Short Answer:

A flitched beam is a type of beam made by combining two or more materials, usually a steel plate placed between wooden joists or beams. This combination increases the strength and stiffness of the beam while keeping its weight low. The steel takes most of the tensile stress, while the wood resists compression, making it an economical and efficient structural member.

In simple words, a flitched beam is a composite beam made of wood and steel used in buildings and bridges to carry higher loads than a normal wooden beam. It offers both strength and cost advantages where full steel beams are not needed.

Detailed Explanation:

Flitched Beam

A flitched beam is a composite structural member made by fastening together materials of different mechanical properties, generally wood and steel, to act as a single unit under bending loads. The term flitch refers to a thin steel plate or strip inserted between wooden members. This arrangement helps in improving the load-carrying capacity and stiffness of the beam without excessively increasing its weight or cost.

In construction and mechanical engineering, flitched beams are used where greater strength is required than that provided by a single wooden beam, but where the use of a complete steel beam is unnecessary or uneconomical.

Construction of a Flitched Beam

A typical flitched beam consists of:

Two wooden joists (or planks) placed parallel to each other.
A steel plate (called the flitch plate) sandwiched between them.
Bolts or rivets used to connect all the parts firmly together so that they act as one beam.

The flitch plate runs along the full length of the wooden beam and is tightly fixed to ensure that both materials bend together as a single unit. This means there is no relative motion between the steel and wood when the beam bends.

The bolts are usually placed at regular intervals along the beam, and sometimes washers are used to distribute the load evenly and prevent damage to the wood.

Working Principle of Flitched Beam

The principle behind a flitched beam is based on the composite action of materials with different strengths.

When a beam bends due to applied loads:

The top fibers are under compression.
The bottom fibers are under tension.

In a wooden beam, wood performs well in compression but is weak in tension. Therefore, by adding a steel plate, which is strong in tension, the overall performance of the beam improves.

The steel plate takes up most of the tensile stresses, while the wood carries the compressive stresses. The result is a stronger, stiffer, and more durable beam capable of carrying higher loads.

Bending Stress Distribution in Flitched Beam

In a flitched beam, the bending stresses are distributed between the wood and the steel in proportion to their moduli of elasticity (E). The steel, being much stiffer than wood, carries more stress for the same strain.

If and are the moduli of elasticity for steel and wood respectively, then:

This ensures that both materials deform equally under the same bending moment.

The neutral axis of the beam is not at the center but shifts toward the steel plate due to its higher stiffness. This shift must be considered while designing such beams.

Advantages of Flitched Beams

Higher Strength: The steel plate increases the load-carrying capacity compared to a single wooden beam.
Lightweight: It is lighter than an equivalent all-steel beam.
Economical: More cost-effective than using only steel, especially in medium-load applications.
Easy to Fabricate: Simple to assemble with common fastening methods like bolts and rivets.
Reduced Deflection: The addition of steel increases stiffness, reducing deflection under loads.
Durability: Properly treated wood and coated steel make the structure long-lasting and resistant to bending failure.

Applications of Flitched Beams

Building Construction: Used in wooden floors and roofs where higher strength is required.
Bridges: Used in small-span bridges where wooden decks need additional stiffness.
Temporary Structures: For supporting heavy loads in temporary frameworks.
Reinforcement Work: Existing wooden beams can be strengthened using flitch plates.

These applications take advantage of both materials — steel’s strength and wood’s lightness and cost-effectiveness.

Design Considerations for Flitched Beams

While designing a flitched beam, engineers consider:

The ratio of moduli of elasticity of steel and wood.
Position of the neutral axis, as it shifts toward the steel portion.
Moment of inertia of the combined section.
The connection strength (bolt or rivet spacing and size).
Load conditions (type, direction, and magnitude).

To make the design simple, an equivalent section method is often used, where the steel plate is converted into an equivalent wooden area by multiplying its width by the modular ratio . This allows the beam to be analyzed as a single material beam.

Example Calculation (Conceptual)

Suppose a wooden beam is reinforced with a steel plate. The modular ratio is typically around 15 to 20.
If the moment of inertia of the wooden section is and that of the steel plate (converted into equivalent wood) is , then the total moment of inertia of the flitched beam is:

This total is used in bending stress and deflection formulas to determine beam performance.

Limitations of Flitched Beams

Not suitable for very heavy or long-span structures.
Prone to corrosion of steel and decay of wood if not protected properly.
More complicated fabrication compared to plain wood beams.
Differential expansion or shrinkage may occur due to temperature or humidity changes.

Conclusion:

A flitched beam is a composite beam that combines wood and steel to achieve greater strength, stiffness, and economy. It efficiently utilizes the tensile strength of steel and the compressive strength of wood to resist bending stresses. Though it requires careful design and protection from environmental effects, flitched beams remain an excellent choice for medium-load structural applications where full steel construction is unnecessary.