The process of designing for earthquake loads has three important distinctions when compared to other structural loads.
- The designer accepts that damage to both non-structural and some structural elements will occur, but collapse is to be avoided.
- Earthquakes are highly variable dynamic events that designers can simplify into equivalent lateral loads .
- Earthquakes have a higher level of uncertainty than other forms of structural loading.
This simplifies design to a simple process of specific design and analysis but leaves a degree of uncertainty that makes it important to introduce a degree of robustness and redundancy into the design.
Working stress design
Prior to 1960, seismic design in New Zealand relied on creating structures that were strong enough to withstand the design earthquake. Designers applied a lateral seismic load that was taken as a fraction of the weight of the building. The structure was then designed so that the gravity load plus the seismic load was less than the total allowable load and the structure was assumed to behave elastically during a design-level earthquake. This design approach is known as the working stress method applied in seismic design. If the building was subjected to a greater than design-level earthquake, it was uncertain how it would behave because the elastic limit was exceeded.
What is a design earthquake?
A design earthquake is a theoretical earthquake event that modern building designers use to check the resilience of a new structure. It is impossible to create a completely earthquake-proof structure. However, a building may be engineered to withstand a design earthquake or at least behave in a predictable way if a design level event should occur. In limit state design, the design earthquake usually refers to the ultimate limit state .
In this section: