Drainage structures in traffic areas must be rated to withstand vehicle loads. The AASHTO load rating system classifies grates, frames, manhole covers, and other castings by their load capacity. Selecting the wrong load rating can result in broken castings, vehicle damage, and liability exposure.
AASHTO Load Classifications
AASHTO M306 defines standard load ratings for drainage structures. The primary classification is based on the design wheel load:
| Load Class | Design Wheel Load | Proof Load (1.5×) | Typical Application |
|---|---|---|---|
| Light Duty | N/A | N/A | Pedestrian areas only — no vehicle traffic |
| H-10 | 8,000 lbs | 12,000 lbs | Sidewalks, pedestrian plazas, driveways |
| H-15 | 12,000 lbs | 18,000 lbs | Parking lots, service roads, light commercial |
| H-20 | 16,000 lbs | 24,000 lbs | Public roadways — most common specification |
| HS-20 | 16,000 lbs* | 24,000 lbs* | Highways, bridges — includes tandem axle effects |
| HS-25 | 20,000 lbs | 30,000 lbs | Heavy-duty highways, industrial areas |
| Airport (FAA) | Varies | Varies | Airports — rated per FAA AC 150/5320 |
* HS-20 uses the same wheel load as H-20 but includes additional requirements for tandem axle load distribution and dynamic impact. Most highway agencies specify HS-20 as the minimum.
Selection Guide by Location
| Location | Minimum Rating | Notes |
|---|---|---|
| Residential yard / landscaping | Light Duty | No vehicle access expected |
| Sidewalk / pedestrian area | H-10 | May need H-20 if service vehicles possible |
| Parking lot | H-15 to H-20 | H-20 if fire trucks / delivery trucks expected |
| Residential street | H-20 | Standard for all public roads |
| Collector / arterial road | H-20 / HS-20 | Check state DOT requirements |
| State highway | HS-20 or HS-25 | Per DOT standard specifications |
| Industrial / port facility | HS-25 or custom | Consider actual vehicle loads |
| Airport apron / taxiway | FAA rated | Based on specific aircraft gear loads |
Inlet Grate Types and Considerations
| Grate Type | Bar Orientation | Bicycle Safe | Hydraulic Efficiency | Clogging Tendency |
|---|---|---|---|---|
| Parallel bar | Longitudinal | No | High | Low |
| Transverse bar | Transverse | Yes | Moderate | Moderate |
| Reticuline | Mesh pattern | Yes | Low | High |
| Curved vane | Curved | Yes | High | Low |
| 45° tilt bar | Angled | Yes | High | Low |
ADA Compliance: Grate openings in pedestrian areas must comply with ADA requirements — openings shall not permit passage of a 1/2-inch sphere, and elongated openings shall be placed perpendicular to the dominant direction of travel.
Manhole and Access Cover Ratings
Manhole frames and covers are rated similarly to inlet grates. AASHTO M306 classifies them by their rated load capacity:
| Cover Type | Typical Rating | Application |
|---|---|---|
| Light-duty (solid cover) | H-10 | Non-traffic areas, access covers in yards |
| Standard round (24" or 30") | H-20 | Public roadways — most common |
| Heavy-duty round | HS-25 | Highways, heavy industrial |
| Watertight (bolted/gasketed) | H-20 or HS-20 | Flood-prone areas, sanitary sewer |
Primary Sources
- AASHTO M306 (2019). Standard Specification for Drainage, Sewer, Utility, and Related Castings. American Association of State Highway and Transportation Officials.
- AASHTO LRFD Bridge Design Specifications (2020). Section 3: Loads and Load Factors. 9th Edition.
- FHWA HEC-22 (2009). Urban Drainage Design Manual, 3rd Edition.
- ADA Standards for Accessible Design (2010). Section 302: Floor or Ground Surfaces.