Home Seismic design

Seismic Design Theory

This section contains information on Earthquake Design Forces, design combinations and Capacity Design.

Earthquake Design Forces

Modern structural design standards provide several methods of calculating seismic design forces.

This section is based on the equivalent static method, which defines a set of static lateral forces which ensure that the structure has adequate strength to resist the ground motion.

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

More sophisticated analysis methods, such as modal analysis, time-history analysis, or displacement-based design are required for more complex structures, and will require specialist attention on a case by case basis. Seismic design forces are specified in NZS 1170.5.

 

Design combinations

The combinations of structural forces specified in AS/NZS 1170.0 are very important for wind and earthquake design, because the strength and behaviour of the structure can be greatly affected by the superimposed gravity loads present at the time of the design event.

For seismic design, the likely superimposed weight must be added to the permanent weight at each floor level.

The overturning moment and the hold-down actions resulting from wind or seismic forces will depend greatly on the permanent and superimposed actions. Permanent and superimposed gravity loads are specified in AS/NZS 1170.1.

 

Capacity Design

The capacity design process requires that structures be designed so that ductile yielding takes place in specially designated parts of the structure, with all other parts having sufficient capacity to prevent inelastic deformations or structural failure.

In timber structures, ductility is usually obtained from non-linear behaviour of steel components in various types of connections, with these connections being designated as the weakest part of the structure.

It is unusual to design timber structures for high levels of structural ductility because the elastic displacements will often be a significant percentage of the drift limit imposed by the design code.