Page 242 - Bridge and Highway Structure Rehabilitation and Repair

CHAPTER 5 LOAD AND RESISTANCE FACTOR RATING AND REDESIGN 217

6. Service III for legal live loads is optional.
7. Both legal and permit live loads vary for each state as given in the state bridge design
manual.
8. Bridges need to be load posted if the requirements are not met.

5.6.2 Detailed Load Combinations
The author has detailed a large number of load combinations recommended in the LRFD

method. AASHTO LRFD specifications address a total of 14 load combinations:
1. Total strength limit state (5).
2. Total serviceability limit state (4).
3. Total fatigue and fracture limit state (1).
4. Total extreme events limit state (2); one each for earthquake and water loads (from fl oods).
Earthquake can be replaced by alternate extreme events such as ice, vehicle collision force,
and vessel collision force. Water load, flood, and scour may be classified as three separate

alternate load cases, earthquake as two cases, and construction as three separate cases as
detailed below. Selection of an extreme event such as a bomb blast, earthquake, scour, vessel
collision force, or ice load would primarily affect the substructure design. Piers and abut-
ments would be most vulnerable. The importance of a bridge being on a military route and
its proximity to a hospital or school also need to be considered.
5. Three independent alternate load combinations are added here for blast loads.
6. AASHTO Sections 3.4.2 and 3.6.1.3.2 recommend load combinations for

• Total deflection limit state (1)
• Total construction limit state (1).
Design moments and forces are based on an initial elastic analysis for each of the load combi-
nations on which load factors are applied. Magnitude of load, positions of loads, and all possible
combinations of loads that may occur in practice are covered. An envelope of maximum values
for girder, bearing, or pier design can be generated, thus covering all possible environmental
conditions that may be experienced during the life of the structure.

5.6.3 Limit State for Substructure (Abutment, Backwall, Bearing, Wingwalls, Pier)
Type of AASHTO primary and secondary loads and specified load factors, Table 5.5.

Permanent Loads
D Downdrag
C Dead load of structural components and nonstructural attachments
W Dead load of wearing surfaces and utilities
H Horizontal earth pressure load
S Earth surcharge load
V Vertical pressure from dead load of earth fi ll
Transient Loads Description LL Vehicular live load
BR Vehicular braking force LS Live load surcharge
CE Vehicular centrifugal force PL Pedestrian live load
CR Creep SE Settlement
CT Vehicular collision force SH Shrinkage
CV Vessel collision force TG Temperature gradient
EQ Earthquake TU Uniform, temperature
FR Friction WA Water load and stream pressure
IC Ice load WL Wind on live load
IM Vehicular dynamic load allowance WS Wind load on structure
Note: The above notations and abbreviations are adopted by AASHTO (see Table 5.5).