Single Girder vs Double Girder Overhead Crane: 5 Key Differences That Determine the Right Choice
Introduction
Single girder or double girder — this is the first structural decision in every overhead crane project. Get it right and you have a crane that fits the building, serves the application, and delivers reliable performance for 15 to 20 years. Get it wrong and you discover the problem during installation — when the hook height is 1.5 metres short of what the application requires, or the building columns cannot carry the runway beam loads.
The decision is not complicated. It follows directly from five application parameters. This guide walks through each parameter, explains how it maps to single or double girder construction, and provides the complete decision logic.
By the end of this guide, you will know which configuration is correct for your specific application — before speaking to a single crane supplier.
Part 1: Structural Definitions
Single Girder Overhead Crane
A single girder bridge crane has one bridge girder spanning the distance between the two runway rails. The hoist trolley runs on the lower flange of the bridge girder — hanging below the girder like a monorail hoist.
Because the trolley hangs below the girder, the hook at its highest position is approximately at the level of the girder’s bottom flange. The hook cannot rise above the girder bottom. This limits the maximum hook height for a given building height.
Key advantages of single girder:
Lower purchase price — less structural steel, simpler fabrication.
Lower weight — the runway beams carry less dead load.
Lower runway beam cost — lighter crane means smaller runway beam sections.
Faster delivery — simpler design allows quicker manufacturing.
Key limitation: the underhung hoist reduces the usable hook height compared to a double girder crane in the same building.
Double Girder Overhead Crane
A double girder bridge crane has two bridge girders running in parallel, spanning the runway. The hoist trolley sits on top of the two girders — the trolley wheels roll on rails mounted on the top flanges of the girders.
Because the trolley sits on top of the girders, the hook at its highest position rises significantly above the bottom of the girders — limited only by the hoist’s minimum headroom from the top rail to the hook in the highest position. This allows the hook to reach much higher in the building than an equivalent single girder crane.
Key advantages of double girder:
Greater hook height in the same building.
Higher maximum span.
Higher maximum capacity.
Lower deflection at large spans and high loads.
Platform between the girders for maintenance access.
Key limitation: higher purchase price, heavier weight, higher runway beam cost.
Part 2: Five Key Differences
Difference 1: Hook Height
This is the most decisive difference for most applications.
Single girder hook height: the hook at its maximum raised position is at approximately the bottom flange of the bridge girder. Subtracting the hoist’s minimum headroom (distance from bottom flange to hook at maximum height): typically 0.3 to 0.6 metres.
Available hook height for a single girder crane = Building clearance height − Bridge girder depth − Runway beam depth − Hoist headroom (0.3 to 0.6 m)
Double girder hook height: the hoist sits on top of the girders. The hook rises to a point determined by the distance from the top rail surface to the hook at maximum height — typically 0.5 to 1.0 metre above the top rail.
Available hook height for a double girder crane = Building clearance height − Runway beam depth − Hoist headroom above top rail (0.5 to 1.0 m)
In practice: in a building with 8-metre clearance height, a single girder crane provides approximately 5.5 to 6.5 metres of hook height. A double girder crane in the same building provides approximately 6.5 to 7.5 metres of hook height.
The difference of 1 to 2 metres of hook height is the single most common reason projects switch from single to double girder during the specification phase.
Rule of thumb: if you need to lift over tall machinery, if you need to stack loads to height, or if the required hook height approaches the building’s clear height — calculate the hook height for both configurations before choosing.
Difference 2: Capacity Range
Single girder cranes: standard commercial range 125 kg to approximately 16 to 20 tonnes.
Above 16 tonnes: single girder designs become structurally marginal. The single girder must carry the full load plus its own weight across the span. At capacities above 16 tonnes and spans above 16 metres, the required girder depth and weight makes single girder less economical than double girder.
Double girder cranes: available from 5 tonnes to 500+ tonnes as standard products. The two-girder configuration distributes the load efficiently. Heavy lifting is the natural home of double girder cranes.
Rule of thumb: for capacities above 16 tonnes or spans above 20 metres — specify double girder.
Difference 3: Span Capability
Single girder cranes: economically practical to approximately 20 to 25 metres span. Above 25 metres, the single girder must be very deep to control deflection within the L/600 limit. A deep single girder consumes the hook height advantage it otherwise provides.
Double girder cranes: economically practical to 35+ metres as standard products. The two-girder system provides inherently higher lateral stiffness and can be designed for longer spans without disproportionate depth increase.
Rule of thumb: for spans above 20 metres — double girder is the preferred structural choice.
Difference 4: Deflection Performance
ASME MH1 and CMAA Specification No. 70 specify maximum bridge girder deflection under rated load: L/600 where L is the span. This limit ensures the hook does not drop significantly during travel — which would create unwanted load movement.
Single girder: at long spans and high loads, meeting L/600 requires a deep girder. A deep single girder is heavy and expensive. The cost of meeting L/600 in single girder at spans above 20 metres approaches the cost of a lighter double girder.
Double girder: the two-girder configuration inherently provides greater stiffness per unit of structural weight than single girder. Meeting L/600 is less demanding on the girder depth at equivalent spans and loads.
For precision positioning applications — where load movement during travel must be minimized — double girder’s superior stiffness is a functional advantage beyond pure cost considerations.
Difference 5: Maintenance Access
Single girder: no maintenance platform between the girder and the building structure. Maintenance on the hoist mechanism, trolley, and electrical systems requires a man-lift or other access equipment. This is not a problem for infrequent maintenance. For cranes requiring frequent access — daily or weekly — the absence of a maintenance platform adds time and cost to every maintenance event.
Double girder: the space between the two girders accommodates a maintenance walkway with handrails. Maintenance personnel can walk on the bridge to access the hoist, trolley, and electrical systems safely without additional access equipment. Required by CMAA Specification No. 70 and OSHA 1910.179 for cranes with bridge-mounted electrical equipment that requires regular maintenance.
For cranes above approximately 10 to 15 tonnes in production service: double girder with maintenance walkway is strongly recommended — both for safety and for maintenance efficiency.
Part 3: Side-by-Side Decision Table
Requirement | Single Girder | Double Girder
Capacity up to 16 tonnes | ✅ Standard choice | Optional (more expensive)
Capacity above 16 tonnes | ⚠️ Marginal | ✅ Standard choice
Span up to 20 metres | ✅ Standard choice | Optional
Span above 20 metres | ⚠️ Marginal | ✅ Standard choice
Maximum hook height priority | ⚠️ Limited | ✅ Advantage
Budget-constrained project | ✅ Lower cost | ⚠️ Higher cost
Maintenance walkway needed | ❌ Not available | ✅ Standard feature
Precision positioning required | ⚠️ Adequate to 16t/20m | ✅ Superior stiffness
Building runway beam cost | ✅ Lower (lighter crane) | ⚠️ Higher
Part 4: Worked Selection Examples
Example A: Automotive Parts Assembly, 8-Tonne, 16-Metre Span, 7m Hook Height Needed
Capacity: 8 tonnes — within single girder range.
Span: 16 metres — within single girder range.
Required hook height: 7 metres in a 9-metre clear-height building.
Calculate single girder hook height: 9m − 0.8m (runway beam) − 0.6m (girder depth) − 0.5m (hoist headroom) = 7.1m. Just sufficient.
Calculate double girder hook height: 9m − 0.8m (runway beam) − 0.8m (hoist headroom above top rail) = 7.4m. More comfortable margin.
Recommendation: single girder is technically adequate but with minimal hook height margin. If any workpiece height estimate is uncertain or future loads might be taller: specify double girder for the 300mm hook height safety margin. If the application is well-defined and the 7.1m hook height is confirmed adequate: single girder is the cost-effective choice.
Example B: Steel Fabrication Shop, 25-Tonne, 24-Metre Span
Capacity: 25 tonnes — above single girder practical range.
Span: 24 metres — above single girder economic range.
Both parameters independently indicate double girder. This is a clear double girder application. No further analysis required.
Example C: Maintenance Bay, 3-Tonne, 12-Metre Span, Low-Frequency Use
Capacity: 3 tonnes — well within single girder range.
Span: 12 metres — comfortably within single girder range.
Use: maintenance lifting, approximately 5 cycles per shift.
This is a straightforward single girder application. Double girder adds cost without functional benefit. Single girder with standard underhung hoist is the correct specification.
Part 5: Price Reference and Cost Comparison
Single girder bridge crane (installed, standard CMAA Class C):
3-tonne, 12m span: $12,000 to $28,000
8-tonne, 16m span: $22,000 to $50,000
16-tonne, 20m span: $45,000 to $95,000
Double girder bridge crane (installed, standard CMAA Class C):
8-tonne, 16m span: $35,000 to $75,000
16-tonne, 20m span: $65,000 to $130,000
25-tonne, 20m span: $90,000 to $185,000
Double girder premium over single girder at equivalent capacity and span: 50 to 80%.
Runway beam cost difference: a double girder crane at 8 tonnes typically weighs 30 to 50% more than a single girder at the same capacity. The heavier dead load requires larger runway beam sections. Runway beam cost increases: 15 to 30% higher for double girder at equivalent span.
Total installed cost premium for double girder: typically 40 to 70% above single girder at equivalent capacity and span.
Frequently Asked Questions
Q: Can I convert a single girder crane to double girder if I need more hook height later?
A: Not practically. Single and double girder cranes are fundamentally different structural designs. The bridge girder, end trucks, trolley, hoist, and runway beams are all different between the two configurations. Converting is essentially replacing the entire crane. If there is any chance the application will require more hook height in the future — specify double girder from the start. The additional upfront cost is far less than a premature crane replacement.
Q: Is a double girder crane always safer than a single girder?
A: No. Both types must meet the same CMAA and ASME B30.2 safety standards. The safety level is determined by the specification, construction quality, and maintenance program — not by the number of girders. A correctly specified and maintained single girder crane is as safe as a correctly specified double girder crane.
Q: What is the maximum span available for a double girder crane?
A: Standard commercial double girder cranes are available to approximately 35 metres span from most major manufacturers. Custom designs extend to 50+ metres. Above 35 metres, the crane typically becomes a specialty product with longer lead times and higher engineering costs. Very long span applications (40+ metres) benefit from consulting the crane manufacturer early in the project design phase to confirm feasibility and lead time.