The runway is part of the crane system
A crane does not stand on the floor. It hangs from a runway, and the runway hangs from the building. Every load the crane lifts passes through the bridge, the end carriages, the wheels, the rails, the runway beams and finally the columns and foundations.
That chain is why a crane quotation and a building drawing have to meet before either is finalised. A crane supplier who quotes only the machine and leaves the runway to someone else is not wrong — the runway is usually outside the crane scope — but the interface has to be defined by someone, and it is usually the crane supplier who has the wheel load data.
Wheel loads: the number the building actually feels
Wheel load is the force one wheel applies to the rail. It is not simply the crane weight divided by four. It changes with the trolley position: when the loaded trolley is at one end of the bridge, the wheels at that end carry much more than the wheels at the other.
A proper wheel load schedule gives the maximum and minimum load for each wheel, with the trolley at both ends and in the middle, and it separates the dead load of the crane from the live load. Dynamic factors for hoisting and travelling are applied on top. The runway designer needs the maximum figure, the corresponding horizontal forces from starting and braking, and the rail gauge.
This is the first thing to ask for when a crane is being quoted against an existing building. If the building was designed for a lighter crane, the wheel loads decide whether the new machine can be installed at all.
Rail size, wheel material and the interface
The rail is not a generic steel bar. Its head profile has to match the wheel, and its size has to carry the wheel load without excessive contact stress. A hard wheel on a soft rail wears the rail; a soft wheel on a hard rail wears the wheel. The supplier should state the rail section it expects, or confirm that the existing rail is suitable.
Rail joints matter as much as the section. A misaligned joint produces impact loading every time a wheel crosses it, which shows up later as cracked rails, loose clips or damaged bearings. End stops, rail clips and the expansion arrangement are part of the same interface.
Tolerances: what goes wrong when they are missed
Four tolerances decide whether a crane runs quietly: rail gauge, rail level, rail straightness and the relative alignment of the two rails. The exact figures belong in the crane supplier's installation drawing, because they depend on span, speed and the crane type. What matters here is what each one does when it is out.
- Gauge too wide or too narrow — the flanges bind against the rail, wheels climb, and the drive motors work against each other.
- Level error along one rail — the crane runs uphill, one end carriage carries more load, and the bridge skews.
- Straightness error — the crane crabs, the wheel flanges wear on one side, and the runway feels noisy.
- Rails not parallel — the span changes along the runway, which is the most damaging of the four and the hardest to correct after the building is finished.
These are checked with a theodolite or a laser, not with a tape measure. The installation drawing should state the method and the acceptance values, and the building contractor should sign off the runway before the crane arrives.
Columns, brackets and bracing
Vertical wheel load is only part of what the building sees. Starting and braking produce horizontal forces along and across the runway; skewing produces lateral thrust at the rail; and an out-of-balance lift adds a torsional component. The building designer needs all of them, not only the vertical figure.
On an existing building, the check is often about the bracket and the column, not the main frame. A bracket designed for a light maintenance crane may be adequate in strength but too flexible in service, which shows up as visible movement when the crane passes. If the runway is on a mezzanine or a light steel structure, ask for a deflection limit as well as a stress check.
What the crane supplier should send
Before the runway is fabricated, ask for a package containing:
- Wheel load schedule: maximum and minimum per wheel, with trolley positions
- Horizontal forces from travel, braking and skewing
- Rail section, rail gauge and the tolerance values for installation
- End stop and buffer positions, and the required approach clearances
- Overall crane dimensions, including the hook approach at both ends
- Power supply requirements and the festoon or conductor rail arrangement
If a supplier cannot produce this before the order, the runway will be designed on assumptions — and the assumptions usually belong to the building contractor, not the crane engineer.
When the building already exists
Replacing a crane in an existing building is a different exercise. The starting point is the existing rail and runway drawing, or a measured survey if the drawing is missing. The new crane's wheel loads are then compared with the original design loads, and the rail and wheel are checked for compatibility.
This is exactly what we did on a 5 t replacement in a Thai stamping plant: the wheel loads were issued before production so the customer could confirm the existing runway beams and brackets. On a 5 t semi-gantry in Malaysia, the load path into the warehouse columns had to be checked because one side of the crane hangs on the building rather than on the ground.
Both projects would have been much more expensive if the interface had been checked after the crane arrived.
Questions about runway design
Who is responsible for the runway design?
Usually the building or steelwork designer, because the runway is part of the building structure. The crane supplier is responsible for providing the loads, rail requirements and tolerances that the runway design is based on. The interface should be agreed in writing before fabrication.
Can I use the existing runway for a larger crane?
Only after comparing the new wheel loads and horizontal forces with the original design, and checking the rail and wheel compatibility. A larger crane on an existing runway is a structural question first and a mechanical one second.
What tolerance is normal for rail installation?
It depends on span, speed and crane type, and the figures should come from the crane supplier's installation drawing. Do not accept a generic number from a contractor without checking it against the crane specification.
Does a semi-gantry crane load the building differently?
Yes. One side runs on a ground rail, the other on a runway beam fixed to the building, so that side imposes vertical and horizontal loads on the columns. The column and bracket capacity has to be checked before the crane is ordered.