Flitch beam analysis

What is a flitch beam?

Flitch beams use a combination of timber and steel (iron in times past) and were commonly in times past as the primary beams on long span floors. SuperBeam checks flitch beam that use a Grade 43 (S275) steel plate flanked on either side by timbers bolted to each other through the plate. Most designers prefer to make the plate depth 20-25mm less than that of the timbers so that the steel will not protrude if the timbers shrinks.

Basis of calculation

In a properly constructed flitch beam the timber and steel members will deflect by exactly the same amount. The loading on the beam will therefore be shared between the timber and steel members in proportion to their EI values. The E value for steel is known and is almost constant. The E value of a piece of timber of a particular grade is much less certain. If a higher value is assumed, a greater proportion of the load will be taken by the timber members and vice versa.

SuperBeam (from version 4.12) uses three different E values when checking flitch beams. The minimum value is used to apportion the loads before checking that the bending stress in the plate does not exceed 180N/mm 2. The mean value is used to check the timber members. Using these figures generates worst case conditions for the steel and timber members respectively. The Emin value x K9 for two members is used to calculate deflection - this is the same E value that would be used to calculate the deflection of an unflitched beam.

Bolting of flitch beams

SuperBeam's flitch beam design is currently based on 'Timber Frame Housing Structural Recommendations' (TRADA publication TBL52:now out of print).

This assumes that in the first instance the load is taken by the timbers, the calculated proportion then being transferred from the timbers to the plate by the bolts over the main span, and then transferred back from the plate to the timbers at the bearings, from which SuperBeam calculates the required bolt spacing. Release 4.55 (April 2009) updated the flitch beam analysis to follow TRADA paper GD9, How to design a bolted steel flitch beam.

Consider a beam spanning 6m carrying a UDL of 10kN/m. The beam is built up of 2no 75 x 300mm C16 timbers flanking a 25mm x 275mm steel plate. The SuperBeam analysis shows that in the worst case the plate carries 0.819 (say 0.8 for ease of calculation) of the total load and the bolts therefore need to transfer 10 x 0.8 = 8kN/m to the plate across the span, and at each bearing transfer 30kN x 0.8 = 24kN back to the timbers. The permissible load on M16 bolts in this situation is 7.57kN. The maximum bolt spacing is therefore 7.57 x 1000/8 = 946mm over the main span. At each bearing 24/7.57 = 3.2 (i.e. 4) bolts are needed.

Having regard to the need to maintain the integrity of the beam, SuperBeam flitch beam printouts add a note suggesting that the maximum bolt spacing should be limited to 600mm where the theoretical figure is greater than this, and will also suggest staggering the bolts, placing them alternately a minimum of 2D from the edge of the plate and 4D from the edges of the timber members.

Flitch beam bearings

The above beam has end reactions of 30kN. Assuming that these are taken by the timber only, the required bearing lengths will be 30 x 1000/2.2 x 150 = 91mm (assuming there is no wane in the timbers). If the beam bears on a wall it will need to sit on padstones or bearing plates, as this stress is considerably in excess of what is permitted on 'normal' brickwork.

Lateral stability

Flitch beams have limited strength when analysed about the minor axis. You MUST ensure that any flitch beam is provided with adequate lateral restraint.

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© 2017 Greentram Software Last updated 23 May 2017