How Drainage Load Ratings Work

What is a load rating?

A load rating describes the traffic a grate or drainage system can support under specified conditions. Manufacturers establish ratings through load testing, then identify the components and installation needed to achieve them.

Think of a grate as a small bridge. It spans an opening, carries a wheel or foot, and transfers that force into its supports. Those supports may be the channel’s edges or a separate metal frame. The channel and surrounding concrete then transfer the load into the ground.

A stronger grate helps only if its supports can carry the force too. This is why a load rating can depend on a concrete surround, a particular frame and the way the grate is secured.

Weight, wheel load and pressure

The vehicle’s total weight is shared among its wheels. The load on one wheel depends on the axle loading, the cargo and the vehicle’s layout. That wheel presses against the surface through its tire’s contact patch.

For a simple example, suppose a wheel carries 1,500 lb. Spread over 30 square inches, the average pressure is 50 PSI. Concentrate the same load onto 6 square inches, and it becomes 250 PSI.

That is why a small, hard forklift tire can be more demanding than a larger air-filled tire carrying the same weight. The figures above illustrate the arithmetic; actual selection uses the vehicle’s wheel loads and tire data.

Pressure is only part of the picture. A wider unsupported span creates more bending in the grate. Rib depth, thickness, material and support geometry affect how it resists that bending. Braking, turning and repeated crossings also load the drain in ways a stationary wheel does not.

How is a load rating determined?

The manufacturer designs the grate and channel to resist bending, crushing and movement, then tests the product using a defined setup. A laboratory applies a measured force through a loading block. The block’s size, the location of the force and the way the sample is supported all affect the result.

1. Set up the sample. Support the grate or complete channel assembly as the test method requires.
2. Apply the load. A testing machine presses through the specified block, bringing the force up to the required test level.
3. Assess performance. Check for damage and deformation against the method’s acceptance criteria. The standard determines the loading sequence and what counts as a pass.

EN 1433, the European standard for drainage channels, includes tests for both grates and complete systems. Its loading blocks vary with channel width so the force acts on the grate. A block that bridges across the opening and rests on the supports tests a different load path.

A test load is the force applied during that procedure. A service load is what the installed drain experiences in use. Test requirements and design margins connect the two. Dividing a test force by four wheels does not give an allowable vehicle weight.

Test diagrams: ACO load-testing guide (PDF, printed pages 132–133).

How do NDS product lines differ?

Dura Slope and Pro Series use NDS’s letter classes and PSI recommendations. BG-FILCOTEN uses EN 1433 classes. The distinction extends to NDS’s commercial trench drains; it is not just a difference between residential and commercial products.

Dura Slope: the frame changes the capacity

Dura Slope has an HDPE channel body and interchangeable grates. NDS rates the DS-232 ductile-iron grate at Class C, up to 325 PSI, when properly installed. With the DS-200H ductile-iron frame and the required installation, NDS rates it at Class D, up to 575 PSI, and identifies that configuration as H-20 rated.

The frame provides a different support arrangement for the grate. Buying a ductile-iron grate alone does not create the framed Class D installation.

NDS also says Dura Slope channels and grates have been tested using AASHTO M306 and EN 1433 methods by an independent accredited laboratory. So Dura Slope does use those test methods, while its published product recommendations remain NDS letter classes and PSI. For a job specifying D400, compare the proposed assembly’s EN classification or test report with that requirement.

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Pro Series: the grate matters more than the family name

Pro Series includes several widths and grate designs. For example, NDS’s 8-inch #838 Load Star grate is glass-reinforced nylon and carries NDS Class C, 176–325 PSI. Its specification calls for four #829 screws per grate. A nonmetal grate can therefore have a higher rating than a metal grate designed for pedestrian use.

Compare the actual grate and channel combination when choosing a Pro Series drain. “Pro Series” alone does not tell you its load capacity.

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BG-FILCOTEN: an EN-rated concrete system

BG-FILCOTEN uses high-performance concrete channels with metal edge rails. NDS’s U.S. catalog identifies the 4-inch tec V100, 8-inch pro V200 and 12-inch pro V300 systems under DIN EN 1433, with configurations up to E600. The selected grate still determines which class the complete combination can achieve.

For these systems, E600 identifies a 600 kN test class. It is the same EN designation used to compare other manufacturers’ EN-rated channels. DIN EN 1433 is the German adoption of that European standard.

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NDS load classes: A through D

NDS pairs these pressure ranges with traffic recommendations. The vehicle recommendations for Classes B, C and D apply at speeds below 20 mph. Its ratings depend on the specified concrete installation.

Manufacturer guide: NDS load rating recommendations.

EN 1433 classes: A15 through F900

The number is the class’s test force in kilonewtons (kN). For example, D400 means 400 kN, equivalent to about 89,900 pounds-force applied in the test. That figure is a force on the test sample; it is not a truck weight or a capacity per foot of drain.

A drain beside a road and a drain across the traffic lane face different loads. Choose the group for the drain’s actual position and traffic, including delivery or emergency vehicles that may use the area.

Can you compare NDS PSI with an EN test force?

You can calculate pressure from force if you know the loaded area: pressure = force ÷ area. But the area must be the contact area used in that calculation. Multiplying NDS’s PSI figure by the entire length and width of a grate treats the whole grate as uniformly loaded, which does not represent a wheel crossing it.

For an EN test, the loading block and test conditions supply that missing context. For NDS’s recommendations, the PSI bands are paired with traffic and installation limits. There is no single PSI multiplier that turns every NDS Class D product into D400.

Where does H-20 fit?

H-20 comes from U.S. highway design loading. Its 32,000 lb rear axle represents 16,000 lb at each side of the axle, often described as a wheel load. That is a design load; a casting’s proof test can apply a higher force. For example, a 2.5 multiplier gives a 40,000 lb proof load.

H-20 describes a vehicle-loading requirement, while AASHTO M306 specifies requirements for drainage castings and EN 1433 classifies drainage channels. When a project calls for H-20 or M306, use the product’s documented rating for that requirement. The DS-232 with DS-200H example above shows where NDS makes an explicit H-20 claim.

Background: Trumbull Manufacturing’s explanation of H-20 loading.

How to choose the right drain

1. Identify what will cross it. Include the heaviest occasional vehicle as well as everyday traffic. For forklifts, use loaded axle or wheel data and the tire type.
2. Locate the drain in the traffic path. A driveway crossing, a loading-dock turn and a protected walkway impose different demands. Account for speed, braking and turning.
3. Select the complete combination. Match the channel, grate, rails or frame and fasteners to the required rating. For Dura Slope Class D, include the specified frame. For FILCOTEN, match the channel and grate to the required EN class.
4. Build the required support. Follow the installation drawing for concrete dimensions and strength, reinforcement, pavement joints and curing. A loosely supported channel cannot deliver the performance of the specified concrete installation.

For an ordinary residential driveway, NDS Class B is a starting point for cars and light trucks within NDS’s stated conditions. A driveway used by refuse trucks, delivery trucks or equipment needs a selection based on those heavier vehicles. Load capacity and drainage capacity are separate checks: the chosen system must carry both the traffic and the water.