Seismic design

This section gives the background to specific engineering design of lateral force resisting systems for timber buildings. When making specific engineering calculations, earthquake forces must be obtained directly from the standards. We also look at the effect the Christchurch earthquake has had on homes based on the preliminary report recently released.

NZ Wood has  an overview of the damage and a preliminary report on the performance of residential houses after the Canterbury earthquake on the 4th Spetember 2010 prepared by Professor Andy Buchanan from Canterbury University.
To read the full report and associated resources, visit the Christchurch/Darfield earthquake recovery rescources page here.

Image: Collapse of double brick veneer house at Homebush.

Specific engineering

All buildings must have resistance to lateral forces. Wind and earthquake forces are the main lateral loads that need to be considered.

Wind forces, which occur frequently, are developed by positive and negative pressures acting on the exposed surfaces of buildings during windy conditions.

Earthquake ground motion, which occurs infrequently, induces horizontal forces in a building as it responds to the ground motion.

Earthquake induced inertial forces increase as the weight of the building increases and as the stiffness of the structure increases. The earthquake design forces also depend on the level of structural ductility selected by the designer.

Timber structures are designed in accordance with NZS 3603, Code of Practice for Timber Design, which is written in “limit state” format.

The design actions and their combinations are prescribed in the Structural Design Actions Standard AS/NZS 1170, for both ultimate strength and serviceability limit states design.

Video of an earthquake simulation on a 6-level wooden building.

During a test in 2009 at Japan’s Hyogo Earthquake Engineering Research Center, researchers used an E-Defense shake table, the largest shake table in the world, to simulate an earthquake measuring 7.5 on the Richter scale. The six-level, million-pound wood condominium that was placed on the table remained standing, only suffering some minor cosmetic damage.

Key Points

• The equivalent static design forces for seismic design in timber are related to the weight of the building, the earthquake zone, the underlying soil, the stiffness of the building, and the structural ductility.

• The ductility of the lateral force resisting system affects the level of earthquake design forces.

• The vertical elements of a lateral force resisting system may consist of frames, cantilevers, diagonal bracing, or vertical shear panels.

• The horizontal elements of a lateral force resisting system are either horizontal diaphragms or diagonal bracing.

Seismic design