Last reviewed 16 Sept 2026 · 5 min read
Why plate girders
When the bending moment or span exceeds what the heaviest rolled beams can carry (roughly beyond 20–25 m spans or large concentrated loads), a plate girder is built up from plates (and sometimes angles) to exactly the required proportions. Uses: railway and road bridges, gantry girders for heavy cranes, transfer girders in buildings, industrial platforms.
Components
- Web plate — deep and thin; carries shear and holds the flanges apart.
- Flanges — flange plates (welded girders) or flange angles with cover plates (riveted/bolted girders); carry most of the bending moment.
- Intermediate transverse stiffeners — prevent shear buckling of the web.
- Longitudinal (horizontal) stiffeners — in very deep webs, in the compression zone, to prevent bending buckling.
- Bearing (load-carrying) stiffeners — at supports and under concentrated loads.
- Connections — flange-to-web welds or rivets/bolts designed for horizontal shear flow; splices in web and flanges.
Proportioning
Economical depth
For a girder of given moment with web slenderness , minimising the total steel area gives approximately
Common practice: = 100–200 for welded girders; depth about 1/10 to 1/12 of span for simply supported girders (deeper for bridges).
At the economical depth, the web area roughly equals the combined flange area (about half the girder's material in the web).
Flanges
Approximately, the flanges carry the moment as a couple:
Flange outstand-to-thickness ratios must satisfy the section classification limits (plastic/compact/semi-compact) to avoid local buckling. For lateral stability the compression flange width is commonly about 1/40 to 1/30 of span (or restrained laterally).
Self-weight estimate
A common first estimate for the weight of a plate girder per metre is about of the total superimposed load (in consistent units) — refined after sizing.
Web slenderness limits (IS 800:2007)
Serviceability requirement:
- Without transverse stiffeners:
- With transverse stiffeners only: when ; when ; when
- With transverse and one longitudinal stiffener (at 0.2 from the compression flange):
- With a second longitudinal stiffener at the neutral axis:
Compression flange buckling requirement (web must be stiff enough so the compression flange does not buckle into it):
- Transverse stiffeners with :
- Transverse stiffeners with :
Shear buckling of the web
A thin web buckles in diagonal compression before yielding in shear.
Simple post-critical method
depends on web slenderness :
| ≤ 0.8 | |
| 0.8 to 1.2 | |
| ≥ 1.2 |
Elastic critical shear stress:
Closer stiffener spacing increases and shear strength.
Tension field method
After the web buckles, it continues to carry load through a diagonal tension field anchored by the flanges and stiffeners — the girder behaves like a Pratt truss with the web as tension diagonals and stiffeners as compression verticals. It gives higher shear capacity when stiffeners are provided at and the end panels are properly anchored.