Introduction To Plate Girder Design

MEng4/MSc

Lectures 3 and 4: Introduction to Plate Girder Design 1 – Basic Principles

Introduction to Plate Girder Design 1 – Basic Principles Introduction The guidance given in these notes is based upon BS 5400: Part 3. Modern plate girders are fabricated by welding together two flanges and a web plate. (PG 1) Plate girders are used where standard rolled sections have insufficient load carrying capacity or stiffness. Typical uses include long-span floors in buildings, bridge girders and crane girders in industrial structures. Each plate girder is designed individually to resist the applied actions using proportions that ensure low self-weight and high load resistance. For efficient design it is common to use a relatively deep girder; this minimises flange area for a given applied moment. A deep girder also provides a deep web whose area may be minimised by reducing its thickness to the minimum required to carry the applied shear. Such a deep web may be quite slender (a high

dw ratio) and may be susceptible to shear tw

buckling and local buckling. Two alternative design approaches may be used: (i)

provide stiffeners to improve the load carrying resistance but with a consequent increase in fabrication cost.

(ii)

provide a thicker web which does not require stiffening but with a consequent increase in self weight.

Girder Types Plate girders may take several forms. (PG2) -

Unstiffened Transversely stiffened Transversely and longitudinally stiffened.

It is also possible to use different flange thicknesses (or breadths) to accommodate the variation in applied moment. (PG3) The web thickness may also be varied to accommodate the variation in applied shear 1 C:\My Structures\MEng4\Collings\Introduction to Plate Girder Design 1.doc

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MEng4/MSc

Lectures 3 and 4: Introduction to Plate Girder Design 1 – Basic Principles

force. The strength grade of the steel used for the flange and web plates may also be varied. This can allow strength to be strategically provided whilst minimising self weight. Plate girders which employ different steel strength grades are called ‘hybrid’ girders. It is possible to closely match the load carrying capacity of the girder to the applied actions and minimise self weight. Minimum weight design is not the most economic design approach in terms of fabrication and erection costs. A balance has to be achieved between excessive stiffening and excessive self weight. (PG4) The current trend is to minimise the provision of stiffeners since fabrication costs are very high in relation to materials costs.

Proportions (PG5) Depth (h):

Typically span/overall depth (h) = 15 to 25

Flange breadth (bfo): Typically

Flange thickness (tfo): Class 3

Class 2

Web thickness (tw): Class 2

h h ≤ b fo ≤ 5 3

(use 25 mm increments)

bfo ≤ 12ε t fo

ε=

355 σy

bfo ≤ 7ε t fo dw ≤ 56ε tw

(for the case of pure bending)

No limits for Class 3 or 4 In heavily stiffened webs,

dw may range from 200-500 in order to tw

minimise weight.

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MEng4/MSc

Lectures 3 and 4: Introduction to Plate Girder Design 1 – Basic Principles

Initial Sizing Under static design loading, the ULS (strength + stability) will govern most plate girder design. The SLS (deflection and vibration) is usually less critical. In addition to designing the girder to carry the loads in the finished structure, it is essential to provide adequate strength and stiffness for the erection and subsequent stages of construction. It will usually be necessary to cross-brace pairs of girders for example. A generally accepted method of initial design is to assume that the flanges carry all the applied moment and the web carries all the applied shear, at any particular section. (PG6) Flanges

-

moment

Web

-

shear

Influence of Buckling A plate girder comprises an assembly of relatively thin steel plates. Two design approaches may be used: (i)

keep the plates stocky and base the design on yield strength with no stiffening the penalty is a relatively high self weight.

(ii)

use thin, stiffened plates and minimise self weight - the penalty is a higher fabrication cost.

In practice a balance between the two approaches is used and the various forms of potential buckling may need to be identified and minimised by using stiffeners strategically. The various forms of buckling are: (PG7) -

Shear buckling of the web

Once the

dw ratio of the web exceeds a limiting value (Fig. 11 BS 5400 Pt.3) the web tw

will buckle before it reaches its shear capacity at yield ( A w .τ y ).

Shear buckling is characterised by diagonal buckles in the web. (PG8)

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MEng4/MSc

Lectures 3 and 4: Introduction to Plate Girder Design 1 – Basic Principles

The introduction of stiffeners improves shear capacity and delays the onset of buckling. The load at which the web buckles in shear is a function of both ratio

a . d

-

Lateral Torsional Buckling of the girder

-

Local buckling of the compression flange

Need to restrict -

dw and the aspect tw

b fo t wo

Compression buckling of the web

Compression buckling can arise due to compressive bending stresses or axial stresses or a combination of both. Check effective section for global analysis in the absence of axial loads to check whether the development of the full moment of resistance is possible. (cl. 9.4.2.5.1 BS 5400Pt.3). -

Flange induced web buckling

If the web is relatively thin it may not offer sufficient support to the flange. It is then possible for the flange to cause the web to buckle like an isolated strut. Transverse stiffness help resist this form of buckling. -

Local web buckling

A combination of vertical loading and overall bending action may cause local buckling failure in the web. By using appropriate

dw ratios and a combination of transverse and longitudinal tw

stiffeners this type of failure can be prevented.

Post Buckling Web Strength (PG9) After a web has buckled elastically in shear it can still carry load. The diagonal pattern of shear buckles allows the development of zones of tension called "tension fields".

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MEng4/MSc

Lectures 3 and 4: Introduction to Plate Girder Design 1 – Basic Principles

Transverse stiffeners allow the tension fields to produce a system of shear resistance that is analogous to an "N" truss.

Summary of Initial Design Considerations Flanges

- resist moment

Web

- resists shear

Web to flange welds

- resist longitudinal shear at interface

Transverse (vertical) stiffeners

- improve shear buckling resistance

Longitudinal (horizontal) stiffeners

- improve shear and bending resistance

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