Buffering for uncertainty; choosing conservative inputs.
Be conservative, but transparent. This page sets typical allowances for speed, angle, and mass, balancing safety with constructability and cost. Build simple tables to stress-test scenarios and support peer review. When site constraints change (216, 422), revise factors and record the rationale. These bands underpin defendable HVM bollard selection and crash rated bollard equivalence claims (414). Include one-sentence context that naturally links upward to the parent hubs (this section and the chapter hub). Add SIRA context with a link to SIRA Bollards (UAE) when relevant. Link installation pages only if helpful: What to Expect and Installation Guide.
Important: This is a general guide. For live projects we develop a tailored
Method Statement & Risk Assessment (MS/RA) and align with authority approvals (e.g., SIRA) where in scope.
228.1 Why apply factors
Inputs are uncertain; factors make outcomes conservative and auditable. They prevent optimistic HVM bollard spacing and over-claiming a crash rated bollard’s capability (414).
In a VDA, small errors in impact speed, angle, or vehicle mass amplify into penetration and clearance risks. Applying an explicit safety factor makes decisions reproducible and reviewable.
Factors also standardize comparisons between rating strings and penetration classes (413), preventing accidental “apples to oranges” when reading certificates or supplier datasheets. Document them once and reuse across similar sites for consistency.
| Aspect | What matters | Where to verify |
|---|---|---|
| Performance | Certified system (bollard + footing), P-value window | How to read ratings (413) |
| Geometry | Array & clear-gap calculations | Clear-Gap (322) |
| Operations | Fail-state, interlocks, safety devices & measures | Interlock Verification (634) |
228.2 Typical allowances (+/−)
Use ± for speed (e.g., +10%), angle bands, and vehicle mass ranges. These shift HVM bollard counts and your margin against the crash rated bollard penetration limit (413).
Speed: add a +5–15% allowance to speeds derived from 224 Speed Estimation Methods to cover timing error and surface friction variability. Angle: assess in 10–15° bands from the 225 vectors page; glancing impacts can increase slip-through risk. Mass: consider unladen to laden spans for the chosen 223 vehicle class.
Where certificates show a narrow acceptance band, choose the next-up rating or tighten arrays via 232 spacing rules to maintain protection across the band.
228.3 Conservatism vs buildability
Balance safety with cost/constructability (338). Over-conservatism inflates HVM bollard numbers; under-conservatism risks a crash rated bollard shortfall. Document trade-offs (445).
Conservative speed/angle choices may push more units or deeper foundations. Where 422 depth/utilities or 216 constraints bite, consider 338 value engineering options such as shallower certified systems or mixed-type arrays.
Record the trade-offs and any temporary mitigations (e.g., 327 event reconfiguration) used to keep risk ALARP until permanent works complete.
228.4 Documenting rationale
State sources, site observations, and reviewer feedback (717, 229). Written rationale stabilizes HVM bollard approvals and defends crash rated bollard choices.
Use an assumptions register referencing photos, 937 photo/redlines, and measurements per 211 checklist. Capture reviewer remarks and decisions inside 717 submittals and summarize them up front in the 229 VDA report.
228.5 Sensitivity tables
Show speed/angle/mass grids with resulting gaps and penetration estimates (322). Tables expose how HVM bollard performance shifts and whether the crash rated bollard still passes.
Start with a base scenario (speed from 222 run-up, vehicle from 223, vector from 225) and vary one input at a time. For each cell, record predicted penetration distance and clear gap status. Highlight the first failing cell to show margins visually.
228.6 Stress-testing scenarios
Vary inputs systematically; confirm choices stay safe across ranges (221). This keeps HVM bollard solutions robust and prevents fragile crash rated bollard claims (431).
Use ± bands from this page and re-check the decision path in the 339 decision flow. Where sensitivity exposes a weak point (e.g., glancing hits at higher mass), adjust 321 array pattern or choose the next rating per 414 standards equivalency.
228.7 Peer review checklist
Independent check of inputs, bands, and outcomes (444). Peer review reduces HVM bollard disputes and catches crash rated bollard misreads.
- Base data: run-ups, vectors, vehicle class envelope match site reality.
- Factors: speed/angle/mass bands stated and justified; evidence attached.
- Tables: sensitivity grids present; failing cells and mitigations noted.
- Ratings: certificate scope and as-tested configuration align with design intent (431, 421).
228.8 When to revise factors
If constraints change—utilities, grades, furniture, or policies (216, 238)—update factors. Re-validate HVM bollard gaps (232) and crash rated bollard applicability (415).
Triggers include new utility conflicts (241–243), changed effective run-up (222, 227), or revised access policy (233, 236). Update the 913 worksheet and regenerate sensitivity tables.
228.9 Reporting format
Place factors early in 229 with tables and notes. Clear reporting accelerates HVM bollard sign-off and smooths crash rated bollard review (938).
In the 229 VDA Report Template, add a one-page summary: input assumptions, chosen factors, sensitivity highlights, and resulting spacing/ratings decisions. Reference the 938 submission pack guidance to keep reviewers aligned.
Related
External resources
- NPSA — HVM specialised guidance
- ASTM F2656 — crash testing standard
- BSI — impact test specifications overview
228 Sensitivity & Safety Factors — FAQ
How big should my speed safety factor be in a VDA?
A practical starting point is +5–15% on derived approach speed to cover timing error, friction, and seasonal variation. Use higher margins for glancing angles or uncertain run-ups, and record the justification in your VDA report (229).
Do safety factors replace certified crash ratings?
No. Ratings and penetration classes define tested performance; safety factors cover project uncertainty. Use them together and cross-check the certificate’s acceptance band against your sensitivity tables (413, 322).
When should we revise our factors during design?
Revise whenever constraints change: utilities/shallow foundations (241–244), altered access policies (233), or new terrain/calming features (227). Update calculations and sensitivity tables, then re-state decisions in the VDA report (229).
What evidence do reviewers expect to see?
Clear input sources, photo/overlay evidence (936–937), stated factors, and sensitivity tables with failing cases highlighted. Map the outcome to spacing decisions (232) and the selected crash rating or equivalency rationale (414).