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Last edited 04 May 2018
Limit state design
A degree of loading or other actions imposed on a structure can result in a ‘limit state’, where the structure’s condition no longer fulfils its design criteria, such as; fitness for use, structural integrity, durability, and so on. Limit states are conditions of potential failure.
LSD involves estimating the subjected loads on a structure, choosing the sizes of members to check, and selecting the appropriate design criteria. LSD requires two principal criteria to be satisfied: the ultimate limit state (ULS) and the serviceability limit state (SLS).
 Ultimate limit state (ULS)
The ultimate limit state is the design for the safety of a structure and its users by limiting the stress that materials experience. In order to comply with engineering demands for strength and stability under design loads, ULS must be fulfilled as an established condition.
The ULS is a purely elastic condition, usually located at the upper part of its elastic zone (approximately 15% lower than the elastic limit). This is in contrast to the ultimate state (US) which involves excessive deformations approaching structural collapse, and is located deeply within the plastic zone.
If all factored bending, shear and tensile or compressive stresses are below the calculated resistances then a structure will satisfy the ULS criterion. Safety and reliability can be assumed as long as this criterion is fulfilled, since the structure will behave in the same way under repetitive loadings.
 Serviceability limit state (SLS)
The servicability limit state is the design to ensure a structure is comfortable and useable. This includes vibrations and deflections (movements), as well as cracking and durability. These are the conditions that are not strength-based but still may render the structure unsuitable for its intended use, for example, it may cause occupant discomfort under routine conditions.
SLS requirements tend to be less rigid than strength-based limit states as the safety of the structure is not in question. A structure must remain functional for its intended use subject to routine loading in order to satisfy SLS criterion.
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