Checks the deflection of a solid-timber or glulam beam at the serviceability limit state: the combinations of SIA 260 from your elastic deflections, creep to SIA 265:2021, and the limits of SIA 260 annex A. Every step shows its formula and the clause it comes from.

What it checks

Creep coefficient φ from SIA 265 clause 3.2.2.2, table 5, for the service class and the moisture of the timber when installed.
Function, incorporated elements (SIA 260 annex A, table 3): for ductile elements, the frequent combination against l/350, instantaneous w_el = w_el,f + w_el,G and long term w = w_el,G · (1 + φ) + w_el,qp · (1 + φ) + w_el,f − w_el,qp. Brittle elements are checked under the rare combination against l/500.
Function, use and operation · comfort: the frequent combination of the variable actions only, w_el = w_el,f and w = w_el,qp · (1 + φ) + w_el,f − w_el,qp, against l/350.
Appearance: the quasi-permanent combination, w_el = w_el,G + w_el,qp and w = (w_el,G + w_el,qp) · (1 + φ), against l/300.
Depth needed at constant width: h_new = h · (w / w_lim)^(1/3).

What you enter and what you get

You enter the member (wood type, strength class, service class, depth h), bending about one axis or two, the type of structure, the reference length l, the moisture when installed, the incorporated elements, and which deflections to verify (instantaneous, long term or both). You then enter the elastic deflections from your own analysis, one per action: w_el,G, w_el,qp, w_el,f and w_el,r. The limits are pre-filled from SIA 260 annex A, table 3, and editable.

You get each combination, the utilisation of each row with a pass or fail, and the depth needed. With the Basic plan, it prints as an A4 calculation note.

Assumptions and limits

The type of structure (building floor or beam, road bridge, footbridge or cycle bridge, rail bridge) selects which SIA 260 annex the limits come from.
A member carries either brittle or ductile incorporated elements, not both, so only the matching row of table 3 is verified.
About two axes, each combination is formed per axis and the limit is compared with the resultant √(w_y² + w_z²).
This verifies deflection only. Bending, lateral torsional buckling and the fire situation are verified in the bending or biaxial-bending module; horizontal displacement has its own module.

Worked example

A solid-timber beam C24, 120 mm deep, in service class 1, installed conditioned, with a reference length of 2.00 m, ductile incorporated elements, and elastic deflections of 0.54 mm (self weight), 4.27 mm (quasi-permanent) and 4.80 mm (frequent). In this example the instantaneous deflections pass, but the long-term deflection does not pass in any of the three rows. These are the module's own results for its worked example, computed when this page was built.

Inputs
Input	Value	Ref.
Wood type	Solid
Strength class	C24
Service class η_w	1	§ 3.2.1.3 t.4
Section depth h	120 mm
Bending acts about	One axis — bending about y-y
Type of structure	Building floor or beam
Reference length l	2000 mm
Moisture when installed φ	Conditioned	§ 3.2.2.2 t.5
Incorporated elements	Ductile elements incorporated	SIA 260 §A t.3
Deflections to verify	Instantaneous and long term
Self weight w_el,G	0.54 mm
Quasi-permanent variable w_el,qp	4.27 mm
Frequent variable w_el,f	4.8 mm
Rare variable w_el,r	0 mm
Function — ductile elements incorporated l / n	350	SIA 260 §A t.3
Function, use and operation · comfort l / n	350	SIA 260 §A t.3
Appearance l / n	300	SIA 260 §A t.3

Calculation: Common values
Step	Value	Ref.
Creep coefficientfrom table 5 for timber installed conditioned in service class 1	φ = 0.60	§ 3.2.2.2 t.5

Calculation: Check 1 — Function — ductile elements incorporated
Step	Value	Ref.
Function — ductile elements incorporated (frequent combination) — instantaneousw_el,f + w_el,G. Elastic value, without creep.	w_el = 5.34 mm	SIA 260 §A t.3
Function — ductile elements incorporated (frequent combination) — instantaneousw_el ≤ l/350	η = 0.935 · Pass93 % of the capacity used	SIA 260 §A t.3
Function — ductile elements incorporated (frequent combination) — long termw_el,G·(1+φ) + w_el,qp·(1+φ) + w_el,f − w_el,qp. Creep added through φ.	w = 8.23 mm	SIA 260 §A t.3
Function — ductile elements incorporated (frequent combination) — long termw ≤ l/350	η = 1.440 · Fail144 % of the capacity used	SIA 260 §A t.3

Calculation: Check 2 — Function, use and operation · comfort
Step	Value	Ref.
Function, use and operation · comfort (frequent combination, variable actions only) — instantaneousw_el,f. Elastic value, without creep.	w_el = 4.80 mm	SIA 260 §A t.3
Function, use and operation · comfort (frequent combination, variable actions only) — instantaneousw_el ≤ l/350	η = 0.840 · Pass84 % of the capacity used	SIA 260 §A t.3
Function, use and operation · comfort (frequent combination, variable actions only) — long termw_el,qp·(1+φ) + w_el,f − w_el,qp. Creep added through φ.	w = 7.36 mm	SIA 260 §A t.3
Function, use and operation · comfort (frequent combination, variable actions only) — long termw ≤ l/350	η = 1.288 · Fail129 % of the capacity used	SIA 260 §A t.3

Calculation: Check 3 — Appearance
Step	Value	Ref.
Appearance (quasi-permanent combination) — instantaneousw_el,G + w_el,qp. Elastic value, without creep.	w_el = 4.81 mm	SIA 260 §A t.3
Appearance (quasi-permanent combination) — instantaneousw_el ≤ l/300	η = 0.721 · Pass72 % of the capacity used	SIA 260 §A t.3
Appearance (quasi-permanent combination) — long term(w_el,G + w_el,qp)·(1+φ). Creep added through φ.	w = 7.70 mm	SIA 260 §A t.3
Appearance (quasi-permanent combination) — long termw ≤ l/300	η = 1.154 · Fail115 % of the capacity used	SIA 260 §A t.3

Calculation: Depth needed
Step	Value	Ref.
Depth needed to satisfy the governing limith_new = h · (w / w_lim)^(1/3), at constant width.	h_new = 135 mm

How this module is verified

Verified clause by clause against SIA 265:2021 and against independent calculations used on real projects, before release.

Questions

Why is creep applied to only part of the deflection?

Creep is added through φ to the permanent and quasi-permanent deflections. The frequent share above the quasi-permanent one, w_el,f − w_el,qp, is taken without creep.

Does timber installed wet change the result?

Yes. Timber installed wet creeps far more; with the service class, it selects φ from table 5.

Can I print the calculation?

The full calculation is shown on screen, free and without an account. A print-ready A4 calculation note is part of the Basic plan.

Norm: SIA 265:2021 · Page updated: 21 October 2026

Timber beam deflection to SIA 265 and SIA 260

Member

About this check