Case Studies

Truck Failure The review of an incident where a container fell off a truck whilst tipping

Executive Summary

Ingenia was contacted and attended site to assist with the investigation of a tipped trailer. Ingenia was requested to provide a verbal and written opinion on what were likely contributing factors in the truck incident.

During the site visit the damaged trailer and tipped bulk solids container were inspected.   The standard discharge procedure of the tipping trailer was outlined.

The incident was described to have occurred as the container was being tipped rear-ward and it began to lean towards the passenger side. The lean was noted, however the tipping operation of the container was not stopped or able to be stopped before collapse. The instability of the trailer reached a critical lean angle and the container toppled from the trailer mounting points and fell to the ground.

During the inspection, significant damage was noted to the container and truck including a broken rear container pivot, buckled lifting ram and attachments.

This report is based on the following assumptions:

  • The truck was loaded within appropriate levels (peak capacity used for analysis).
  • Failed pin material properties are consistent with Carbon Steel 20MnV6 or EN 10294-119MnVS6 Hollow Bar.
  • The activities leading to the incident are assumed to be consistent with those discussed on site. Alternate methods may increase vehicle loads or reduce stability of the trailer and container.
  • The use of shunting or jolting the trailer to release resistant product from the container was not undertaken. This can provide a large impulse loading through the truck, pivots and supports.
Failure Mode

The most likely main cause of the failure is an off-centre load leading to instability of the container on the trailer. This is based on calculation of the load capability of the supports, along with a review of witness marks on damaged parts. From examination the material in the container was “sticky” preventing it flowing properly out of the container.  This contributed to the instability by raising the centre of gravity. The driver’s side rear container twist lock also appear (from witness marks) to not have retained the container effectively. This would have further decreased stability of the trailer as the container began to lean, since the container could easily lift away from the trailer at this corner. Witness marks show only minor damage to the rear driver’s side twist lock, indicating that it pulled out easily without deforming the trailer bracket.  It was not possible to determine if the lock was defective or if was not engaged fully.  (Please note that if a better lock had been fitted, initial stability would have been improved, but we cannot determine if it would have prevented the accident.  Also, had a more adequate lock been fitted, the severity of the collapse may have been worse, causing the prime mover to tip over as well, rather than the container separating from the trailer.) The breaking of the passenger side rear support pivot, Rotating Rear Lifting Transverse Bolster, along with the collapse of the lifting ram were, in our assessment,  consequences of the instability and not the cause.

Recommendations
  1. Implement procedures and processes to minimise or eliminate sticky material to ensure balanced loads and quick discharge.  We cannot advise how easy this would be to implement and manage, but if loading can be managed to prevent imbalance and ensure flow-ability then accidents will be prevented.   Alternatively, implement adequate inspection prior to lifts to determine if loads are safe to lift. Again, we are not sure if this can be implemented effectively.
  2. Improved hold down anchors will help resist the toppling of the container, at least up to a critical lean angle. This will increase the initial stability of the containers. This increased stability is likely to lead to a reduction in the incident rate, however we cannot guarantee that accidents will be prevented. Also if a significant instability occurs the severity of the accident could be worse.  Therefore we recommend that a risk assessment is conducted o determine the value of improved locks.  
  3. Investigate device/s to detect imbalance early and warn or prevent further lifting.  Without detailed analysis, we believe that the monitoring of differential suspension air bag pressure or strain gauges on the rear support posts may effectively measure imbalance.
  4. A fatigue crack was found in the failed passenger support pivot, Rotating Rear Lifting Transverse Bolster.  This did not cause the accident but contributed to the support failing during the event.  Left un-detected this crack would have led to a collapse eventually. Mag-particle crack inspection of supports should be implemented on an annual basis.
  5. Check trailer suspension components for damage and repair appropriately.  Determine if any defect is present in the suspension that could have contributed to imbalance under load.

Introduction

Ingenia was contacted and attended site to assist with the investigation of a tipped trailer accident. Ingenia was requested to provide a verbal and written report on the likely contributing factors to the truck incident.

The incident occurred within an offloading shed. During Ingenia's inspection, the local site was roped off to restrict access. The trailer was in a stable condition and supported on jacks to allow for the removal of the truck.

After partial tipping and offloading of the trailer, the container had fallen to the passenger side of the trailer. A proportion of the sand remained in the container.  This indicates that the sand had not been fully unloaded from the container prior to the collapse and the system was under load at point of failure.

An inspection of the damaged truck and tipped container was undertaken, followed by an onsite discussion of the truck operation, methods and practices, and potential factors leading to the failure. During the visit a local trailer manufacturer attended site to provide their opinion on the failure. Potential failure methods were debated and potential improvements in design were discussed.

During Ingenia's site visit a recent previous trailer tipping incident was discussed. This failure occurred nearby in an open and exposed off-loading station. The current failure was investigated as an independent incident, however some similarities were noted.

Definitions

The main components of the trailer and tipped container discussed within the report are summarised in the following points. Reference terms and names associated with each item are also discussed. 

  • Corner Blocks or Corner Castings are the locations on the container provided for the locating and constraint of the container to the tipping trailer. Four corner blocks are constrained during standard operation of the tipper.
  • Twist Lock Assemblies are the assemblies used to constrain the container corner blocks at the truck tipping locations. References to tie down mechanisms, lock down pins, corner constraints or fasteners refer to the container twist lock assemblies.
  • Rotating Rear Transverse Bolsters are the pivots on which the constrained container rotates during the lifting of the container. References to rear pivots, rear pins, or similar within this report refer to the rear bolsters on which the container pivots.
  • Front Lifting Transverse Bolsters are the front framework supporting the raised end of the container during tipping of the container. References to the front lifting frame, or ram connecting frame refer to the front lifting bolsters.
  • Hydraulic Tipping Ram is the means of lifting the front lifting transverse bolster during the tipping of the container. References to the lifting ram, tipping ram or similar refers to the hydraulic tipping ram used to tip the container during discharge.
  • Cylinder Clevis is the connection of the hydraulic lifting ram to the trailer. References to the cylinder mount or trailer ram connections refer to the cylinder clevis.

Container Tipping Process

A typical container tipping video was viewed, however no photos or video were captured of the trailer tipping during the incident. The following sequence of events was identified through observation of site, and during discussions.

  • The container was filled off site and was brought to the off-loading station by rail, and on sealed, bitumised or concrete roadways.
  • The truck and trailer were driven to the off-loading shed (closed by walls on 3 sides) and reversed into the levelled tipping location.
  • From accounts and documentation, container clampdowns were checked by the operators and no defects where identified.
  • Operators moved themselves to the designated safety area for load tipping.  The container was lifted by the driver sitting in the cabin of the Prime Mover.
  • Tipping of the container truck was initiated by the operator. (The container is raised by a hydraulic ram on a pivot).
  • The operator noticed that the container was not tipping straight and that the container was leaning towards the passenger side.
  • Tipping of the container continued as the operator deemed it was safe to continue.
  • The operator observed that the driver side trailer wheels lifted from the ground as the container continued to tip further to the passenger side as it was being raised.
  • Restraint of the container was lost from the trailer (broke free from driver's side Rotating Rear Lifting transverse Bolster) as it was partially released. 
  • The operator observed that the hydraulic cylinder broke as the container fell to the passenger side of the trailer.
  • The trailer wheels returned to the ground after the collapse.
  • The trailer jacks were operated to allow the Prime Mover to be disconnected and removed from the shed.
  • The incident area was cordoned off.

Site Observation and Discussion

General Site Observations

The site had been inspected by other parties prior to Ingenia’s arrival. Alterations to the site from its “as failed” condition included:

  • The movement of a broken rear container pivot to allow inspection of the broken component.
  • The discharged sand and the sand still within the tipped trailer had been walked on and compacted from its original un-disturbed levels.

 

All tyres appeared to have consistent tyre pressures (visual inspection). No significant lean was evident in the trailer following the incident.

Bulk Solids Container

From the remaining sand in the container and the dumped load at the rear of the trailer, we conclude that a relatively small amount of product (~10-15%) had been discharged from the container before failure. This is consistent with a “clumpy” or “sticky” load remaining within the container. Our view is that the load was not free flowing, and caused the side toppling loads on the pivot structures as the container was lifted. the centre of gravity was raised significantly during lifting, and the sticky load was off-centre causing overbalancing forces.

Within the tipped container there was a visible outline of the product on the overturned walls. This was from sand which stuck to walls of the container before the container toppled. This provides evidence of moisture within the sand during the container tipping incident. Accurate moisture content of the sand within the toppled trailer is unknown.  From observation it was clearly moist and sticky.

Trailer and Container Damage

Damage included:

  • Rear of truck (Container landing on truck)
  • Container - bottom (Container landing on truck)
  • Container - top (impact of load and “slosh” of sand against roof).
  • Failure of each hydraulic ram mount
  • Buckling of hydraulic ram cylinder 
  • Impact damage to cement ground
  • Overloaded pivots and components

This documented damage list is not exhaustive. Inspection of the truck and container shall be undertaken to determine the full extent of damage and required repair/replacement.

Trailer Identification and Name Plate

The name plate and registration data was documented to identify the trailer type and key details, and to aid the traceability for any future reviews.  (Figures removed)

Trailer Pivot (Rotating Rear Lifting Transverse Bolster) Failure

The passenger side pivot was broken off at the welded connection to the triler frame. Further detail of the filure surface was undertaken.

The pivoting shaft remained attached to the locating spring as the container mount toppled with the container. This is consistent with functioning passenger side container lockdowns as the shaft remained locked to the toppling container during an excessive loading event.  In our assessment, this damage was a symptom of the container falling and was not the cause.

The container was released by the driver side rear twist lock mount during the event. The damage to the corner casting at the driver side rear pivot was significantly less than appeared in photos viewed of similar events. The bracket is burred and not bent indicating that the pin pulled out before bending the housing.  This indicates that the container was not retained effectively.

Rotating Rear Transverse Bolsters Review

Review of Loss Area

The approximate shape of the failed pin was sketched with modelling software to accurately approximate the lost area at failure. The lost (dark) area is approximately 1,700 mm2 of the complete 9,425 mm2 cross sectional area. This equates to approximately 18% material loss at failure.

The sketch was also used to determine the section modulus of the final area. This was used within the manual calculations of capacity.

Pivot Failure Surface

The surface of the pivot failure was not uniform. Various areas of failure are evident in the failure profile. Oil was released onto the failed shaft during the failure. This has contaminated the failure surface. This may be observed from the blue tint and darker lower section of the pivot.

Areas evident in failure cross section:

  • Initial fatigue - Upper dark smooth surface, forming approximately 5% of section area
  • Secondary Fatigue - Appears rough, dark, and rusted, approximately 15% of section area
  • Abrupt Rupture - Rough and light colour, approximately 80 % of area.
  • No bending (plastic deformation) of the pivot was obvious, indicating high yield strength and low ductility, consistent with high strength steel or sudden very high load.
Manual Calculations of Capacity

Various assumptions were made to complete a manual analysis of the pivot pin. The assumptions include:

- Combined container and product design load of 15 tonne.

- Approximate loading at peak tipping angle and the load shared evenly between the rear pivots (68kN vertical and 3.25kN horizontal (6.9 tonne)).

- Bending loading is applied at the centre of the pivot and pin interface.

- Pivoting pin material is Carbon Steel 20MnV6 or EN 10294-119MnVS6 Hollow Bar.

- Yield stress of 420 MPa and an Ultimate Tensile stress of 635 MPa.

Note: alternate loadings or material properties will alter the calculated capacity of the pin.

The bending moment load capacity of the bolster in “as new” condition is  22.5 tonnes which is 3.3 times lifted load. The load capacity of the cracked bolster is 16.2 tonnes which is 2.3 times lifted load.

In summary, the load of the container and contents would need to be significantly greater than the assumed loads under standard lifting conditions to generate the loadings required to fail the pivot.

Dynamic Load Condition

Shunting to assist in the release of resistant product from the container, or rapidly raising and lowering the load, would significantly increase the loads experienced. Under the assumed load and unfavourable operation (dynamic factor of 2G- maximum brake force), the safety factor of the "as new" bolster would be reduced to 1.7 and the fatigued bolster, 1.15. Operation within this level of stress would increase the risk of fatigue. This is to be avoided and is inconsistent with the procedure and description of the failure event. Dynamic loading of the pivot is not considered to be the cause of the failure in this case.

Material Welding

Steel material data sheets show Carbon Steel 20MnV6 or EN 10294-119MnVS6 Hollow Bar may be welded without preheating, up to a thickness of 25mm. The 30mm wall of the hollow bar may require additional weld preparations and treatment to reduce the impact of the heat effected zone and reclaim material strength. Welding the material without consideration of heat input may weaken or reduce the local area about the welds. This would reduce the capacity of the bar from its ultimate tensile capacity. This will not cause the failure alone, under standard loadings.

Finite Element Analysis of Bolster

A model of the Rotating Rear Transverse Bolsters was developed. The model was analysed for discussion of loads andassessment of stress concentration.

Various assumptions were made to complete analysis of the bolster:

- Combined container and product design load of 15tonne.

- Approximate loading at peak tipping angle and the load shared evenly between the rear pivots (68kN vertical and 3.25kN horizontal (6.9 tonne)).

- Bending loading is applied at the centre of the pivot and pin interface.

- Pivoting pin material is Carbon Steel 20MnV6 or EN 10294-119MnVS6 Hollow Bar.

- Yield stress of 420 MPa and an Ultimate Tensile stress of 635 MPa.

Note: alternate loadings or material properties will alter the calculated capacity of the pin.

This analysis shows the stress concentration at the connection to the stiffening gusset between the bolster and frame. It can be seen that, under standard loadings, the stress within the pivot body connection is less than half of the yield limit of the material (210 MPa). 

Analysis suggests that the failure of the Rotating Rear Transverse Bolster in its fatigued condition is not possible under standard loading conditions. The failure of the Rotating Rear Transverse Bolster is a consequence of the instability event, and is not the cause.

Hydraulic Ram

A single ram was used to lift the load during tipping of the container (see Figure 1). The ram is centrally mounted to the trailer through two clevis mounts and a pin. As the ram is extended the container is lifted and the ram pivots to maintain the connection to the Front Lifting Bolster. Lateral movement of the hydraulic ram is limited by the clevis mounts.

Witness marks confirm sudden rupture which can only be caused by significant overloading. Side loading (prying) applied to the cylinder mounts increased until bolt failure, clevis failure and constraint of the hydraulic ram was lost. This side load required to break the mounts is, in our view, a result of the excessive lean of the container. at critical lean angle, the cylinder collapsed.

The driver side tension and the passenger side compression of the clevis is consistant with the lean of the trailer.

The driver side ram pivot appears to have failed under tension as the bolt hole is raised from the trailer mount surface. This is consistant with resisting the rotation of the ram pivot.

The passenger side ram mount failed as the bolt was pulled out through the thin side of the mount. A single failure surface is visible on each of the failure surfaces. Following the failure of the side wall of the mount, the ram pivot had freedom to move and allowed the ram to release and fall with the container.

It is our view that the failure of the ram was not the lead cause of the tipped trailer. A functioning ram lifted the load and resisted the fall during toppling, but the mounts could not resist side and prying loading as the container toppled.

Fault Tree Analysis

A fault tree analysis was undertaken to investigate the likely root cause of the fault. Various conditions leading to the outcome were assessed, based on theevidence collected and the witness marks. The most likely contributing factors for this failure are highlighted in red, while yellow indicates a reduced likelihood and green indicates a low likelihood of the factors being the direct cause of the failure.

The most likely failure mode was determined to be the off-centre load leading to instability of the trailer. It is our view that the risk of failure was increased by a “sticky” load and an ineffective rear driver side twist lock allowing the pull out of the twist lock pin form the corner container latch. This had the secondary effect of overloading the hydraulic cylinder clevises and the passenger side rear bolster.

Improved hold down anchors will resist toppling of the container. This will increase the stability of the containers tipping trailer. The increased stability is likely to lead to a reduction in the incident rate, however an increased severity of failure may be introduced.

Regular inspection and maintenance can assist in capturing fatigued and broken or damaged components. This alone would not have prevented the trailer from tipping, however it can prevent future catastrophic failure of components.

Recommendations

  1. Implement procedures and processes to minimise or eliminate sticky material and obtain balanced loads.  We cannot advise how easy this would be to implement and manage but if loading can be managed to prevent imbalance and ensure flow-ability then accidents will be prevented. Alternatively, implement adequate inspection prior to lifts to determine if loads are safe to lift. Again we are not sure if this can be implemented effectively.
  2. Improve hold down anchors to help resist the toppling of the container at least up to a critical lean angle. This will increase the initial stability of the containers. This increased stability is likely to lead to a reduction in the incident rate, however we cannot guarantee that accidents will be prevented. Also if a significant instability (uneven load) occurs the severity of the accident will be worse. Therefore we recommend that a risk assessment is conducted to determine the value of improved locks.  
  3. Investigate device/s to detect imbalance early and warn or prevent further lifting.  Without detailed analysis, we believe that the monitoring of differential suspension air bag pressure or strain gauges on the rear support posts may effectively measure imbalance.
  4. Implement mag-particle crack inspection on an annual basis. A fatigue crack was found in the failed passenger support pivot. This did not cause the accident but contributed to the failure of this support during the event.  Left un-detected this crack would have led to a collapse.
  5. Check trailer suspension components for damage and repair appropriately.  Determine if any defect is present in the suspension that could have contributed to imbalance under load.