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STRUCTURA ACADEMIC · LESSON AREA

Engineered Timber and Advanced Structural Elements

Chapter 05 · Timber Design

Approved course
StandardEN 1995-1-1:2004+A2:2014 with applicable product standards, declarations and National Annex
Source1 source file
Review stateApproved · 2026-08-19
LEARNING OUTCOMES

After this chapter, you should be able to

  • Select traceable engineered-timber product data.
  • Design straight glulam/LVL members with product-specific properties.
  • Identify special tapered and curved-member checks.
  • Analyse justified full and partial composite action.
  • Explain built-up member and flitch-beam requirements.
  • Use declared CLT resistance and effective stiffness in the correct direction.

5.1 Purpose of this ChapterSource §5.1

This chapter extends the earlier member, connection and building-system routes to glulam, LVL, composite elements, built-up members and CLT.

5.2 Learning OutcomesSource §5.2

The learner selects the correct product data and analysis model, checks representative engineered members, and identifies where declared or manufacturer-specific methods govern.

5.3 Engineered Timber Product FamiliesSource §5.3

Glulam, LVL, I-/box beams, built-up elements and CLT have different manufacturing, axes, declared properties and structural behaviour; product identity precedes calculation.

Figure 5.R1 — engineered-timber product families and the analysis route selected for each.Approved original STRUCTURA academic diagram

5.4 Product Data Before AnalysisSource §5.4

Record the product standard, DoP/ETA or grade, lay-up/orientation, strength, stiffness, density, dimensions, connection limitations, service conditions and fire/durability data before choosing equations.

5.5 Glued Laminated Timber (Glulam)Source §5.5

Glulam is assembled from bonded laminations and can form straight, tapered or curved members. Homogeneous and combined lay-ups use their declared class data and manufacturing geometry.

5.6 Straight Glulam and LVL MembersSource §5.6

Straight members use the Chapter 2 resistance, stability and SLS structure with product-specific properties, partial factors, size effects and declared orientation.

Engineered-timber design strength
fd = kmod fk / γM

Use the property, product class, orientation, modification factors and partial factor declared for the selected engineered product.

5.7 LVL: Direction, Size Effect and Cross LayersSource §5.7

LVL properties depend on loading direction and whether cross veneers are present. Use only the declared property for the selected product, thickness and axis.

5.8 Tapered Glulam and LVL BeamsSource §5.8

Locate the critical varying-depth section and apply the source taper treatment. The compression-edge and tension-edge routes are not interchangeable.

Taper angle and reduced bending strength
α = arctan[(hap − hs) / (L / 2)]; fm,α,d = km,α fm,d

The sign of stress on the tapered edge and the source special-member route determine the applicable reduction.

5.9 Curved, Double-Tapered and Pitched-Cambered MembersSource §5.9

Curvature and apex geometry introduce special bending, radial tension, stressed-volume and lamination-radius checks beyond a prismatic M/W calculation.

5.10 Composite Timber Elements: Structural IdeaSource §5.10

Flanges, webs or separate members share bending only to the extent permitted by glue lines or mechanical connectors; shear transfer and slip determine the valid model.

5.11 Full Composite Action — Transformed SectionSource §5.11

Where a qualified structural bond develops full composite action, transform material areas to a reference modulus and calculate stiffness and material stresses about the transformed neutral axis.

Transformed-section second moment of area
Itr = Σ ni[Ii + Ai(yi − ȳ)2]; ni = Ei / Eref

This route requires justified full composite action and a consistent reference modulus.

5.12 Partial Composite Action and the Gamma MethodSource §5.12

Mechanically connected layers slip, reducing composite stiffness. The Gamma method relates connector stiffness and spacing to member length and layer axial stiffness.

Gamma-method connection factor
γi = [1 + π2 Ei Ai si / (Ki l2)]−1

The full Annex B assembly also needs the layer geometry and effective stiffness terms; gamma below one records connection slip.

5.13 Built-Up ColumnsSource §5.13

Built-up compression members require effective stiffness and connector-force checks; gross inertia is not available unless the assembly can develop it.

5.14 Flitch Beams and Timber–Steel Built-Up MembersSource §5.14

A flitch system needs compatible stiffness, load sharing, fastener transfer, stability, local bearing, steel design and fire/durability coordination.

5.15 Cross-Laminated Timber (CLT): Structural BehaviourSource §5.15

CLT is a layered anisotropic plate. Major-layer bending and axial action interact with cross-layer rolling shear, panel direction, joints and support conditions.

Figure 5.R2 — cross-laminated timber lay-up with major layers, cross layers and rolling-shear direction.Approved original STRUCTURA academic diagram

5.16 CLT Effective Stiffness — Adapted Gamma MethodSource §5.16

An adapted layer model may estimate effective stiffness when permitted, but declared effective properties must not be mixed with a second shear-flexibility allowance that double counts the same behaviour.

5.17 CLT Floor and Roof PanelsSource §5.17

Use the governing panel direction and declared resistance/stiffness to check bending, shear, rolling shear, bearing, deflection and vibration; joints and openings can control.

One-way CLT strip actions
MEd = qd L2 / 8; VEd = qd L / 2; uinst = 5 qSLS L4 / [384(EI)eff]

Use declared product resistance and effective stiffness for the governing panel direction, support condition and lay-up.

5.18 CLT Wall Panels and OpeningsSource §5.18

Wall panels can carry axial, out-of-plane and in-plane actions. Product-specific panel, opening, pier, lintel and connection methods must accompany building-system racking and anchorage checks.

Declared-resistance wall interaction used in WE-05F
η = NEd / NRd + MEd / MRd ≤ 1

This is the source product-specific teaching interaction, not a universal CLT formula; the selected product design guide governs.

5.19 Moisture Movement, Tolerances and Product DetailingSource §5.19

Delivery moisture, cross-grain movement, erection tolerance, bearing interfaces, protection during construction and manufacturer-approved service penetrations remain structural design inputs.

5.20 Integrated Design Workflow for Engineered TimberSource §5.20

Identify the exact product, import traceable data, choose the valid analysis model, calculate actions/resistance/SLS, design connections and then close fire, durability, movement and execution requirements.

5.21 Interactive Calculator Suite SpecificationSource §5.21

The source suite separates straight glulam/LVL, special glulam, composite members, Gamma-method elements and CLT so the selected method and product data remain visible.

APPROVED ACADEMIC CALCULATOR

Engineered Timber Design Calculator

Transparent straight-glulam beam check benchmarked to WE-05A, with product-specific exclusions and unresolved checks reported.

Inputs
Product class and properties · Section and span · ULS/SLS line loads · kmod and γM · Restraint assumption
Outputs
Actions and section properties · Bending and shear utilisations · Elastic deflection · Warnings and governing state
Status states
PASS · FAIL · INVALID INPUT
Validation
Approved against the supplied worked-example results; project-specific verification remains required
APPROVED SOURCE-BENCHMARKED CALCULATOR · WE-05A

Engineered Timber Design Calculator Suite

Straight GL28h beam strength and instantaneous-deflection mode reproducing WE-05A; proprietary engineered products require their declared data.

Teaching inputs
STRENGTH PASS · SLS REVIEW

Bending governs the illustrated strength checks at 0.76; instantaneous elastic deflection is 23.9 mm.

Design moment MEd
64.0 kNm
Design shear VEd
32.0 kN
Section modulus W
4.725 × 10⁶ mm³
Bending strength fm,d
17.92 N/mm²
Bending stress
13.54 N/mm²
Bending utilisation
0.76
Shear utilisation
0.34
Instantaneous deflection
23.9 mm
Show source calculation trail
  1. MEd = qd L2 / 8 = 64.0 kNm; VEd = qd L / 2 = 32.0 kN
  2. fm,d = kmod fm,k / γM = 17.92 N/mm2
  3. σm,d = MEd / W = 13.54 N/mm2
  4. winst = 5 q L4 / (384 E I) = 23.9 mm

5.22 Worked Example WE-05A — Straight Glulam BeamSource §5.22

The source example checks bending, shear and elastic deflection for a straight continuously restrained glulam beam.

WORKED EXAMPLE

WE-05A · Straight glulam beam

Check a continuously restrained GL28h beam 140 × 450 mm over 8.0 m under qd = 8.0 kN/m, with the source design data.

  1. Actions

    MEd = qd L2 / 8; VEd = qd L / 2

    MEd = 64.0 kNm; VEd = 32.0 kN
  2. Section

    W = b h2 / 6

    W = 4.725 × 106 mm3
  3. Bending

    fm,d = 0.80(28) / 1.25 = 17.92; σm,d = MEd / W

    σm,d = 13.54 N/mm2; ηm = 0.76
  4. Shear

    fv,d = 0.80(3.5) / 1.25

    fv,d = 2.24 N/mm2; ηv = 0.76
  5. Elastic deflection

    uinst = 5 q L4 / (384 E I)

    uinst = 23.9 mm

Result. The simplified bending and shear checks pass; bearing, restraint, connections and final deformation remain project checks.

5.23 Worked Example WE-05B — Tapered Glulam Geometry and Taper ReductionSource §5.23

This example applies the source compression-edge taper factor and identifies the remaining special-member checks.

WORKED EXAMPLE

WE-05B · Tapered glulam geometry and taper reduction

Evaluate the taper angle and compression-edge bending reduction for the source GL28h roof beam.

  1. Taper angle

    α = arctan[(750 − 320) / 4,500]

    α = 5.46°
  2. Reduction

    km,α = 0.887

    fm,α,d = 0.887(17.92) = 15.90 N/mm2

Result. The small compression-edge taper gives a modest reduction; the critical section and any tension-edge route must still be located correctly.

5.24 Worked Example WE-05C — Glued OSB-Webbed I-Girder, Full Composite ActionSource §5.24

The transformed-section example keeps timber flange and OSB web moduli and stresses distinct.

WORKED EXAMPLE

WE-05C · Glued OSB-webbed I-girder, full composite action

Use a transformed section for the source symmetrical I-girder with 45 × 70 mm C24 flanges and a 15 × 500 mm OSB web.

  1. Modular ratio

    nweb = 4,500 / 11,000

    nweb = 0.409
  2. Transformed section

    Itr = Σ ni(Ii + Ai ai2)

    Itr = 578.2 × 106 mm4
  3. Material stresses

    σflange = M y / Itr; σweb = nweb M y / Itr

    σflange = 22.14; σweb = 7.08 N/mm2

Result. The transformed section establishes the stress basis only for justified full composite action; web shear/buckling, glue transfer and SLS remain required.

5.25 Worked Example WE-05D — Partial Composite Stiffness by Gamma MethodSource §5.25

The source Gamma calculation shows how finite connection stiffness reduces composite action.

WORKED EXAMPLE

WE-05D · Partial composite stiffness by Gamma method

Calculate the explanatory gamma factor for the source connected timber layer.

  1. Flexibility term

    χ = π2 E A s / (Kser l2)

    χ = 1.303
  2. Gamma factor

    γ = 1 / (1 + χ)

    γ = 0.434

Result. The connection is not infinitely stiff. The full Annex B layer geometry is required to assemble (EI)eff.

5.26 Worked Example WE-05E — CLT Floor Panel Using Declared Effective PropertiesSource §5.26

Declared product bending resistance and effective stiffness are used without inventing a generic lay-up capacity.

WORKED EXAMPLE

WE-05E · CLT floor panel using declared effective properties

Check a 1 m strip of a 160 mm five-layer CLT floor over 5.5 m using the supplied declared resistance and stiffness.

  1. ULS actions

    MEd = 5.0(5.5)2 / 8; VEd = 5.0(5.5) / 2

    MEd = 18.91 kNm/m; VEd = 13.75 kN/m
  2. Bending

    ηM = 18.91 / 52

    ηM = 0.364
  3. Elastic deflection

    uinst = 5 qSLS L4 / [384(EI)eff]

    uinst = 14.9 mm

Result. The declared bending resistance passes; rolling shear, final deflection, vibration, joints, bearing and openings still require verification.

5.27 Worked Example WE-05F — CLT Wall Interaction Using Declared ResistancesSource §5.27

A product-specific axial-plus-moment interaction illustrates traceable use of declared wall capacities.

WORKED EXAMPLE

WE-05F · CLT wall interaction using declared resistances

Check a 1 m wall strip carrying NEd = 180 kN/m and MEd = 4.5 kNm/m against the source declared resistances.

  1. Axial utilisation

    ηN = 180 / 700

    ηN = 0.257
  2. Moment utilisation

    ηM = 4.5 / 25

    ηM = 0.180
  3. Interaction

    η = ηN + ηM

    η = 0.437

Result. PASS for the selected product-specific teaching interaction; racking, connectors, hold-downs, bearing and opening zones remain separate checks.

5.28 Design Decisions When an Engineered Element FailsSource §5.28

Change the mechanism that governs: depth/span for stiffness, taper/radius for special-member stress, web/connector layout for composite action, lay-up/support for CLT, or the connection/opening detail where local behaviour controls.

5.29 Common MistakesSource §5.29

Common errors include treating engineered products as solid C24, using generic stiffness, assuming full composite action, ignoring rolling shear and direction, mixing manufacturer data, and cutting unapproved openings.

5.30 Chapter SummarySource §5.30

Engineered-timber design starts with product identity and declared data, then applies the analysis model that matches geometry, orientation and composite behaviour.

Key points

  • Identify the exact product before analysis.
  • Use product-specific strength, stiffness, axes and factors.
  • Apply special rules to tapered and curved members.
  • Distinguish full from partial composite action.
  • Treat CLT as a layered direction-dependent panel.
  • Keep declared properties and manufacturer limitations traceable.

Source references recorded by the supplied chapter

  • University of Moratuwa timber product lectures.
  • EN 1995-1-1:2004+A2:2014 and Annex B.
  • EN 14080, EN 14374 and EN 16351 product frameworks.
  • IStructE/TRADA engineered-timber guidance.
  • Porteous & Kermani Chapters 6–8.
  • Swedish Wood Volumes 1–3 and the COFORD handbook.