FIGURE 25.17 Lateral torsional buckling: (a) strong axis bending (torsional buckling) and (b) weak axis bending (no torsional buckling).

3. Area of flanges. After the sizes of the web are determined, the flanges are then designed. The flanges work mostly in bending and the required area is calculated approximately using equilibrium conditions imposed on the internal and external bending moments. A selection of strength for the steel material is principally made at this stage in the design process.

4. Width and thickness of flanges. The width and thickness can be determined by ensuring that the area of the flanges falls under the limiting width-to-thickness ratio, b/t (Figure 25.16), as specified in design codes. If the flanges are too thin (i.e., the width-to-thickness ratio is too large), the compression flange may buckle or the tension flange may be distorted by heat of welding. Thus, the thickness of both flanges must be checked. Since plate girders have little torsional resistance, special attention should be paid to lateral torsional buckling. To prevent this phenomenon, the compression flange must have sufficient width to resist "out-of-plane" bending. Figure 25.17 shows the lateral torsional buckling that may occur by bending with respect to strong axis.

After determining the member sizes, calculations of the resisting moment capacity are made to ensure code requirements are satisfied. If these fail, the above steps must be repeated until the specifications are met.

A few other important factors in the design of girder bridges will be explained in the following:

1. Web stiffeners. The transverse (vertical) and longitudinal (horizontal) stiffeners are usually needed (Figure 25.18) when a web is relatively thin. Bending moment produces compression and tension in the web, separated by a neutral axis. The longitudinal (horizontal) stiffener prevents bending buckling of web and is therefore attached to the compression side of the web (the top half for a simply supported girder). Since the bending moment is the largest near the midspan for a simply

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