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stresses in service conditions, and thus, the serviceability requirements
such as deflections, cracking, slip, and deformations were not generally
critical.
3.7.3 Limit State Design Approach
By gaining better understanding of the behavior of different loads acting on
structures and material properties, which is accompanied by improved cal-
culation and construction techniques, limit state design approach replaced
the traditional allowable stress design approach in most current codes of
practice. Limit state design approach considers that the structure should sus-
tain all loads and deformations liable to occur during its construction, per-
form adequately in normal use, and have adequate durability. For most
structures, the limit states can be classified into two main states that are
the ultimate and serviceability limit states. The ultimate limit states are
related to a collapse of the whole or a substantial part of the structure.
On the other hand, the serviceability limit states are related to the disruption
of the normal use of the structure. Ultimate limit states should have a very
low probability of occurrence since they are considered failure situations.
Examples of ultimate limit states include loss of static equilibrium of a part
or the whole of the structure, loss of load-bearing capacity of a member due
to its material strength being exceeded or due to buckling, or a combination
of these two phenomena, or fatigue, and finally overall instability. While the
serviceability limit states depend on the function of the structures and for
bridges, they correspond to excessive deformation of the structure, or any
of its parts, affecting the appearance and functional use or drainage or causing
damage to nonstructural components like deck joints and surfacing; exces-
sive local damage like cracking, splitting, spalling, yielding, or slipping,
affecting appearance, use, or durability of the structure; and finally excessive
vibration causing discomfort to pedestrians or drivers.
3.7.4 Limit State Design Codes
To provide design methods in the current codes of practice achieving the
basic design requirements of structures, a reliability approach was commonly
adopted. Design values are determined such that they have a known statis-
tical probability of being achieved. The values of actions (loads) have a
known (low) probability of not being exceeded and the values for strength
have a known (high) probability level of being achieved. The design proce-
dure is then to model and evaluate the behavior of a structural model in
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