This is the decision buyers spend the most time on and get the least clear advice about. One supplier says single girder because it is cheaper. Another says double girder because it is stronger. Both are answering a question you did not ask. The real answer comes from four numbers in your building: capacity, span, required lift height, and cycles per hour. Get those on paper and the choice usually makes itself. Below is the honest comparison — where each design wins, where each one quietly costs you, and a framework you can apply before your next quote.
The Basic Difference in One Paragraph
How the hoist mounts on each
A single girder crane has one bridge beam, and the hoist hangs from its bottom flange, travelling underslung. A double girder crane has two beams, and the trolley runs on rails laid on top of them. That is the whole structural difference. Everything else — lift height, capacity ceiling, cost, maintenance access — follows from it.
Why that one detail changes everything else
Hanging the hoist below the beam saves vertical space but puts the load off the beam’s shear centre, which introduces torsion. Running the trolley on top of two beams removes the torsion, raises the hook, and gives you a platform to walk on. Simple geometry, large consequences.
Side-by-Side Comparison Table
| Factor | Single Girder | Double Girder |
| Typical capacity | Up to 20 t | 5 t to 500 t+ |
| Practical span | Up to 25–30 m | Up to 40 m+ |
| Hook height under same roof | Lower | Higher |
| Headroom needed above rail | Less | More |
| Dead weight | Lighter | 25–50% heavier |
| Wheel loads on building | Lower | Higher |
| Purchase price | Baseline | 30–60% more |
| Maintenance walkway | Not standard | Standard on larger units |
| Suits continuous heavy duty | Moderate | Yes |
| Auxiliary hoist possible | Rarely | Yes |
Lift Height and Hook Approach
Where single girder gains headroom
Because the hoist hangs below the beam rather than above it, a single girder crane sits lower in the roof space. In low sheds — under about 6 m to the truss — this is often the deciding factor. You keep vertical clearance for the load instead of spending it on the crane.
Where double girder gains lift
Counterintuitively, the double girder usually gives you more usable hook height in a tall building. The trolley sits between the girders and the hook can rise into that space, and the hoist drum sits above the beam rather than below it. On a 9 m building, the difference in achievable lift is commonly 0.6 to 1.2 m — sometimes the exact margin that lets you lift a tall vessel clear of a machine.
Hook approach runs the other way. Single girder cranes typically get closer to the runway on the side approach because the underslung hoist has less bulk. If you need to serve machines hard against the wall, that matters.
Capacity and Span Limits
Practical single girder ceiling
Single girder cranes are built up to 20 tonnes and beyond, but economics turn against them well before that. Past roughly 12–15 tonnes, or spans over 25 m, the beam section needed to control deflection and torsion becomes so heavy that a double girder costs about the same and performs better.
When span forces the decision
Deflection scales sharply with span. A single girder at 28 m needs a very deep section to stay within limits, which eats the headroom advantage that justified choosing it. If your bay is over about 25 m, treat double girder as the default and ask the supplier to prove otherwise.
Duty Cycle and Service Life
Fatigue behaviour of each design
The torsional loading on a single girder is real and cyclical. On light and moderate duty it is nowhere near the fatigue limit. On continuous heavy duty it becomes the governing design case, and the weld details at the girder-to-end-carriage joint are where attention goes.
What continuous operation does to a single girder
Here is the pattern we see in service records: single girder cranes running two and three shifts at high average load develop end-carriage connection issues years before an equivalent double girder shows anything at all. It is not that single girder cranes are weak. It is that they are usually specified by buyers who chose them on price and left the duty class at the bottom option.
Cost — Purchase, Building, and Lifetime
The purchase price gap
A double girder crane of the same capacity and span typically costs 30–60% more, driven by extra steel, a heavier end carriage, a top-running trolley, and access walkways.
The hidden building cost
This is the part that flips the maths. A double girder crane is heavier, so wheel loads are higher, so runway beams and columns are heavier, so foundations are heavier. In a new-build shed that structural uplift can approach the price difference of the crane itself. In an existing shed it can mean strengthening work that was never in the budget.
Maintenance and downtime over ten years
Single girder cranes have fewer components and cost less to maintain per year. Double girder cranes cost more per service visit but need fewer emergency ones under heavy duty, and the walkway means routine work happens without hiring a boom lift every time.
Maintenance Access and Walkways
On a single girder crane, every service task needs access equipment from the floor, which means clearing the bay. On a larger double girder crane, the walkway between the girders lets a technician reach the hoist, festoon, and panel while production continues underneath. Over a fifteen-year life this quietly removes dozens of shutdowns.
Add-Ons Each Design Supports
What only a double girder can carry
- Auxiliary hoist for lighter, faster secondary lifts
- Cabin control for the operator
- Magnet or grab power packs mounted on the bridge
- Rotating or tilting below-the-hook devices
- Maintenance walkway and mainline panel on the bridge
If any of these are on your list, the decision is already made.
A Decision Framework You Can Apply Today
Choose single girder if…
- Capacity is under 12 tonnes
- Span is under 25 m
- Duty is light to moderate — one or two shifts, average load well below rated
- Headroom is tight
- Budget and building strength are both constrained
Choose double girder if…
- Capacity is over 15 tonnes
- Span is over 25 m
- You run continuous or heavy-duty cycles
- You need maximum hook height in a tall building
- You need an auxiliary hoist, cabin, magnet, or walkway
The grey zone and how to break the tie
Between 12 and 15 tonnes with a 20–25 m span, both work. Break the tie with duty class. If the crane runs more than one shift at above 50% of rated load, take the double girder. If it does not, take the single girder and spend the saving on a higher duty class and VFD controls.
Three Real Factory Scenarios
A 5-tonne crane in an 18 m fabrication bay, single shift. Single girder, comfortably. Spending on double girder here buys nothing except heavier foundations.
A 20-tonne crane in a 30 m machine shop, two shifts, heavy average load. Double girder without debate. Span and duty both rule out the alternative.
A 10-tonne crane in a 22 m shed with a 5.5 m roof, two shifts. The grey zone. Low headroom pushes towards single girder; two-shift duty pushes towards double. Resolve it by measuring required hook height against the load’s tallest item — if single girder clears it with margin, take single girder at Class III duty.
FAQs
Is a double girder crane always safer? No. Safety comes from correct design, correct duty class, and correct maintenance. A properly classed single girder crane within its limits is entirely safe. An under-classed double girder crane is not.
Can I convert a single girder crane to double girder later? Practically, no. The end carriages, runway loads, and often the building structure are all sized for the original design. A conversion costs more than a new crane and rarely gets approved.
Which one is quicker to install? Single girder, usually by a few days. It is lighter, needs smaller lifting equipment on site, and involves fewer assemblies at height.
Does double girder always give more hook height? In tall buildings, generally yes. In very low sheds, no — the extra structural depth above the rail can cost you more than the hoist arrangement gains. Measure your specific building rather than applying the rule.
What about spare parts availability? Both use standard hoists, motors, gearboxes, and drives. Availability depends on the hoist brand and your supplier’s stocking policy, not on the girder configuration.
Conclusion
Write down four numbers — capacity, span, required hook height, cycles per hour — and the answer is usually obvious within a minute. Where it is not, duty class breaks the tie. Do not let the choice be made by whichever supplier quoted first.
Heben Crane builds both, and recommends whichever the numbers support. Every proposal we send states the girder configuration with the reasoning behind it — capacity, span, deflection, duty class, and the wheel loads your building will have to carry.
Share your bay dimensions and duty cycle with Heben Crane and get a straight recommendation, not a default.
