Plate Girders

Image source: Wikipedia

Introduction:
Plate girders are generally used in the plate girder bridges.
These are used when standard rolled sections do not provide the required section modulus. The image on the left shows a plate girder bridge. Building up a beam by riveting plates and angles increase the modulus of section. Plate girders are adopted specifically for short spans and heavy loads.
James Millholland built first tubular, wrought iron plate girder in  1846-47.

Components in a riveted plate girder:
a) web and Flange plates.
b) Flange angles.
c) Stiffeners
d) Splices.

Design of web and flange plates:
Web and flange plates are the main components of a plate girder and are designed to resist the shear and bending respectively.

(a) depth of Girders:
Approximate depth of girder is usually taken as 1/8 to 1/2 of the effective span of the girder.

(b) Width of flange: Width of flange would be within a range of L/40 to L/45.

(c) Deflection: Maximum deflection should not exceed 1/325 of the span.

Plate girder, source : wikipedia

Allowable Stresses: 

Upto 20mm web thickness :
 Bending stress = 1575 kg/cm2
Shear Stress = 945 kg/cm2

Over 20 mm thick web : Bending stress = 1500 kg/cm2 ; Shear Stress = 865 kg/cm2

Minimum thickness of the plates: Fr girders exposed to weather but accessible for painting, it is 6 mm; For girders exposed to weather but not accessible for painting, it is 8mm.


References:
1.Wikipedia
2. GATE 2013 civil Engineering - GK Publishers