood the NBCC provides several ways to design structures for seismic events. One of the most promising of these is an emerging technology known as base isolation. Base isolators are specially designed structural components that effectively act as a suspension system, isolating the structure from the ground motion effects. While this technology has proven effective at protecting structures and their occupants, the current design methodology often requires custom base isolator designs and comprehensive engineering. This creates a significant cost barrier that disincentivizes application of base isolation to common Part 9 designed single family residential structures that normally avoid engineering involvement.To eliminate this cost barrier to widespread adoption of base isolation, a program was developed to perform the engineering seismic design for a base isolated single family residential structure. Currently the NBCC does not provide a design methodology for base isolated structures, and so the program utilizes the ASCE design method adapted for NBCC requirements. The program performs the design of the base isolated structure using a catalogue of pre-certified isolators, and structural characteristics and seismic data available to non-engineer Part 9 designers. This makes the program usable by non-technical experts who will be aided through a series of recommendations and provisions. The program produces key design data and performance metrics such as base shear, maximum deflection, distribution of loads, and the number of isolators required and their placement. The program was validated by time history analysis of common single family residential structure designs. Elastomeric isolators were proposed for the case study structures, and discussions and recommendations for the design of elastomeric bearings for Part 9 structures are proposed. It was determined that the developed program provides a suitable representation of the design characteristics and seismic response of single-family residential structures.This research serves to address and remove many of the barriers which have prevented widespread adoption of base isolation in residential structures. Through the development of this design methodology base isolation will move towards becoming available for widespread use in Part 9 single family residential structures in vulnerable regions.

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