D-Load (Concrete Pipe Strength)
The load a concrete pipe carries in a three-edge-bearing test.
Quick Answer
D-load is the strength rating of reinforced concrete pipe, expressed as the test load in pounds per linear foot per foot of inside diameter in a three-edge-bearing test. Higher D-loads mean stronger pipe. It sets the pipe class for storm drains, and an estimator prices pipe by LF according to class.
The Full Picture
D-load comes from the three-edge-bearing test, in which a pipe section is supported along two edges at the bottom and loaded along one at the top. The test load is divided by the pipe's length and its inside diameter to give a number in pounds per linear foot per foot of diameter. Engineers use the result to select a pipe class that can carry the soil cover and traffic expected over the line. Reinforced concrete pipe is commonly manufactured to ASTM C76, whose title is Standard Specification for Reinforced Concrete Culvert, Storm Drain, and Sewer Pipe, and that standard organizes pipe into numbered strength classes.
The class, not the D-load number, is what usually appears on the plans. A storm sewer profile or pipe schedule lists diameter, material, class and length for each run, and cover depth plus bedding type govern what class the engineer picks. Deep runs or shallow ones under heavy traffic can need stronger pipe than the typical run, so a single class should not be assumed for the whole system.
For takeoff, D-load itself is not a unit. The quantity is LF of pipe by diameter and class, with structures such as manholes, inlets and headwalls counted by EA. Joint type, gaskets, fittings, bedding and backfill are carried as separate items. The pipe class shifts the unit price, because stronger pipe has more reinforcement and is more expensive and heavier to ship and set.
Pricing pitfalls include quoting the typical class for runs the profile shows with deeper cover, counting pipe length from structure center to center instead of the net length between structure walls, and forgetting that heavier pipe needs larger equipment to set. Specification sections for storm drainage, usually in Division 33, state the pipe standard, class and installation requirements. Pipe class is a design decision, so questions go to the engineer of record.
Real Examples
Common Misconceptions
People assume: D-load is the maximum load the pipe can ever carry in the ground.
Actually: It is a laboratory test rating. Field capacity depends on installation, bedding, cover and loads, which is why engineers choose the class based on the design conditions.
People assume: The same pipe class works for an entire storm system.
Actually: Class can change with cover depth, traffic loads and bedding along the run, so each segment should be priced from the pipe schedule or profile.
Frequently Asked Questions
What does D-load stand for?
It is the three-edge-bearing test load per foot of pipe length per foot of inside diameter, expressed in pounds per linear foot per foot of diameter.
How does D-load relate to pipe class?
ASTM C76 groups reinforced concrete pipe into numbered classes with defined strength levels. Plans normally name the class rather than the D-load number.
Is D-load a takeoff unit?
No. Pipe is measured in LF by diameter and class, and structures are counted by EA. D-load only affects which class and price applies.
Where do I find the pipe class?
In the storm drain profile, pipe schedule or general notes on civil sheets, and in the Division 33 storm drainage specification.
Why does pipe class change the price?
Higher-strength pipe uses more reinforcement and wall material and often weighs more, which affects the pipe price, hauling and the equipment needed to set it.