The Design of Modern Steel Bridges - TEDI

Chapter 4
Aims of Design
4.1 Limit state principle
The aim of design is that the structure should:
(1) sustain all loads and deformations liable to occur during its construction,
use and also foreseeable misuse or accident
(2) perform adequately in normal use
(3) have adequate durability.
When a structure or any of its components infringes one of its criteria for
performance or use, it is said to have exceeded a limit state. For most structures
the limit states can be placed in two categories:
(1) the ultimate limit states which are related to a collapse of the whole or
a substantial part of the structure
(2) the serviceability limit states which are related to disruption of the normal
use of the structure.
Ultimate limit states should have a very low probability of occurrence, since
they may cause loss of life, amenity and investment. The common ultimate
limit states are:
(1) loss of static equilibrium of a part or the whole of the structure considered
as a rigid body (e.g. overturning, uplift, sliding)
(2) 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
(3) overall instability, leading to very large deformation or collapse, caused
by, for example, aerodynamic or elastic critical behaviour or transformation
into a mechanism.
The serviceability limit states depend on the function of the structures; for
bridges they correspond to:
(1) excessive deformation of the structure, or any of its parts, affecting the
appearance, functional use or drainage, or causing damage to nonstructural
components like deck joints, surfacing, etc.
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