# How to Design a Concrete Pipe

Concrete pipe is often a good alternative to corrugated steel for highway culvert applications.  Although it is more expensive it is significantly stronger.  The American Concrete Pipe Association’s advertising slogan “Relax with Concrete Pipe” certainly holds true;  I definitely can relax with the concrete pipes I’ve installed.

There are two design methods in existence for concrete pipe:

1. Indirect Design:  This is the traditional design method.  It is a simplified method over Direct Design and requires you to use a concrete pipe that comes from a standardized list who’s strength has been tested (using the 3 edge bearing test).  This method involves calculating the loads on the pipe and selection of a factor of safety.  The applicable design standard is ASTM C76.
2. Direct Design:  This is the full pipe design method, and is normally used for larger pipes to minimize reinforcement costs, or where a simplified design method is not preferred.  The applicable design standard is ASCE 15-98.

In this article, I will outline the design procedure for concrete pipe using the indirect design method.  This procedure should be considered an overview;  I cannot provide enough information in this article to perform a full design.  The procedure involves the following seven steps:

4. Select Standard Installation
5. Choose Bedding Factor
6. Factor of Safety
7. Select Pipe

Because we are focused on culverts in a highway setting, we will be using the embankment condition instead of the trench condition.  To calculate the Prism Load (PL) in lbs/ft the following formula is used:

$PL = w(H + \frac {D_{o}(4+\pi)} {8})D_{o}$

w = soil unit weight (lbs/ft3)
H = height of fill (ft)
Do = outside diameter of pipe (ft)

The prism load is then multiplied by an arching factor to account for varying settlement from the fill above the pipe versus that beside the pipe.

$W_{e} = VAF x PL$

The vertical arching factors are taken from standard installation methods, which are chosen in step 4.  The only way to do this properly is to choose an installation type and come back to this step if you have to.

 Standard Installation Vertical Arching Factor, VAF Type 1 1.35 Type 2 1.40 Type 3 1.40 Type 4 1.45

The AASHTO Standard Specifications for Highway Bridges states that a pipe design should use a fluid weight of 62.4 lbs/ft3.

This is done via the AASHTO standard design trucks translated down to the pipe with 1:1 to 2:1 angles, as specified.  Please consult the AASHTO LRFD Bridge Design Specifications or the American Concrete Pipe Association’s Design Data #1.

## Select Standard Installation

Standard installation types range from 1 to 4, and each has a varying specification for backfill compaction at various locations within the backfill.  Type 1 is the highest level of construction quality and type 4 is the lowest.  Thus, type 4 will require the highest strength pipe and type 1 the lowest.  We have posted a more detailed description here.

## Determine the Bedding Factor

The bedding factor is the ratio of the strength of the pipe under the field conditions of loading and bedding to the strength of the pipe in plant tests.  The following tables lists bedding factors for earth loads.  For pipe diameters other than the ones listed here, simply interpolate between the values on the table.

 Pipe Diameter Standard Installations Type 1 Type 2 Type 3 Type 4 12 in. 4.4 3.2 2.5 1.7 24 in. 4.2 3.0 2.4 1.7 36 in. 4.0 2.9 2.3 1.7 72 in. 3.8 2.8 2.2 1.7 144 in. 3.6 2.8 2.2 1.7

Live load bedding factors are located here (the table is too big to include manually).

## Factor of Safety

To determine the factor of safety, the design criteria must be consulted.  If the design is for a 0.01-inch crack D-load (the usual scenario), the following factors of safety are used.

 0.01-inch crack D-load Factor of Safety < 2000 1.5 2000 – 3000 interpolate > 3000 1.25

You can also design for ultimate strength, in which case a factor of safety of 1.0 is used.

## Select Pipe

To select the pipe, we first choose a pipe from standard size tables (from suppliers or see our list) and then calculate the D-load it must withstand.  The suppliers produce pipe in different D-load levels, like 50-D, 65-D, 100-D, and 140-D.  The pipe designer must specify which D-load level is needed, and this should be the next higher D-load level from the following calculation.

$D-load = [\frac{W_E}{B_f} + \frac{W_L}{B_{fLL}}] x \frac{F.S.}{D}$