Plate girders gained popularity in the late 1800s when they were utilised in railroad bridge construction. To make plate girders of the desired size, angles and rivets were used to link the plates together. Welded plate girders had largely supplanted riveted and bolted plate girders in the developed world by the 1950s, owing to their superior quality, aesthetics, and cost. The cross sections of two common types of plate girder bridges are shown in.The use of plate girders for the two major girders rather than rolling beam sections allows the designer to choose the most cost-effective girder for the construction. If extensive embankment fills are necessary in the bridge approaches to meet the specified minimum headroom clearance, the half through bridge is preferable.
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Plate girders (main girders)
When the highest allowable approach gradient for the track is modest, this configuration is widely utilised in railway bridges. A moment resisting U-frame consisting of a floor beam and vertical stiffness, which are coupled together with a moment resisting joint, prevents lateral buckling of the compression flange in this scenario. A deck-type bridge, as shown, is a superior choice if the construction depth is not necessary, as the bracings offer restraint to the compression flange against lateral buckling. Plate girders (main girders)In the chapters on Plate Girders, the design criteria for main girders as employed in buildings were examined. Additional elements to consider in the design of plate girders in bridges are discussed in the following sections.
To boost efficiency, main girders typically require web stiffening (either transverse or both transverse and longitudinal). The functions of these web stiffeners are covered in detail in the plate girder chapters. To achieve an inexpensive design, variations in bending moments in major girders may need variations in flange thickness. This can be accomplished by welding extra cover plates or utilising thicker flange plate in the area of greater moment. Variable depth plate girders may be more cost-effective in very long continuous spans (> 50 m). The initial design of the main plate girder is usually based on experience or rules of thumb like the ones listed below. Such principles also provide a solid assessment of the bridge structure’s dead load.
Main plate girders in bridges: detailed design
The length between places of zero moment is denoted by l. The comprehensive design procedure can subsequently be carried out to maximise girder efficiency while satisfying strength, stability, stiffness, fatigue, or dynamic criteria, as applicable. Recent advancements in optimum design methodologies have made it possible to construct girder bridges directly while minimising weight and expense. Main plate girders in bridges: detailed design Individual and un-factored load scenarios are used to determine load effects (such as bending moment and shear force). On the basis of these, the sum of load effects due to various load combinations for various load factors is calculated. Because bridges are subjected to cyclic stress and hence are prone to fatigue, redistribution of forces owing to the production of plastic mechanisms is prohibited by BS 5400:
Local buckling limits the shape of the object. The change of stress over the depth at failure varies depending on the type of cross section (compact or non composite). A compact section, such as the rectangular stress block shown in, can develop a full plastic moment. Local buckling of individual component plates should not occur before the establishment of this entire plastic moment. As a result, elements on the compression zone of the compact section should have a minimum thickness so that they do not buckle locally before the entire compression zone yields in compression. shows the minimum element thickness for a typical compact section, where fy is to be substituted in SI units (MPa).
Lateral torsional buckling
Non-compact section is defined as a section that does not meet the compact section’s minimal thickness condition. Before the entire section plastic capacity is reached, a non-compact section may buckle locally. As a result, the design of such a section is based on a triangle stress block, with yielding at the extreme fibre limiting the design moment, as shown in. The following formulas can be used to calculate the moment capacity of compact and noncompact cross sections: The use of plastic modulus in the compact section does not indicate that plastic analysis with moment redistribution is applicable. Plastic analysis is not permitted under BS 5400: Part 3 and no moment redistribution is allowed. This is to avoid plastification during cyclic loading and the resulting low plastification rate.
A typical bridge girder with a section of the span over which the compression flange is laterally unrestrained. This girder is prone to lateral torsional buckling. A laterally buckled view of a part of the span is shown in Fig. The displacements at midspan, where the beam is laterally constrained, will only be vertical, as shown in. However, the beam between the restraints can translate downwards and sideways, as well as spin around the shear centre. Lateral torsional buckling may then be the determining factor in failure. The unconstrained length of the compression flange, the geometry of the cross section, the moment gradient, and other factors all influence this sort of failure. In the chapters on, the process for calculating the value of the limiting compressive stress is described in depth.
Modes of instability of plate girders
Plate girders have a high ratio of major axis to minor axis moment of inertia and a low torsional rigidity. As a result, when they bend around their primary axis, they are extremely vulnerable to lateral-torsional instability, as demonstrated in. During construction, adequate resistance to such instability must be supplied. The compression flange is normally stabilised by the deck in the finished structure. Technology Madras Distortional buckling, Fig, is a probable form of failure if the unconstrained flange is in compression, and such cases must be suitably braced. In order to provide lateral stability, lateral bracings are a system of cross frames and bracings situated in the horizontal plane at the compression flange of the girder.
Loads acting transversely on the plate girders also generate lateral bending, with wind loads playing a prominent role. Increased girder depth creates a wider surface area over which wind loads can act since plate girders can be very deep. In addition to creating lateral bending, this contributes to the girder’s compression flange’s instability. As a result, lateral bracing should be designed to account for this effect as well.
Plate Girder Bridges
To strengthen the lateral stability of the compression flange, triangulated bracing, as shown. is supplied for deck type plate girder bridges. However, it cannot be used on half-through or through girder bridges since it interferes with the bridge’s operations. The deck is designed as a horizontal beam in these circumstances.
Steel plate girder bridges are a versatile bridge type that can sustain a wide range of roadway structures, including train and highway traffic, so it’s no surprise that they’ve been one of the most popular bridge types since the late 1800s. Steel plate girder bridges are often utilised for spans of 100 to 400 feet. Top and bottom flange plates are welded to narrower web plates with fillet welds to form steel plate girders. Stiffened or un-stiffened web plates will be used.
The most important parameter for the design of steel plate girder bridges is the selection of near practical optimum web depth.
By using excel spreadsheet, the author compares the weight of the plate girder bridge designed.
A plate girder is typically an I-beam cross-section made up of separate structural steel plates which are welded.
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