Understanding Load Ratings: Selecting the Right Manhole Covers for Traffic Areas

When manhole covers live under cars, delivery trucks, or forklifts, the right load rating isn’t optional—it’s safety, compliance, and lifecycle cost all rolled into one. Below is a clear guide to H-20/HS-20, how AASHTO M306 proof-load testing works, what HL-93 means in modern bridge design, and how EN 124 classes map to real-world applications.

The quick primer: H-20 vs. HS-20 (what those codes actually mean)

H-20 refers to a 20-ton highway truck model with a single 16,000-lb wheel load (32,000-lb axle). This shorthand is still commonly used in utility casting specs. Manhole Covers Direct
HS-20 represents a tractor–semitrailer loading: a front axle of ~8,000 lb followed by 32,000-lb axles spaced to simulate tandem/trailer effects—harsher than basic H-20. lockesolutions.com

Bottom line: both labels describe vehicular live loads that covers in traffic areas must withstand, but HS-20 is the more demanding configuration.

AASHTO M306: the proof-load test most utilities require

For U.S. municipal castings (frames, rings, grates, and covers), AASHTO M306 sets the performance bar. The standard requires a centered proof load applied through a 9-in × 9-in platen; for H-20/HS-20 traffic, manufacturers demonstrate capacity by meeting a 40,000-lb proof load, a conservative factor above the nominal wheel load. Municipal Castings Association+1ejco.com

Why the numbers don’t match 1:1

M306 uses a safety factor (~2.5×) between the design wheel load (16,000 lb for H-20) and the required proof load (40,000 lb). If a cover passes the test without cracks or permanent deformation beyond limits, it’s considered suitable for traffic applications. Schier Products

“But my bridge engineer says HL-93” — here’s the translation

Modern bridge design uses AASHTO LRFD HL-93 (a design truck, a design tandem, plus lane load). That’s a bridge design model, not a utility casting spec. Many jurisdictions still specify H-20/HS-20 for manhole covers while the roadway itself is designed to HL-93—both can be true at once. If your project documents say HL-93 for the bridge, you can still call out M306 proof-load requirements for the castings. Federal Highway AdministrationStructure MagazineEngineeringCivil.org

International/alt spec: EN 124 classes (A15–F900)

If you work with global suppliers or composite covers, you’ll also see EN 124:2015 load classes:
A15 (pedestrians), B125 (light vehicles/parking), C250 (curbs), D400 (roadways), E600 (industrial), F900 (airfields/ports). These are test loads in kN, not service loads. For typical roads, D400 is the common selection. jdpipes.co.ukmcovers.com

Selection matrix: match the cover to the traffic above

Location / Use Case	Typical Spec Language	Why it Fits
Sidewalks, plazas, landscaped areas	EN 124 A15–B125 (project dependent)	Pedestrian/occasional light-vehicle service only. jdpipes.co.uk
Parking lots & drive aisles	AASHTO M306 traffic-rated (H-20/HS-20 proof load), or EN 124 C250–D400	Covers may see delivery vans and light/medium trucks. Municipal Castings Association+1
Public streets & heavy drive lanes	M306 traffic-rated (H-20/HS-20 proof load) or EN 124 D400	Designed for repeated truck wheel loads. Municipal Castings Association
Industrial yards, distribution hubs	EN 124 E600 or higher; confirm with local agency	High-frequency heavy axles and forklift point loads. Castings Services
Airfields, ports, crane paths	Airport-rated hardware; consult owner’s standard (often >200,000-lb proof loads)	Far beyond street traffic; special duty castings or composites. NPCA

Always verify with your jurisdictional standards and the owner’s approved materials list. Some DOTs and cities have proprietary notes for rim thickness, bearing area, bolting, and gasket details even when M306 is met. Municipal Castings Association

Five mistakes that cause failures (and how to avoid them)

Confusing proof loads with wheel loads. A “40,000-lb” proof load per M306 does not mean a 40,000-lb vehicle; it’s the test load on a 9×9 platen at the cover center. Municipal Castings Associationejco.com
Specifying H-20 for true heavy-industrial sites. Where lift trucks or off-highway equipment dominate, step up to E600/F900 or owner-approved airport/port ratings. Castings Services
Ignoring frame–cover fit and bearing. Load rating assumes the cover is supported as tested; poor seating or rocked frames concentrate stress. (Check your agency’s frame details alongside M306.) Municipal Castings Association
Assuming bridge HL-93 replaces casting specs. Use HL-93 for bridge design, M306 (or EN 124) for castings. Federal Highway AdministrationStructure Magazine
Overlooking security & sealing. In traffic areas, bolted or locked covers and gaskets can prevent blow-offs and inflow—small details, big risk reduction. Municipal Castings Association

Materials & durability: cast iron, ductile iron, composite

Ductile iron dominates traffic covers for strength and fatigue resistance; it’s the “default” for M306 traffic-rated products. Municipal Castings Association
Composites/FRP (often tested to EN 124) offer corrosion resistance and lower weight; choose the correct class (D400+ for roads) and confirm agency acceptance. mcovers.com

Spec language you can adapt

Manhole Frames & Covers – Traffic Areas

Frames and covers shall conform to AASHTO M306; proof-load test per Section 7 with 40,000-lb load applied via 9 in × 9 in platen at cover center (H-20/HS-20 traffic).
Provide bolted or locking lids where required; gaskets as detailed on plans.
Casting surfaces true to plane; no rocking on frame; bearing surfaces machined as specified. Municipal Castings Association

Alternative (International Projects)

Covers and frames shall conform to EN 124:2015, Class D400 (streets) unless noted otherwise; seat and clear openings per drawings. jdpipes.co.uk

FAQs

Is H-20 “enough” for city streets?
Yes, for typical municipal streets, M306 traffic-rated (H-20/HS-20 proof load) covers are standard unless the owner requires higher classes (e.g., truck routes or industrial corridors). Municipal Castings Association

What does the 9×9 platen mean for my project?
It’s the standardized test footprint used to apply the proof load at the cover center—ensuring apples-to-apples performance among suppliers. ejco.com

Why do I see HL-93 on roadway plans but H-20 on casting submittals?
Different standards for different components: HL-93 guides bridge/live-load design; M306 governs utility castings. Both can be correct on the same project. Federal Highway AdministrationStructure Magazine

How do European EN 124 classes compare to H-20?
There’s no exact conversion, but EN 124 D400 is the usual choice for roadways—broadly comparable in use case to M306 traffic-rated covers. Always follow the owner’s standard. jdpipes.co.uk

References & further reading

AASHTO M306-10: Standard Specification for Drainage, Sewer, Utility, and Related Castings (proof-load procedures and acceptance). Municipal Castings Association
EJ: Product resource on M306 proof-load method and 9×9 platen test setup. ejco.com
Municipal Castings Association: Overview of M306 proof loads (40k H-20; 50k H-25) and safety factors. Municipal Castings Association
Locke Solutions: Plain-language HS-20 axle configuration explanation. lockesolutions.com
FHWA LRFD Reference Manual: Context for HL-93 in modern bridge design. Federal Highway Administration
Structure Magazine: HL-93 design truck/tandem and lane load description. Structure Magazine
EN 124 guides: Class definitions and applications (A15–F900). jdpipes.co.ukmcovers.com
NPCA: Load-bearing designations (traffic vs. airport-rated). NPCA