Single Girder vs Double Girder Overhead Crane
Introduction
This is the decision that quietly sets the ceiling on your entire crane investment — capacity, hook height, span, and how much steel your building needs to carry it all. Choose it once, and you live with the consequences for the crane’s full 20-to-30-year service life.
Single girder and double girder are the two fundamental bridge configurations for an overhead crane. A single girder crane runs the hoist below one bridge beam. A double girder crane carries the hoist on a trolley that rides on top of, and between, two bridge beams. That structural difference sounds minor. It isn’t — it drives capacity, span, hook height, wheel loads, runway and building steel, and the price on every line of the quote.
This guide breaks down the structural load paths, the engineering reasons the two diverge, their practical capacity and span ranges, the hook height comparison, the building steel impact, and a clean decision framework — so you specify the right configuration before the first drawing is finalized.
What you’ll take away:
- How each configuration carries load, and why that limits or unlocks capacity
- The real capacity, span, and duty-class ranges for each (CMAA and FEM/ISO)
- How hook height genuinely compares — with a worked example
- Where each configuration saves or costs building steel
- A decision framework you can apply before you call a supplier
Part 1: Structural Definitions and Load Paths
Single Girder
A single girder crane has one bridge beam spanning the runway. The hoist and trolley hang below the girder, running along its bottom flange — the same underhung principle as an under running trolley, applied to the bridge itself.
Load path: hook → hoist → trolley on bottom flange → single girder (bending + some torsion) → end trucks → runway wheels → runway → building.
SINGLE GIRDER (end view)
┌────────────┐ ← one bridge girder
│ │
│ [trolley] │ ← hoist hangs BELOW girder
│ │ │
│ ▼ │
hook
══════ ══════ ← runway rails (each end)
Because the hoist hangs off the girder’s bottom flange rather than over its web, the load line sits below and slightly off the girder’s shear center, adding a torsional component to the bending. This is why single girder bridges use closed box sections or reinforced top-flange beams — the closed section resists twist far better than an open one.
Double Girder
A double girder crane has two parallel bridge beams. The trolley rides on top of both girders, on rails fixed to each, with the hoist mounted on the trolley between them.
Load path: hook → hoist → trolley on top of both girders → two girders (clean vertical bending, split between them) → end trucks → runway wheels → runway → building.
DOUBLE GIRDER (end view)
┌────[ trolley ]────┐ ← hoist rides ON TOP, between girders
╞═══════════════════╡
│ │ │ ← two bridge girders
│ ▼ │
hook
══════ ══════ ← runway rails (each end)
The load splits between the two girders, and because the trolley sits centered on top, the torsion that penalizes a single girder largely cancels. Each girder mostly sees clean vertical bending — the loading condition steel handles most efficiently.
Part 2: Why the Engineering Differs
One Beam Carrying Torsion vs Two Beams Sharing Bending
The core difference is how each configuration handles the load’s line of action. On a single girder, the hoist hangs below the beam, so every lift applies bending and torsion to a single member. The girder must be sized to resist both — and torsional stress rises quickly as capacity and span increase, forcing a disproportionately heavy, deep box section to keep the twist in check.
On a double girder, the trolley sits on top, between the two beams. The vertical load divides between them, and the symmetric arrangement cancels most of the torsion. Each girder resists clean bending only, which is the efficient use of a steel section. Two moderately sized girders sharing a split load handle far more than one girder fighting bending and torsion together.
Why This Caps Single Girder Capacity
The torsion penalty is exactly why single girder cranes have a practical ceiling. To carry heavy loads over long spans, a single box girder would need to become so deep and thick — purely to resist twist and deflection — that it stops being economical. At that point, splitting the load across two girders is both lighter in total steel and structurally cleaner. This crossover is the engineering reason double girder configurations dominate the heavy, long-span, high-duty end of the market.
The deflection limits governing both — commonly around L/700 to L/1000 of the span — apply to each, but the single girder hits its usable limit sooner because it’s fighting bending, torsion, and deflection in one member.
Part 3: Practical Capacity and Span Ranges
Single girder configurations are the economical default for lighter, shorter-span, lower-duty work. Double girder configurations take over where capacity, span, hook height, or duty class push past what a single beam can carry efficiently.
| Configuration | Practical capacity | Practical span | Typical duty (CMAA / FEM-ISO) |
|---|---|---|---|
| Single girder | Up to ~20 t | Up to ~20–25 m | A–C / M3–M5 |
| Double girder | 5 t to 500+ t | Up to 35+ m standard | C–F / M5–M8 |
A few practical notes on the ranges:
- Single girder stays economical up to roughly 20 tonnes and moderate spans. Push capacity or span beyond that and torsion plus deflection force an oversized, costly girder.
- Double girder overlaps single girder in the 5–20 tonne band, so both are options there — the choice then turns on hook height, duty, and cost (covered below).
- Duty class matters as much as tonnage. High-cycle, heavy-duty service (CMAA Class D and above, FEM M6+) favors double girder even at moderate capacity, because the two-girder structure and top-running trolley handle fatigue and maintenance access better.
Choose on the binding constraint, not just the load. A 10-tonne crane at Class C on a 15-metre span is comfortable single girder territory; the same 10 tonnes at Class E over 28 metres is a double girder job.
Part 4: Impact on Building Structural Steel
The configuration you pick ripples straight into the runway and building steel — and this is where the total-cost picture often surprises buyers.
Single Girder — Lighter Loads Down the Whole Structure
A single girder bridge is lighter than a double girder bridge of the same span, and that dead-weight saving compounds all the way down. Lighter bridge means lower wheel loads on the runway, which means a lighter runway beam, lighter brackets, and often lighter columns and foundations. For light-to-moderate capacities, single girder delivers a genuinely cheaper building structure, not just a cheaper crane.
Single girder cranes are also usually top running on the runway but can be under running, giving flexibility to integrate with lighter building framing where capacity allows.
Double Girder — More Bridge Steel, Heavier Runway
A double girder bridge is two girders plus the connecting end trucks and a top-running trolley — more steel, more dead weight, higher wheel loads. Those higher wheel loads drive a heavier runway beam, more substantial brackets, and stronger columns and foundations to carry the combined loads. The building structure works harder at every level.
Where the Comparison Balances Out
At light capacities, single girder wins clearly on total steel cost. As capacity, span, and duty climb, the single girder’s own section must grow so heavily to fight torsion and deflection that the double girder’s split-load efficiency closes the gap — and, at the heavy end, the double girder is simply the only structurally sound option. The building-steel advantage of single girder is real at the light end and fades as you move up the range.
Part 5: Hook Height Comparison
Hook height is often the deciding variable in a low-clearance building — and here the two configurations differ in a way that catches buyers out.
The Headroom Tradeoff
On a single girder crane, the hoist hangs below the bridge girder. The girder depth sits above the trolley, and the hook’s highest position is limited by how far the underhung trolley can rise toward the girder’s bottom flange. You lose the girder depth from your available lift.
On a double girder crane, the trolley rides on top of the two girders, so the hoist can pull the hook up between the girders rather than staying below them. This recovers the girder depth as usable hook height — the structural reason double girder cranes often give more headroom despite being the bigger, heavier machine.
Worked Comparison — 9-Metre Clear Height Building
Single girder configuration (10 t, top running):
- Building clear height: 9.0 m
- Less: runway beam + bracket: ~0.8 to 1.0 m
- Less: bridge girder depth (hoist hangs below it): ~0.6 to 0.8 m
- Less: hoist headroom (underhung trolley): ~0.5 to 0.7 m
- Available hook height: ~6.5 to 7.1 m
Double girder configuration (10 t, top running):
- Building clear height: 9.0 m
- Less: runway beam + bracket: ~0.8 to 1.0 m
- Less: hoist headroom (hook pulls up between girders): ~0.5 to 0.7 m
- Available hook height: ~7.3 to 7.7 m
In this comparison, double girder delivers roughly 0.6 to 0.8 m more hook height in the same building — because the hook rises between the girders instead of hanging below a single one.
The Practical Takeaway
If hook height is your binding constraint in a fixed-height building, don’t default to single girder assuming it’s more compact. Run the numbers on both. A double girder crane frequently recovers enough headroom to justify its cost — and gives you higher capacity in the bargain. For any project where hook height is critical, calculate both using the actual girder depths from the structural design, not a rule of thumb.
Part 6: Decision Framework
Choose Single Girder When:
- Capacity is at or below ~20 tonnes and comfortably within the single girder range.
- Span is at or below ~20–25 metres, before torsion and deflection force an oversized section.
- Duty is light to moderate (CMAA Class A–C / FEM M3–M5) — intermittent or single-shift service.
- Budget is the priority and the lighter bridge, runway, and building steel deliver a genuine total-cost saving.
- Hook height is not the binding constraint, or the calculated single girder headroom meets the task.
Choose Double Girder When:
- Capacity exceeds ~20 tonnes, where the single girder section becomes uneconomical.
- Span is long (beyond ~20–25 metres), where split-load efficiency outperforms one heavy girder.
- Duty is heavy or continuous (CMAA Class D–F / FEM M6–M8) — high cycle counts and multi-shift service.
- Maximum hook height matters in a fixed-height building and the hook must pull up between the girders.
- You need a crane walkway, multiple hoists, or auxiliary lift — the top-running bridge accommodates these where a single girder can’t.
- Future capacity growth is realistic — double girder gives more headroom to upgrade within the same structure.
Ask yourself which constraint is truly binding — tonnage, span, duty, or hook height. That single answer usually settles the configuration before cost even enters the conversation.
Part 7: 2026 Structural Cost Reference
These figures are planning benchmarks for the crane plus its runway and building-steel impact, compared across configurations at representative capacities and spans. Actual pricing varies with region, duty class, span, and civil scope.
| Configuration | Capacity / Span | Crane equipment (USD) | + Runway & building steel impact |
|---|---|---|---|
| Single girder, top running | 5 t / 15 m | $12,000 – $28,000 | Lower — lighter runway, brackets, columns |
| Single girder, top running | 10 t / 18 m | $20,000 – $45,000 | Moderate |
| Double girder, top running | 10 t / 18 m | $35,000 – $70,000 | Higher — heavier runway & columns |
| Double girder, top running | 32 t / 25 m | $90,000 – $180,000 | Substantial |
| Double girder, heavy duty (Class E–F) | 50 t+ / 30 m | $160,000 – $400,000+ | Major civil and structural scope |
Cost drivers to build into the budget:
- Double girder over single girder at the same capacity/span: typically +40% to +70% on the crane, plus the heavier runway and building steel it demands.
- Higher duty class (e.g., CMAA C to E): often +20% to 50%, reflecting heavier structure, upgraded hoists, and more robust drives.
- Runway beam and brackets: a real, often-separate civil cost — heavier for double girder because of higher wheel loads.
- VFD control: smooths starts and stops, reducing dynamic impact factors on the runway — sometimes allowing a lighter runway beam for the same capacity.
Procurement tip: normalize every quote to the same capacity, span, duty class, and runway/building-steel scope. A cheaper single girder quote that ignores the hook height shortfall — or a double girder quote that excludes the heavier runway — isn’t a like-for-like comparison. Hold each supplier to the same numbers before you compare prices.
Frequently Asked Questions
Q: We need 10 tonnes over an 18-metre span — both configurations are possible. How do I decide?
A: In the overlap band, let hook height, duty class, and total cost decide. If your building clear height is tight and you need maximum headroom, double girder recovers hook height by letting the hook pull up between the girders — often worth its premium. If duty is light-to-moderate (CMAA A–C) and headroom is comfortable, single girder is cheaper across the crane, runway, and building steel. If the service is heavy-cycle (Class D+) or you’ll want a walkway or future capacity growth, lean double girder even at 10 tonnes.
Q: Is a single girder crane always cheaper than a double girder crane?
A: On the crane itself, yes, at capacities and spans where both are viable — typically 40% to 70% less. But “cheaper crane” isn’t “cheaper project.” Factor in the runway and building steel (lighter for single girder) and hook height. If a single girder can’t deliver the headroom you need and forces a taller building, or can’t carry the duty and fails early, the apparent saving evaporates. Compare total installed cost against the requirement, not sticker price alone.
Q: Does duty class affect the single vs double girder choice?
A: Significantly. Single girder cranes suit CMAA Class A–C / FEM M3–M5 — intermittent to moderate service. For CMAA Class D–F / FEM M6–M8 heavy or continuous duty, double girder is standard because the two-girder structure and top-running trolley handle high cycle counts, fatigue, and maintenance access far better. A moderate-capacity crane in a demanding duty cycle can justify double girder on duty class alone.
Q: Can a single girder crane carry more than one hoist or an auxiliary lift?
A: Rarely, and not comfortably. The underhung trolley and single bottom flange limit what a single girder bridge can accommodate. Double girder cranes carry a main and auxiliary hoist on the trolley, support multiple trolleys, and provide room for a maintenance walkway along the bridge — all reasons heavy and complex lifting applications specify double girder regardless of raw tonnage.
Q: Which configuration gives better hook height in a low-ceiling building?
A: Usually double girder, counterintuitively. Because the hook pulls up between the two girders rather than hanging below a single one, double girder recovers the girder depth as usable lift — often 0.6 to 0.8 m more in the same building. If hook height is your binding constraint, calculate both using actual girder depths before defaulting to single girder.