Timber bearing reinforced with fully threaded screws
Checks a solid-timber or glulam bearing reinforced with fully threaded screws or threaded rods against compression perpendicular to the grain at the ultimate limit state, to SIA 265:2021 with the factor k_c,90 of EN 1995-1-1. Every step shows its formula and the clause it comes from.
Pro SIA 265:2021
Part of the Pro plan
This module is included in the Pro plan and above, with its calculation note.
About this check
clauses, worked example and questions
Checks a solid-timber or glulam bearing reinforced with fully threaded screws or threaded rods against compression perpendicular to the grain at the ultimate limit state, to SIA 265:2021 with the factor k_c,90 of EN 1995-1-1. Every step shows its formula and the clause it comes from.
What it checks
- Design strength fc,90,d · ηt · ηw (SIA 265 clauses 2.2.6 and 3.2.1.3, table 4). The capacity of one screw Fc,90,Rd is a design value from its assessment and is not modified.
- Timber and screws together: kc,90 · B · lef,1 · fc,90,d + n · Fc,90,Rd, with lef,1 = l, the contact length, and n = n0 · n90.
- Factor kc,90, EN 1995-1-1 §6.1.5, from the expression you choose. For a support near the end, kc,90 = (2.38 − l/250) · (1 + h/(12·l)); the module shows that h > 2.5·b makes expression (6.10) the applicable one.
- Timber alone at the screw tips: B · lef,2 · fc,90,d. For an end bearing, lef,2 = lef + (n0 − 1) · a1 + min(lef ; a1,CG), spread at 45° from the screw group, not from the plate edges.
- Reinforced bearing: F90,Ed ≤ F90,Rd, the smaller of the two branches.
What you enter and what you get
You enter the member (wood type, strength class, load duration, service class, b, h); end or centre bearing; F90,Ed, the bearing width B and the contact length l; fc,90,d from your copy of SIA 265 (clause 3.3.2.1, table 8, or clause 3.4.2.2, table 9); the expression for kc,90; and the screws: angle α, n0, n90, a1, a1,CG, the penetration lef and Fc,90,Rd from the technical assessment of the screw.
You get both branches, the capacity and the utilisation with a pass or fail, each with its source. With the Pro plan, it prints as an A4 calculation note.
Assumptions and limits
- Screws at 45° < α < 90° to the grain, with the force shared evenly between all of them.
- A steel plate is required between timber and support, with the screws driven flush so they bear on it. Reinforcing screws for wood-based panels are outside the assessment.
- The contact covers the full width of the member, as (6.6) and (6.10) of EN 1995-1-1 require.
- kc,90 is capped at 4.0. With a raised value the module warns that the deformation at the ultimate limit state reaches about 10% of the member depth (EN 1995-1-1 §6.1.5(2)).
- The unreinforced bearing is its own module.
Worked example
A glulam beam GL24h, 160 × 600 mm, in service class 1, on an end bearing 160 × 160 mm with fc,90,d = 2.5 N/mm² and a bearing force of 180 kN, reinforced with 3 × 2 screws at 90°, spaced 60 mm, 80 mm from the end, 400 mm deep, 18 kN each; the timber at the screw tips governs. These are the module's own results for its worked example, computed when this page was built.
| Input | Value | Ref. |
|---|---|---|
| Wood type | Glulam | |
| Strength class | GL24h | |
| Load duration ηt | Short to permanent | § 2.2.6 |
| Service class ηw | 1 | § 3.2.1.3 t.4 |
| Section width b | 160 mm | |
| Section depth h | 600 mm | |
| Type of bearing | End bearing | |
| Design bearing force F90,Ed | 180 kN | |
| Bearing width B | 160 mm | |
| Contact length l | 160 mm | EN 1995-1-1 §6.1.5 |
| Design compressive strength ⟂ grain fc,90,d | 2.5 N/mm² | § 3.3.2.1 t.8 / 3.4.2.2 t.9 |
| Factor kc,90 from | Beam on a support near the end, a ≤ h/3 (6.4) | EN 1995-1-1 §6.1.5 |
| Angle of the screws to the grain α | 90 ° | |
| Screws in a row parallel to the grain n0 | 3 | |
| Screws in a row perpendicular to the grain n90 | 2 | |
| Spacing parallel to the grain a1 | 60 mm | |
| End-grain distance of the screw centre a1,CG | 80 mm | |
| Point-side penetration length lef | 400 mm | |
| Design compressive capacity of one screw Fc,90,Rd | 18 kN |
| Step | Value | Ref. |
|---|---|---|
| Load duration factorfrom the load duration chosen in the Member panel | ηt = 1.00 | § 2.2.6 |
| Moisture factorfrom the service class chosen in the Member panel | ηw = 1.00 | § 3.2.1.3 t.4 |
| Design strength usedfc,90,d · ηt · ηw = 2.500 · 1.00 · 1.00. This is what both branches below are formed with. The capacity of one screw, Fc,90,Rd, is a design value from the assessment of the screw and is not modified. | fc,90,d · ηt · ηw = 2.500 N/mm² | § 2.2.6 · 3.2.1.3 t.4 |
| Effective length of the contact arealef,1 = l, the contact length. EN 1995-1-1 (6.3) measures the compressive stress on the contact area; how far the load spreads inside the member is carried by kc,90 below, not by a longer area. | lef,1 = 160.0 mm | EN 1995-1-1 §6.1.5 |
| Compression factor ⟂ grainkc,90 = (2.38 − l/250)·(1 + h/(12·l)), for a support near the end | kc,90 = 2.2837 | EN 1995-1-1 §6.1.5 |
| Section proportionh > 2.5·b, so expression (6.10) is the applicable one | h / b = 3.750 | EN 1995-1-1 §6.1.5 |
| Distribution length at the screw tipslef,2 = lef + (n0 − 1)·a1 + min(lef ; a1,CG), for an end bearing — the spread towards the end is cut off by whichever is smaller. The spread is measured at 45° from the screw group, not from the edges of the plate: the screws are what carry the load down to that plane, and the timber directly under the plate is already in the first branch. | lef,2 = 600.0 mm | |
| Number of screws | n = n0 · n90 = 6 | |
| Timber and screws togetherthe timber under the bearing plus every screw, 146.16 kN from the wood and 108.00 kN from the 6 screws. fc,90,d is taken as 2.500 N/mm², the entered value with ηt · ηw applied. | kc,90·B·lef,1·fc,90,d + n·Fc,90,Rd = 254.160 kN | |
| Timber alone at the screw tipsat the plane of the tips the load has spread to lef,2, but nothing carries it there except wood. This is what caps the arrangement. fc,90,d is taken as 2.500 N/mm², the entered value with ηt · ηw applied. | B·lef,2·fc,90,d = 240.000 kN | |
| Capacity of the reinforced bearingthe smaller of the two above | F90,Rd = 240.000 kN | |
| Reinforced bearingF90,Ed ≤ F90,Rd | η = 0.750 · Pass75 % of the capacity used |
How this module is verified
Verified clause by clause against SIA 265:2021 and EN 1995-1-1 and against independent calculations used on real projects, before release.
Questions
Why does adding screws not raise the capacity?
When the timber at the screw tips governs, more screws do nothing. Spread them further along the grain to lengthen lef,2, drive them deeper, or widen the bearing.
Why is l_ef,1 not entered?
It is the contact length l. EN 1995-1-1 (6.3) measures the stress on the contact area; the spreading inside the member is carried by kc,90.
Which plan includes this module?
The Pro plan, with its calculation note.
Related
- Bearing at a support, ultimate limit state
- Reinforcement of a curved or tapered beam
- Bar in compression, ultimate limit state
- SIA 265: modules and clauses
Norm: SIA 265:2021 · Page updated: 21 October 2026