Monopoutre ou bipoutre : comment choisir

One of the first decisions when specifying a bridge crane is whether to use a single girder or double girder design. Both are proven, standards-compliant configurations — the right choice depends on capacity, lifting height, duty cycle and budget.

What is the difference?

A single girder overhead crane uses one main beam, with the hoist trolley running on the lower flange (underslung). A double girder overhead crane uses two beams with the trolley running on rails on top, between the girders.

Lifting capacity and span

Single girder cranes are the economical choice up to about 20 tons and spans up to 30 m. Above that — or for very long spans — the double girder design provides the rigidity you need. Double girder cranes are practical all the way up to 500 tons.

Lifting height (hook height)

Because the trolley sits on top of the girders, a double girder crane gains extra hook height — valuable when you need to lift close to the roof. A single girder crane, with its underslung trolley, gives up a little lifting height but reduces total structural weight.

Duty cycle

For light and medium duty (A3–A5) — typical of workshops and warehouses — a single girder crane is ideal. For heavy, high-frequency operation (A6–A8) such as steel mills and foundries, choose double girder.

Cost

Single girder cranes have lower purchase, installation and runway costs because they are lighter and impose smaller wheel loads on the building. Double girder cranes cost more but deliver capacity, height and durability that single girder designs cannot.

Runway and building impact

The comparison does not stop at the crane. A single girder crane is lighter, so it puts smaller vertical and lateral wheel loads into your runway beams and columns. In an existing building that difference often decides the question outright: a double girder crane of the same capacity may require the runway to be strengthened, and that civil work can cost more than the price gap between the two cranes.

Ask your supplier for the maximum wheel load and the wheel base early. Your structural engineer needs both numbers before you commit to a design, and a manufacturer who cannot produce them quickly is a warning sign.

Maintenance access

On a double girder crane the trolley runs on top of the girders, so a walkway can be fitted along the bridge and the hoist machinery is reachable at floor-of-crane level. On a single girder crane the hoist hangs below the beam, and routine service usually means a man-lift or a scaffold. If your crane runs two or three shifts, that access difference adds up over a twenty-year life.

Auxiliary hoists and special attachments

If you need a second, smaller hook for tilting or turning a load — common in steel and metallurgy and in machine shops — a double girder bridge gives you the room to mount it. Grabs, magnet beams, rotating hooks and C-hooks are also far easier to accommodate. Single girder cranes can carry attachments, but the payload and headroom budget is tighter.

Where each one fits in practice

Most of the overhead cranes we ship into distribution centres and general workshops are single girder, typically 5–16 t on a 15–25 m span. Almost everything above 32 t, and anything feeding a furnace or a casting line, is double girder. The European FEM/DIN crane sits between the two: a single girder design with a compact end carriage and a low-headroom hoist that recovers much of the hook height a conventional single girder gives up.

Quick rule of thumb

• Choose single girder if: capacity ≤ 20 t, light/medium duty, budget-sensitive, standard headroom.
• Choose double girder if: capacity > 20 t, heavy/continuous duty, need maximum hook height, or need an auxiliary hoist.

Questions to answer before you request a quote

• What is the heaviest single load, and how often is it lifted?
• What is the rail-to-rail span, and is the runway existing or new?
• How much clear height is there under the roof steel?
• How many lifts per hour, over how many shifts?
• Will you ever need a second hook or a special attachment?

Answer those five and the girder decision usually makes itself. Send us your capacity, span and duty requirements and our engineers will recommend the most cost-effective configuration — including whether a European-style single girder can do the job a double girder was quoted for.