LOADS ON BRIDGES: LIMIT STATES Flashcards
AASHTO codes intended to provide buildable and serviceable bridge capable of safely carrying design loads for specified life span
LIMIT STATES
Bridge Life span
75 years
❖ A condition which represents the limit of “structural usefulness”
(AISC LRFD, 1968)
❖ A boundary between desired and undesired performance of structure (Nowak and Collins)
❖ A condition beyond which the bridge or component ceases to satisfy the provisions for which it was designed
LIMIT STATE
TYPES OF LIMIT STATES:
STRENGTH LIMIT STATE
SERVICE LIMIT STATE
FATIGUE & FRACTURE LIMIT STATE
EXTREME EVENT LIMIT STATE
▪Ensure that (local and global) strength and stability are
capable to resist statistically significant load combinations experienced during its design life
STRENGTH LIMIT STATE
used to control stress, deformation, deflection,and crack width under normal service conditions to ensure structure serviceability
SERVICE LIMIT STATE
Restrictions on stress range under regular service
conditions reflecting number of expected stress range
excursions
FATIGUE & FRACTURE LIMIT STATE
Ensure structural survival of bridge component or system during rare events
EXTREME EVENT LIMIT STATE
defines Safe operation and adequacy of structure under normal or extreme load condition
Strength
Strength Defined by:
Yielding strength
ultimate strength
buckling
overturning
Defines performance and behaviour of structure under
nominal service condition
Serviceability
Serviceability Defined by:
Stress
Fatigue
Deflection
Vibration
Crack width
AASHTO LRFD SPECIFICATIONS:
▪Strength I
▪Strength II
▪Strength III
▪Strength IV
▪Strength V
▪Service I
▪Service II
▪Service III
▪Service IV
▪Fatigue
▪Extreme Event I
▪Extreme Event II
▪Basic load combination
▪Relates to normal vehicular use of structure without
wind or any extreme event loads (Earthquake)
▪Mostly applied to control superstructure member
Strength I
▪Used for owner-specified special design vehicles or
permit vehicles
▪No wind or any extreme event loads considered
▪Not commonly used
▪Reduced dynamic load allowance may be allowed
Strength II
▪Used on bridges exposed to maximum wind velocity
Limit states
▪No live loads is assumed present on the bridge
Strength III
▪Used for structures with very high dead-live load force
effect ratios
▪Control load combination for certain elements if
▪structure has long span length and/or large dead
load
▪Bridges under construction
▪Make sure that various type of bridges have similar
failure probability
Strength IV
▪Relate to normal vehicular use of bridge with wind
velocity of 55mph (90kph)
▪Live and wind load combined – both values reduced
because probability is very low to experience very
heavy live load and extremely high wind load
Strength V
▪Used for normal operational use with 55mph wind
▪All loads are taken at nominal values
▪Extreme loads are excluded
▪Used to control deflection, crack width (for RC),
compressive stress (prestress), and soil slope
stability
Service I
▪Is for preventing yielding of steel due to vehicle live
load
▪Live load used is approximately halfway between
service I and strength I
Service II
▪Relates only to tension in prestressed concrete
superstructure
▪Load factor of 0.80 is applied to live load → If nominal
design live load is used, superstructure is overdesigned
for concrete tensile areas
Service III
▪Relates only to tension in prestressed concrete
substructure to control cracks
▪Load factor of 0.70 for wind (represents 84mph)
▪Reflects probability of PC substructure will be
subjected to tensile stress once every 11years
Service IV
▪Fatigue and fracture load combinations
▪Relate to repetitive gravitational vehicular live load
and dynamic responses
▪Live load factor of 0.75 represents cumulative effects
of majority truck population
▪Only a single 32k loads (30ft constant spacing) is
applied
Fatigue
▪Related to earthquake
▪Live load considered shall be based on daily traffic
volume of bridge
▪Normal bridge: Live load Factor, 0.5
▪Implies low probability of maximum live load at
time of large earthquake may occur
Extreme Event I
▪Used for extreme conditions (ice load, collision by
vessels and vehicles)
▪Only one should be considered at a time
▪reduced live loads are considered
Extreme Event II
