Precast concrete element fails during erection
WorkSafe is issuing a reminder of the risks associated with design, manufacture and erection of precast concrete elements following a recent incident.
Two workers narrowly avoided being seriously injured or killed when a nearly 10 tonne precast concrete element snapped in two during rotation whilst suspended from two cranes.
Project structural engineers (PSE) typically only design precast concrete elements for their final permanent position in the building or structure, however these elements are exposed to many forces during manufacture, storage, transport, lifting, rotation and erection.
If an element is not designed and manufactured to be robust enough to resist these forces, it may fail during rotation, lifting or erection. Workers and others near the failed element may be struck by falling sections of concrete and steel. These sections could weigh several tonnes each.
Several factors that have the potential to cause a precast concrete element failure during rotation include:
- poor design of element shapes and element 'break-up'
- insufficient concrete strength at the time of lift
- inadequate, incorrect or missing reinforcement in the element and lifting inserts
- inadequate specification of the strong-backs and associated fixings
- using strong-backs and associated fixings that don’t align to the engineer's specification
- incorrectly rigging the element
Recommended ways to control risk
The builder or principal contractor should ensure that precast concrete elements are appropriately designed to resist all forces they will likely be exposed to.
A suitably qualified person, such as a structural engineer experienced in erection design, should be engaged to develop a safe system of work for the handling, transport, lifting, rotation and erection of precast concrete elements. This person is referred to as the erection design engineer (EDE).
The EDE should review the precast concrete element designs and specify any additional reinforcing or strong-backs required to ensure the element is safe for transport, lifting, rotation and erection into its final position.
The EDE may not always be directly engaged by the builder or principal contractor. However, both the builder or principal contractor and precast erector should ensure that a suitably qualified person has been engaged and has undertaken the functions of the EDE before erection works begin. More information on the role of the EDE can be found in WorkSafe's Information about: Precast and tilt-up concrete construction.
The following critically important factors should be addressed to reduce the risk of a precast concrete element failing during rotation. Please note that this is a non-exhaustive list:
Prior to element manufacture
The PSE and builder should consult with the EDE and precast element manufacturer to ensure that the precast element ‘break up’ design achieves element shapes that are robust and can be handled safely, where possible without the need for strong-backs and additional reinforcement.
The EDE should carefully consider all forces that the element will be exposed to during lifting, rotation and placement and:
- review the design of the element to determine whether additional reinforcement or strong-backs are required to ensure that it can be handled safely, especially where the element has cut-outs or penetrations
- provide the precast manufacturer with marked up structural or shop drawings that accurately and clearly show the necessary lifting and bracing insert locations and any additional reinforcement or strong-backs required
- where strong-backs are required, the strong-back type and associated fixings should be accurately and clearly specified
- produce rigging designs for the various lifting stages required (demoulding, pre-transport, rotation, installation)
- review and approve the updated shop drawings (prior to them being issued for construction), to ensure that the lifting and brace inserts and any additional reinforcement and strong-backs are correctly detailed on the drawings
- following completion of the above steps, issue the EDE's certificate of compliance
During element manufacture
The precast manufacturer should ensure:
- the shop drawing for the element has been reviewed and approved by the project design engineer (PDE) and EDE prior to commencing manufacture
- a suitably competent person, who was not involved in the original set-up, has ensured all reinforcement and components comply with the approved shop drawings before casting
- minimum specified concrete strength is achieved before initial lift of the casting bed
- all strong-backs are installed as per the EDE's specification prior to lifting or transport
- non-specified and non-structural strong-back components and fixings are not used
- minimum specified concrete strength has been achieved prior to transporting the element to site
- they have completed and issued the manufacturer’s certificate of compliance (birth certificate) to the builder or principal contractor
- any variations to the original design (including changes to strong-back or component specifications) are approved in writing by the EDE
Prior to lifting the element on-site
When the precast concrete element arrives at site, but prior to lifting, an appropriately licenced rigger should:
- inspect the element to ensure it aligns with the shop drawing (dimensions, cast-in plates, inserts and ferrule locations all match the drawing) and that the element and components are free from damage
- confirm the lifting inserts match the shop drawing specification and are free from damage
- confirm the concrete adjacent to the lifting inserts is in good condition and free from significant cracking, honeycombing or damage (if in doubt seek advice from the EDE)
- ensure strong-backs are all in place and as per the shop drawing specification (type, size, location, orientation, fixings)
- review the birth certificate for the element to confirm the casting date
- confirm the lifting clutches are compatible with the lifting inserts, and inspect all rigging gear prior to use
- ensure the element is rigged in accordance with the relevant rigging design, being sure to use the correct lifting inserts where there are multiple inserts present
- stop the lift and seek advice from the EDE if any of the above safety critical information is missing or if there are any issues identified during inspection of the element.
Under the Occupational Health and Safety Act 2004:
- employers have a general duty to provide and maintain for employees a working environment that is safe and without risks to health. This includes a duty to provide systems of work that are so far as reasonably practicable safe and without risks to health. This duty to employees includes contractors engaged by the employer, where the employer has control over their work
- employers and self-employed persons also have a duty to ensure other persons are not exposed to health or safety risks from the conduct of their undertaking