Transfer slabs
Transfer slabs are typically used where there is a change in building use above and below a slab leading to a change in column layout between the floors. A transfer slab takes the load from the columns above (planted columns) and transfers the load to the columns below (supporting columns), without the use of transfer beams.
Although transfer slabs offer flexibility when determining grid layouts, they have significant structural implications which should be considered when selecting them as a design solution. Transfer slabs are structurally inefficient, requiring a thick slab across a large area when compared to typical flat slabs or an arrangement using transfer beams and slabs. The thickness of a transfer slab will typically be governed by high shear forces in sections of the slab between planted and supporting columns, and much of the slab will be underutilised as a result.
Significant load path uncertainty is another challenge when designing transfer slabs, regardless of the chosen design approach. The actual load path within the structure will depend on the construction and propping sequence of the building, which is unlikely to be known during the detailed design phase.
Transfer slabs are not widely covered by design guidance, which leads to different approaches being taken by designers. “Design of transfer slabs”, published in November 2024 by the Institution of Structural Engineers (IStructE), aims to address the lack of guidance. The guidance was developed through a collaboration between AECOM, IStructE, MPA The Concrete Centre, and Imperial College London. The guide is available to download for free from the IStructE website: https://www.istructe.org/resources/guidance/design-of-transfer-slabs/