Flitch Beams
Whilst the ease of assembly and customizability of timber beams allow for convenient problem-solving solutions, sometimes the implementation of more heavy-duty hardware is required. In certain situations where timber alone just doesn’t cut it, but a full-blown steel is considered overkill, flitch beams make an appearance to provide a happy medium.
What is a flitch beam?
A flitch beam typically consists of an engineered steel flitch plate sandwiched in between two timbers, in a similar assembly style of a bolted timber beam. Some variations can include multiple steel flitch plates. A flitch beam is often much lighter and cheaper than a full depth steel beam/RSJ. They possess many of the desirable qualities of an RSJ, namely an increased load bearing capacity, whilst still providing a solid timber fixing for other components to connect into. The single plate is much easier to manoeuvre than a large steel, and in most instances can be assembled into a beam in situ, making the process much easier in contrast to lifting heavy steels.
Common uses for flitch beams include ridge beams for vaulted ceilings, long trimming or trimmer beams for the construction of roof light openings, as well as load bearing beams that support joist ends or structural studwork in large floor systems.
What are the criteria for the assembly and installation of a flitch beam?
Flitch beams can sometimes be a little tricky to assemble correctly in that the bolt placement within the beam is dictated by the pre-drilled holes in the flitch plate. A regular flitch plate will arrive from the fabricators with a typical 600mm staggered drilling pattern. That being said, a template can be taken to the fabricators to allow them to drill the holes in a more desirable location for our layout. This template can also be used to indicate any specific cut-outs on the flitch plate, for instance where the flitch beam may receive in the web of a larger RSJ. The cutting and drilling of the plate can taken place on site, though this can be unnecessarily time consuming, and typically requires the implementation of speciality tools. With a little planning and correct procedure most of the complications involved can be overcome, though it's still important to be precise during assembly. With the plate residing in between the timbers it's important to ensure that the alignment of the timbers stays true to each other as with a regular timber beam. Care should be taken to ensure the plate remains within the width of the timber and is not projecting out of the end of the beam, unless provisions have been made for this. The timbers should be crowned upwards in the same direction, as should the plate itself. Due to the manufacturing process of the steel plate, one might find that it has a crown to it, and this should be aligned with the crown of the timbers. Single sided timber dog tooth connectors should be used if required to provide stability to the holes bored through the timber. As with all structural components, an appropriate bearing must be available for the flitch beam during assembly.
How are flitch beams assembled and installed?
To assemble a flitch beam, take the two timbers whose sizes are specified on the engineer's drawing, where the use of a flitch plate is also specified. The steel flitch plate itself will have been pre-measured and cut to the appropriate length. The length of the timbers should be cut to accommodate the length of the flitch plate, as well as an extra bearing required to support the assembled beam. The width of the steel plate should be slightly less than the finished width of the timber specified on the drawings. Having cupped the timbers together in the correct orientation in accordance with the procedure for assembling regular timber beams, lay one timber down with the outside facing up and lay the steel plate on top of it. Make sure the plate is lined up end to end with the timber and isn't protruding past the end of the timber. Care should also be taken to ensure that the plate is centred along the width of the timber, so as to ensure that if any shrinkage occurs in the future, the steel will not protrude past the timber. Using a pencil, mark the holes through the steel onto the timber. Remove the plate and screw the timbers together as if you were making a double. Drill the holes, disassemble, then reassemble with the steel plate in the middle. The dual sided timber dogs don't work in this application, though varieties are available with teeth only on one side of the dog. This helps to reinforce the hole drilled in the timber, and prevent any of the components of the beam, from coming out of alignment with one another in the event of the shrinkage or movement in the future. The flitch beam is then ready to be installed in the desired location.
Key Notes:
The flitch plate must be round-sided in the same direction as the timbers.
Regular timber dog tooth connectors do not work in this scenario.