Limit state design

[edit] What is limit state design

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.

Limit state design (LSD) is a structural engineering design method. All actions likely to occur during a structure’s design life are considered to ensure that the structure remains fit for use with appropriate levels of reliability. Limit state design involves estimating the subjected loads on a structure, choosing the sizes of members to check, and selecting the appropriate design criteria.

Limit state design requires that two principal criteria are satisfied:

• Ultimate limit state (ULS).
• Serviceability limit state (SLS).

[edit] Ultimate limit state (ULS)

Ultimate limit state (ULS) is design for the safety of a structure and its users by limiting the stress that materials experience.

The ultimate limit state 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.

BS EN 1990 Eurocode – 'Basis of structural design' describes four ultimate limit states:

• EQU: Loss of static equilibrium of the structure.
• STR: Internal failure or excessive deformation of the structure.
• GEO: Failure or excessive deformation of the ground.
• FAT: Fatigue failure of the structure.

[edit] Serviceability limit state (SLS)

Servicability limit state (SLS) is 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. It might also involve limits to non-structural issues such as acoustics and heat transmission.

Servicability limit state 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.

[edit] Related articles on Designing Buildings

• Adaptive structures.
• Anticlastic structures.
• Biaxial bending.
• Braced frame.
• Building science.
• Concept structural design of buildings.
• Defects in construction.
• Lateral loads.
• Material utilisation (MUT).
• Shear force.
• Structural engineer.
• Structural steelwork.
• Structural vibration.
• Synclastic.
• Types of structural load.

[edit] External resources

• BGStructural Engineering - LSD
• Handbook of structural steelwork, Eurocode Edition, 2013.

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