LOADS ON BRIDGES: LIMIT STATES

Question 1

AASHTO codes intended to provide buildable and serviceable bridge capable of safely carrying design loads for specified life span

Answer 1

LIMIT STATES

Question 2

Bridge Life span

Answer 2

75 years

Question 3

❖ 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

Answer 3

LIMIT STATE

Question 4

TYPES OF LIMIT STATES:

Answer 4

STRENGTH LIMIT STATE
SERVICE LIMIT STATE
FATIGUE & FRACTURE LIMIT STATE
EXTREME EVENT LIMIT STATE

Question 5

▪Ensure that (local and global) strength and stability are
capable to resist statistically significant load combinations experienced during its design life

Answer 5

STRENGTH LIMIT STATE

Question 6

used to control stress, deformation, deflection,and crack width under normal service conditions to ensure structure serviceability

Answer 6

SERVICE LIMIT STATE

Question 7

Restrictions on stress range under regular service
conditions reflecting number of expected stress range
excursions

Answer 7

FATIGUE & FRACTURE LIMIT STATE

Question 8

Ensure structural survival of bridge component or system during rare events

Answer 8

EXTREME EVENT LIMIT STATE

Question 9

defines Safe operation and adequacy of structure under normal or extreme load condition

Answer 9

Strength

Question 10

Strength Defined by:

Answer 10

Yielding strength
 ultimate strength
 buckling
 overturning

Question 11

Defines performance and behaviour of structure under
nominal service condition

Answer 11

Serviceability

Question 12

Serviceability Defined by:

Answer 12

 Stress
 Fatigue
 Deflection
 Vibration
 Crack width

Question 13

AASHTO LRFD SPECIFICATIONS:

Answer 13

▪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

Question 14

▪Basic load combination
▪Relates to normal vehicular use of structure without
wind or any extreme event loads (Earthquake)
▪Mostly applied to control superstructure member

Answer 14

Strength I

Question 15

▪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

Answer 15

Strength II

Question 16

▪Used on bridges exposed to maximum wind velocity
Limit states
▪No live loads is assumed present on the bridge

Answer 16

Strength III

Question 17

▪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

Answer 17

Strength IV

Question 18

▪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

Answer 18

Strength V

Question 19

▪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

Answer 19

Service I

Question 20

▪Is for preventing yielding of steel due to vehicle live
load
▪Live load used is approximately halfway between
service I and strength I

Answer 20

Service II

Question 21

▪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

Answer 21

Service III

Question 22

▪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

Answer 22

Service IV

Question 23

▪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

Answer 23

Fatigue

Question 24

▪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

Answer 24

Extreme Event I

Question 25

▪Used for extreme conditions (ice load, collision by
vessels and vehicles)
▪Only one should be considered at a time
▪reduced live loads are considered

Answer 25

Extreme Event II