Round
Elliptical
Arch
Box/Rectangular
These pipe shapes can work in an incredible variety of situations. Round being the most simple and widely used of all the shapes, box is used where cover is a concern or flow is too high, and arch and elliptical are used where there are additional issues from outside sources. But even so, many of these pipe shapes can be used in lieu of one another. We need to be armed with the best information possible to make the most prudent choice.
When reviewing your options, understanding the environment and the volume the conduit will have to carry in the worst-case scenario will be key. Many considerations have to be taken into account, including the land’s topography, water table, existing roads, utilities, and other considerations all play into the selection of pipe shape.
Flow volume will differ from shape to shape, which will be another factor in determining if you need a specific size or shape and if you need multiple runs to safely discharge the stormwater.
The most widely accepted formula for evaluating the hydraulic capacity of non-pressure sewers is the Manning Formula.
This formula is:
Where:
Q = discharge in cubic feet per second
n = Manning’s roughness coefficient
A = cross-sectional area of flow in square feet
R = hydraulic radius in feet (equals the area of the flow divided by the wetted perimeter)
S = slope of pipe line in feet of vertical drop per foot of horizontal distance
Since the designer is usually concerned with selecting a sewer size for a given design flow and pipe slope, the Manning Formula is more conveniently expressed as:
By evaluating the values of 1.486/n x A x R2/3 for the various types and shapes of pipes available, a pipe size can be selected for any Q/S1/2value. Under any given flow condition, the area A and hydraulic radius R are constant for a particular size and shape of pipe. The hydraulic capacity of a pipe on a given slope is primarily dependent on n, the roughness coefficient. Table 1 lists recommended values for roughness coefficients of various pipe materials.
These values are substantiated by extensive research and have been adopted for use by most government agencies.
Results of numerous test programs conducted under laboratory conditions have shown the roughness coefficient of concrete pipe to range between 0.009 and 0.011. The design values of 0.012 and 0.013, shown in Table 1, have been generally used to account for the possible build up of slime or grease in sanitary sewers and foreign debris in storm sewers.
Round pipe is the most commonly used pipes for all pipe materials for many reasons. It allows for the most easily reproducible shape as many of the machines used for the manufacture of a reinforced concrete pipe are set up for round pipes. These machines can manufacture a range of sizes, from 12” diameter to 96” diameter. Many regions can make even larger sizes than that!
A round pipe also has benefits in design and installation, too. The national standards and specifications for the design of reinforced concrete pipe (ASTM C76) are accepted, at minimum, throughout all municipalities and Departments of Transportation (DOT’s) of the United States. There are regions that have additional strength requirements for any RCP that will be installed, such as TxDOT’s additional 15% compressive strength requirements. Of every batch of concrete pipes that are manufactured, a pipe will be removed and tested to ensure these minimums are met. These tests will happen multiple times a day, documented, and reported to every agency to ensure the quality is consistent.
When installing a round pipe, the benefit of round is it is not important to keep the crown (inside top) or flowline (inside bottom) in a specific spot. If the pipe rotates, it does not matter due to the pipe being consistent all around. But a circle needs support on the outside! Therefore, when installing a round pipe, it is important to have the installation meet the national and local requirements for supporting the pipe in the ground.
Read size of pipe required from appropriate table corresponding to values of 1.486/n x A x R2/3 equal to or larger than 1400.
The following type and size of pipe will carry the design flow: Circular Concrete Pipe Table 2.
Elliptical pipes are unique; the shape is similar to a circle, but it isn’t uniform all the way around. Elliptical pipes get to do something special that a circular pipe can’t, they can fit in places where there isn’t enough room for a circle! Is there an issue above the pipe where there isn’t the clearance the pipe needs? Then use an equivalently sized elliptical pipe to convey the water. What about the horizontal clearance? Do you need to squeeze a pipe in a narrow opening? The elliptical pipe can go in that space with the flowline being in the narrow part of the pipe.
Elliptical pipes are synonymous with the solution to utility conflicts. In many urban areas, utilities are ubiquitous. It is hard to develop an area without needing to consider the placement of other utilities. Elliptical pipes really shine where there is also an issue with low cover AND you must maintain a certain amount of flow. That’s because these pipes have equivocal sizes with round pipes. Therefore, it is an easy task for either then engineer or the contractor to determine if an elliptical pipe is an acceptable alternative to RCP.
Read size of pipe required from appropriate table corresponding to values of 1.486/n x A x R2/3 equal to or larger than 1400.
The following type and size of pipe will carry the design flow: Horizontal Elliptical Concrete Pipe Table 3.
Arch pipes have a very similar benefit to a system as elliptical pipe, but there is one main difference: arch pipes have a flat side on the bottom of the pipe. This means the arch pipes are more beneficial only if there is vertical clearance issues for a round pipe. However, given that an arch pipe has a flat, non-rounded bottom, there are less possible complications for installation purposes. An arch pipe does not need support from structural soil to prevent any rolling that might occur. The flat bottom only needs a flat foundation to sit upon.
Arch pipes and elliptical pipes can be very similar to one another, but the manufacturing process might limit a region or manufacturer from creating both types of pipe. It is important to check with your local manufacturer on whether you can do the type you prefer. But remember, an arch pipe is an elliptical pipe with a flat footing.
Read size of pipe required from appropriate table corresponding to values of 1.486/n x A x R2/3 equal to or larger than 1400.
The following type and size of pipe will carry the design flow: Concrete Arch Pipe Table 4.
Now let’s think INSIDE the box. Round, and its variations, has its own benefits and the incredibly ubiquitous use in development. However, it isn’t a universal answer to all situations. Round pipes do have a minimum cover requirement. They do still have a reliance, no matter minimal, on the structural backfill up to the springline of the pipe. Round pipes don’t lend themselves to a “T” connection nor a bend or corner without the use of an additional structure. Round pipes also don’t tend to allow for a size change without the use of a structure.
That is where a box culvert comes to center stage. A round pipe needs at least 12” of backfill cover over the top of the pipe. A box culvert needs no cover. Absolutely nothing. The contractor can place the road directly on top of the box. A box culvert is designed to be traffic rated; that is how the no-cover is possible. Also, due to having a flat bottom with no haunches, a box culvert doesn’t need any structural backfill on the sides. It simply needs an appropriately compacted backfill on the sides and top.
A unique benefit of a box culvert is the culvert can be used as a structure itself. This means that any lateral pipes coming in or even a vertical attachment, such as a manhole opening, can be installed directly into the box. This ease means that as long as the contractor installs the box culverts according to manufacturer and national specifications, any branching lines or structures can be located and tied into the main alignment without having to worry about locating a junction box. This can save time in installation and save money for the client. One could even have a transition of a round pipe into a box culvert! A box culvert also has a higher hydraulic capacity than pipe.
Therefore, a theoretical design and development of a storm system could look as such: a main line of culverts that has transitions from a round RCP’s to RCB’s of varying sizes (as needed according to the hydraulic design), laterals and vertical structures connecting to the main line without a junction box, then going directly to the outfall. This simple, clean design is great for serving master planned communities around the US!
Under certain conditions the hydraulic or structural characteristics of reinforced concrete box sections offer advantages over the circular and non-circular pipe shapes commonly used for sewers and culverts. The cost-effective advantages of precast concrete pipe productions and construction methods are available in a product manufactured in accordance with the ASTM Standard C1433, Precast Reinforced Concrete Monolithic Box Sections for Culverts, Storm Drains and Sewers and Standard C1577, Precast Reinforced Concrete Monolithic Box Sections for Culverts, Storm Drains, and Sewers Designed According to AASHTO LRFD. The American Concrete Pipe Association’s CP Info, Precast Concrete Box Sections, presents the development and verification of the design method and standard sizes.
The standard precast concrete box section produced under Standards C1433 and C1577 is shown in Figure 1.
The standard sizes and wall thicknesses are shown in Tables 1 and 2. The standard sizes have 45-degree haunches with a leg dimension equal to the wall thickness. The availability and construction details of box sections should be discussed with local concrete pipe producers. Precast box designs other than standard are available through American Concrete Pipe Association member companies.
Rinker has the capability to manufacture Mega box culverts up to 24’ span by 8’ rise in select locations. Please note that not all products are made at all our facilities. For more information, specifications, and product availability, please contact us or reach out to your local sales rep.
The hydraulic characteristics of precast concrete box sections are similar to those for circular, arch and elliptical pipe. The most widely accepted formula for evaluating the hydraulic capacity of non-pressure conduit is the Manning Formula.
This formula is:
Where:
Q = discharge in cubic feet per second
n = Manning’s roughness coefficient
A = cross-sectional area of flow, square feet
R = hydraulic radius in feet (equals the area of the flow divided by the wetted perimeter)
S = slope of conduit, feet of vertical drop per foot of horizontal distance
Since the designer is usually concerned with selecting a box size for a given design flow and slope, the Manning Formula is more conveniently expressed as:
By evaluating the values of 1.486/n x A x R2/3 for the various box sizes available, a size can be selected for any Q/S1/2 value. Table 3 lists the area A, hydraulic radius R, and C (1.486/n x A x R2/3) a constant for the full flow condition. Based on Manning’s Formula, these tabular values are equal to Q/S1/2 for full flow. For any Q/S1/2 value, the size of box required can be read directly.
* Values have been rounded due to the empirical nature of the terms used to calculate the constant.
It is important to note that in sewer design, a hydraulic comparison between various shapes cannot be made solely on the basis of cross-sectional areas or peripheries.
Box sections used for culverts are evaluated by the major factors affecting the hydraulic capacity as illustrated in Figure 2. For any given headwater depth, these factors interact to control the hydraulic capacity by one of the following means:
a. Geometry of the inlet;
b. Combined influence of size, shape, slope and surface roughness of the culvert.
c. Tailwater conditions at the outlet.
