Span and lift height are the two dimensions you cannot change after the crane is built. Capacity can sometimes be worked around. A hoist can be swapped. But a girder cut to 18 metres will never serve a 20-metre bay, and a hoist that stops 400 mm short of the required height will stop short every day for the next twenty years. Most of the disappointment we see on commissioning day traces back to a tape measure and an assumption. This guide shows you exactly how to measure both, what building constraints cap them, and the checklist to send your supplier.
The Two Numbers That Define Your Crane’s Working Area
Span in plain terms
Span is the horizontal distance between the centres of the two runway rails. It sets how wide a strip of your floor the crane can serve. It is not the width of the building, and it is not the distance between column faces.
Lift height in plain terms
Lift height is the vertical travel of the hook — from its lowest position to its highest. It is not the height of your building, and it is not the same as hook height above floor level. Mixing these up is the single most common source of quoting errors.
How to Measure Span Correctly
Rail centre to rail centre — not wall to wall
Stand at one runway beam, find the centreline of the rail, and measure across to the centreline of the opposite rail. If rails are not yet installed, measure between the planned rail positions on the corbels. Wall-to-wall dimensions include cladding, columns, and services, and they will overstate your span by anywhere from 300 mm to well over a metre.
Measuring an existing building vs designing a new one
In an existing shed, the building dictates the span and you measure it. In a new build, you choose it — and the choice should follow your machine layout and material flow, not a round number. Set the runway so the hook reaches every load and unload point with margin, then build to that.
Measure at three points, then design for the worst
Industrial sheds settle, columns lean, and erection tolerances accumulate. Measure span at the two ends and the middle of the bay. A variation of 30–50 mm across a 20 m span is completely normal and is handled by end carriage design — but only if the supplier is told about it. Report all three figures.
The end approach dead zone nobody budgets for
The hook cannot reach the last stretch at each end of the bay, because the end carriages and trolley have physical width. On a typical crane you lose 700 mm to 1.2 m at each end and a similar amount on each side. Across a 20 m × 40 m bay that is often 8–12% of floor area the crane cannot serve. Plan your storage and staging in those zones deliberately, rather than discovering them later.
How Span Affects Cost and Performance
Deflection and why it grows faster than span
Girder deflection under load rises with the cube of the span. Doubling the span does not double the sag — it multiplies it eightfold for the same section. This is why long-span cranes get disproportionately heavy: the steel is fighting deflection, not just load.
Wheel loads, columns, and foundations
Longer span means a heavier bridge, which means higher wheel loads, which means heavier runway beams and stronger columns. The crane price is only the visible part. On new builds, structural steel for the runway commonly runs a significant fraction of the crane cost itself, and it rises with span.
The cost curve — where span gets expensive
Cost per metre of span is close to flat up to about 20 m, then starts to climb, and climbs steeply past 30 m. If your layout is flexible, this is where money is saved: trimming a 32 m span to 28 m can cut more cost than dropping a full tonne of capacity.
How to Calculate Lift Height
The stack-up method, step by step
Work upward from the floor and add every element:
- Floor to top of the highest machine or obstruction the load must clear
- Safety clearance over that obstruction — 300 to 500 mm
- Height of the tallest load you will carry
- Height of the lifting tackle — sling legs, spreader beam, magnet, C-hook
- Hook block and hook height — the physical depth of the hook assembly
Total that stack. That is your minimum hook height above floor. Your lift height is that figure minus the lowest hook position you need — which for pit work or basement loading may be below floor level.
Worked example with real numbers
A press shop moving dies over a 2.4 m machine.
- Machine height: 2,400 mm
- Clearance over it: 400 mm
- Tallest die: 1,100 mm
- Sling legs at working angle: 900 mm
- Hook block: 350 mm
Total required hook height: 5,150 mm. Add the hoist’s own height above the hook and you now know the minimum rail level — and whether the existing shed can take it.
Lift height vs hook height vs headroom
- Lift height — total hook travel
- Hook height — distance from floor to the hook at its top position
- Headroom — the vertical space the crane itself occupies between the rail and the roof
Quote all three separately to your supplier. A supplier who receives only one of them will assume the other two.
Building Constraints That Cap Lift Height
Roof trusses, purlins, lighting, ducting, sprinklers
The roof structure is rarely the lowest obstruction. Light fittings, cable trays, exhaust ducting, sprinkler lines, and roof-mounted fans usually hang below the truss bottom chord. Survey the actual lowest point along the full crane travel, not just at the point where you are standing.
Hoist type and its effect on the top hook position
Hoist configuration changes achievable height by hundreds of millimetres for the same building. Low-headroom monorail hoists, double-reeved arrangements, and top-running trolleys all sit at different heights. If you are short by 200–400 mm, a hoist configuration change usually solves it — and costs far less than raising a roof.
Getting Both Right Together
The layout trade-off between span and height
Span and lift height compete for budget through the building. A wider span needs deeper girders, which sit lower into the roof space and cost you hook height. In a fixed building, every extra metre of span you request costs you a little lift.
When one long bay beats two short ones
Buyers often ask whether to run one 30 m crane or two 15 m cranes side by side. Two cranes cost more in total and add a column line, but they cut span dramatically, allow independent operation in two areas, and remove the queuing that a single crane creates in a busy shop. Where two departments compete for the same crane, the throughput gain frequently outweighs the extra capital.
Common Mistakes
- Measuring span wall to wall instead of rail centre to rail centre
- Quoting building height and calling it lift height
- Forgetting sling and hook block depth in the stack-up
- Ignoring the end and side approach dead zones during layout
- Measuring at one point and assuming the bay is square
- Surveying to the roof truss and missing the ducting below it
The Measurement Checklist to Send Your Supplier
- Span, measured rail centre to rail centre, at three points
- Bay length and total runway length
- Floor to underside of roof truss
- Floor to lowest obstruction along the crane’s travel, with the obstruction named
- Existing rail top level, if rails are installed
- Required hook height, with the stack-up shown
- Lowest hook position needed, including any pit
- Column spacing and existing corbel details
- Photographs down the bay from both ends
FAQs
Can I increase lift height after installation? Sometimes, by changing hoist configuration or re-reeving, but only within the building’s clearance. Raising the runway is a structural project, not a crane modification, and it almost always costs more than getting it right initially.
How accurate does the span measurement need to be? Give it to the nearest 10 mm and report the variation across the bay. End carriage and wheel flange design absorbs normal building tolerance, but only when the designer knows the actual figures.
Does a longer span reduce capacity? Not the rated capacity, which is fixed by design. It does increase deflection and the steel needed to control it, so at long spans the same capacity costs considerably more.
What clearance is needed above the load? Plan a minimum of 300–500 mm over the highest obstruction the load must pass. Anything tighter turns every transit into a slow, hand-guided operation and defeats the purpose of the crane.
Should I add spare lift height for the future? Add it in the building, not the crane. Setting the runway a little higher in a new build is inexpensive. Retrofitting height later is not.
Conclusion
Take the tape measure to your bay this week. Get span at three points, the lowest obstruction on the travel path, and a written hook-height stack-up. Those three items turn a rough enquiry into a quote you can actually rely on.
Heben Crane starts every project with a measured site survey — span, headroom, obstructions, and structural check — so the crane that arrives fits the building you have. No assumptions carried into fabrication.
Book a Heben Crane site survey and get your span and lift height confirmed before you order.
