Concrete transfer slabs – getting them right

The Institution of Structural Engineers’ Technical Director – and recent CABE Webinar Wednesday presenter – Patrick Hayes explains why it is crucial to understand the science behind concrete transfer slabs.

The public reflection in the aftermath of the Grenfell tragedy hasn’t focused solely on the single issue of flammable building cladding; all areas that could affect the safety of buildings have been placed under the microscope. One concern that has been raised relates to the use of concrete transfer slabs, particular in the construction of mixed use high-rise buildings. 

In a fascinating recent CABE Webinar Wednesday presentation (see below), the Institution of Structural Engineers’ Technical Director, Patrick Hayes, guided members through the technical issues that need to be considered when using concrete transfer slabs. We spoke to Patrick about why it is so important that these issues need to be considered.

"Concrete transfer slabs are a relatively new technology and were really driven by two things," explains Patrick.

"One was the change in planning policy around 2000, which drove people more towards mixed use high-rise developments, with commercial or retail at ground floor and then residential towers above them. The grids in those buildings don’t really align: generally you need quite open grids in commercial and retail, and smaller and more irregular grids in residential. The columns from residential can’t really run through.

"Traditionally we would have put a whole grillage of beams in to resolve that but, around about the same time, quite complex software packages were developed – finite element packages – which could analyse those as flat plates. That allowed people to take all the beams out and put a flat concrete plate between the changing grids. That is desirable to contractors because the formwork for putting beams in is very time consuming and expensive – it’s much simpler and cheaper to put flat formwork in and then pour a flat plate of concrete."

Tech failings

While such an approach appeared simpler and cheaper, it is now apparent there were significant weaknesses around the robustness of the technology involved.  

"There are a number of issues," Patrick notes. "The first is with the standards and the way they are written. They are not really written for this type of structure.

"Then there are two further issues. One is that people have utilised whole building models – they have put the entire building, all the frame, into a computer analysis and used that. Whilst that appears accurate it doesn’t model the way we actually build structures – which leads itself to redistribution – or the way that we actually design individual elements. That leads to inaccuracy in how the loads are applied to these slabs.

"Then, secondly, the software analysis packages haven’t traditionally handled the shear stresses involved properly. There are two types of stresses: bending stresses which cause the plates to deflect; and shear stresses which basically cause them to break. The computer packages haven’t traditionally accurately modelled that breaking effect."

However, as we mentioned, it was only in the aftermath of Grenfell that these weaknesses became apparent.

"Following the Grenfell tragedy, developers started to remove flammable cladding from buildings. In a number of cases, they noticed that buildings with transfer slabs had deflected and some of them were cracked. Concerns were raised about that and that some of these slabs appeared quite thin," says Patrick.

"The difficulty is that the type of failure involved is a brittle failure which shows very little sign before it actually fails, so you won’t necessarily see a problem. However, once assessed properly, you might be aware that you don’t have the level of safety that you really require in a structure. The concern is that there are structures out there that don’t have an adequate factor of safety and may be susceptible to long-term problems, or problems if they are modified in some way."

Beyond the new

That leads us to the importance of Patrick’s Webinar Wednesday presentation, which isn’t just the need to be aware of the factors around transfer slabs in the design stage – an issue that Patrick says has largely been successfully addressed – but in ensuring those buildings that were previously constructed with transfer slabs are safe.

"What we wanted to do first with the Institution of Structural Engineers’ guidance is really draw a line in the sand and ensure that buildings are properly designed going forward. This is a phenomenon that isn’t covered by standards. However, the building control profession and CABE have been really good in helping alert us to these issues and then acting as a gateway to building control," says Patrick.

"Now we are looking at existing buildings and, when it comes to those buildings, there is a slightly different approach. Whilst we can set the benchmark for how new buildings should be designed, with existing buildings we need to look at them on an individual basis and carry out more thorough assessments, which can be quite time consuming. That means our attention now is on how we can adapt the guidance to make it applicable to existing buildings. 

"Concrete transfer slabs are a very popular form of construction. If we get this wrong – and we have set out how this can go wrong – then this really is a potential safety issue.

"Watching the webinar (below) alerts people in building control for what they should be looking out for and tells designers and contractors how they should be executing these structures to ensure that they are safe because, most often, they have entire buildings resting on them."

The Institution of Structural Engineers’ FREE guidance to transfer slabs is available here.