RSJ Flitch Beam Calculator: Combining Steel and Timber for Stronger Supports

Flitch beams offer an innovative solution when you need extra strength but want to maintain timber aesthetics or reduce steel dimensions. By sandwiching a steel plate or RSJ between timber members, you create a composite beam stronger than either material alone while keeping costs lower than a full-size steel

beam.

What is a Flitch Beam?

A flitch beam is a composite structural member consisting of:

Traditional Flitch Beam:

• Steel plate (typically 6-12mm thick)
• Sandwiched between two timber joists
• Bolted together at regular intervals
• All components act together to resist loads

RSJ Flitch Beam:

• RSJ or universal beam section
• Timber members bolted to one or both flanges
• Creates wider, more stable composite section
• Better lateral stability than steel alone

Advantages of Flitch Beams

Cost Savings: Using timber alongside steel reduces the required steel size, potentially saving 20-40% on steel costs while achieving similar or better performance.

Easier Installation: Flitch beams are easier to handle and maneuver than equivalently strong solid steel beams, especially important for DIY projects or sites with access constraints.

Better for Fixing: Timber components allow easy fixing of plasterboard, architraves, and other finishes using standard woodworking techniques rather than specialized masonry anchors.

Thermal Performance: Timber provides better insulation than solid steel, reducing thermal bridging effects.

Acoustic Benefits: The timber-steel composite naturally dampens vibration better than steel alone, improving acoustic performance.

Disadvantages

More Complex Installation: Requires careful bolting to ensure full composite action, taking longer to install than simple steel beams.

Moisture Sensitivity: Timber components must be protected from moisture, limiting use in damp environments.

Fire Protection: While timber provides some natural fire resistance, the steel plate can heat rapidly, potentially compromising the timber. May require additional fire protection.

Building Control Scrutiny: Less common than simple RSJ beams, so Building Control may require more detailed engineering verification.

Flitch Beam Design Principles

Composite Action

For a flitch beam to work effectively, steel and timber must act together rather than sliding past each other. This requires:

Sufficient Bolting:

• Bolts at 300-600mm centers (closer for higher loads)
• Minimum M12 bolts (M16 for heavy applications)
• Washers under all nuts to prevent pull-through
• Torque to specified values

Full Contact:

• Timber must sit flush against steel throughout length
• No gaps or air pockets
• Kiss packs (thin shims) to ensure perfect fit if needed

Load Sharing

In an ideal flitch beam:

Steel carries: 60-75% of total load (due to higher stiffness) Timber carries: 25-40% of total load

The exact split depends on:

• Relative size of steel vs. timber
• Steel grade and timber species/grade
• Degree of composite action achieved

Calculations for Flitch Beams

Step 1: Determine Required Moment Capacity

Same as standard beam design:

M = (w × L²) / 8

For uniform load on simply supported beam.

Example:

• Span: 4.5m
• Uniform load: 8 kN/m
• Required moment: (8 × 4.5²) / 8 = 20.25 kNm

Step 2: Calculate Composite Section Properties

This requires determining the transformed section where timber is converted to equivalent steel area.

Modular Ratio (n):

n = E_steel / E_timber

Where:

• E_steel = 210,000 N/mm² (Young’s modulus for steel)
• E_timber = 8,000-12,000 N/mm² (depending on species and grade)

For typical C24 softwood: E = 11,000 N/mm²

n = 210,000 / 11,000 = 19.1

This means steel is ~19 times stiffer than timber.

Step 3: Transform the Section

Convert timber width to equivalent steel:

Example Configuration:

• Central steel plate: 250mm deep × 10mm thick
• Two timber joists: 250mm deep × 75mm wide (each side)

Transformed to steel equivalent:

• Steel plate: 250 × 10 = 2,500 mm²
• Timber (each side): 75mm / 19.1 = 3.9mm equivalent steel thickness
• Total equivalent: 250 × (10 + 3.9 + 3.9) = 250 × 17.8mm = 4,450 mm² equivalent section

Step 4: Calculate Section Modulus

For rectangular section:

Z = b × d² / 6

Where:

• b = 17.8mm (transformed width)
• d = 250mm (depth)

Z = 17.8 × 250² / 6 = 17.8 × 62,500 / 6 = 185,417 mm³ = 185 cm³

Step 5: Check Capacity

M_capacity = Z × σ_allowable

Using allowable stress of 165 N/mm² (with safety factors):

M_capacity = 185,417 × 165 = 30.6 × 10⁶ Nmm = 30.6 kNm

Versus required 20.25 kNm: Adequate ✓

Step 6: Check Deflection

More complex for composite beams but generally needs verification by structural engineer.

Common Flitch Beam Configurations

Type 1: Steel Plate Core

Configuration:

• 8-12mm steel plate
• 75-100mm timber joists each side
• Total width: 160-220mm

Best for:

• Floor beam replacements
• Matching existing timber joist depths
• Hidden installations

Typical Capacity:

• 200mm depth: 5-8 kN/m over 3-4m span
• 250mm depth: 8-12 kN/m over 4-5m span

Type 2: RSJ with Single Timber Flange

Configuration:

• 152-203mm RSJ
• 75-100mm timber member bolted to bottom flange
• Creates ‘T’ section

Best for:

• Ceiling support (timber provides fixing surface)
• Reducing steel section size
• Architectural exposed steel with concealed timber

Typical Capacity:

• 152mm RSJ + timber: Equivalent to next size up RSJ
• 203mm RSJ + timber: 20-30% capacity increase

Type 3: RSJ with Timber Both Flanges

Configuration:

• 152-254mm RSJ
• Timber members on both top and bottom flanges
• Maximum composite benefit

Best for:

• Maximum load capacity
• Wide spans
• Easy fixing top and bottom

Typical Capacity:

• Can achieve 40-60% greater capacity than RSJ alone

Type 4: Double RSJ with Timber Infill

Configuration:

• Two parallel RSJ sections
• Timber spacers between
• Bolted through web

Best for:

• Very heavy loads
• Wide bearing requirements
• Maximum strength

Typical Capacity:

• Nearly double that of single RSJ

Bolt Sizing and Spacing

Bolt Selection

Light Applications:

• M12 bolts (12mm diameter)
• Suitable for loads <10 kN/m
• Spacing: 450-600mm

Medium Applications:

• M16 bolts
• Loads 10-20 kN/m
• Spacing: 300-450mm

Heavy Applications:

• M20 bolts
• Loads >20 kN/m
• Spacing: 200-300mm

Bolt Pattern

Stagger bolts to avoid weakening timber:

Timber    Steel     Timber
  |         |          |
  O    ========== ======O
  |         |          |
    O       |        O
  |         |          |
  O    ========== ======O

This distributes stress more evenly.

Washers and Tolerances

Always use washers:

• Large plate washers (50mm diameter minimum)
• Prevents bolt pulling through timber
• Distributes compression stress

Torque values:

• M12: 80-100 Nm
• M16: 180-220 Nm
• M20: 350-400 Nm

Don’t over-tighten (crushing timber) or under-tighten (no composite action).

Timber Selection for Flitch Beams

Preferred Species

Softwoods:

• European Redwood/Whitewood (most common in UK)
• Douglas Fir (higher strength, better choice)
• Larch (good durability)

Hardwoods (premium applications):

• Oak (excellent strength and durability)
• Ash or Beech (high strength)

Grading Requirements

Minimum grade: C24 (structural grade softwood)

• Characteristic bending strength: 24 N/mm²
• Modulus of elasticity: 11,000 N/mm²
• Widely available

Preferred: C27 or GL24h glulam

• Higher and more consistent properties
• Better long-term performance

Moisture Content

Critical: Timber must be at equilibrium moisture content for the environment:

Internal applications: 12-16% MC External (protected): 16-20% MC

Never use “green” (unseasoned) timber – will shrink and lose bolt tension.

Installation Guide

Preparation

1. Cut Components:

• Steel plate or RSJ to exact length
• Timber to match (allow 2mm tolerance)
• All cuts square and clean

2. Drill Bolt Holes:

• Mark hole positions accurately
• Drill pilot holes in timber (slightly undersized)
• Drill final holes to bolt diameter + 1mm clearance
• Deburr all holes

3. Treat Timber:

• Apply preservative treatment (exterior grade)
• Especially important at bolt locations
• Allow to dry fully

Assembly

1. Dry Fit:

• Assemble without bolts to check fit
• Make any necessary adjustments
• Verify no gaps between components

2. Apply Moisture Barrier:

• Place DPC (damp-proof course) between steel and timber
• Prevents moisture transmission and corrosion
• Use bitumen felt or plastic membrane

3. Bolt Up:

• Insert bolts with washers
• Hand-tighten all bolts first
• Then torque to specification in sequence (center outward)
• Check all bolts after 24 hours and re-torque if needed

4. Installation:

• Lift into position (usually lighter than solid steel)
• Set minimum 100mm bearing each end
• Level and shim as required
• Fix joists or other members as normal

Cost Comparison

Example: 4.5m Span, 8 kN/m Load

Option 1: Pure Steel Beam

• Required: 254x146x31 RSJ
• Material: 4.5m × £75/m = £338
• Weight: 140kg (requires 2-3 people + equipment)

Option 2: Flitch Beam

• Steel plate: 250 × 10mm, 4.5m = £180
• Two 75 × 250mm C24 joists: £65
• 20 × M16 bolts, washers, etc: £35
• Total material: £280
• Saving: £58 (17%)
• Weight: 95kg (easier handling)

Plus easier ceiling fixing, better insulation, potential weight savings.

When Flitch Beams Cost More

Very short spans (<2.5m): Simple RSJ is cheaper and faster.

Very heavy loads: Large steel section alone is more efficient.

Damp environments: Timber durability becomes issue – pure steel better.

Building Regulations Considerations

Engineering Certification

Building Control generally requires:

• Structural engineer’s stamped calculations
• Materials specifications (steel grade, timber grade)
• Bolt schedule (size, spacing, torque)
• Connection details

Fire Protection

Unprotected flitch beams: Typically 30-minute fire resistance

To achieve 60 minutes:

• Plasterboard encasement (two layers 12.5mm)
• Intumescent paint on steel components
• Increased timber thickness

Inspection Points

Building Control will usually want to inspect:

• Materials before assembly (verify grades)
• Bolt pattern before tightening
• Final installation before loading

Common Mistakes

1. Inadequate Bolting

Problem: Too few bolts or excessive spacing Result: Steel and timber act independently, losing strength Solution: Follow engineering specifications exactly

2. Green Timber

Problem: Using unseasoned timber Result: Shrinking causes bolt loosening and loss of compression Solution: Use kiln-dried C24 at correct moisture content

3. No Moisture Protection

Problem: Direct steel-to-timber contact in damp areas Result: Corrosion of steel, rot in timber Solution: Always use DPC membrane between materials

4. Over-Drilling Holes

Problem: Bolt holes too large (sloppy fit) Result: Poor load transfer, slip at service loads Solution: Drill holes only 1mm larger than bolt diameter

5. Assuming Simple Addition

Problem: Thinking flitch beam = full strength of steel + full strength of timber Result: Overloading and potential failure Solution: Use proper composite section analysis or engineer’s calculations

Conclusion

Flitch beams offer an excellent compromise between pure steel and pure timber solutions, providing high strength at moderate cost with better handling and fixing characteristics. The key to success is proper design using composite section theory, careful material selection, and meticulous installation with correctly spaced and torqued bolts.

For spans of 3-6m with moderate to heavy loads, flitch beams can save 15-30% compared to equivalent solid steel while providing superior fixing opportunities and thermal performance.

Use our calculator above to estimate flitch beam sizing, then engage a structural engineer for detailed design and Building Regulations compliance.

Disclaimer: Flitch beam design requires structural engineering expertise. This article provides general guidance only. Always consult a chartered structural engineer and comply with Building Regulations.