ISO 18900:2025

Overview

ISO 18900:2025 - Steel structures - Structural bolting - Test method for determining the slip factor for faying surfaces of slip-resistant connections defines a standardized laboratory procedure to measure the slip factor of faying surfaces used in slip‑resistant (friction) connections in structural steelwork. The method is applicable mainly to carbon steels and to uncoated or coated steel components with various surface treatments and coatings.

Key topics and technical requirements

• Purpose: Determine a representative slip factor for a given surface treatment/coating to be used in structural design and specification of bolted friction connections.
• Specimens and materials: Prescribed compression‑type and tension‑type specimens; steels with specified yield strengths (350–700 MPa range are covered); coated contact surfaces must be applied as in intended use and measured at ≥25% above nominal dry film thickness.
• Bolt pretension and measurement: Bolts tightened to within ±5% of specified pretension (Fp,C). Pretension must be measured with equipment accurate to ±4%.
• Test sequence: Four static compression tests (load–slip recorded, ~10–15 min duration each), then a fifth specimen loaded in a creep (tension) test at 90% of the mean slip load from the first four specimens.
• Slip definitions and thresholds: Individual slip load Si defined at 0.15 mm displacement or peak before 0.15 mm. Delayed slip (difference between slip at 5 minutes and 3 hours) must not exceed 0.002 mm - otherwise extended creep testing is required.
• Extended creep testing: Procedures for chain‑linked tension specimens and use of “displacement vs log(time)” curves to ensure long‑term displacement < 0.30 mm for a 50‑year design life (or as specified).
• Statistical evaluation: Individual slip factor μi = Si / (4·Fp,C). Compute mean μm, standard deviation and take the 5% fractile as the characteristic slip factor. If the standard deviation of the first ten values exceeds 8% of the mean, more specimens are required per the standard’s formula.
• Documentation: Record significant variables (bolt class, washer configuration, coating composition & thickness, surface preparation, curing, time intervals, etc.) as part of the test report.

Applications and users

• Structural and connection designers specifying frictional bolted joints.
• Fabricators and coating suppliers validating surface treatments for slip resistance.
• Testing laboratories performing certification, quality control, or product development.
• Inspectors, code authorities and procurement teams needing verified slip factors for design codes and execution specifications.

Related standards

• ISO 17607‑1 - Steel structures - Execution of structural steelwork - General requirements and terms.
• ISO 17607‑3 - Steel structures - Execution of structural steelwork - Fabrication.

Keywords: ISO 18900:2025, slip factor, faying surfaces, steel structures, structural bolting, slip‑resistant connections, friction connections, creep test, bolt pretension, surface coating testing.