Description
6″ × 24″
ADA Manhole
Frame & Grate
Where 4″ frames reach their vertical limit, the 6″ body takes over. Engineered for installations demanding deeper grade-differential accommodation — new paving overlays, reconstructed arterials, elevated utility vault collars — while retaining the same 24″ clear bore and full ADA-compliant grate geometry. Cast to ASTM A48 Class 30B, proof-load qualified per AASHTO M306 H-20.
Assembly Geometry &
Dimensional Reference
The drawing below represents the plan view, cross-sectional profile, and complete dimension schedule for the 6″ × 24″ ADA assembly. Dimensional designators B through X follow standard municipal casting conventions and are consistent across Amercast's 24″ bore product family, allowing direct substitution between frame heights without structure modification.
Full Specification
Schedule
| Parameter | Dim. (C–H) | Value (inches) | Engineering Note |
|---|---|---|---|
| Grate Outside Diameter | C | 24″ | Nominal clear opening designation; consistent across 24″ bore family |
| Frame Height | D | 6″ | Extended riser height — accommodates greater grade-differential without added ring courses |
| Clear Seat Diameter | E | 23.5″ | Interior bearing ledge; grate seats within this diameter |
| Frame Seat Diameter | F | 24″ | Bearing surface; interfaces with precast cone seat |
| Overall Assembly O.D. | G | 33″ | Outer flange diameter; governs opening size in pavement surface |
| Grate / Cover Thickness | H | 1.5″ | Structural depth of grate casting at bearing plane |
| Slot Width (Typical) | — | ½″ max | ADA § 302.3 sphere-passage limit — perpendicular to travel direction |
| Material | — | Gray Iron, Class 30B | ASTM A48 / A48M-22 |
| Proof Load | — | 40,000 lbf | H-20 × 2.5 safety factor via 9″ × 9″ plate per AASHTO M306 |
| Design Wheel Load | — | 16,000 lbf | AASHTO H-20 single-wheel equivalent; full traffic service |
| ADA Compliance | — | § 302.3 Conforming | Slot long-axis parallel to dominant pedestrian travel direction |
| Finish Options | — | Raw / Bituminous | Custom inscriptions and markings available on request |
When Frame Height
Is the Design Constraint
The frame height designation — 4″, 6″, or 8″ — is one of the least-discussed but most consequential decisions in drainage system design. It determines how much vertical working room exists between the finished casting elevation and the top of the precast structure below. Get it wrong and the result is either a protruding hazard or costly field re-work.
Extra elevation capacity over the 4″ frame. Where pavement overlays, resurfacing programs, or grade transitions demand additional vertical clearance without replacing the underlying structure, the 6″ body provides the buffer. Grade adjustment guidance →
Precast structure modifications required. The seat diameter (F = 24-1/4″) and overall O.D. (B = 34″) are identical to the 4″ variant, enabling height-class substitution on existing cone sections without re-drilling, shimming, or resetting the manhole barrel.
Single-casting solution for applications where two separate 4″ grade rings would otherwise be stacked — an approach many municipal specs limit to two maximum and that introduces additional mortar joints susceptible to settlement and I/I infiltration.
Gray Iron Mechanics
Under Load
The 6″ frame body carries a greater moment arm between the grate seating plane and the precast bearing surface than shorter variants. This makes the casting's section modulus — governed by wall thickness and the distribution of iron mass through the frame height — the critical structural variable, not merely minimum tensile strength.
ASTM A48 Class 30B gray iron delivers the necessary section properties: a pearlitic matrix reinforced by uniformly distributed Type A graphite flakes that simultaneously provide compressive strength exceeding 620 MPa — roughly three to four times the tensile minimum — and outstanding vibration attenuation under dynamic vehicle loading. The graphite network also functions as a passive corrosion barrier, forming a stable graphitic passivation layer on exposed cast surfaces without supplemental coating.
Why the 6″ frame is not simply a taller 4″ frame: Increased frame height shifts the neutral axis of bending, requiring the foundry to calibrate wall thickness distribution to maintain equivalent structural performance under the same 40,000 lbf proof load. The casting geometry — not just the alloy — is engineered for the height class.
Gray iron's compressive dominance is the decisive material advantage for cyclic wheel loading: vehicles generate compressive impulses far exceeding any tensile component, and the alloy's compressive-to-tensile ratio maps directly onto this loading signature.
Every Applicable Code,
Met Without Variance
A48
Class 30B qualification requires minimum 207 MPa tensile strength verified via B-type test bars — the tier calibrated to castings with controlling wall sections between 0.32″ and 0.64″. Surface integrity requirements prohibit adherent sand, scale, cracks, or hot tears.
→ ASTM A48/A48M-22M306
The current and most stringent proof-load standard for municipal castings in traffic service. H-20 compliance mandates a 40,000 lbf static load applied via 9″ × 9″ platen at the casting centroid, sustained for one minute with zero cracking or deformation exceeding 1/8″.
→ AASHTO M306 Overview§ 302
Section 302.3 of the 2010 ADA Standards limits ground surface openings to ½″ maximum in the dimension perpendicular to pedestrian travel. The 6×24 ADA grate achieves conformance through slot orientation, not reduction in open area — maintaining drainage capacity while satisfying the sphere-passage threshold.
→ U.S. Access Board ADA § 3022023
The Public Rights-of-Way Accessibility Guidelines extend ADA surface requirements to sidewalks, shared-use paths, and pedestrian street crossings. The 6×24 ADA grate's compliant slot geometry satisfies PROWAG drainage surface provisions in addition to standard ADA requirements.
→ PROWAG 2023 GuidelinesC478
Frame seat diameter F = 24-1/4″ is dimensioned for direct bearing on standard 24″ nominal I.D. precast concrete manhole sections per ASTM C478. No adapter ring or shimming required for typical installations on conforming precast cone sections.
→ ASTM C478 Precast Standard§ 5:21
Conforms to NJ Administrative Code § 5:21-7.4, which mandates ASTM A48 Class 30 material and H-20 load capacity for manhole frames in public ROW. Grate slot geometry satisfies bicycle-safe and pedestrian-safe requirements simultaneously.
→ NJ Admin Code § 5:21-7.4Slot Geometry &
§ 302.3 Conformance
The critical accessibility dimension for any drainage grate is not the overall opening area but the cross-slot measurement — the span perpendicular to the predominant direction of pedestrian travel. ADA § 302.3 caps this at ½″ (12.7 mm), the dimension derived from wheelchair caster wheel diameter research to prevent wheel entrapment.
The 6×24 ADA grate achieves conformance by orienting slot long-axes parallel to the direction of travel — allowing elongated openings that substantially increase hydraulic throughput compared to equivalent circular perforations while keeping the critical cross-slot dimension within the ½″ threshold. This dual benefit is the engineering rationale for the ADA grate geometry over solid covers in surface drainage applications.
Field compliance can be verified without instrumentation: a standard ½″ ball bearing traversed across any axis of the grate surface will not pass through any slot or gap. This test, outlined in the ADA drainage design literature, provides instant confirmation of § 302.3 conformance at the installation site.
- Cross-slot dimension ≤ ½″ (12.7 mm) per ADA § 302.3 — passes sphere-passage ball test
- Slot long axis parallel to dominant pedestrian travel; perpendicular to bicycle tire contact
- Frame-to-pavement transition height ≤ ¼″ per ADA § 303 — flush installation required
- Surrounding surface slope not to exceed 1:48 (2.08%) per ADA site accessibility provisions
- Compatible with PROWAG pedestrian circulation route requirements
- Suitable for ADA Transition Plan projects and Title II public accommodation compliance
- Bicycle-compatible: long slot axis eliminates road-bike and wheelchair tire entrapment risk
- Markings available — "DUMP NO WASTE · DRAINS TO WATERWAYS" and custom legends
Where the 6″ Frame
Is the Right Specification
When a roadway receives a new asphalt overlay, the finished surface rises — often by 2″ to 4″ — while the manhole barrel elevation stays fixed. The 6″ frame absorbs this differential in a single casting, eliminating the need to stack precast grade rings or pour mortar transition courses before paving. Industry guidance limits grade ring stacks to two maximum, making the 6″ frame the cleaner single-piece solution.
Downtown streetscape reconstructions, transit-oriented development corridors, and pedestrianized zones demand drainage solutions that satisfy both hydraulic engineers and ADA compliance officers. The 6×24 ADA grate handles the additional vertical relief typical of raised crosswalks, decorative pavers, or permeable surface installations while remaining accessible-compliant across all pedestrian circulation routes.
Subsurface telecommunications chambers, electrical duct banks, and water/wastewater access structures frequently require greater collar height above the precast cone to maintain flush grade through varying pavement thicknesses across the structure's service life. The 6″ frame provides that depth reserve without special adapter castings, while the open-grate configuration delivers passive ventilation to the vault below.
Principal arterials, collector roads, signalized intersections, and DOT-administered highway drainage structures where sustained heavy vehicle volumes demand full H-20 proof-load qualification and long-service-life materials. The 6″ frame height accommodates road reconstruction projects that restore original pavement section depths, placing the manhole rim at the correct final grade without interim adjustment.
Ordering & Specification
Reference Data
- Item Reference6×24-ADA
- Description6″ × 24″ ADA Manhole Frame & Grate
- Grate O.D. (C)24″
- Frame Height (D)6″
- Clear Seat Dia. (E)23.5″
- Seat Dia. (F)24″
- Overall O.D. (G)33″
- Grate Thickness (H)1.5″
- MaterialGray Iron, ASTM A48 Class 30B
- Load RatingAASHTO H-20 (M306 qualified)
- ADA Status§ 302.3 Conforming
- FinishRaw or Bituminous Coat
- AvailabilityIn Stock
Insert the following into Division 33 — Utilities or the appropriate specification section for your project delivery method:
Manhole frame and grate shall be 6″ × 24″ round configuration, gray cast iron conforming to ASTM A48 / A48M Class 30B, rated to AASHTO H-20 wheel loading with proof-load per AASHTO M306 (40,000 lbf minimum). Grate shall be ADA compliant per 2010 ADA Standards § 302.3 with slot width ≤ ½″ perpendicular to travel. Furnished by Amercast or approved equal.
Related Assemblies &
Height Variants
The shallow-frame sibling. Specifies when grade differential is limited and vertical clearance needs are modest. Same B, C, D, F, and X dimensions — only E differs.
→ View ProductMaximum frame height in the 24″ bore family. Specifies for deep grade differential applications, thick pavement sections, or structures with significant cover buildup over service life.
→ View ProductAlternate ADA configuration with a fully circular frame profile. Specifies where project geometry or local standard details call for round-on-round assembly.
→ View ProductSame 6″ frame body with a solid cover plate. Specifies where subsurface ventilation is not required and watertight or tamper-resistant access is preferred.
→ View ProductBolt-down cover with rubber gasket for inflow/infiltration prevention. Stainless steel locking hardware. Matches the 6″ frame height for seamless I/I-controlled system design.
→ View ProductCast iron and steel riser rings for installations requiring height adjustment beyond what frame height alone accommodates. Load-tested and DOT-approved in multiple states.
→ View Category
