Design SPAX Screws to Eurocode 5

SPAX (SPAX International GmbH & Co. KG, Ennepetal, Germany) manufactures self-tapping screws and threaded rods for timber structures, assessed under European Technical Assessment ETA-12/0114 of 2020-01-07, issued by ETA-Danmark on the basis of EAD 130118-01-0603. SPEC Toolbox designs the connections to Eurocode 5, using the characteristic values declared in that assessment.

The integration covers the SPAX screws and threaded rods of the assessment, grouped by coating and material, in partially threaded, fully threaded and dual-threaded forms.

Zinc-coated carbon steel

• Wirox and Delta-Seal screws in countersunk, washer and cylinder heads, nominal diameters 5, 6, 8, 10 and 12 mm, in lengths to 800 mm; partially threaded, fully threaded and dual-threaded variants.

Stainless steel

• A2 (1.4567) and A4 (1.4578) screws in countersunk and washer heads, 6 to 12 mm, and HCR (1.4539) screws in 10 mm, for higher corrosion exposure.

Threaded rods

• Hexagon-head and headless threaded rods of 16 mm nominal diameter, in lengths to 3,000 mm, in Wirox and A2 stainless.

The assessment covers screws of 2.5 to 12.0 mm (carbon steel) and 3.0 to 12.0 mm (stainless steel) in lengths from 20 to 1,000 mm, and threaded rods of 16.0 mm. Screws are supplied with the CUT and 4CUT drill tips and the T-STAR drive.

In the EU and ANZ scope the SPAX screws and threaded rods support:

• Connections between members of solid timber, glued laminated timber, cross-laminated timber, LVL and similar glued members, in softwood and hardwood.
• Timber-to-timber, panel-to-timber and steel-to-timber connections, with panels and steel plates on the screw-head side.
• Tensile, compressive and shear reinforcement perpendicular to grain, and the fixing of thermal insulation on top of rafters.
• Service classes 1 and 2 for zinc-coated screws, and additionally service class 3 for stainless steel.

• One assessment across carbon steel, stainless and threaded rods. The same ETA covers zinc-coated carbon steel screws, stainless screws in four steel grades (1.4567, 1.4578, 1.4539 and others) and the 16 mm threaded rods, so a project’s standard connections, its corrosion-exposed details and its heavy reinforcement rods are all designed from one source of characteristic values.
• Reinforcement is assessed, not only connections. Annexes C, D and E give design rules for compression reinforcement of bearing areas, tensile reinforcement of connections loaded perpendicular to grain, notched supports and holes in beams, and shear reinforcement, with the screws used as reinforcement rather than as fasteners.
• CUT and 4CUT drill tips allow the pre-drilled spacings. Screws with the CUT or 4CUT drill tip may use the smaller minimum spacings and distances given for nails in pre-drilled holes, without pre-drilling, provided the member thickness and the spacing conditions of the assessment are met.
• Declared buckling capacity for screws in compression. The assessment gives a buckling verification for fully threaded screws and threaded rods embedded in timber, with the elastic foundation of the timber and tabulated buckling capacities, which Eurocode 5 does not cover.
• Assessed CLT rules. A dedicated embedment strength is declared for screws parallel to the plane of cross-laminated timber, together with separate spacing rules for the plane and edge surfaces of CLT.

The SPAX screws are available in the SPEC Toolbox screw connection module and in the connection detail calculators, worked to Eurocode 5:

• panel-to-panel joints (spline, butt joint and half-lap);
• CLT floor-to-glulam beam, CLT floor-to-floor and CLT wall-to-wall connections.

Screw behaviour is taken from the integrated database with the characteristic values of ETA-12/0114 for the selected article, keyed to the connected member’s characteristic density, with the assessment’s minimum spacings, end and edge distances and penetration requirements applied. The same values are used for design to the Australian and New Zealand standards.

Design follows Eurocode 5 (EN 1995-1-1). The screws are assessed under ETA-12/0114, which supplies the characteristic values used in design: the tensile capacity (from 7.9 kN at 5 mm to 38 kN at 12 mm in carbon steel, and 63 kN for the 16 mm threaded rod), the yield moment (0.15 x 600 x d^2.6 Nmm for carbon steel and 0.15 x 400 x d^2.6 Nmm for stainless), the withdrawal parameter f_ax,k (14.0 N/mm2 below 6 mm, 12.0 N/mm2 from 6 to 8 mm, 11.5 N/mm2 at 10 mm and 11.0 N/mm2 at 12 mm) and the head pull-through parameter by head or washer diameter. Lateral capacity is calculated to Eurocode 5 using the outer thread diameter, with the assessment’s embedment strengths for non-pre-drilled and pre-drilled holes and for CLT, and the rope effect per Eurocode 5. Withdrawal is taken at angles from 15 degrees to the grain (30 degrees in LVL) with the effective number of screws, and inclined screws in shear connections use the assessment’s effective-number rule. Combined axial and lateral loading uses the quadratic interaction of Eurocode 5. Characteristic values are reduced to design values with the Eurocode load-duration and service factor and the material partial factor from the National Annex. Assessed service classes are 1 and 2 for zinc-coated screws and additionally 3 for stainless steel; the assumed working life is 50 years, and reaction to fire is class A1.