The difference is not just one girder or two. It changes your hook height, the load on your building, the span you can reach, and whether you can ever have two hooks on one crane. Here is how to decide.
The structural difference, in one paragraph
A single girder crane has one box girder with a compact wire rope hoist running on its bottom flange, so the hoist hangs below the bridge. A double girder crane has two girders with an open winch trolley travelling on top, so the hoisting drum and the hook sit above the bridge line. Everything else that follows — cost, hook height, capacity, maintenance access — comes from that one arrangement.
Hook height: the argument buyers underestimate
Because the double girder hoist sits on top of the girders rather than under them, the hook can be brought much closer to the underside of the bridge — often 1–2 m closer than a single girder machine of the same capacity. In a low building where the crane must lift a load over a machine, a wall or a neighbouring process, that difference is decisive.
Compare your options honestly. If the single girder crane leaves you 300 mm short of the height you need, the fixes are: a low-headroom hoist (usually a few hundred dollars), a double girder crane (a different cost bracket), or raising the roof (usually the most expensive of the three by a wide margin). Ask a supplier for the hook approach figure in writing before you choose.
Capacity, span and duty
Single girder cranes are normally offered from about 1 t to 20 t, with spans to 30 m. Double girder cranes run from about 5 t to 100 t and beyond, with spans to 31.5 m in standard series and wider by engineering. Two thresholds matter:
- Above roughly 20 t, the single girder arrangement stops being the sensible choice — the girder becomes deep and heavy, wheel loads climb, and the price advantage disappears.
- Above about 30 m span, the deflection and stability of a single girder bridge get difficult, and a double girder design is the honest answer.
Duty class runs in the same direction. A single girder crane is at its best in intermittent duty — A3 to A5. If the crane runs continuously, if the hook is in the air all shift, or if the load swings because the operator cannot position it precisely, a double girder machine with VFD control and a heavier structure will outlast it and cost less over its life.
Wheel loads and your building structure
Every force the crane exerts on the building arrives through the wheels. A double girder crane is heavier for the same capacity, and its wheel loads are correspondingly higher — sometimes enough to require stronger runway beams and columns. That civil work is real money and belongs in the comparison.
The practical sequence is: get the wheel loads from the supplier, give them to your structural engineer, and let them confirm the runway and columns. Two useful levers if the loads come back too high: a longer wheel base (wheels further apart) or more wheels per corner, both of which reduce the load per wheel. Ask for these in the first quotation rather than after the structure has been designed.
Maintenance, and the day something breaks
On a single girder crane, the hoist hangs under the girder, which makes routine checks slightly awkward but the hoist itself easy to remove and replace as a unit. On a double girder crane, the winch trolley sits on top with open access to the drum, brake, reducer and motor, and many bridges are supplied with a walkway and handrail so maintenance can be done properly. Over a fifteen-year life in a hard-working plant, that access is worth more than the purchase-price difference.
Also count the second hook. On a double girder crane it is normal to have a main and an auxiliary hoist on the same bridge — a heavy hook for the big lifts and a small fast hook for everything else, which is exactly what most steel yards, precast plants and heavy machine shops actually need. On a single girder crane you cannot do that.
The short version
Choose single girder if your loads are under about 20 t, your span is under 30 m, duty is intermittent, your building has normal headroom, and the hook approach is adequate. You will pay less, load the building less, and install faster.
Choose double girder if you need more than about 20 t, a span beyond 30 m, a hook that lifts high in a low building, two hooks on one crane, a grab / magnet / C-hook arrangement, or heavy continuous duty. You will pay more and may need structure work, but you will get a crane that suits the job.
If you are close to the boundary, send the process description rather than the crane type — number of lifts per hour, shifts per day, load weights, and a photo of the bay. A supplier who quotes the cheaper machine when it will do the job is worth keeping.
Questions about your own project? Send the capacity, span, lifting height and duty and we will tell you honestly what we would specify — send an inquiry or message us on Zalo.