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Section 4: Structural Characteristics

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Flexible pipe and rigid pipe have some common structural characteristics. The following information provides general guidance on selecting appropriate strength of conduit. However, you may need to coordinate efforts with structural designers to ensure structural adequacy and compatibility.

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Corrugated Metal Pipe Strength

Corrugated metal pipe (CMP) is structurally designed in accordance with AASHTO Section 12. Fill height tables are presented in the Conduit Strength and Durability document. These fill height tables are based on the following minimum parameters:

For structures not represented by tables and conditions outside of above referenced conditions, contact the Bridge Division, Structures Section.

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Concrete Pipe Strength

The final design of reinforced concrete pipe walls is not specified in detail on the plans. The required strength of the concrete pipe is indicated on the plans by the D-load that the pipe will be required to support in the test for acceptance. With this designated loading, the manufacturer can determine the most economical structural design of the pipe walls and reinforcement that comply with the applicable American Society for Testing and Materials (ASTM) specification.

The D-load is written as a number followed by (-D). For example, consider the shorthand notation of 1350-D, which represents 1350 lb./ft. of pipe length per foot of pipe diameter (lb./ft./ft.). For this example, multiply 1350 by the pipe diameter (in ft.) for the total allowable loading per foot of pipe length. (65-D represents 65 N/m of pipe length per millimeter of diameter (N/m/mm). For this example, multiply 65 by the pipe diameter in mm to obtain the total allowable loading per meter of pipe length.)

Design load (D-load) values have been computed for a range of conditions and are tabulated in the Conduit Strength and Durability document. The D-load values depend primarily on the following:

The soil weight used for preparing the tables is 120 lb./cu.ft. (18,857 kN/m3). Live loads are determined using AASHTO methods, and the design loads for the various pipe diameters and corresponding fill heights are based upon the American Concrete Pipe Association Design Manual (Rev. 1978).

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High Strength Reinforced Concrete Pipe

When the required pipe strength exceeds a D-load of 3000 lb./ft./ft. (140 N/m/mm), the structural design of the pipe can fall into a special design category. This can increase the cost because such pipe is usually not a standard stock item with the manufacturer.

Often, refinement of parameters for high-strength pipe, such as bedding, soil weight, and/or trench width, is warranted because the cost of stronger pipe justifies a more refined analysis. For such cases, even the use of Class A bedding may prove to be cost-effective.

Contact the concrete pipe manufacturer for assistance with estimates for the various design alternatives when earth loads require pipe strength greater than 3000 lb./ft./ft. (140 N/m/mm).

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Recommended RCP Strength Specifications

Pipe strengths should be specified, as indicated in table below, to reduce the number of bid items and to simplify the administration of the project.

Anchor: #i1017957Recommended RCP Strength Specifications (Metric)

For D-loads (lb./ft./ft.) from...


...or Equivalent Class

0 to 80



801 to 1,000



1001 to 1,350



1,351 to 2,000



2001 to 3,000



Anchor: #i1017902Recommended RCP Strength Specifications (Metric)

For D-loads (N/m/mm) from…


…or Equivalent Class

0 to 40



40.1 to 50.0



50.1 to 65.0



65.1 to 100.0



100.1 to 140.0.



For some projects, it may be justified to indicate the actual computed D-load for bidding purposes without adhering to the suggested increments above. Generally, deviate from the suggested specification increments only when sufficient quantity of a pipe size warrants the special manufacturer of a specific D-load. Manufacturing conditions vary from company to company. Therefore, potential manufacturers should be contacted to confirm any suspected advantage.

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Strength for Jacked Pipe

Pipe that must be jacked under an existing roadway embankment must endure an additional loading not considered for pipe that is simply placed during roadway construction. For jacked pipe, there is the additional load of the axial or thrust load caused by the jacking forces applied during the construction.

Often, ordinary reinforced concrete pipe will serve for the purpose of jacked pipe. Under some conditions, it may be worthwhile to consider specially fabricated fiberglass or synthetic material pipe for jacked pipe. Become acquainted with the availability of various special pipe types in the project area.

For axial loads, the cross-sectional area of a standard concrete pipe wall is adequate to resist stresses encountered in normal jacking operations, if the following construction techniques are used. To prevent localized stress concentrations, it is necessary to provide relatively uniform distribution of the axial loads around the periphery of the pipe. This requires the following:

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  • pipe ends be parallel and square for uniform contact
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  • jacking assembly be arranged so that the jacking forces are exerted parallel to the pipe axis

If excessive jacking pressures are anticipated due to long jacking distances, intermediate jacking stations should be provided.

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Reinforced Concrete Box

The Bridge Division issues and maintains culvert standard details for cast-in-place and precast reinforced concrete culverts. These accommodate a range of fill heights from direct traffic up to as high as about 30 ft. (9 m) for some boxes. Consult the Bridge Division for conditions not covered by the standards.

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Plastic Pipe

Consult the Bridge Division concerning strength requirements for plastic pipe.

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