Contents:

**Main plate girders**

The design criterion for main girders as used in buildings, was discussed in chapters on Plate Girders. In the following sections some additional aspects that are to be considered in the design of plate girders in bridges, are discussed.
Generally, the main girders require web stiffening (either transverse or both transverse and longitudinal) to increase efficiency. The functions of these web stiffeners are described in the chapters on plate girders.
Sometimes variations of bending moments in main girders may require variations in flange thickness to obtain economical design. This may be accomplished either by welding additional cover plates or by using thicker flange plate in the region of larger moment.
In very long continuous spans (span> 50 m) variable depth plate girders may be more economical. Initial design of main plate girder is generally based on experience or thumb rules such as those given below. Such rules also give a good estimate of dead load of the bridge structure to be designed.
For highway and railway bridges, indicative range of values for various overall dimensions of the main girders are given below:
Overall depth, D: l/18 **Fig.1 Common types of plate girder bridge**

**Detailed design of main plate girders in bridges**

The load effects (such as bending moment and shear force) are to be found using individual and un-factored load cases. Based on these, the summation of load effects due to different load combinations for various load factors are obtained. Since bridges are subjected to cyclic loading and hence are vulnerable to fatigue, redistribution of forces due to plastic mechanism formation is not permitted under BS 5400: Part - 3. The design is made based on Limit State of collapse for the material used considering the following:
- Shape limitation based on local buckling
- Lateral torsional buckling
- Web buckling
- Interaction of bending and shear
- Fatigue effect

**Shape limitation based on local buckling**

Depending on the type of cross section (compact or noncompact) the variation of stress over the depth at failure varies. A compact section can develop full plastic moment i.e. rectangular stress block as shown in fig.
Before the development of this full plastic moment, local buckling of individual component plates should not occur. Thus the compact section should possess minimum thickness of elements on the compression zone such that they do not buckle locally before the entire compression zone yields in compression.
The minimum thickness of elements for a typical compact section is shown in Fig, where fy is to be substituted in SI units (MPa).
**Design stresses**

**Shape limitations for plate girder**

The section that does not fulfill the minimum thickness criterion of compact section is defined as non-compact section. A non-compact section may buckle locally before full section plastic capacity is reached.
Therefore the design of such section is based on triangular stress block wherein yielding at the extreme fibre, as shown in, limit the design moment. The moment capacity of the compact and noncompact cross sections can be evaluated by the following formulae:
**Lateral torsional buckling**

A typical bridge girder with a portion of the span, over which the compression flange is laterally unrestrained, is shown in Fig Such a girder is susceptible to lateral torsional buckling. Fig shows a laterally buckled view of a portion of the span.
The displacements at midspan, where the beam is laterally restrained, will be only vertical, as shown in Fig A part of the beam between restraints can translate downwards and sideways and rotate about shear centre. Failure may then be governed by lateral torsional buckling.
This type of failure depends on the unrestrained length of compression flange, the geometry of cross section, moment gradient etc. The procedure in detail for calculating the value of the limiting compressive stress is given in chapters on laterally unrestrained beams.
**Lateral bracing for plate girders**

**Fig. Modes of instability of plate girders**