What are Flitch plates?

What are Flitch plates?

Flitch plates are steel plates that are bolted or welded to the sides of wooden beams to increase their load carrying capacity and stiffen the beams. They are typically used in situations where the wooden beam alone is inadequate to support the design loads.

Wooden beams have been strengthened with steel plates since centuries. Traditional timber framing techniques used heavy timber sections which are not sustainable today. Flitch plating provides an efficient way to enhance strength of smaller wood sections for modern construction.

Flitch plates are commonly made from structural steel grade S355 and available in thickness from 5mm to 20mm. They are usually about 50mm wider than the beam on each side. The higher grade steel provides increased strength to resist tension and compression forces. Thicker plates offer more rigidity against beam buckling and lateral torsional forces.

Installation involves bolting or welding steel plates on two opposite sides of the timber beam length. This converts the flexible wooden beam into a rigid composite member for higher loading. The flitch plates also protect the inner wood core from damage.

For bolted connections, the beam is sandwiched between the steel plates with bolts spaced at regular intervals. Welded flitch plates are joined through intermittent fillet welds or shear connectors like nails, screws or dowels. Periodic tightening of bolts may be needed to maintain rigidity.

Flitch plated beams are commonly used in long span structures like bridges, residential floors, railway sleepers, mine tunnel supports and retaining walls. They provide increased strength at reduced timber volumes compared to un-plated beams.

In bridges, flitch beams can support heavier vehicle loads and wider deck spans. They also suit restoration of older bridges with undersized beams for modern loads. Skew flitch plated beams at abutments accommodate bridge decks at curved alignments.

Buildings employ flitch beams for heavier loads in long-span floors and flat roofs. They allow longer distance between support columns for open spaces. Vibration serviceability performance also improves in floors with flitched beams.

Flitched columns provide enhanced compressive strength for high-rise buildings and tall industrial structures. They offer ductile failure unlike brittle crushing of timber columns. This additional safety is useful in earthquake zones.

Curved flitched beams can support unique architectural shapes and spaces. Pre-cambered versions allow better deflection control in long spans. Weathering steel grades on exterior flitch plates improve durability.

Advanced connection methods include shear tread plates for friction grip, fanged bolts that grip the inner timber core and serrated intumescent paints for fireproofing. Optimized positioning of bolts or welds reduces stress concentrations.

Design challenges for flitch beams include connection slippage, difference in stiffness of wood and steel, natural timber shrinkage over time and corrosion protection. Thicker gauge plates, tighter bolts spacing and gap filling adhesives can counter these effects for composite action.

In summary, flitch plates provide a lightweight and sustainable strengthening solution for undersized wood beams across diverse structural applications. They efficiently supplement the beam stiffness using relatively small amounts of steel. Flitched wood-steel beams thus optimize material usage and costs in modern construction. With appropriate connection detailing, flitch plating facilitates long span designs from an engineering perspective while retaining the aesthetic qualities of exposed timber.

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