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Flitched Beams

Updated: Oct 10, 2022

A client has asked me about Flitched Beams and I thought it might be an interesting topic for others. As always, I am not going to be too technical and so this is going to be a quick introduction to the wonderful world of the Flitched Beam. As you will know, steel is stronger than wood because it can withstand compression, ( being squeezed ) much more than wood and it can withstand tension ( being stretched ) more than wood. However, a long slender piece of steel will still buckle and so if you can stop it from buckling under load, its ability to withstand the compressive forces will be greatly enhanced. So, if you bolt a fat piece of wood on either side of a piece of steel so that they act together, the steel will take the compressive forces and the wood will stop the buckling of the steel. Most typically a flitched beam consists of a plate of steel about 10mm thick and 150mm deep with a timber plate also 150mm deep and about 50mm wide on each side to form a sandwich. The assembly is bolted together with bolts along its length at about 300mm spacings.

In any beam spanning across an opening, the top of a beam is in compression and the bottom is in tension. The wood clamped on either side of the steel will help stop the top half of the steel buckling, thus enhancing the strength of the assembly much more than the wood could do on its own. Clearly, a piece of steel of the overall size of the assembly would be much stronger but much heavier too. Furthermore, the wood element is easier to fix too as you can screw or nail into it and the assembly can be assembled on-site with can be a blessing if access is restricted. Another very useful characteristic is that in a fire steel loses a lot of strength and collapses quite quickly, whereas timber initially burns until the surface becomes carbonized and chars which creates a protective layer. Also, wood does not conduct heat as well as steel, for these two reasons the Flitched beam performs better in fire than a steel beam. To ensure this we often make the timber constituent a little bigger, creating what is known as a sacrificial layer, so that the beam can be exposed to fire for say half an hour without collapsing.

I am not sure when they were first used but I am aware of Victorian flitched beams. I am also not sure where the name comes from and that would be interesting to know as the word flitch is used to describe many ancient items.

Written by Tony Keller – Building Tectonics Ltd.

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