Plate Girders - Steel Structures.

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

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

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