Corrugated Structural Plate (also known as Structural Plate Corrugated Steel Pipe – SPCSP) products are used where the size of culvert exceeds the constructable size of Corrugated Steel Pipe (CSP), around 3.6 m or 7 feet. They can also be a good alternative when structural concerns require a thicker culvert than CSP. Because CSP is rolled from single pieces of sheet metal, there is a thickness limit – usually 4.2 mm or 0.168 inches.
I will be using CulvertPro, our in-house design software. If you choose to use other software (or do them by hand) the mechanics will be different but generally the steps should be the same.
STEP 1: Hydrology. This step produces a design flow. If you have hydrology software, you can use it here. If not, CulvertPro’s calculators can do some a hydrological analysis sufficient for most applications.
Firstly, navigate to the Rational Method Calculator and enter the following inputs:
- Runoff Coefficient (C): This coefficient takes into account all of the hydrological abstractions, soil types and antecedent conditions and all other site parameters. Click on “C-Table” in the Information Panel to get a table listing various values for this coefficient. Also, use the Hybrid Runoff Coefficient Calculator if you need to.
- Rainfall Intensity (i): The intensity of the rainfall assumed to occur over the entire drainage basin. This should be equivalent to at least the time of concentration of the watershed. The Time of Concentration is the time it takes for the last drop of water in the drainage basin to reach the site, and the handy Time of Concentration Calculator in the Information Panel uses the three most popular methods.
- Drainage Area (A): The total size of the drainage basin.
The calculator will produce a flow rate in m3/s or ft3/s. The Rational method is best suited for small drainage areas because it assumes a uniform rainfall over the entire area. Larger than about 15 mi2, you will need to reduce the rainfall intensity value due to the unlikelihood of such rainfall occurring over such a large area.
We want to use two different methods to calculate the design flow (for comparison), so navigate to the SCS Method Calculator and enter the following inputs:
- Drainage Area: The total size of the drainage basin.
- Curve Number: A coefficient that takes into account the permeability of the soil. The “CN Table” in the Information Panel gives you a handy reference table.
- Time of Concentration: Same as for the Rational Method. The Information Panel contains a handy Time of Concentration Calculator.
- Rainfall Intensity: The design storm is defined as the rainfall intensity and duration (next). These two variables are usually obtained from Intensity-Duration-Frequency curves for the area, which are in turn usually produced by the relevant government agency. The designer must choose a return period to design for (such as a 1:50 year storm) and the axes represent each of the values, intensity and duration.
- Rainfall Duration: See above.
The SCS Method Calculator will produce a flow rate in m3/s or ft3/s, and the designer must choose a value after considering these results.
STEP 2: Channel Hydraulics. This step produces a channel flow depth, which in the next step will be called “Tailwater.” Navigate to the Tailwater Calculator and enter the following inputs:
- Flow: The design flow, calculated above.
- Channel Slope: The average channel slope in the reach of the culvert. You will need to have some type of channel survey to get an accurate channel slope. If you don’t, you can estimate it but this has a decent impact on the results. In the open prairies, you’re likely to see less than 0.005 ft/ft. In hilly terrain, 0.005 – 0.020 ft/ft, and above 0.02 ft/ft is generally only found in steep, hilly terrain or mountains.
- Manning’s n: The roughness coefficient of the stream channel. Use the handy Manning’s n table in the Information Panel for reference.
- Channel Shape and cross-section properties: This is where you tell CulvertPro the channel geometry parameters. Try to make it as average as possible, and don’t use cross-section properties for a reach that has been affected by man made features (like an existing culvert).
The Tailwater Calculator will produce the natural flow depth of the channel.
STEP 3: Culvert Hydraulics and Culvert Sizing. Armed with the design flow and channel flow depth, we will proceed to the meat and potatoes of the culvert design. The channel flow depth is also called the “tailwater,” because it is assumed at the downstream end of the culvert. Navigate to the Culvert Hydraulics Calculator and enter the following inputs:
- Culvert Shape: Self Explanatory. If you’re using a structural plate corrugated steel pipe, this would be either Round, Horizontally Ellipsed, or Pipe Arch, as they don’t come in box shapes.
- Culvert Length: This has very little impact so you can assume it, or you can use the Culvert Length Calculator (the 4th Calculator from the left).
- Flow: The design flow, calculated above.
- Tailwater Depth: The tailwater calculated above.
- Upstream Invert Elevation: The elevation at the upstream invert of the culvert.
- Downstream Invert Elevation: The elevation at the downstream invert of the culvert.
- Culvert Burial: The depth that the inverts will be buried below the streambed. Most round pipe are buried to allow them to conform to the channel better.
- Manning’s n of Culvert: The roughness coefficient of the culvert. Use the handy table in the Information Panel for reference.
- Entrance Loss Coefficient: The coefficient used for calculating entrance losses. Use the handy table in the Information Panel for reference.
- End Section: The type of end section. Either conforming to the slope (bevelled), projecting from the slope (straight end), or headwall (flat vertical surface around culvert opening).
The culvert hydraulics calculator will produce a headwater elevation and inlet/outlet extreme velocities, which can be compared against design criteria.
STEP 4: Structural Design. CulvertPro does not have a structural design calculator. Most jurisdictions have design tables which specify the culvert thickness given its size and height of fill above it. You can also see my overview of structural design of CSP culvert using the AASHTO LRFD Bridge Design Specifications here.
This was a brief overview, but all of these steps are covered in more detail in the CulvertPro Manual. Good Luck!