314.1 Definitions (deflection/set)

Dynamic deflection is peak movement; permanent set is post-test offset. Both affect clear gaps (232). Understand terms to apply a crash rated bollard rating correctly (412–413).

In impact testing, deflection (elastic) describes the maximum displacement at the instant of impact, whereas permanent set is the residual offset once rebound and vibrations settle. Both influence the clear-gap left after an incident and therefore whether an array still performs its protective function.

Certificates typically state a rating string plus measured deflection and set. Designers should read these alongside site tolerances (315) and spacing rules (232, 322) to avoid over-promising performance in tight frontages.

314.2 Measurement in tests

Tests record markers on post/ground and penetration into stop-lines (411). Use reported values when spacing HVM bollard arrays.

Impact programs (e.g., IWA 14-1 / ASTM F2656) use ground and post reference points to track peak displacement and the residual set. Penetration is measured to a defined vehicle penetration distance beyond a notional defend line; plotting this consistently against the array centreline helps compare models and foundations. Always rely on the test report’s stated methodology—do not substitute ad-hoc site measurements when making product-to-product comparisons.

314.3 Design allowances

Add deflection and set allowances to nominal gaps (322). This ensures a crash rated bollard still blocks target vehicles after impact.

Practical designs must reserve a “movement budget” that combines peak deflection with likely permanent set. Start from the certificate values, add installation tolerances (315, 626), then check serviceability under repeated bumps (636). For doors and corners, pair the allowance with local acceptance bands so inspectors can apply a simple Go/No-Go judgement on day one.

314.4 Effect on clear gaps

Combine spacing tolerances (612) with deflection bands to check worst-case pass (232). Gap control maintains HVM bollard intent.

For every critical opening, compute a worst-case gap after impact: (initial gap ± set-out tolerance) − (left post movement + right post movement). Compare that to the project’s clear-gap rule from Spacing rules and Clear-Gap Calculations. Record these checks so operations teams understand how much “gap drift” remains acceptable post-incident.

314.5 Implications for foundations

Greater deflection increases socket rotation demand (331–333). Verify the chosen crash rated bollard base can resist.

Large dynamic deflections amplify moments at the base and can increase rotation at socket embedment, especially with shallow systems. Cross-check your array against Impact loads & foundations and the project soil conditions (333). Where margins are tight, consider deeper sockets or a grade-beam tie to share rotation.

314.6 Adjacent finishes tolerance

Paving must tolerate post-impact shifts (629). Detail joints so HVM bollard heads don’t spall edges.

Allow for post-impact misalignments in paving modules, cover frames, and tactile bands. Provide slip joints or compressible interfaces around collars so a slightly displaced head does not crack adjacent stone. Coordinate reinstatement details with Surface reinstatement & finishes, and log pre-/post-works photos in the Photo/Redline Logbook.

314.7 Reporting & acceptance

Capture as-built gaps and acceptance checks (626, 638). Records defend the crash rated bollard performance claim (431).

At handover, record actual clear gaps with a calibrated gap gauge, include survey shots to benchmarks (612), and file them in the SAT / witness pack. Cross-reference certificate values in the Documentation & certificates page so reviewers can trace the assumptions.

314.8 Inspection methods

Use calibrated gauges and survey shots (714). Repeatable methods aid HVM bollard QA and satisfy reviewers.

Define repeatable checks in the ITP: (a) set-out checks to benchmarks; (b) pre-impact as-built gaps; (c) post-incident or periodic re-checks. For arrays with credentialed access, include an operational sanity check—heads must rise within the defined window and stop at the correct height (632, 636). Store results in the project’s evidence templates.

314.9 Examples from ratings

Show two rating strings and typical deflection/set ranges (413). Convert to spacing tables for everyday HVM bollard design.

Example: An IWA 14-1 N2A @ 48 km/h test might report peak deflection ~120–200 mm with a permanent set of ~40–90 mm (illustrative ranges). For a 1,200 mm initial centre-to-centre gap, the worst-case post-impact opening could still pass the site’s clear-gap rule when allowances and tolerances are applied (232, 322). Use your project’s acceptance band and Go/No-Go gauges to make this check field-friendly.

314 Deflection vs permanent set in Crash-Rated Bollards — FAQ