The structural difference
An EOT (electric overhead travelling) bridge crane spans the bay it operates in, supported on a runway at each side. The bridge — the part spanning the bay — is the structural component that carries the lifting load. Its configuration is what we're comparing.
A single-girder bridge uses one main beam, typically a wide-flange I-section. The trolley (carrying the hoist) runs along the lower flange of this beam. The hook hangs below the trolley.
A double-girder bridge uses two parallel main beams set apart. The trolley runs on rails fixed to the top of these beams. Because the trolley sits between the beams (not below), the hook can be raised higher relative to the runway level — gaining hook height for the same total bridge depth.
Side-by-side comparison
| Capacity range | Single-girder: 500 kg to ~15 t typical, up to 20 t with optimised design. Double-girder: 5 t to 100 t+ with custom engineering. |
| Span range | Single-girder: economical to ~20-22 m. Beyond this, deflection and weight push toward double-girder. Double-girder: economical from 5 m to 35 m+. |
| Hook height (under-hook to runway) | Single-girder: limited by trolley + hoist hanging below the beam. Double-girder: significantly more — trolley sits between beams, not below. |
| Duty class capability | Single-girder: A2-A5 typical, A6 with care. Double-girder: comfortable to A6-A8. |
| Bridge weight | Single-girder: lower — less steel, smaller end-carriages, lighter runway loads. Double-girder: higher — two beams plus crab structure. |
| Runway loads | Single-girder: lower wheel loads — runway can be lighter section. Double-girder: higher wheel loads — runway must be sized for it. |
| Cost (supply + install) | Single-girder: typically 20-40% less than equivalent double-girder. Double-girder: higher capex, but mandatory above ~15 t. |
| Maintenance access | Single-girder: trolley accessed from below. Double-girder: walkway between beams possible — full top access to drives, motors, hoist. |
| Auxiliary hoist | Single-girder: typically not feasible. Double-girder: easily accommodates auxiliary hoist on the same trolley or separate. |
When single-girder is the right answer
- Capacity 15 tonnes or under — most workshop and light-industrial applications
- Span 20 m or under — typical of smaller manufacturing bays
- Duty class A2–A4 — moderate use, not running near rated load all day
- Hook height not critical — you have enough clearance below the runway
- Cost-sensitive procurement — single-girder gets you the same lifting capability for less capex
- Existing building with light overhead structure — lighter crane = lighter runway = lower steel load on the building
The classic single-girder application: a 5 t bridge crane in a 15 m span fabrication shop running 30 lifts a day at A3 duty. Single-girder is unambiguously the right answer.
When double-girder is the right answer
- Capacity over 15 tonnes — single-girder structurally limited above this
- Span over 22 metres — single-girder deflection becomes problematic
- High duty class (A5–A8) — double-girder handles high-cycle better
- Maximum hook height required — gaining 0.5-1.5 m of under-hook height matters
- Auxiliary hoist needed — double-girder accommodates it
- Walkway needed for maintenance — double-girder supports a between-beam walkway
- Heavy industrial environments — steel mills, foundries, port handling, mining maintenance
The classic double-girder application: a 25 t double-girder in a 28 m span steel mill bay running A7 duty, with auxiliary 5 t hoist, walkway access, and the hook needing to clear coiled product on the floor. Single-girder isn't an option.
The grey zone — 10 to 15 tonnes
Capacities between 10 and 15 tonnes are where the engineering judgement comes in. Both configurations work. The decision usually comes down to:
- How tall does the lift need to go? If hook height matters, lean double-girder.
- What's the duty class? A5+ leans double-girder. A2-A4 can stay single-girder.
- Is auxiliary hoist needed now or in future? Future-proof with double-girder.
- What's the span? Over 20 m leans double-girder.
- What's the capex constraint? Tight budget pushes single-girder if it just fits.
This is exactly the kind of question we work through during engineering review at quote stage.
What this means when buying
- Don't over-spec. Many specifications get written as "double-girder" by default. If your application is 5 t at A3 in a 14 m span, single-girder saves significant cost without compromising performance.
- Don't under-spec. If you're at 15 t with high duty cycle, paying less for single-girder will cost more in shorter design life and earlier replacement.
- Get an engineering review. Ask the supplier to justify the configuration choice in writing. "We always do double-girder" or "we always do single-girder" is a sign of catalogue thinking, not engineering.
- Consider future requirements. Going from single to double-girder later is effectively a new crane. If capacity might grow, factor that into the original spec.
Talk to an engineer
Sorian designs both single and double-girder EOT bridge cranes — and we'll tell you which one suits your application based on the actual numbers. Send through your span, capacity, duty cycle and headroom and we'll come back with a configuration recommendation.
Frequently asked questions
What's the difference between a single girder and a double girder crane?
A single-girder crane uses one main beam with the trolley running along its lower flange. A double-girder crane uses two parallel beams with the trolley on top. Double-girder gives more hook height, better load distribution, and supports higher capacities and duty classes.
When do I need a double-girder crane?
Typically for: capacities over 10-15 tonnes, spans over 20 metres, high-duty applications (A5+), where maximum hook height is critical, where the crane needs maintenance walkways, or where auxiliary hoists are needed.
Which is cheaper — single or double girder?
Single-girder cranes are typically 20-40% cheaper than equivalent-capacity double-girder for capacities under 15 tonnes. Above that capacity, single-girder becomes structurally limited.
Can I retrofit a single-girder to a double-girder?
Generally no — runway, end-carriages, drives and electrical are sized for the original configuration. Upgrading is effectively a new crane. If future capacity increase is likely, design the original with that in mind.