SOME IMPORTANT DEFINITIONS:
Load: It is the effect of acceleration, including that due to gravity, imposed deformation or volumetric change.
Nominal Load: An arbitrary selected design load level.
Load Factor: A coefficient expressing the probability of variations in the nominal load for the expected service life of the bridge.
Permanent Loads: Loads or Forces which are, or assumed to be, constant upon completion of construction.
Force Effect: A deformation or a stress resultant, i.e. thrust, shear, torque/moment, caused by applied loads, imposed deformation or volumetric changes.
LIMIT STATES
“A limit state is a condition beyond which a structural system or structural component cease to full-fill the function for which it is designed”.
Bridges are designed for specified limit states to achieve the objectives of constructibility, safety and serviceability.
Generally the limit states that are considered in bridge design are;
- Service limit state
- Fatigue and fracture limit state
- Strength limit state
- Extreme Event limit state
Service Limit State
This limit state refers to restrictions on stress, deflections and crack widths of bridge component that occur under regular service conditions, There are three limit conditions given in the table to cover different design situations.
Service I: This service limit state refers to load combination relating to the normal operational use of the bridge with 90 km/h wind.
Service II: This service limit state refers to the load combination relating to steel structures and is intended to control yielding and slip of slip critical connections.
Service III: This service limit state refers to the load combination relating only to tension in pre-stressed concrete structures with the objective of crack control.
Fatigue and
This limit state refers to restrictions on range caused by a design truck. The restrictions depend upon the stress range excursions expected to occur during the design life of the bridge.
This limit state is used to limit cracks growth under repetitive loads and to prevent fracture due to cumulative stress effects in steel elements, component, and connections.
For the fatigue and fracture limit state, Ф = 1.0
Since, the only load that causes a large number of repetitive cycles is the vehicular live load; it is the only load effect that has a non-zero load factor.
Strength Limit State
This limit state refers to providing sufficient strength or resistance to satisfy the inequality
ФRn ≥ η ∑ γi Qi
The statically determined resistance factor Ф will be less than 1.0 and will have values for different materials and strength limit states.
Strength-I: This strength limit is the basic load combination relating to the normal vehicular use of the bridge
without wind.
Strength-II: This strength limit is the basic load combination relating to the use of the bridge by permit vehicles without wind.
Strength-III: This strength limit is the basic load combination relating to the use of the bridge exposed
to wind velocity exceeding 90 km/h.
Strength-IV: This strength limit is the basic load combination relating to very high dead load/live load force effect ratios.
Strength-V: This strength limit is the basic load combination relating to the normal vehicular use of the bridge with wind of 90 km/h velocity. It differs from the Strength-III limit state by the presence of the live load on the bridge, wind on the live load and reduced wind on the structures.
Extreme Event Limit State
This load effect refers to the structural survival of a bridge during a major earthquake or floods or when collided by a vessel, vehicle, or ice flow. These loads are specified to be applied separately, as the probability of these events occurring simultaneously is very low.
Extreme Event-I: This extreme event limit state is the load combination relating to earthquake. This limit state also includes water load and friction.
Extreme Event-II: This extreme event limit state is the load, to ice load, collision by vessels, vehicles and to certain hydraulics events with reduced live loads.
BEHAVIOR OF BRIDGE SUPER STRUCTURE
The typical bridge superstructure consists of the deck slab placed on girders spaced at a certain interval and placed parallel to the direction of traffic flow. The centre to centre spacing of the girders and the slab thickness are of main concern in predicting the response of the bridge under vehicle loads.
Load Transfer Path
The vehicle moves over the deck slab, which distributes the load to the girders, and the girders in turn transfer the load to abutments.
Distribution Factor
The fraction of load transferred to each girder is called the distribution factor for that girder. The distribution factor is a function of proximity of the load to the girder and relative stiffness of deck and girder. In general, the girder near to the load will support greater fraction of the load hence having a greater distribution factor, while the girder away from the load will support a smaller fraction of load and thus having a smaller distribution factor.
Effect of High Relative Stiffness
The relative stiffness of girder and deck plays an important role in distributing the load to the girders. A relatively thin slab, resulting in a higher relative stiffness of girder to deck will not distribute the load evenly to all girders. The demand on the girders will be highly localized, i.e., the closest girder will support a significant fraction of load, resulting in higher deflections. Such a system utilizes the structural system inefficiently. It reduces the amount of concrete in the deck, thereby saving some cost and reducing the self weight of deck, but on the other hand, induces a higher live load demand on the critical girder.
Note that this not only increase demand on the deck but also on the critical girder, while other girders are contributing less to support the load. This demonstrates inefficient utilization of structural system.
Effect Of Low Relative Stiffness
The relatively thick slab results in a low relative stiffness of girder to deck and will distribute the load more evenly to all girders. The demand on the girders will be more uniform, I.e. all girders will support almost same fraction of load, resulting in lower deflections. Such a system utilizes the structural capacity efficiently. Although, it increases the amount of concrete in the deck, thereby increasing cost of concrete and increasing self weight of deck, but on the other hand, requires a lighter girder section due to reduced distribution of live load demand on the critical girder. Note that this not only decreases demand on the deck but also on the critical girder, while other girders are also contributing equally to support the load. This demonstrates efficient utilization.
CATEGORIZATION OF BRIDGES ACCORDING TO MATERIAL OF CONSTRUCTION
- Steel bridges
- Concrete bridges
- Hybrid bridges
- Wood bridges
- Stone / Brick bridges
CATEGORIZATION OF BRIDGES ACCORDING TO SPAN
1.Small Span Bridges (Up to 15m)
- Culvert bridges
- Slab bridges
- T-Beam bridges
- Wood Beam bridges
- Pre-cast concrete Box Beam bridges
- Pre-cast concrete I-Beam bridges
- Rolled steel Beam bridge
2.Medium Span Bridges (Up to 50m)
- Pre-cast concrete Box Beam bridges
- Pre-cast concrete I-Beam bridges
- Composite Rolled Steel Beam bridges
- Composite Steel Plate Girder bridges
- Cast in RCC Box Girder bridge
- Cast in Place Post-Tensioned Concrete Box Girder
- Composite Steel Box Girder
3.Large Span Bridges (50m to 150m)
- Composite Steel Plate Girder bridges
- Cast in place Post-Tensioned Concrete Box Girder
- Post Tensioned Concrete Segmental Construction
- Concrete Arch and Steel Arch
- Cable Stayed bridges
- Suspension bridges
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