Element design
How to design timber members
Glulam beam and column design to Eurocode 5 explained: kmod, γM, bending, shear, lateral-torsional buckling and column buckling, with worked GL24h examples.
Timber is light, strong along the grain and quick to build with — but it behaves differently from steel and concrete. Its strength depends on how long the load lasts and how damp it gets, it is weak across the grain and in shear, and it creeps. Eurocode 5 handles this with a few extra factors. This tutorial designs a glulam beam and a glulam column.
Design strength in Eurocode 5
Every characteristic strength (bending fm,k, compression fc,0,k, shear fv,k…) becomes a design strength the same way:
| Load duration | Example | kmod (service class 1–2) |
|---|---|---|
| Permanent | Self-weight | 0.60 |
| Long-term | Storage | 0.70 |
| Medium-term | Imposed floor load, snow | 0.80 |
| Short-term | Snow (some countries), maintenance | 0.90 |
| Instantaneous | Wind gusts, impact | 1.10 |
A combination takes the kmod of its shortest-duration load. Outdoors (service class 3) the values drop by about 15 %.
Worked example 1: a glulam roof beam
A GL24h beam, 140 × 450 mm, spans 8.0 m simply supported at 4.0 m centres. Roof dead load 1.0 kPa (including self-weight), imposed roof load 0.4 kPa. GL24h: fm,k = 24 MPa, Emean = 11.5 GPa, fv,k = 3.5 MPa. Take kmod = 0.8 (medium-term) and γM = 1.25, so kmod/γM = 0.64.
Loads and forces
gk = 4.0 kN/m, qk = 1.6 kN/m. EN 1990: wEd = 1.35 × 4.0 + 1.5 × 1.6 = 7.8 kN/m.
Bending (6.1.6)
Shear (6.1.7)
Cracks reduce the effective width: bef = kcr b with kcr = 0.67.
Lateral-torsional buckling (6.3.3)
If the roof deck holds the top edge, kcrit = 1. If not, over the whole 8 m (E0.05 ≈ 0.833 E = 9.58 GPa):
Utilization 0.86 / 0.98 = 0.88 ✓. Deep, narrow beams lose much more — keep h/b moderate or restrain the compression edge.
Deflection
Under gk + qk = 5.6 kN/m with I = 140 × 450³ / 12 = 1.063 × 10⁹ mm⁴:
Timber creeps. The final deflection is the instantaneous one times (1 + kdef) for the permanent part — kdef = 0.6 for glulam in service class 1, 0.8 in class 2. Here that would add about 0.6 × 17.4 mm ≈ 10 mm.
Worked example 2: a glulam column
A GL24h column, 200 × 200 mm, 4.0 m high and pinned at both ends, carries NEd = 150 kN (fc,0,k = 24 MPa).
With bending as well, EN 1995-1-1 6.3.2 combines them: σc/(kc fc,d) + σm,y/fm,d + km σm,z/fm,d ≤ 1, with km = 0.7 for rectangles.
Design it in Struxt3D
- Open a template such as the Portal frame shed and, in Materials, choose Timber. Pick the glulam grade (GL24h, GL28h, GL32h in Eurocode; the code's own glulam grades elsewhere).
- Pick sections in Sections — glulam rectangles from GL 90×270 beams up to GL 300×1260.
- Run the design (F6). Every timber member is checked for tension, compression with buckling (kc), bending with LTB (kcrit from Lb), shear with kcr and the combined rules, plus deflection against span/250.
- Inspect a member and open its Calculation report to see each check with its EN 1995-1-1 clause.
Struxt3D designs timber members with a simplified EN 1995-1-1 method for every design code: kmod = 0.8 (medium-term) and γM = 1.25 for all combinations, no size factor kh and no creep factor kdef. Timber connections are not designed. Treat the results as a first sizing.
Common mistakes
- One kmod for everything — a permanent-only combination can govern because kmod = 0.6.
- Forgetting creep in deflection.
- Notches and holes at the supports — they slash shear capacity.
- Ignoring fire: timber chars at roughly 0.7 mm/min; size for the residual section where fire resistance is required.