This is not the truck

WORK PLACE HEALTH AND SAFETY BULLETIN ALBERTA

Weld Failure Concrete Boon Pumping Truck

AL031 - Alerts 1 April 2005 Concrete Pumping Truck - Boom Failure
An incident recently occurred in which a worker was struck and killed by the boom of a concrete pumping truck. This style of concrete pumping truck conveys concrete to a desired delivery point through a delivery pipe that is permanently attached to a versatile hydraulic boom. This equipment is considered to be powered mobile equipment under Alberta's Occupational Health and Safety (OHS) Code.

The concrete pumping truck involved in the incident was rigged up and the boom on the concrete pumping truck was fully extended in a horizontal position. The maximum length of the boom in this position was 38 m. The hopper was filled and the concrete line was primed. The concrete pour had just commenced when the boom suddenly collapsed and fell to the ground striking one worker. The king post tube in the turret assembly sheared off, resulting in a catastrophic failure. The king post tube is a cylinder approximately 1 m long, 33 cm in diameter with a wall thickness of 5.5 cm

A typical turret assembly
The king post tube was top welded to the turning column, which is what supports the base of the boom. The lower plate of the turning column was welded around its perimeter approximately 34 cm form the top of the king post tube. The assembly then fit into the turning base, allowing the boom to swivel. The resulting investigation by Workplace Health and Safety revealed that:

[A] The fracture of the king post tube occurred along the weld below the lower plate of the turning column.
[B]The turret assembly containing the king post tube was manufactured offshore presenting problems with identification of the following;

Boom
Turning Column
Lower Plate
Point of Fracture
King Post Tube
Turning Base

AL031 - Alerts 2 April 2005
Materials and physical testing. Tensile tests conducted on the fractured material showed that the material used had a strength less than the capacity required by the original design. Concrete Pumping Truck - Boom. Proper welding procedures were not used while welding the king post tube to the lower turning column plate, resulting in brittle fracture, fatigue cracking and eventual failure of the king post tube. The concrete pumping truck involved in the incident had been in service for 10 months. Further investigation revealed that a similar king post failure occurred several months prior to this incident during a load test by the manufacturer in their yard. A few weeks after the fatal incident another similar unit was found in Alberta with a six-inch long crack in the king post tube below the lower plate on the turning column.

The following is a summary of incidents involving concrete pumping trucks in Alberta and British Columbia in the past three years: A total of 20 incidents involving truck-mounted concrete pumping units with integral placing booms have been investigated in the past three years. There were 17 incidents involving equipment failures and three incidents involving power line contacts. The equipment failures consisted of 14 failures from design or manufacturing deficiencies, two failures due to inadequate inspection and maintenance and one failure was reported to be the result of unsafe operating practices. The failures occurred in rotation drive components, an outrigger, a boom linkage, elbows, boom rods, cylinders, welded connection points, a pedestal and a king post tube failure.

Most of the equipment failures were on machines that were less than one year old with many only a few months old. It should also be noted that the incidents reported are not limited to one manufacturer of this type of equipment. In addition, a survey of recent reported incidents elsewhere in Canada and the United States shows that concrete pumping trucks are involved in overhead power line contacts and loss of stability due to improper placement of outriggers on unstable soil. Currently there is no CSA Standard for concrete pumping trucks. The information obtained from the incidents identifies the need for a

AL031 - Alerts 3 April 2005
standard for this equipment from which appropriate design criteria could be established. This will ensure there is a consistent, acceptable level of safety for those who work around and under the equipment. Until such time that a standard specific to concrete pumping trucks is recognized in Alberta, the employers and suppliers must comply with the Occupational Health and Safety Act, Regulation and Code. The equipment supplier has an obligation to ensure that any equipment it supplies is in safe operating condition. The employer must also ensure the equipment is maintained, inspected and certified as required by this legislation.

AL031 - Alerts 4 April 2005
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