rounding up to the next whole pipe. For this example, the number of pipe required is 32/27.5 = 1.2 or two 36-inch
STEP 7. The next step is determining the length of pipe to be ordered. The order length is calculated by
multiplying NP (Step 6) times LIP (Step 3) times a waste factor (WF). Since pieces of material will be damaged
in manufacturing, handling, transporting, and assembling, an additional amount over the actual in place length
will be required. This value has been determined to be 15 percent of the total length of pipe required. For this
project, the pipe selected will be 36 inches in diameter with a length in place of 238 feet. The length to order will
NP x LIP x WF
2 x 238 x 1.15 = 547.4 ft (round up to 548 ft)
The pipe comes in 2-foot increments; therefore, the value of 547.4 is rounded up to the next even value, or 548
feet of pipe.
PART G - DESIGNING A CULVERT FOR A ROADWAY
The procedure for designing a culvert for a roadway is much like that of designing a culvert for an airfield with
one important difference: computing the pipe diameter.
To compute the pipe diameter for an airfield culvert, the calculation is based on aircraft loads, pipe gauges, and
cover requirements. When making this determination in the field, you will rely on tables.
To compute the pipe diameter for a roadway culvert, determine the maximum diameter of a culvert that can be
placed in the minimum fill section without the danger of crushing. This will determine the subsequent size of all
culverts that can be used at that section without danger of crushing. Due to a difference in the application of
loading, there will be a difference between the amount of cover required to protect culverts under vehicle and
The maximum pipe diameter used for roads or other systems carrying vehicle traffic is two-thirds of the minimum
fill depth (Dmax = 2/3Fmin).
The following is a design method for a CMP under a roadway.
Gather known data. Make a cross section drawing.
Design Q - From the Rational Method.
Fill depth - From construction site.
Slope - From elevations at top and bottom.
Load (for airfields).
Compute pipe diameter.
Dmax = 2/3Fmin.