18m Cat Ladder Design Engineering Singapore

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Why 18 metres is a watershed height

A cat ladder of 3–6 m is essentially a maintenance prop: easy to specify, easy to install, easy to certify. At 18 m total climb, the design problem multiplies in three directions simultaneously:

1. Climber fatigue and fall-arrest physics — a single uninterrupted vertical climb beyond ~6 m is unsafe even for fit users. EN ISO 14122-4 and OSHA both demand staged climbs with rest landings.
2. Buckling and dynamic deflection — a slender 18 m steel column wants to whip laterally. Stiffness, not strength, drives stile sizing.
3. Anchorage demand — 18 m of self-weight + climber + wind load + fall-arrest reaction transmits into wall fixings that must each take 5–10 kN repeatedly over a 25-year life.

The reference project for this blog is the Aggreko / 8B Buroh Street cat ladder — designed by DWB CS-Engineering Consultants, fabricated in Aluminium AA 6063-T6, with a middle landing platform, full safety cage, and chemical-anchor wall fixings. The QP report runs 133 pages of STAAD.Pro analysis to clear a single ~8.6 m ladder for endorsement under the Singapore Building Control Act. Scaling that thinking to 18 m is what this blog is about.

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1. Anatomy of an 18 m cat ladder

An 18 m cat ladder is never one continuous run. It is a stack of three 6 m flights (or two 9 m flights with a single mid-rest), separated by rest platforms / landing platforms. A typical layout:

  ── Roof exit hand-grab + transition barrier ──
  ┃                                              ┃
  ┃  Top flight  6,000 mm                        ┃
  ┃                                              ┃
  ── Upper rest platform (≥ 700 × 700 mm) ──────
  ┃                                              ┃
  ┃  Middle flight  6,000 mm — flights staggered ┃
  ┃                                              ┃
  ── Lower rest platform (≥ 700 × 700 mm) ──────
  ┃                                              ┃
  ┃  Bottom flight  6,000 mm                     ┃
  ┃                                              ┃
  ── Floor / grade level + retractable section ──

Why staggered flights? EN ISO 14122-4 §4.4.2.2 requires that if a climber falls, the fall is intercepted at the rest platform within ≤ 6 m — a continuous straight 18 m drop is not survivable even with a fall arrester. Modern designs offset adjacent flights laterally by ≥ 300 mm so that a fall ends on the platform edge, not the rung below.

Standard component sizes (taken directly from the Buroh Street drawings, scaled):

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2. The design loads to satisfy

For an 18 m ladder serving an industrial roof in Singapore, the QP must run loads from four codes simultaneously:

These are characteristic loads. The QP applies partial factors per EC0:

• γG = 1.35 for permanent (dead)
• γQ = 1.50 for variable (climber, wind)
• γM0 = 1.00 for carbon-steel cross-section resistance (EN 1993-1-1); γM0 = 1.10 for stainless cross-section (EN 1993-1-4); γM0 = 1.10 for aluminium cross-section (EN 1999-1-1); γM1 = 1.10 for member buckling. The 1.10 factor reflects manufacturing variability and local-buckling sensitivity — not, as is sometimes informally said, the lower elastic modulus.

The Buroh Street report explicitly cites:

"EC0 – Eurocode 0 SS EN 1990 & NA – Basis of Structural design; EC1 – SS EN 1991-1-1 & NA; EC3 – SS EN 1993-1-1 & NA"

For an 18 m ladder this is the non-negotiable code stack.

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3. The structural model — what an 18 m STAAD.Pro run looks like

The Buroh St QP modelled an 8.6 m ladder in STAAD.Pro V8i with:

• 205 nodes
• ~200 frame elements (one per rung + stiles + cage hoops + brackets)
• Multiple load cases combined per EC0 fundamental combination

Scaled to 18 m, you would expect:

• ~430 nodes
• ~430 elements
• 2 staggered rest platforms modelled as horizontal frames
• 8–12 wall-anchor points (4 per platform + 4 per top/bottom)

Critical model details

(a) Boundary conditions

• Wall brackets are modelled as fixed in 3 translations, free in 3 rotations (pinned to the wall plate). Some QPs model them as fully fixed (rigid moment connection) — this overestimates capacity. The Buroh St report uses pin-fixed which is the conservative and correct approach.

(b) Stile-rung joints

• All rungs are welded (3 mm fillet weld all round, per general note 6 of the drawings). Modelled as rigid in the frame analysis.

(c) Cage as a structural element?

• Most QPs exclude the cage from primary structural action — it is treated as fall-arrest hardware only, and the stiles must take 100% of the climber load. This is the safe assumption.

(d) Buckling

• For aluminium stiles 6 m long, slenderness λ = L/i ≈ 6000 / 11 ≈ 545, which is far above the EC9 limit (λmax = 200 for compression members). Lateral restraint at every rung is what makes the assumption work — the rung-to-stile welds force buckling to occur over a 300 mm rung pitch, not 6 m.

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4. Worked utilisation check on the stile

Using the Buroh Street section (70 × 30 × 3 mm aluminium C-channel, AA 6063-T6, fy = 190 N/mm²) over a 300 mm rung-to-rung span:

Section properties (C-channel 70 × 30 × 3):

• Area A = 270 mm²
• I (strong axis) = 8.6 cm⁴ ≈ 86,000 mm⁴
• Wel (strong axis) = 2.46 cm³ = 2,460 mm³

Design moment from F1 = 1.5 kN at mid-rung (treating stile as continuous beam):

\[ M_{Ed} = \frac{F_1 \cdot L}{8} = \frac{1.5 \cdot 0.300}{8} = 0.056 \text{ kN·m} = 56 \text{ N·m} \]

Design bending resistance (γM1 = 1.10 for aluminium):

\[ M_{Rd} = \frac{f_y \cdot W_{el}}{\gamma_{M1}} = \frac{190 \cdot 2{,}460}{1.10} = 425 \text{ N·m} \]

Utilisation: M_Ed / M_Rd = 56 / 425 = 13% ✓ — comfortable for the rung-to-rung span.

Now check the stile between brackets — 1,500 mm bracket spacing, F2 = 1.5 kN at mid-span:

\[ M_{Ed} = \frac{F_2 \cdot L}{4} = \frac{1.5 \cdot 1.500}{4} = 0.563 \text{ kN·m} = 563 \text{ N·m} \]

Utilisation: 563 / 425 = 132% ✗ — fails if no intermediate restraint.

That is exactly why the 18 m design needs brackets at ≤ 1.5 m spacing, and ideally ≤ 1.0 m for aluminium (because of the lower modulus E = 69 GPa vs 210 GPa for steel). A "skinny" aluminium ladder with sparse brackets is the most common QP rejection reason in Singapore submissions.

The Aggreko drawing addresses this by specifying brackets at 1.45 m, 1.50 m intervals — and that drawing is for a 5 m ladder. For 18 m the bracket pitch would need to drop further or the stile section sized up to 70 × 30 × 4 mm or even a closed SHS 50 × 50 × 4 mm for the longest spans.

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5. The wall connection — the highest-failure subsystem

Every cat-ladder forensic report points to the same conclusion: the wall fixings are the failure-prone subsystem, not the ladder itself. The Buroh Street commenting reviewer flagged exactly this — visible on Sheet 5 of the COMMENTS PDF:

• "BOLT (M16) DIAMETER NOT MENTIONED" — drawing didn't specify the rod diameter for the chemical anchor
• "EMBEDMENT 85 mm" — flagged as inadequate for M16 chemical anchor (rule of thumb is 8d = 128 mm for M16)
• "NOT TALLY" — bracket detail and assembly view didn't match
• "SUPPORT MISSING" — the warehouse front elevation showed an unsupported run
• "NOT TALLY PLS UPDATE" — section dimensions inconsistent across sheets

These are the five most common reviewer comments on cat-ladder submissions in Singapore, and at 18 m every one of them becomes a structural defect rather than a paperwork fix.

Anchor design for an 18 m ladder

A representative bracket on an 18 m ladder might carry:

For a Hilti HIT-RE 500 V4 + M12 stainless threaded rod in C25/30 cracked concrete with hef = 110 mm and edge distance c1 = 150 mm, the ETA-tabulated tension capacity is approximately 15–20 kN — comfortable. But drop to M10 with hef = 80 mm (the embedment shown on the Buroh St drawing) and capacity falls to roughly 6–8 kN — failing the top-bracket case.

This is precisely why the reviewer flagged the 85 mm embedment. For 18 m ladders the rule is: M12 minimum rod, hef ≥ 110 mm, ETA Option 1 cracked-concrete approval, A4 stainless rod for outdoor.

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6. The middle landing platform — design specifics

The Buroh St ladder includes a single middle landing platform, configured as a 740 mm Ø circular platform inside the safety cage. Detail callouts include:

• 75 × 75 × 6 mm SHS as primary platform frame
• 38 × 38 × 6 mm angle bar as in-fill
• 50 × 75 × 3 mm hollow section as platform support to the wall
• 6 mm thick galvanised mesh wire as tread / fall-through protection
• Hand-grabs continuing through the platform per EN 14122-4 §5.6

For an 18 m ladder you need at least two such platforms (one every 6 m or every 12 m if a fall arrester is fitted, per EN 14122-4 §4.4.2.4):

Design loads for each platform:

• 1.5 kN/m² distributed (EC1) — standard floor live load
• 1.5 kN concentrated per person at the unfavourable corner
• Same fall-arrester reaction at the upper-flight start anchor

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7. Geometry, clearances and OSHA-compatible layout

Per EN ISO 14122-4 (most-cited values for a Singapore submission):

The Aggreko drawing dimensions (1450 mm cage projection, 750 mm cage standoff, 5000 mm flight, 450 mm exit width) all sit safely inside these envelopes.

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8. The QP submission package — what 18 m takes

A submission to BCA/SCDF for an 18 m cat ladder under Section 5A of the Building Control Act typically includes:

1. Architectural drawings — front elevation, side elevation, plan, all with key dimensions
2. Structural drawings — bracket details, weld details, anchor bolt schedule, bearing plate sizes
3. Material schedule — every bar size, grade, finish, fastener spec
4. Design report — typically 80–150 pages:

• Introduction & code references
• Design loadings (dead, live, wind, climber, arrester)
• Material strengths
• STAAD.Pro 3D model output (geometry, load cases, member forces, deflections)
• Stile bending check at most onerous span
• Bracket utilisation check (welded connection capacity)
• Anchor design per EN 1992-4 / Hilti PROFIS / Fischer FiXperience output
• Deflection check — typically L/200 for industrial access ladders
• Wind / dynamic check if H > 12 m

1. PE endorsement — name, registration number, signature, date, declaration that the design complies with the Building Control Act
2. Site test results — anchor proof-load tests at 1.5 × design tension per BS 8539, signed test sheets

The Buroh Street files in workspace illustrate this exactly: a separate 133-page warehouse design report (20260225-QP-REPORT-FOR-CAT-LADDER-WAREHOUSE-For-Buroh-Street-ENDORSED.pdf) plus a drawing set with a reviewer's mark-up PDF (20260223-Buroh-St-Cat-ladder-COMMENTS.pdf) — and at this scale the workflow is draw → STAAD model → calc report → PE endorse → reviewer comments → revise → re-endorse → BCA submission.

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9. Common failure modes the QP must catch

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10. The 18 m design checklist

Boil down everything above into a one-page QC sheet that goes into the project file alongside the QP report:

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11. Bottom line

An 18 m cat ladder is not a bigger version of a 5 m ladder. It is a structural system with two rest platforms, full fall-arrest, staggered flights, twelve-plus wall anchors, and a calculated dynamic envelope that no single rule of thumb covers. The Buroh Street project files in this workspace — drawings, comments and 133-page QP report — show every one of those moving parts in a real-world Singapore submission.

The four levers that determine whether an 18 m design succeeds or fails reviewer comment are:

1. Structural geometry — bracket pitch tight enough for the chosen stile material's modulus; rest platforms at ≤ 6 m / ≤ 12 m (with arrester); cage hoops ≤ 1.5 m.
2. Material grade — aluminium 6063-T6 fy = 190 N/mm² or 6061-T6 fy = 240 N/mm²; SS304 fy = 210 N/mm²; galvanised S275 carbon steel where service environment permits. Match the section size to the modulus, not just the yield.
3. Anchor system — ETA Option 1 chemical anchor, ≥ M12, hef ≥ 110 mm, A4 stainless rod, 4 anchors per plate. The most-cited reviewer comment is anchor underspec.
4. Drawing-to-report consistency — every dimension, grade, fastener spec, embedment depth and bracket detail must match exactly across the drawing set and the calculation report. "NOT TALLY" is the easiest-to-avoid rejection reason.

Get those four right, and the PE endorsement is a formality. Get any one wrong, and the submission bounces — usually with red ink on the bracket detail.

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References inline. Project files in workspace: 20260108_Aggreko_Catladder-on-roof-D.pdf, 20260223-Buroh-St-Cat-ladder-COMMENTS.pdf, 20260225-QP-REPORT-FOR-CAT-LADDER-WAREHOUSE-For-Buroh-Street-ENDORSED.pdf, 20260225-QP-REPORT-FOR-CAT-LADDER-REPIR-CENTRE-For-Buroh-Street-ENDORSED.pdf. Codes cited: SS EN 1990, SS EN 1991-1-1, SS EN 1991-1-4, SS EN 1992-4, SS EN 1993-1-1, SS EN 1999, EN ISO 14122-4:2016, BS 8539, OSHA 29 CFR 1910.23.

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