1. EAT 415 :ADVANCED STEEL BUILDING DESIGN PLATE GIRDER
Prepared by :
Shamilah

2. Plate girder with haunches, tapers and cranks
Introduction
Plate girders are fabricated sections employed to support heavy vertical loads
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.
Plate girder with haunches, tapers and cranks

4. Fabrication of Plate Girder

5. Types of Plate Girder Connection of flange to web by welding
Welded Plate girder

6. Types of Plate Girder (cont’)
Splicing of plate girder by bolting
Bolted Plate girder

7. Main components of Plate Girder

8. Elevation and cross-section of a typical plate girder

9. Principal Functions of main components in Plate Girders
Flange
Resist moment
Web
Resist Shear
Web/Flange Welds
Resist longitudinal shear at interface
Vertical Stiffeners
Improve Shear Buckling Resistance
Longitudinal Stiffeners
Improve shear & bending resistance

10. Initial choice of cross-section for plate girders : Span-to-depth ratios
Advances in fabrication methods allow the economic manufacture of plate girders of constant or variable depth.
Traditionally, constant-depth girders have been more common in buildings.
However, this may change as designers become more inclined to modify the steel structure to accommodate services.
Recommended span-to depth ratios are given in Table 1.

12. maximum depth-to-thickness ratio of the webs (cw / tw) of plate girders in buildings is usually limited to:
The outstand width-to-thickness ratio of the compression flange (cf / tf) is typically limited to:
Where fyw and fyf is web and flange strength.
For non-composite girders the flange width is usually within the range 0.3–0.5 times the depth of the section (0.4 is most common).

13. Design of plate girders to BS EN 1993-1-5 : Dimensions of webs and flange
A minimum web thickness is needed to prevent the compression flange buckling into the web (see Clause 8(1) of BS EN ) and to ensure satisfactory serviceability performance including, for example, avoiding unsightly buckles developing during erection and in service.
The buckling resistance of slender webs can be increased by the provision of web stiffeners. In general, the webs of plate girders used in buildings are either unstiffened or have transverse stiffeners only.
Minimum web thickness values to avoid serviceability problems are not prescribed in BS EN , but the following values, taken from BS , are recommended.

15. Minimum web thickness
The following minimum web thickness values are prescribed in Clause 8(1) of BS EN to avoid the compression flange buckling into the web (i.e. flange induced web buckling).

16. Moment Resistance

17. Moment Resistance (cont’)

18. Shear resistance : Web not susceptible to shear buckling

19. Shear resistance : Web susceptible to shear buckling
Webs of slender proportions (i.e. height-to-thickness ratio exceeding specified limiting values) are susceptible to shear buckling.

21. Shear buckling resistance

23. Shear buckling resistance (cont’)

24. Design Procedure Calculation of loading
Design the shear force and bending moment diagrams
Initial sizing of plate girder
Cross section classification
Dimension of web and flanges
Bending moment resistance
Shear Buckling resistance

25. Example 1
The fully restrained plate girder is showed below. The span is 36m and carries two concentrated loads. Design a plate girder
36000 mm

26. Example 2
A simply supported plate girders shown in Figure below is laterally restrained throughout its length subjected to the loading tabulated below.
Check the classification of the web and flange of the plate girder
Check the moment capacity of the section assuming all bending taken by the flanges
Check the buckling resistance of the stiffener at F by assuming flange restraint against rotation in the plate of the stiffener (use 2/200mm x 12mm)

27. Unfactored imposed load
Unfactored dead load
Unfactored imposed load
Uniformly distributed load, UDL (kN/m) 15 30
Point Load, PL (kN)
170
260

28. FAILURE TYPES OF PLATE GIRDER
Shear buckling of web
compression buckling of web
Lateral-torsional buckling of girder
Flange induced buckling of the web
Local buckling of web due to vertical load
Local buckling of compression flange

29. Buckling of slender web under shear
Unstiffened web
stiffened web

30. THE IMPORTANT OF STIFFENERS
Stiffeners strengthened a very thin web of plate girder which may buckle laterally or cripple under the heavy concentrated load.
Normally, depth of plate girder is design to be large for economic reason and it is made thin to reduce the self weight of the girder.

31. EXAMPLE 3- Design of stiffeners
The stiffeners are spaced as shown in Figure below. The spacing of stiffeners is taken as 3000mm. The spacing can be increased towards the centre of the span for economy reason.
Check critical shear strength, qcr
Since panel AB is the most critical panel (maximum shear zone), check for moment capacity of end panel AB
Check stiffener against buckling

32. THANK YOU