Load and Resistance Factor Design (LRFD) Philosophy
The design of new structures and new
elements of rehabilitated bridge structures shall be completed with the use of
the AASHTO LRFD Bridge Design Specifications.
The LRFD bridge design philosophy is based
on the premise that four Limit States are stipulated to achieve the basic
design objectives of constructability, safety and serviceability. All Limit
States are given equal importance.
The four Limit States are:
Service Limit State: Stress, deformation
and crack width are limited under service conditions
Fatigue and Fracture Limit State: Fatigue
stress range is limited for the expected number of stress cycles due to a
single design truck in order to control crack initiation and propagation, and
to prevent fracture during the design life of the bridge.
Strength Limit State: Strength and
stability are provided to resist the significant load combinations that a
bridge is expected to experience in its design life.
Extreme Event Limit States: Structures are
proportioned to resist collapse due to extreme events, such as, major
earthquake, flood, ice flow, collision by a vessel, etc.
Equation 1.3.2.1-1 of the AASHTO LRFD
Bridge Design Specifications, unless otherwise specified, must be satisfied for
each Limit State:
Equation 1.3.2.1-1 of the AASHTO LRFD
Bridge Design Specifications, unless otherwise specified, must be satisfied for
each Limit State:
Where η = ηDηRηI ≥ 0.95
η = A factor relating to ductility, redundancy and
operational importance.
ηD = A factor relating to ductility
ηR = A factor relating to redundancy
ηI = A factor relating to importance
ɤI = Load factor: A statistically based multiplier
ɸ = Resistance Factor: A statistically based multiplier
QI = Force Effect
Rn = Nominal Resistance
Rr = Factored Resistance: ɸ Rn
Subsection 1.3 of the LRFD Specifications
may be referred to for additional commentary concerning the philosophy of the
Specifications’ development.
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