What causes the main chords on a lattice crane boom to buckle?

by | Aug 14, 2026 | Crane Repair

Lattice crane boom section lying horizontally in a workshop, main chord steel tubes visibly bowed from compression stress, with metal shavings and welding equipment nearby.

A lattice crane boom is an engineering marvel built to handle enormous loads over long spans. But like any structure under repeated stress, it has failure points. One of the most serious is buckling of the main chords. When a chord buckles, the entire boom loses its structural integrity, and what started as a manageable repair can quickly become a full-blown safety incident or a costly equipment loss. Understanding why this happens is the first step toward preventing it.

This article walks through the structural logic behind main chord buckling, the conditions that cause it, and what to look for during inspections before a small deformation turns into a lattice boom structural failure.

Structural role of main chords in a lattice boom

The main chords are the four longitudinal members that run the length of a lattice crane boom, typically arranged in a square or rectangular cross-section. They carry the primary compressive and tensile forces generated when the boom lifts a load. Think of them as the spine of the structure: everything else, the lacings, bracing members, and connecting nodes, exists to keep those chords aligned and stable under load.

When a crane lifts, the boom bends slightly under the combined effect of the load and its own weight. The lower chords experience tension while the upper chords take compression. It is the compressed chords that are most vulnerable to buckling. The longer the boom section and the higher the load, the greater the compressive force acting on those members. This is why lattice boom structural failure almost always begins at the main chords rather than the secondary bracing.

The most common causes of main chord buckling

Main chord buckling rarely happens without a reason. In most cases, it results from one or more of the following causes:

  • Overloading: Exceeding the rated capacity, even briefly, puts compressive forces on the chords beyond their design limit. This can cause immediate buckling or create micro-deformations that worsen over time.
  • Impact damage: A collision during transport, a dropped load, or contact with a structure can dent or bend a chord locally. Even a small local deformation dramatically reduces the buckling resistance of the entire member.
  • Weld defects or repairs of insufficient quality: Poorly executed welds at chord connections introduce stress concentrations. Under repeated loading, cracks initiate at these points and weaken the cross-section.
  • Corrosion: Surface corrosion reduces the effective wall thickness of chord members. On high-grade steel booms, even modest material loss can significantly reduce load-bearing capacity.
  • Fatigue from cyclic loading: Cranes that lift and lower loads thousands of times accumulate fatigue damage in the chord material itself, even when individual lifts are within rated capacity.

It is worth noting that high-grade steel booms, those made from steel up to 960 or 1100 N/mm², are particularly sensitive to heat input during any welding or cutting work. Incorrect repair procedures on these materials can reduce the steel’s strength locally, creating a weak point that behaves very differently under load than the surrounding material.

How operating conditions accelerate chord failure

The environment a crane operates in plays a significant role in how quickly boom chord damage progresses. Harsh conditions do not cause buckling directly, but they accelerate the underlying processes that lead to it.

Offshore environments are a clear example. Salt air and moisture accelerate corrosion on chord surfaces, particularly in areas where protective coatings have been scratched or worn away. Combined with the dynamic loading that comes from wave-induced vessel movement, offshore cranes experience a much more aggressive fatigue cycle than land-based equipment.

Temperature extremes also matter. In cold climates, high-grade steel can become more brittle, lowering its resistance to sudden impact loads. In hot environments like the Middle East, thermal expansion and contraction over daily cycles add stress to welded connections.

Operating on uneven ground or with poorly set outriggers introduces side loads into the boom structure. These lateral forces are not accounted for in standard load charts and can push compressed chords toward buckling much sooner than expected. Even seemingly minor issues, like consistently picking loads at the edge of the permitted radius, compound over time and accelerate boom chord damage.

Inspection signs that indicate chord buckling risk

Catching chord problems early requires knowing what to look for. A visual inspection alone is not always enough, but there are clear warning signs that warrant immediate closer examination.

Visual indicators

Any visible bow or lateral deviation in a chord member is a serious red flag. Chords should be straight along their full length. Even a slight curve, particularly in the upper chords under compression, indicates that the member has already begun to yield. Dents, flat spots, or wrinkles in the chord wall are equally concerning and should never be dismissed as cosmetic.

Cracking or flaking paint along a chord can indicate that the steel beneath has deformed. Rust streaking from weld zones suggests that protective coatings have failed at stress concentration points, exactly where fatigue cracking tends to initiate.

Measurement and testing indicators

Straightness checks using a taut string or laser alignment tool can reveal deviations that are not obvious to the naked eye. Magnetic Particle Inspection (MPI) is particularly effective at detecting surface and near-surface cracks in chord material and weld zones. Ultrasonic testing goes deeper, identifying internal flaws that visual inspection cannot reach.

Any crack detected in a main chord, regardless of its size, should be treated as a critical finding. Cracks in compressed members propagate rapidly under cyclic loading and can lead to sudden, catastrophic failure.

Repair vs. replacement: restoring a buckled boom chord

When a main chord shows signs of buckling or damage, the question becomes whether to repair it or replace the entire boom section. The answer depends on the extent of the damage, the grade of steel involved, and the quality of the repair process available.

For many operators, sourcing a new boom section from the original manufacturer involves long lead times and significant cost. A properly executed lattice boom repair can restore the chord to its original structural value at a fraction of that cost, provided the repair is carried out under certified conditions with the correct welding procedures for the specific steel grade.

This is where the steel grade becomes important. Repairing a boom made from 960 or 1100 N/mm² steel requires specialist knowledge and tightly controlled welding procedures. The heat input must be carefully managed to avoid reducing the steel’s strength in the heat-affected zone. A repair carried out without the correct Welding Procedure Specification (WPS) for that material can leave the chord weaker than before, not stronger.

A credible repair process should include a full preparation phase with material strength checks, precise measurements, and a documented repair plan. After the repair, 100% visual inspection and Magnetic Particle Inspection of all new welds are the minimum standard. Where the damage is more extensive, ultrasonic or X-ray testing by a third-party Notified Body provides an additional layer of verification. A repair that meets these standards keeps CE certification valid and gives operators confidence that the boom is fit for service.

How Rusch Cranes helps with lattice boom chord damage

We specialize in the repair of lattice and telescopic crane booms, including booms made from high-grade steel up to 1100 N/mm². We are one of only three companies in Europe with the capability to repair booms of this grade, and we carry out repairs both in our workshop in the Netherlands and on-site at client locations worldwide.

When you bring a buckled or damaged boom chord to us, here is what the process looks like:

  1. We assess the material strength and document every repair point with photographs.
  2. We prepare a Welding Procedure Specification and a full Repair Plan before any work begins.
  3. We carry out the repair under controlled conditions, whether in our workshop or at your location.
  4. We perform 100% visual inspection and MPI on all new welds, and arrange third-party testing where required.
  5. We deliver the repaired boom with CE certification intact and a one-year guarantee on the repair.

The result is a boom restored to its original structural value, at significantly lower cost and faster turnaround than sourcing a new section. If you are dealing with a damaged boom or want to assess the condition of your lattice crane, get in touch with our team to discuss what we can do for you.

Frequently Asked Questions

How often should lattice crane booms be inspected for chord buckling risk?

At a minimum, lattice crane booms should undergo a thorough visual inspection before each shift and a detailed structural inspection at regular intervals defined by the manufacturer and applicable standards such as EN 13000. For cranes operating in harsh environments — offshore, coastal, or in extreme temperatures — more frequent inspections are strongly recommended. Any crane that has experienced an overload event, impact, or unusual operating condition should be taken out of service and inspected immediately, regardless of its scheduled maintenance interval.

Can a buckled main chord be repaired without removing the entire boom from the crane?

In some cases, on-site repair is possible, and experienced specialists like Rusch Cranes do carry out chord repairs at client locations worldwide. However, whether an on-site repair is appropriate depends on the extent of the damage, the accessibility of the affected chord section, and whether controlled welding conditions can be maintained at the location. For high-grade steel booms (960–1100 N/mm²), workshop conditions are often preferable because the heat input and post-weld testing requirements are more difficult to control in the field.

What is the difference between fatigue cracking and impact damage in a main chord, and does it change how the repair is approached?

Fatigue cracking typically originates at stress concentration points — weld toes, notches, or areas of geometric change — and grows progressively under cyclic loading. Impact damage, by contrast, causes immediate local deformation that reduces the chord’s buckling resistance right away. Both are serious, but they require different repair strategies: fatigue cracks must be fully excavated and the root cause addressed, while impact damage often involves reshaping or replacing the deformed section. In both cases, the steel grade dictates the welding procedure, and a proper Welding Procedure Specification (WPS) is non-negotiable regardless of the damage type.

How do I know if my crane boom is made from high-grade steel, and why does it matter for repairs?

The steel grade used in your boom is typically documented in the original manufacturer’s documentation, the CE declaration of conformity, or the crane’s load chart booklet. If you are unsure, a material strength check — which should be part of any credible repair assessment — can confirm the grade. It matters enormously for repairs because high-grade steels (960–1100 N/mm²) are highly sensitive to heat input: incorrect welding procedures can reduce the steel’s yield strength in the heat-affected zone, leaving the repaired chord structurally weaker than before the damage occurred.

Will repairing a buckled boom chord affect the crane's CE certification?

A properly executed repair, carried out with the correct WPS for the specific steel grade and followed by full inspection and third-party verification where required, should keep the CE certification intact. The key is documentation: the repair must be traceable, with a documented repair plan, inspection records, and MPI or ultrasonic test reports. A repair that lacks this documentation trail — for example, one carried out by an uncertified workshop without proper NDT — can invalidate the CE marking and expose the operator to significant liability.

What are the most common mistakes operators make that accelerate boom chord damage over time?

The most frequent mistakes include consistently operating at or near the maximum rated radius, neglecting to check outrigger setup on uneven ground (which introduces unintended side loads), and ignoring minor paint cracking or surface corrosion on chord members. Many operators also underestimate the cumulative effect of transport impacts — small dents from loading and unloading may seem cosmetic but can meaningfully reduce a chord’s buckling resistance. Establishing a culture of pre-shift inspection and acting on early warning signs before they escalate is the most effective way to extend boom service life.

Is it ever more cost-effective to replace a boom section rather than repair it?

Yes, in certain situations replacement is the better choice — specifically when the damage is so extensive that the required repair volume would approach or exceed the cost of a new section, or when the boom section is so old that its overall fatigue life is nearly exhausted. A credible repair specialist will assess both options honestly and provide a clear comparison of cost, turnaround time, and long-term structural confidence. In most cases involving localised chord damage on a structurally sound boom, a certified repair delivers equivalent structural value at significantly lower cost and with a faster return to service.

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