Steel Designers Manual - TheBestFriend.org

This material is copyright - all rights reserved. Reproduced under licence from The Steel Construction Institute on 12/2/2007
To buy a hardcopy version of this document call 01344 872775 or go to http://shop.steelbiz.org/
Steel Designers' Manual - 6th Edition (2003)
Chapter 17
Plate girders
by TERENCE M. ROBERTS and RANGACHARI NARAYANAN
17.1 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 compressive
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: 2000 1 and BS 5400: Part 3 2 respectively. This chapter
explains current practice in designing plate girders for buildings and bridges; references
to the relevant clauses in the codes are made.
17.2 Advantages and disadvantages
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.
470