322.1 Defining clear gap

Clear gap is the minimum free width a vehicle could exploit at effective height. Treat it as a pass/fail metric for any HVM bollard layout and re-check against crash rated bollard deflection/set values (314, 232).

In practice, the “clear gap” is the tightest effective width remaining between adjacent elements at the effective height. You should compute it under a credible worst case that includes deflection, permanent set, and all tolerances.

For acceptance, use an objective Go/No-Go method (see SAT / Witness Procedure) and record the result as pass/fail with measurements and photos (716). Linking to rating context early prevents disputes—see How to read HVM/Crash-Rated Bollards ratings.

322.2 Vehicle width baselines

Adopt baseline widths by class (223) and include tire bulge/climb effects. These baselines drive HVM bollard center spacing; pick a crash rated bollard that still blocks after residual set (314).

Use the Vehicle Classes page to choose width envelopes (cars, vans, HGVs, buses). Add a conservative allowance for tire bulge and curb climb—especially at door throats and pinch points. Where legitimate access must be credentialed, combine width with a keepered opening to maintain control.

322.3 Deflection allowances

Add dynamic deflection to nominal centers (314). HVM bollard drawings show worst-case gap; acceptance uses a gauge. The crash rated bollard chosen must report deflection at your speed line (413).

From the product’s rating sheet, take the dynamic deflection and permanent set at the relevant impact speed/angle (see ratings). Subtract the worst-case combined movement from nominal centers to get the post-impact clear gap. Where multiple vehicle classes are credible, run sensitivity bands and pick the governing case.

322.4 Tolerance stacking

Combine fabrication, set-out, paving, and measurement tolerances (315, 612, 629). Stack-up protects HVM bollard compliance under variance. Verify the crash rated bollard socket/plate tolerances too (333).

Build a simple tolerance stack-up: (a) fabrication (straightness, roundness), (b) foundation/plate positions, (c) paving levels/crossfall, (d) measurement error. Document each source and apply signed directions (e.g., both posts leaning outward increases the gap). See Tolerances & Manufacturability and Datum & Alignment Checks.

322.5 Slopes/curbs adjustments

Project gaps along grades and around curbs (232). On downslopes, reduce centers. The nearby crash rated bollard may need a higher head to prevent ride-up (312, 313).

On slopes and near curbs, project the effective-height line parallel to the local grade and re-measure the opening. Downhill direction often increases usable width due to bumper rake and tire climb; apply a deduction to nominal centers. Where curb climb is likely, consider a taller head profile (see Height setting and Heads & Attachments).

322.6 Measurement methods

Use rigid bars/step gauges and survey shots (626). Record failures with photos (716). Consistent methods protect HVM bollard acceptance and crash rated bollard evidence (431).

Measure at the stated effective height using a rigid bar or a calibrated Go/No-Go gauge. Supplement with total-station shots to confirm centerline spacing and alignment (626). Log each check with a photo/redline logbook entry (716).

322.7 Compliance acceptance bands

Define Green/Amber/Red bands to guide fixes. Amber triggers local HVM bollard adjustments; Red requires rebuild. The crash rated bollard at the failure location is rechecked for alignment.

Typical site bands: Green (≥ target clearance), Amber (within a small negative margin that can be corrected without structural works), Red (beyond corrective tolerance; rebuild). Make bands explicit in your ITP (714) with a clear escalation path and NCR trigger—see Nonconformance & Defects.

322.8 Recording on ITP

Tie each check to ITP lines and drawings (714, 931). Evidence closes HVM bollard QA loops and supports crash rated bollard submittals (938).

Reference the drawing number, chainage, gauge type, effective height, measured value, acceptance band, and photo IDs. Cross-index to the sheet in CAD/BIM Standards (931) and attach evidence per Submission-Pack Guidance (938).

322.9 Worked examples

Show door, corner, and island cases with inputs → stacked gap → result. Each includes the crash rated bollard deflection line used (413).

(a) Door protection array: Inputs—vehicle class: van, baseline width 2.1 m; nominal centers 1.2 m; deflection 40 mm each; permanent set 15 mm each; paving tolerance +/-8 mm. Stacked deduction ≈ 110 mm; post-impact gap ≈ 1,090 mm. Result—Amber; reduce centers by 50 mm or raise effective height gauge by 50 mm per Frontage/Door Protection Arrays (323).

(b) Corner pinch point: Inputs—car class 1.85 m envelope; inside curb with 120 mm upstand; downhill 3%. Adjusted projection adds ~30 mm usable width; apply head height increase (312) or add an island nose per Corners, Islands & Pinch Points (324).

(c) Island cluster: Inputs—staggered posts at 1.4 m/1.0 m alternating; HGV class check; deflection/set 60/25 mm. Compute pairwise minimum at effective height; worst clear gap 0.98 m (Red). Remedy—shift the inner post 80 mm or add a mid-island post; re-verify per Array Patterns (321).

322 Clear-Gap Calculations — FAQ