Question 1

What is a flitch beam?

Accepted Answer

A flitch beam consists of a steel plate (the flitch plate, typically 6-12mm thick mild steel) sandwiched between two timber members (typically structural hardwood or laminated veneer lumber, LVL) and the whole assembly bolted together with high-strength bolts at regular centres. The steel plate provides the tensile and compressive strength that the beam needs; the timber provides bearing surfaces, allows nailing of floor joists directly to the beam face, and provides fire resistance around the steel. The composite action of the bolted assembly gives a beam that is deeper but narrower than an equivalent all-steel section, making it easier to conceal within a floor zone and easier to connect joists to.

Question 2

When would you use a flitch beam instead of an RSJ?

Accepted Answer

A flitch beam is preferred over a steel RSJ (universal beam) in situations where: the beam needs to sit within the floor depth so that the floor is level (an RSJ requires the floor structure to sit on top of it, raising the floor level unless the beam is housed in a pocket in the wall); joists need to be nailed or screwed directly to the beam face (easy with the timber faces of a flitch beam, difficult with bare steel); the beam needs to fit within the same depth as the joists (an RSJ is typically deeper than the surrounding floor joists); or where the flitch plate can be cut from standard steel plate more economically than a rolled section. RSJs are preferred for very heavy loads or long spans where a flitch beam would require an impractically large steel plate.

Question 3

How is a flitch beam designed?

Accepted Answer

A flitch beam must be designed by a structural engineer. The design determines: the steel plate thickness and grade (typically S275 mild steel); the timber member size and species or grade (structural grade softwood, hardwood, or LVL); the bolt size, grade, and spacing (the bolts must transfer shear between steel and timber to achieve composite action); and the total beam depth and width needed to carry the specified loads within the allowable deflection limits. The timber members do not significantly increase the bending strength of the assembly (their contribution is small compared to the steel) but they are essential for bolt bearing, for connecting the joists, and for protecting the steel in a fire. A structural engineer's calculation and Building Control approval is required before installation.

Question 4

What are the fire resistance requirements for a flitch beam?

Accepted Answer

The timber flanges of a flitch beam provide inherent fire resistance by charring in a fire - the char layer insulates the steel plate behind it and slows the rate of temperature rise in the steel. The fire resistance of the flitch beam depends on the timber thickness available to char before the steel plate overheats and loses strength. For 30-minute fire resistance (the standard requirement for floors in a two-storey house), the minimum timber thickness on each side of the steel should be calculated by the structural engineer based on the timber species and density. If the timber faces are insufficient for the required fire resistance period, intumescent paint or fire protection boards can supplement the inherent timber fire protection.

Flitch Beam

Just so you know...