Engineering and Construction

Tweetear

The development of highly automated workshops in recent years has reduced the fabrication costs of plate girders very considerably; box girders and trusses still have to be fabricated manually, with consequently high fabrication costs. Optimum use of material is made, compared with rolled sections, as the girder is fabricated from plates and the designer has greater freedom to vary the section to correspond with changes in the applied forces. Thus variable depth plate girders have been increasingly designed in recent years. Plate girders are aesthetically more pleasing than trusses and are easier to transport and erect than box girders.

There are only a very few limitations in the use of plate girders. Compared with trusses they are heavier,more difficult to transport and have larger wind resistance.

The provision of openings for services is also more difficult. The low torsional stiffness of plate girders makes them difficult to use in bridges having small plan radius.

Plate girders can sometimes pose problems during erection because of concern for the stability of compression flanges.

Tweetear

Introduction
Plate girders are employed to support heavy vertical loads over long spans for which the resulting bending moments are larger than the moment resistance of available rolled sections. In its simplest form the plate girder is a built-up beam consisting of two flange plates, fillet welded to a web plate to form an I-section (see Fig. 17.1).

The primary function of the top and bottom flange plates is to resist the axial com- pressive and tensile forces caused by the applied bending moments; the main function of the web is to resist the shear. Indeed this partition of structural action is used as the basis for design in some codes of practice.

For a given bending moment the required flange areas can be reduced by increasing the distance between them.Thus for an economical design it is advantageous to increase the distance between flanges. To keep the self-weight of the girder to a minimum the web thickness should be reduced as the depth increases, but this leads to web buckling considerations being more significant in plate girders than in rolled beams.

Plate girders are sometimes used in buildings and are often used in small to medium span bridges. They are designed in accordance with the provisions contained in BS 5950: Part 1: 20001 and BS 5400: Part 32
respectively. This chapter explains current practice in designing plate girders for buildings and bridges; refer- ences to the relevant clauses in the codes are made

Fig. 17.1 Elevation and cross-section of a typical plate girder