EAT 415 :ADVANCED STEEL BUILDING DESIGN PLATE GIRDER Prepared by : Shamilah Anudai@Anuar

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

Fabrication of Plate Girder

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

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

Main components of Plate Girder

Elevation and cross-section of a typical plate girder

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

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.

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).

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 1993-1-5) 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 1993-1-5, but the following values, taken from BS 5950-1, are recommended.

Minimum web thickness The following minimum web thickness values are prescribed in Clause 8(1) of BS EN 1993-1-5 to avoid the compression flange buckling into the web (i.e. flange induced web buckling).

Moment Resistance

Moment Resistance (cont’)

Shear resistance : Web not susceptible to shear buckling

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.

Shear buckling resistance

Shear buckling resistance (cont’)

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

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

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)

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

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

Buckling of slender web under shear Unstiffened web stiffened web

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.

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

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