Jan 16, 20222 min read

Why is a wood structure better at withstanding an earthquake than steel?

Wood vs Steel

Wood buildings are lighter and less expensive to build. Based on current building codes wood structures are not more ductile than steel structures however their mass being lighter attracts less seismic load for the same ground motion than a typical steel structure. (Seismic force is related to mass and ground acceleration and some other inherent building dynamic parameters).

The mass factor alone can be its claim to being better in seismic-prone regions. Additionally, wood framing labor is considerably cheaper than steel framing which would require highly trained, union-based, erectors and welders working under strict schedules and safety protocol. Wood framing can be performed by anyone who’s been on the job no more than a month under proper supervision.

The typical building wood structure in California is what you would characterize as “wall stud framing”. The seismic resistance is provided primarily by plywood shear walls. Sometimes steel frames are added (Special Moment Frames) or Masonry Shear Walls. Structures built up to the 60s combined all kinds of systems. Afterward, the trend became in not mixing the seismic force-resisting systems (SFRS) much.

The steel structures are typically columns, beams, and girders with composite concrete metal decks. The SFRS is provided by a medley of systems, BRBF (Buckling Restrained Braced Frames, SCBF (Special Concentric Braced Frames), SMF (Special Moment Resisting Frames). In some cases Masonry shear walls (or even Concrete shear walls). Each provides benefits and drawbacks however the steel system’s ductility is beyond what can be achieved from a well-detailed wood building. The benefit of steel structures is that they permit the achievement of the architect's vision in a way that is more economical than say concrete or wood construction.

Each material offers its benefits but personally, I would much rather be in a wood building than a steel building during an earthquake. Having witnessed seismic tests on a shake table performed on a wood building I can attest to their resilience even after the failure of the seismic resisting system.

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The difference between Buckling, Compression & Shear