Overhead Crane Bridge Girder Design: Span-to-Depth Ratios, Camber Specifications & CMAA Compliance
Published by: Weiyuan Crane Engineering Team | Last Updated: August 2026 | Reading Time: 10 min
Introduction
The bridge girder looks like the most straightforward part of an overhead crane. It is the beam that spans your bay, carries the trolley, and holds up the load. Pick a beam strong enough to hold the rated tonnage, weld end trucks to each end, and you have a bridge. That is how many buyers picture girder design.
That picture leaves out almost everything that matters.
A bridge girder is not sized by strength alone. It is sized by how far it bends under a moving load, how it resists twisting when the trolley pushes sideways, and how it holds a built-in upward curve — called camber — that keeps the trolley from rolling into a sag at mid-span. It carries loads that move back and forth across the span thousands of times a year, so fatigue drives the connection details as much as the peak load does. And the difference between a single girder and a double girder is not a cosmetic choice — it changes your hook height, your capacity ceiling, and your total cost of ownership.
This guide gives procurement decision-makers a clear framework for evaluating bridge girder design. You will learn the two main girder types, the dynamic loads that drive sizing, the deflection and camber limits from CMAA Specification No. 70, the section selection method, the web and flange details, the end connections, and the design errors that quietly raise your lifetime cost. Let’s start with the two configurations.
Part 1: Two Main Girder Types
The first decision in any bridge crane is single girder or double girder. This choice affects capacity, span, hook height, and price more than any other single specification.
Single Girder Bridges
A single girder bridge uses one beam spanning the bay, with the hoist and trolley running on the bottom flange (an under-running trolley).
- Best for: capacities up to about 20 tonnes and spans up to roughly 25 metres.
- Advantage: lower cost, lighter dead weight, simpler runway loading, and faster installation.
- Limitation: lower headroom efficiency at higher capacities, and the trolley hangs below the girder — reducing available hook height.
Single girder bridges follow CMAA Specification No. 74, the standard covering top running and under running single girder cranes.
Double Girder Bridges
A double girder bridge uses two parallel beams, with the trolley running on rails mounted on top of both girders (a top-running trolley).
- Best for: capacities from 20 tonnes upward, longer spans, and higher duty classes.
- Advantage: greater hook height (the trolley sits above the girders, not below), higher capacity ceiling, better lateral stiffness, and easier mounting of walkways, festoon systems, and maintenance access.
- Limitation: higher cost, greater dead weight, and heavier runway and column loads.
Double girder bridges follow CMAA Specification No. 70, the standard covering top running bridge and gantry cranes across the full capacity range used in industry.
The procurement takeaway: single girder rewards you with lower upfront cost and lighter structure. Double girder rewards you with more hook height, higher capacity, and longer service under heavy duty. Your required capacity, span, and needed lift height decide which pays off. When capacity climbs above 20 tonnes or you need maximum hook height in a fixed building, double girder is usually the more cost-effective choice over the crane’s life.
Part 2: Dynamic Load Inputs
Girder sizing starts with the loads the girder actually carries — and those loads are dynamic, not static. A moving, lifting crane generates more force than its dead weight and rated load suggest.
Vertical Load and the Dynamic Impact Factor
The vertical load is the rated load plus the trolley and hoist weight, multiplied by a dynamic impact factor that accounts for the extra force from lifting, acceleration, and vibration.
Design vertical load = (Rated load + Trolley/hoist weight) × Dynamic impact factor
CMAA Specification No. 70 dynamic (hoist load) factors, rising with duty class:
- Class A–B (infrequent use): 1.10
- Class C–D (moderate to heavy production): 1.15 to 1.20
- Class E–F (severe, continuous duty): 1.20 to 1.25
Maximum Bending Moment from the Trolley
The girder sees its highest bending moment when the trolley sits at or near mid-span. For a simply supported girder of span L with a single concentrated trolley load P at the center:
Maximum bending moment = P × L ÷ 4
Add the girder’s own dead weight as a uniformly distributed load, which contributes an additional moment of w × L² ÷ 8 (where w is the girder self-weight per unit length). The total design moment combines both.
Horizontal Loads — Lateral and Longitudinal
A crane does not only push down. CMAA Specification No. 70 requires the girder to resist two horizontal load cases:
- Lateral (trolley side thrust): generated when the trolley accelerates and brakes across the span. Standard requirement: 20% of the combined rated load plus trolley weight, applied horizontally at the top of the rail, split between the two girders (or resisted by the single girder’s lateral stiffness). This load drives the girder’s horizontal bending strength and top-flange design.
- Longitudinal (bridge tractive force): generated when the whole crane accelerates and brakes along the runway. Standard requirement: 10% of the maximum wheel loads on the driven wheels, applied at the rail. This transfers into the end trucks and runway.
Why this matters to you: a girder sized for vertical load only may pass a “does it hold the tonnage” check and still fail under repeated side thrust — showing up as trolley tracking problems, weld cracking, and premature wear that eats into equipment uptime.
Fatigue Load Cycling
The trolley crosses the girder on every lift. CMAA Specification No. 70 classifies cranes by load cycle count (Class A through F), from under 20,000 cycles over the design life to well over 2,000,000. Welded details — flange-to-web welds, stiffener welds, end connections — must be designed to the correct AISC fatigue category (Appendix 3 of the AISC Specification) so the allowable stress range matches the expected cycle count. Higher duty class means stricter fatigue detailing.
Part 3: Deflection Limits and Camber Design
This is where girder design departs most sharply from ordinary beam design. Two limits govern: how far the girder is allowed to bend down, and how much upward curve is built in to compensate.
Vertical Deflection Limit — L/888
A floor beam might be held to L/240 or L/360. A crane bridge girder is held far tighter. CMAA Specification No. 70 limits vertical deflection under the rated load (without impact) to L/888 of the span.
The reason: the trolley must not roll into a dip at mid-span. Excessive deflection makes the trolley want to “run downhill” toward the center, increases drive load, accelerates wheel and rail wear, and makes precise load spotting difficult. For applications needing precise positioning, some specifications tighten this further.
The Deflection-to-Depth Relationship
Deflection is inversely proportional to the moment of inertia (I) of the section, and I grows roughly with the cube of the girder depth. In practical terms: a modest increase in girder depth produces a large reduction in deflection. This is why crane girders are noticeably deeper than a pure strength calculation would require — the L/888 deflection limit, not the bending stress limit, usually decides the section depth.
Camber — The Built-In Upward Curve
Because the girder will deflect down under load, it is fabricated with a slight upward curve so that under normal working load it sits close to level rather than sagging.
- Standard camber: CMAA Specification No. 70 recommends fabricating vertical camber equal to the dead-load deflection plus one-half of the live-load deflection. A common practical value is span ÷ 1000.
- Horizontal camber (sweep): for the girder’s horizontal axis, to offset lateral deflection under side thrust, typically applied to the top flange or the girder as a whole.
The buyer’s checkpoint: camber is a mark of a properly engineered girder. A girder delivered dead-straight with no specified camber is a warning sign — under load it will sag below level, and no amount of field adjustment fully corrects it. Always confirm camber is specified on the fabrication drawings.
Part 4: Section Selection Method
With loads and deflection limits defined, the next step is choosing the girder section. Crane girders are almost always fabricated (welded) sections rather than standard rolled beams, because the required depth and stiffness exceed what rolled shapes offer economically.
Common Section Types
- Welded box girder: two vertical web plates, a top flange, and a bottom flange forming a closed rectangular section. The closed shape gives excellent torsional (twisting) resistance — important because the off-center trolley load and side thrust twist the girder. This is the standard choice for double girder cranes.
- Welded I-girder (with reinforced top flange): an open I-section, sometimes with a channel or plate cap on the top flange for lateral stiffness. Used on lighter single girder cranes.
Span-to-Depth Ratio Guidelines
The girder depth is commonly set as a fraction of the span. As a starting-point reference for double girder box sections:
- Class A–C (light to moderate): depth ≈ span ÷ 18 to span ÷ 20
- Class D (heavy): depth ≈ span ÷ 16 to span ÷ 18
- Class E–F (severe to continuous): depth ≈ span ÷ 14 to span ÷ 16
Heavier duty and longer spans push toward deeper girders to control both deflection and fatigue.
Three-Stage Verification
Every candidate girder section must pass three checks in sequence:
- Stage 1 — Bending stress: the combined vertical and horizontal bending moments checked against the allowable stress for the section.
- Stage 2 — Deflection: vertical deflection under rated load checked against L/888.
- Stage 3 — Local stability: web buckling, flange buckling, and torsional stability under the combined load.
In most crane girder designs, Stage 2 (deflection) governs the section depth, and Stages 1 and 3 then verify the section chosen for deflection.
Reference Sizing (Double Girder Box Sections)
These are preliminary starting points only. Final selection requires the full load and deflection calculation for your specific crane.
| Capacity | Span | Typical Girder Depth |
|---|---|---|
| 10 tonnes | 15 m | 700–850 mm box |
| 20 tonnes | 20 m | 900–1,100 mm box |
| 32 tonnes | 22 m | 1,100–1,300 mm box |
| 50 tonnes | 25 m | 1,300–1,600 mm box |
Part 5: Web and Flange Design Details
Inside the girder section, the web plates and flange plates each carry a distinct part of the load — and each has its own failure mode to guard against.
Flange Design
The flanges carry the bending forces: the top flange in compression, the bottom flange in tension under vertical load.
- Top flange: on a double girder, it also carries the crane rail and the trolley wheel loads directly, adding local bending. It resists most of the lateral side-thrust bending, so it is often wider or reinforced.
- Bottom flange: carries the tension from vertical bending. On single girder under-running cranes, the bottom flange is also the trolley running surface and must be wide enough for the trolley wheel tread with clearance.
Web Design
The webs carry the vertical shear and hold the flanges apart at the correct depth.
- Web thickness is set by shear stress and, critically, by buckling resistance. A tall, thin web can buckle under shear or under the concentrated wheel load before it reaches its stress limit.
- Web slenderness (depth-to-thickness ratio) is limited by CMAA and AISC provisions to prevent buckling.
Stiffeners
Because crane girder webs are deep and relatively thin, they need stiffeners:
- Transverse (vertical) stiffeners: placed at intervals along the web to raise its shear buckling capacity, and always at points of concentrated load (end connections, wheel load points).
- Longitudinal (horizontal) stiffeners: on deeper girders, to raise bending buckling capacity.
Stiffener welds are fatigue-sensitive details — their category and quality directly affect the girder’s fatigue life under repeated trolley passes.
Part 6: End Connection and Splice Details
The girder must transfer everything it carries into the end trucks, and — for long or shipping-constrained girders — hold together at field splices without becoming a fatigue weak point.
Girder-to-End-Truck Connection
The connection between the girder end and the end truck transfers vertical wheel reactions, lateral side thrust, and longitudinal tractive force. It is a fatigue-critical joint.
- Bolted connections using high-strength friction-grip bolts are common and allow field assembly and future adjustment.
- Welded connections give a stiffer joint but require careful detailing to avoid fatigue-prone weld terminations.
CMAA-compliant designs specify the connection type, bolt grade or weld detail, and stiffener requirements based on the crane’s duty class and the resulting fatigue category.
Field Splices
Long girders often cannot ship in one piece. A field splice joins girder segments on site.
- Bolted splices (spliced webs and flanges with cover plates and friction-grip bolts) are the preferred approach for crane girders, because a properly designed bolted splice avoids the fatigue risk of a field weld made in less-controlled conditions.
- Splices should be located away from the mid-span high-moment region wherever possible, and detailed to develop the full strength of the section.
The buyer’s checkpoint: ask where splices are located and how they are made. A field-welded splice at mid-span on a high-duty crane is a fatigue risk; a bolted splice positioned away from peak moment is the mark of a well-engineered design.
Part 7: Common Design Errors
These errors rarely show up on day one. They surface as tracking problems, weld cracks, and early replacement — costs that land squarely on your maintenance budget and equipment uptime.
Error 1: Sizing for Strength, Ignoring Deflection
The girder is chosen to hold the rated tonnage but never checked against the L/888 deflection limit. It passes a strength check and still sags visibly under load, causing trolley drift toward mid-span, higher drive loads, and difficult load spotting.
Error 2: Omitting Camber
The girder is fabricated dead-straight with no camber. Under working load it sits below level from day one. The trolley runs “downhill” to the center, wheel and rail wear accelerate, and the sag cannot be corrected in the field.
Error 3: Ignoring Lateral Side Thrust
The girder is designed for vertical load only, with no allowance for the 20% lateral trolley thrust. The top flange and end connections are under-designed horizontally, leading to trolley tracking issues and fatigue cracking at the connections within a few years of cyclic operation.
Error 4: Under-Stiffened or Over-Slender Web
A thin web is chosen to save weight and cost, without enough stiffeners. The web buckles under shear or under the concentrated trolley wheel load, distorting the section and initiating fatigue cracks at the web-to-flange weld beneath the rail.
Frequently Asked Questions
Q: How do I decide between a single girder and a double girder crane?
A: Use capacity, span, and required hook height as your guide. For capacities up to about 20 tonnes on moderate spans where cost matters most, single girder is usually the efficient choice. Above 20 tonnes, on long spans, or where you need maximum hook height in a fixed building, double girder delivers better capacity, stiffness, and long-term value. Ask your supplier to run both options against your actual lift height and duty cycle so you can compare total cost, not just purchase price.
Q: Why is the crane girder so much deeper than a normal building beam of the same span?
A: Because a crane girder is sized by deflection, not just strength. The L/888 deflection limit is far stricter than the L/240 to L/360 used for floor beams, and the trolley applies a moving concentrated load rather than a spread static one. Meeting that tight deflection limit requires more depth than strength alone would need — which is exactly what keeps the trolley running level and your positioning accurate.
Q: What is camber, and why should I insist on it?
A: Camber is a slight upward curve built into the girder during fabrication so it sits close to level under working load instead of sagging. A girder delivered dead-straight will droop below level as soon as it is loaded, and that cannot be fixed in the field. Confirming camber is specified on the drawings is a simple, high-value check that separates a properly engineered girder from a cut-rate one.
Q: Can I increase the capacity of an existing bridge girder later?
A: Rarely as a simple bolt-on. Raising capacity increases the trolley load, bending moment, deflection, and fatigue stress on a girder that was designed for the original rating. An upgrade usually means reinforcing or replacing the girder and rechecking the end trucks, runway, and columns. Treat any capacity increase as a system-level engineering review, not a minor modification.
Q: How long should a well-designed bridge girder last?
A: A girder designed to the correct duty class, with proper camber, adequate stiffening, and sound connection details, is engineered for the crane’s full service life — commonly 20 years or more under its rated duty. Fatigue failures almost always trace back to under-designed loads, missing camber, or poor weld detailing rather than the steel simply wearing out.
Conclusion
Bridge girder design is governed by a specific rule set that ordinary beam design does not share: deflection limits far tighter than a floor beam, built-in camber to keep the trolley level, horizontal side-thrust loads on top of the vertical tonnage, and fatigue-driven detailing at every weld and connection. The single-versus-double girder choice sets your capacity ceiling and hook height, and the section, web, flange, and connection details decide whether the crane runs true for its full service life.
Match the girder type to your capacity, span, and hook height. Let the L/888 deflection limit drive the section depth. Insist on specified camber. Design for lateral side thrust, not vertical load alone. And confirm the end connections and splices are detailed for your duty class. Do this, and you gain a crane that tracks straight, spots loads precisely, and delivers reliable equipment uptime with low lifetime cost.
Ready to evaluate a bridge girder matched to your exact capacity, span, and duty cycle? Contact our engineering team for a free bridge girder design review and a cost-reliability assessment for your facility’s crane system.