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Technical GuideSingle Girder or Double Girder?
The differences between single-girder and double-girder overhead travelling cranes: capacity, span, lifting height, hook approach, cost and maintenance.
Where the difference between the two types comes from
On an overhead travelling crane, the bridge runs lengthwise on runways along both sides of the facility, and the trolley moves crosswise on the bridge. The split between single and double girder comes down to where the trolley runs.
On a single-girder crane there is one main girder, and the trolley hangs from its bottom flange. On a double-girder crane there are two main girders, and the trolley sits on top of them.
This one simple difference shapes the crane's whole character: its weight, the load it can carry, how high the hook can reach, and what can be added on top of it.
Lifting height: often the real deciding factor
On a single girder, because the trolley sits underneath, the hook can't rise above the girder's underside. On a double girder the trolley sits on top of the girders; the hook passes between them and can be pulled up much higher.
In practice, that means noticeably more lifting height from a double-girder crane at the same building height. In facilities with a limited ceiling that still need to lift long parts, the decision is usually made right here.
Which one, when
The table below isn't an absolute rule, it's a starting point. Borderline projects are decided by calculation.
| Criterion | Single girder | Double girder |
|---|---|---|
| Typical capacity | Low to medium capacity | Medium to high capacity |
| Span | Efficient at medium spans | Advantageous at wide spans |
| Lifting height | Limited by the girder's underside | Noticeably more |
| Crane weight | Light — low wheel loads | Heavy — high wheel loads |
| Maintenance platform | Usually not practical | Can be added |
| Auxiliary hook | Not practical | A standard option |
| Operator cabin | Not practical | Can be added |
| Initial investment | More economical | Higher |
| Fit to an existing building | Easy | Building statics must be checked |
These are typical tendencies; the final choice comes from calculating span, lifting height and duty class together.
The cost difference isn't just the crane's price
A double-girder crane is heavier; its wheel loads are higher. That means the runway and the building's load-bearing system also need to be stronger. If you're fitting a crane into an existing building, moving to double girder can sometimes trigger a building-strengthening cost.
Conversely, forcing a single girder at the borderline isn't cheap either: a crane running above its class fails often, and downtime cost overtakes the crane's price quickly.
Technical terms
| Term | What it means |
|---|---|
| Main girder | The girder that carries the load of the bridge. One in single-girder cranes, two in double-girder cranes. |
| End carriage | The beam at the ends of the main girder carrying the wheels and travel gearboxes. It's what lets the crane move on the runway. |
| Trolley | The unit that carries the hoist and moves crosswise on the bridge. |
| Auxiliary hook | A second, lower-capacity hoist alongside the main hook, used for light, fast work. |
| Bottom flange | The lower flange of an I- or H-profile. On a single-girder crane the trolley wheels run on it. |
Frequently asked questions
Can I convert a single-girder crane to double girder later?
In practice that's not a conversion, it's a new crane. The bridge, end carriages and trolley are all replaced; the runway and building loads are recalculated. If a capacity increase is on the horizon, starting with double girder from the beginning is almost always more economical.
Why is double girder more expensive at the same capacity?
It uses more steel, two main girders, a larger end carriage and a stronger travel drive. Its weight also raises the demand on the runway and the building.
What should I do if my building has a low ceiling?
There are two routes: move to double girder to lift the hook higher, or use a special low-headroom (underslung) trolley. Which one fits is decided by measuring the existing column height and the length of the part being lifted.