Structural Design Basic Requirements & Limit States
There are basic requirements that govern the design of a structure or its elements. These requirements are governed by the applicable code being put to use. Difference countries have their own applicable codes. Common codes used in Nigeria include the British Code and Eurocode.
Most codes have certain things in common but may vary by certain small degrees. While one code may be stiff and more conservative on certain requirements, another may be looser or more liberal.
The following are basic requirements of the Eurocode and British Standard in structural design.
- During its intended life, with appropriate degrees of reliability and in an economical way, sustain all actions and influences that are likely to occur during execution and use, and also remain fit for the intended use.
- Have adequate durability, serviceability and mechanical resistance
- In the event of life, have adequate resistance for the required period of fire exposure
- Not be damaged by accidents (e.g. explosion, impact and consequences of human error) to an extent disproportionate to the original cause.
The requirements above are self explanatory. This brings us to the subject of limit state.
In order to meet the above requirements, we employ limit states in structural design.
So what are limit states?
Limit states as the name implies, are states beyond which the structure no longer satisfies an agreed relevant design criteria.
Two limit states are commonly employed in civil engineering viz:
- Ultimate Limit State (ULS)
- Serviceability Limit State(SLS)
Ultimate Limit State (ULS): This is associated with collapse or failure of a structure. It corresponds to the maximum load carrying capacity of a structure or its elements and hence, govern the strength of the members or the structure. This also includes the loss of stability or equilibrium of the structure as a whole.
Serviceability Limit State (SLS): This is associated with the conditions beyond which specified service conditions of a structure or its members are no longer met. SLS may be reversible or not. It involves conditions like cracking, deformation and vibration which are conditions that can:
- Damage the structure or its elements (which may include finishes, partitions, and other members), or the contents of the buildings e.g. machinery and/or sensitive equipment.
- Cause discomfort to the occupants of the building
- Affect adversely, appearance, durability or water tightness.
