While precast components can benefit project delivery, there is a significant amount of planning required for the designer. Early engagement with a specialist designer is essential for the successful delivery of the project. The safe lifting of precast elements has developed over many years, but in the Eurocodes for the design of structures, such as EN 1990(1) and EN 1992-1-1(2) (Eurocode 2), there is limited information available to the designer on how to design lifting assemblies – in particular, the lifting socket placed in the concrete. Chains, slings and lifting keys for the lift operation are all covered under the Machinery Directive 2006/42/EC (https://bit.ly/2NLEJYr) but it only covers external metal parts and not the components cast into the concrete.
A number of leading manufacturers formed a working group to clarify working practices and Standards for the production of lifting inserts to be placed into precast components. This resulted in a standard working procedure for manufacturers to produce equipment for the market and overcome this mismatch between Standards and EU law. The guideline produced was the VDI/BV-BS 6205(3). The document sets out the rules for the manufacturer of lifting components and testing regimes. A key point made by all of the manufacturers was that there should be no mix and matching of pieces from separate suppliers.
The VDI/BV-BS 6205 guideline has the objective of encouraging the sharing of information through the life cycles of precast elements. It goes on to cover fabrication work in the precast plant and recommendations for precast designers. It is not uncommon for a precast element to be rotated many times before it is ready to be lifted into place.
The planning of all these operations needs careful consideration, primarily to understand the forces imparted to the lifting inserts. A lift in the factory will see different load actions compared to those on-site. By employing a specialist in this field, all of these load actions should be considered and planned for the work’s programme. Colour coding of parts, introduced by leading manufacturers, allows compatible parts to be connected without risk. Colour coding and load ratings can be added to the model and tabulated on the parts drawing.
As more project designers consider the precast option for construction, so the complexity of the shape and form of precast elements increases. In many cases, it is still a simple process to determine the centres of gravity of precast elements, and the point of instability also needs to be considered.
Whereas beams and columns still form the bulk of components lifted, the consideration of components forming the whole structure is now the norm.