PLATE GIRDER
When span and load increases, the available rolled section may not be sufficient, even after strengthening with cover plates. Such situations are common in the following:
1. Large column free halls in the lower floor of a multistory building..
2. In a workshop, where girders are required to carry crane beams.
3. In road or railway bridges.
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Fig.1: Typical Plate Girder
Before welding technology advanced it was common practice to use riveted/bolted plate girders. Flange and web plates are connected to each other using angles and rivets/bolts. Many railway bridges of 24 m to 46 m were built like this. This practice of using riveted/bolted plate girder is given up in 1960's. Now-a-days only welded plate girders are built which are aesthetically good and at the same time light compared to riveted/bolted plate girders. Hence, in this chapter, design of only welded plate girder is discussed.
ELEMENTS OF PLATE GIRDERS
The following are the elements of a typical plate girder :
1. Web
2. Flanges
3. Stiffners
LEARN WITH 3D PRESENTATION
1.WEB
Web of required depth and thickness are provided to :
(a) Keep flange plates at required distances
(b) Resist the shear in the plate girder
2.FLANGE
Flanges of required width and thickness are provided to resist bending moment acting on the beam by developing compressive force in one flange and tensile force in another flange.
3.STIFFENERS
Stiffeners are provided to safeguard the web against local buckling failure. The stiffeners provided may be-classified as
(a) Transverse (vertical) stiffeners and
(b) Longitudinal (horizontal) stiffeners
Transverse stiffeners are of two types :
(i) Bearinglend stiffeners
(ii) Intermediate stiffeners
End bearing stiffeners are provided to transfer the load from beam to the support. At the end certain portion of web of beam acts as a compression member and hence there is possibility of crushing of web. Hence web needs stiffeners to transfer the load to the support. If concentrated loads are acting on the plate girder (may be due to cross beam) intermediate stiffeners are retired.
To resist average shear stress, the thickness of web required is quite less. But use of thin webs may result in to buckling due to shear. Hence when thin webs are used, intermediate transverse stiffeners are provided to improve buckling strength of web..
Many times longitudinal (horizontal) stiffeners are provided to increase the buckling strength of the web. If only one longitudinal stiffners is provided, it will be at a depth of 0.2 d from the compression flange where d' is the depth of the web. If another longitudinal stiffener is to be provided it will be at mid depth of the web.
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