Selecting Channel Drain Components
To write a proper specification for a trench drain or a slotted drain you must review the load transfer path and the fluid transfer path and carefully choose all components of the trench drain system.
The load transfer path starts with a load exerted on the trench grate. The grate then transfers these forces to the frame that supports that grate. That load is then passed to either the concrete or the drain body depending on the design of the channel drain system. Finally the load is transferred to the soil underneath the channel drain assembly. The component with the least strength will determine the service life of the channel drain system.
The fluid transfer path follows the liquids through the channel drain system. Liquids first pass through the grating, then into the body of the channel drain, and finally into the channel drain outlet and piping system. The component with the least flow capacity will be the limiting factor of the flow path design.
Once the design parameters of the channel drain have been determined one must consider the channel drain anatomy or each piece of the system to ensure it accommodates each of the loading requirements and flow requirements of the channel drain design.
Finally, once all components have been selected write a specification that completely defines each element. This will ensure that a proper channel drain gets installed.
The load path follows the channel drain system loading from where the load enters the system to where it exits the channel drain system. For a trench drain the load is first exerted on the grating, then passed to the trench drain frame, channel body, concrete, and finally the earth or soil. For a trench drain to last each of these components must be carefully selected.
The grate should be selected based on the wheel loading expected to be exerted on the trench drain grate. Typically for pneumatic tire traffic we recommend a minimum of 3X safety factor of the actual load to account for dynamic loading conditions. For hard tire traffic it is better to use a minimum of 5-8X safety factor.
SAMPLE PROBLEM: A hard tire forklift with combined weight of the machine and it's load weighs 30,000 lbs. The forklift will be driving across the trench drain. Determine the loading requirements of the trench drain system.
SOLUTION: The forklift exerts the 30,000 lb load on 4 tires. If we assume this is distributed evenly on all four tires then each tire will exert 7,500 lb load on a single grate. Because of the hard tire dynamic load we will assume an 8X safety factor. The trench drain grate needs to be able to carry 60,000 lbs. Using the EN 1433 (formerly DIN) standard this would require a load class D grate to carry this load.
Next determine the required frame. The frame will take loading from the grate and should be capable of the same loading as the grate. The frame will also receive direct loading as the hard tires cross onto the grate. First the frame thickness needs to be checked for the load applied. From the DIN standards we know that for a D class load the frame needs to be at least 0.157" thick. This is required to keep the top of the frame from deforming as the wheel load exerts a force on the top of the frame. The forklift load in this problem warrants the use of a Dura Trench heavy duty frame style at minimum.
Now we need to verify that the frame has sufficient bearing area and concrete strength to transfer the wheel load into the concrete. We will assume we have selected a 24" long grate for this problem. Let's select the HDBP15ZSA heavy duty frame to start this discussion. This frame has a listed bearing area of 31.5 square inches per linear foot. We will also assume that a rigid iron grate with a length of 24" long has been selected for this application. From this we can calculate the minimum concrete strength required to accept this loading.
concrete strength = applied load / bearing area
Previously we determined that the applied load was 60,000 lbs per tire. Each tire can exert this load on one grate that is 2 feet long. The bearing area is 31.5 sq in/ ft x 2 ft = 63 sq in. Therefore...
concrete strength = 60,000 lbs / 63 sq in.
= 952 psi
This tells the minimum acceptable concrete strength is 952 psi for this combination of load, frame, and grate. Note that the 60,000 lb loading already has a 8X safety factor so no additional safety factor needs to be considered.
Note that the Dura-Trench family of channel drains pass the load directly to the concrete surround and do not place loads on the channel body. With some other systems the next step would be to evaluate the load that is then transferred into the wall of the channel body. Compression and buckling would need to be evaluated.
Since we are using the Dura-Trench products we can move our discussion to the concrete and soil. The final step is to move the loads through the concrete to the earth. While we know that this problem only requires 952 psi concrete for the compressive load there are other considerations. The frame will pass the load to the concrete but the concrete bears on the soil. If the soil is not compacted properly it can be softer under the trench in certain areas and possibly settle. When this happens the concrete can be placed in tension and cause tensile failures. The tensile strength of concrete is only about 10% of the compressive strength so it takes a fairly small load to create a tensile failure due to poor compaction under the channel drain. While concrete design are out of the scope of this discussion, longitudinal reinforcing bars and dowels into surrounding concrete are means of increasing the tensile strength of the concrete encapsulation and transferring loads. Proper consolidation of the soil under a trench drain is also a critical item that cannot be overlooked. Properly designed concrete around a trench drain acts as a beam and spreads the load. Ground bearing pressure should be checked but is rarely the cause of a trench drain failure. Typically the concrete surround fails from lack of expansion control on the sides of the trench drain and from surface impact that causes sheer failures of the concrete.
NOTE: By specifying grates, frames, and even concrete that significantly exceeds the design requirements it will significantly increase the cost of the system with little long term benefit.
The flow path follows the fluids from the surface through the grating, down the channel, and out the piping system at the end of the drain. If any one of these components is not properly sized the system will not work to capacity. It is very common to see drains improperly sized because one of these three items was not properly evaluated. A correctly sized system is much more economical and can save significant cost on any project.
The grate should first be evaluated for inflow capacity. The grating is rarely the limiting factor in a design. The grating can be the limiting factor when the trench run is short, the grate has small openings, or large debris loads are expected to clog the grating. Here is a link to the grate open area page.
The channel body can be a limiting factor. The flow rate should be checked against the design flow. Note that trench width, depth, and slope of a Dura Trench system can be altered to ensure the flow rates match the design flow. Here is a link to the channel flow capacities page.
The outlet system is often overlooked and can be the limiting factor on the flow capacity of a channel drain system. In most cases, the outlet piping system should be designed to accept the design flow. The only time this is not the case is when the trench is used for settling out solids or as a storage / containment vessel.
SAMPLE PROBLEM: A parking lot will be sloped to a trench drain that will need to capture the rainfall during a 50 year storm. The parking lot is 1.8 acres in size and the trench drain will be 70 feet long. The rainfall intensity is 6.2 inches per hour. The drain will be near a cross walk so we will need an ADA compliant grate. There is minimal debris in the parking lot area and it is not a concern. What size trench drain should be used?
SOLUTION: Let's start by simply determining the volume of water that needs to be captured. In this simplistic problem we will use Q = C I A where Q is the required flow in CFM, C is the runoff coefficient for asphalt, I is the rainfall intensity, and A is the area in acres.
Q = (0.9)*(6.2 in/hr) * (1.8 acres)
= 10.0 cfs
Following the flow path we will first select a grate that can capture the required hydraulic load. We need to calculate the required open area of the grate. To do this we will use the equation Q = Cd * A * (2*g*h)^0.5 where Q is the flow rate that is captured in cfs, Cd is a discharge coefficient usually assumed to be 0.6, A is the open area of the drain system, g is a gravitational constant (32 ft/sec/sec), and h is the head above the floor in feet. We will rearrange this formula to solve for the grate open area.
A = Q / (Cd * (2*g*h)^.5)
= 10 cfs / (0.6 * (2*32ft/sec/sec*.01ft)^0.5)
= 20.8 sq ft
=2995 sq in
Now we divide this number by the linear feet of trench drain to determine the open area required per linear foot of trench.
OA = 2995sq in/ 70 ft
= 42.8 sq in/ft
We can now use the tables of grates and look up a grate that has sufficient open area and is also ADA compliant. For this example we will assume we want a ductile iron grate for the vehicular load and need a "C" style grate to meet the ADA compliance. The smallest grate that will have the open area, load rating, and ADA compliant opening is the 12C24DI grate.
Next we must determine the channel flow capacity to ensure the channel can carry the required flow. The channel for a 12" wide grate is 10" wide. No trench body material was specified so we will utilize a standard prefabricated polymer trench. Using the channel flow capacity charts for 10" wide trench drains we can look for a slope and depth combination that will handle the design flow (in this case 10 cfs). By using the channel flow data we see that a 10" trench must be 36.5" deep at 0.5% slope or 28" deep at 1% slope to handle the flow volume of 10 cfs. In this case no depth restriction was given so either solution would work. If depth restrictions were a concern the width could be increased to reduce the depth.
Finally, the outlet pipe must be appropriately sized for the 10 cfs flow requirement. For this design we will assume the pipe has a 1% slope. With this assumption the pipe would need to be a minimum of 18" in diameter. Dura Trench products can create pipe adapters for any size pipe to any size trench. The pipe can either be directly connected to the trench drain or the trench drain could flow into a catch basin that housed the pipe connection.
Anatomy of a trench drain
A complete trench drain system is made up of several components that together make a functional trench drain system. If each component is properly selected the trench drain will last a lifetime. If any component is not properly selected the trench drain may prematurely fail. The components that must be determined are the trench drain body, load bearing frame, trench drain grate, grate locking mechanism, channel joint sealing, and outlet connection.
The load bearing frame should be sized based on the load rating and bearing area. After the size has been determined the material and finish should be selected based on chemical resistance, corrosion resistance, and aesthetic design of the project.
Grates that are small, light, in high speed traffic, or have theft concerns should be locked down to the trench frame. The locking mechanism must be selected based on the pullout resistance, dynamic loading, and chemical loading that the system will see.
In some cases it is imperative that the trench joints be sealed water tight. If this is a requirement make sure the system selected has a large flange for applying sealant or the proper welding is specified.
Finally the outlet pipe should be sized for the proper flow. Also specify the proper pipe material to ensure it can mate with other piping, has proper load rating at the given cover depth, and has the proper corrosion resistance.