Limit state design

The modern approach to the structural design of buildings requires consideration of all loads that could potentially act on the building over its life. These range from those that the building is likely to be subjected to frequently, through to very rare events such as severe earthquakes. It is recognised that the building can perform less reliably the lower the likelihood of the load .

While the working stress design method is appropriate for in-service loads, it cannot deal well with large events. In these events, it is recognised that deformations in the building that are well beyond yield can be tolerated. In many situations, they are a necessary part of the justification for a building to survive the event. This has led to the development of the limit state design method where various limit states are defined and checked as part of the design process.

A limit state is a condition of a building beyond which it no longer meets defined design criteria. The condition may refer to a degree of loading or other actions on the structure. The criteria refer to provisions that deal with structural integrity, fitness for use, durability, serviceability or other design requirements.

Two basic limit states are defined by AS/NZS 1170 Structural design actions – the serviceability limit state (SLS) and the ultimate limit state (ULS).

Serviceability limit state (SLS)

The SLS represents a level of stress or strain within the building below which there is a high expectation the building can continue to be used as originally intended without repair. As a consequence, the limiting level of stress or strain defined for this limit state is low.

This is a state that is expected to be reached several times in the life of most buildings in New Zealand. As a result, the level of design load that is used to check the SLS design criteria is relatively low. For example, the load used to check the SLS for seismic conditions for importance level 2 (IL2) buildings is the defined 1-in-25-year earthquake shaking. 1-in-25-year shaking is expected to be exceeded over a long period, on average, once every 25 years.

Other serviceability limit states have been defined for buildings that are required to be functional in the post-earthquake period, for example, fire stations, hospitals and the like. These limit states represent a level of stress and strain in the building and associated loads that, when considered together, are expected to be consistent with an ongoing operational state. There is an expectation that return to a full operational state may take minutes to hours rather than days.

Ultimate limit state (ULS)

Design for the ULS represents a defined process that is aimed at ensuring the probability of collapse of a building (and therefore the risk to human life) is at an acceptable level. The ULS process is therefore primarily associated with consideration of large (severe), relatively rare events.

In AS/NZS 1170, compliance for the ULS for typical buildings (those with a design life of 50 years) is confirmed using a single level of load based on between a 1-in-100 and 1-in-2500-year event. This is dependent on the assigned IL and the particular environmental effect under consideration. For example, the design seismic load used for ULS checks for a typical IL2 building is based on the defined 1-in-500-year earthquake shaking. These loads are consistent with those used internationally, which is the reason they have been adopted in New Zealand. The ULS design criteria, when checked using the defined load, are set to provide the required level of confidence that the life safety objective has been met across all rare events.