Ultimate Strength design
It is based on the ultimate strength
of the structural members assuming a failure condition, whether due to the
crushing of concrete or due to the yield of reinforced steel bars. In the
strength design method, actual loads or working loads are multiplied by load
factor to obtain the ultimate design loads. The load factor represents a high
percentage of factors for safety required in the design and the material
properties are kept same.
This design concept is based on
elastic theory and assuming a straight line stress distribution along the depth
of the concrete where the actual loads or working loads acting on the structure
are estimated. The allowable stresses are fractions of the crushing strength of
concrete (fc') and the yield strength (fy). Because of the differences in
realism and reliability over the past several decades, the strength design
method has displaced the older stress design method.
Limit state design
It is a further step in the strength
design method. It indicates the state of the member in which it ceases to meet
the service requirements, such as, losing its ability
to withstand external loads or local damage. According to limit state design,
reinforced concrete members have to be analyzed with regard to three limit states:
- Load carrying capacity (involves safety, stability and durability)
- Deformation (deflection, vibrations, and impact)
- The formation of cracks
The aim of this analysis
is to ensure that no limiting sate will appear in the structural member during
its service life.
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