A girder is a longitudinal structural member — running along a ship’s length, bow to stern — providing the fore-and-aft strength that resists bending as the vessel moves through waves.
Girders vs Frames: Perpendicular Partners
- Girders — run longitudinally (bow to stern), resisting fore-and-aft bending stress
- Frames — run transversely (side to side), giving the hull its cross-sectional shape
Both are essential and work together, perpendicular to each other, as the two main categories of the hull’s primary structural framework.
Hogging and Sagging: What Girders Actually Resist
As a ship moves through waves, its hull is subjected to bending forces:
- Hogging — the hull bends with the middle raised relative to the ends (as when a wave crest is amidships and troughs are at bow/stern)
- Sagging — the reverse, with the middle lower relative to the ends (wave trough amidships, crests at bow/stern)
Girders, particularly the deck and bottom girders at the extremes of the hull’s cross-section, are what primarily resist this bending — similar to how the top and bottom flanges of a large beam resist bending in that beam.
Common Girder Locations
- Deck girders — running along the underside of the deck
- Bottom girders — running along the ship’s bottom structure, often including the centerline girder directly beneath the keel, a particularly significant structural member
- Side girders — additional longitudinal strength members in the bottom structure, spaced across the width of the hull
Longitudinal Framing Systems Rely Heavily on Girders
Vessels using longitudinal framing (common on large tankers and bulk carriers, particularly in deck and bottom structure) rely on closely spaced girders as the primary strength members, with fewer, heavier transverse frames — the opposite emphasis from transverse framing systems.
Exam Relevance
Girder function, hogging/sagging stress, and the relationship between girders and framing systems are core Ship Construction topics in deck and engineering officer competency exams.