Capacity is the first filter between a single girder and a double girder crane, but it is rarely the one that decides. Plenty of requirements sit in the range where both configurations are technically possible — and there the decision comes down to hook height, what your existing structure can carry, and what you need to hang off the crane.
The basic difference
A single girder crane runs an electric wire rope hoist on the lower flange of one bridge girder. Fewer members, less steel, lower dead weight.
A double girder crane carries a crab trolley on rails on top of two girders. More steel, more capability, more weight.
Where single girder wins
Lower wheel loads on your existing structure
This is the argument that decides more projects than any other. A single girder crane weighs less, so it transmits lower wheel loads into the gantry girder and the building columns.
If you are putting a crane into an existing building, the structure you already have sets a wheel load budget. Coming in under it means you install a crane. Going over it means you strengthen the structure first — and that civil work often costs more than the crane.
Cost, at light and moderate duty
Below roughly 20 tonnes and at M3 to M6 duty, a single girder crane does the same job for less money and less installed weight.
Where double girder is the only answer
Hook height
On a double girder crane the crab sits on top of the girders, so the hook can be raised much closer to the underside of the roof. On a single girder crane the hoist hangs below the girder, and that difference is lost headroom on every lift.
If your building clearance is fixed and you need maximum lift, double girder recovers height that single girder cannot.
Capacity and duty
Above about 20 tonnes, or at M7 to M8 continuous duty, single girder stops being viable. The girder section required becomes uneconomic and the hoist arrangement will not carry the fatigue.
Anything you need to mount on the crane
Walkways, an operator cabin, magnet or grab attachments, a second auxiliary hoist, festoon systems for heavy power feed — all of these need the deck and the structural capacity that two girders provide.
A practical way to decide
Work through it in this order:
- Capacity and duty class. Above 20 T or above M6, specify double girder and stop here.
- Required hook height against building clearance. If you cannot make the lift with the hoist hanging below the girder, you need double girder.
- Wheel load limit of the existing structure. If it is tight, single girder may be the only option that avoids strengthening works.
- Attachments and access. Cabin, magnet, grab, walkway or auxiliary hoist all point to double girder.
- Cost. Only once the four above are satisfied does price decide.
What to send us
If you are unsure which side of the line your requirement sits on, send us the capacity, span, required lift height, the clear height under your roof structure, and — if the building already exists — the wheel load the gantry girder is rated for. That is enough for us to tell you which configuration works and why the other one does not.