Page 183 - Bridge and Highway Structure Rehabilitation and Repair

158 SECTION 2 STRENGTHENING AND REPAIR WORK

3. Type of surfacing.
4. Bearing conditions.
5. Railings.
6. Modeling of random live loads.
There are many possible varying locations and numbers of people on a bridge to be applied
and many ways to model them, all of which will offer different results. While one person walk-
ing across the bridge will cause minimal vibration, several individuals jumping on the deck can
create higher oscillations. A vibration analysis will be needed, especially for longer spans. Use
of tested computer software is required.

4.9.9 Definitions Based on Structural Behavior
Various physical parameters which represent theoretical concepts govern analytical results.
There should be no ambiguity about their definitions given below.

1. Redundant and non-redundant sections: As defined by the theory of indeterminate struc-
tures, use of a two-girder system or through trusses will cause the least redundancy or least
indeterminacy. The disadvantage is that the probability of failure is increased against envi-
ronmental loads. The formation of plastic hinge in a single girder in an indeterminate system
will only cause failure local to that girder. Other girders will be able to distribute moments
thereby offering combined resistance and avoiding failure. Hence, the greater the number
of girders, the higher the degree of redundancy. A minimum of four girders is permitted by
most states.
Similarly, a single span system will theoretically require only one plastic hinge to form
at the location of maximum positive moment (midspan) for failure. A continuous girder
system will require two or more plastic hinges to be formed at the locations of maximum
positive and negative moments. Hence, there is a need to adopt redundant systems.
2. Composite and non-composite sections: Use of shear connectors makes it possible for
composite action between the deck slab and the top flange of the girder. This provides a

tremendous advantage in preventing global buckling of the top flange. During construction

stages and until the full strength of concrete is achieved, the girder section will behave as
non-composite and will be designed as such.
3. Compact and non-compact sections: To prevent local buckling, the top fl ange needs to be
braced laterally by the composite slab. Flange shapes and sizes which meet long column
effect and local buckling requirements of slenderness and lateral support are called compact
sections.
The advantages of compact sections are that they are capable of generating full plastic
behavior leading to improved strength, with local buckling resistance capability compared
to the non-compact sections. Non-compact sections can develop yield moment, but only
partially develop a plastic response.
Non-compact sections are generally safe but are uneconomical compared to compact sec-
tions. Hence, it is important to try and achieve a compact section while sizing a plate girder.

One method of reducing local buckling in compression flange of a non-compact section is to
provide transverse diaphragms. Long column effect still needs to be checked for global buckling
for total compression flange length between bearings.

4.9.10 Selection of Cross Section/Shape for Steel and Prestressed Girders
For beam bending, an I-shape (wide flange section) or box section has maximum moment of

inertia and gives minimum deflection and bending stress compared to other open shapes, such