The year 1975 is significant. That was about the time that modular polymer concrete channels entered the U.S. trench drain market. Prior to that, only the frame and grate systems and cast-iron channel body products were available in the trench drain marketplace. These products were made by iron foundries.
Over the past 50 years, there has been a consolidation of the foundry market in the United States. According to the American Foundry Society, the quantity of foundries has been reduced from an estimated 4500 in 1975 to 1750 in 2025. That is a reduction of over 61% in all foundries, ferrous and non-ferrous. When considering iron foundries, the numbers are worse.
Where before, many regional foundries were supplying trench frames and grating to the immediate plumbing and civil customers, today we see market changes due to new technologies and global consolidation. The frame and grate systems once so prevalent in the construction market have been largely replaced by modular systems that are both superior in engineering and cost. Meanwhile, competition within the foundry market has led to fewer, but larger companies selling a handful of specialized, large trench systems.
Large trench grating systems still play a vital role in the infrastructure of the world. Modular systems don’t make sense when trench systems approach widths of 24” and beyond. In addition, some old school contractors prefer to build trench drains using traditional wood-framing installation techniques, especially when their labor rates are low.
Some of the more popular large trench gratings come in standard sizes of 12 and 14 inches in width. The standard length of a trench grate has become 24 inches, largely due to manufacturing ease and product integrity issues. And for grating edge thickness, the marketplace seems to have settled on a nominal 1.5-inch depth for most applications. Heavy duty applications and some foreign products are still made with a 2” nominal grating edge thickness.
The number of U.S. foundries that make large trench drain grating systems has been reduced to a handful. Neenah Foundry and EJ (formerly East Jordan Iron Works) currently seem to be the overall favorite product choice when surveying construction specification sheets. However, US Foundry and D&L foundry are two other respected companies with a full line of trench grating. (Remember my comment on consolidation? US Foundry was purchased by Neenah Foundry in 2021.)
Below is a chart comparing dimensions and properties of some popular trench grating. As mentioned previously, 12” and 14” wide foundry grates are the most popular sizes seen in the marketplace. They are used to cover drain channels that are 10” and 12” in width, respectively. In this chart, we’ve selected 12-inch-wide trench grating products (of similar dimensions) to demonstrate the subtle differences between these common municipal gratings.
Physical properties of common 12-inch-wide trench grating (nominal 1.5” thickness)
An additional product added to the above list is from Trench Drain Systems (TDS). They have a growing product line of replacement grating (RG) that is beginning to be noticed in the trench drain marketplace. One big difference stands out the TDS grate. It is made with ductile iron (80-55-06) rather than the traditional 35B cast iron. Ductile iron being more resilient, we know the comparison is not “apples to apples”, but it will prove a point in the end. And, for the record, Class 35B indicates a material modulus of rupture (stress limit) with a minimum of 35,000 psi (pounds per square inch) or 35 kips per square inch. Ductile Iron (80-55-06) indicates a product with a minimum of 80,000 psi modulus of rupture.
Not all grating is created equal! That should be the first thing noticed from this table. Dimensions and weights vary between the grates even though they are marketed in a class of 12” x 24” x 1.5” grating. Three of the products (see asterisk *) are designed with “leveling pads” on the bottom which make up a portion of the nominal 1.5” thickness. All products seem to have a similar percentage of open area for drainage. The big take-away is in the weight differences.
Foundries like to look at their output in terms of dollars per pound. Larger and less complicated shapes tend to have lower $/lbs. cost than smaller, more intricate shapes. Large trench grating is considered a relatively simple shape to make. And, with dimensions being similar, an easy way to make a quick comparison is by looking at the weight of the part. Lower weight parts can indicate a thinner design or higher open area in the grating which can have implications to its load rating capability. Likewise, heavier parts suggest a more robust product or fewer openings for drainage.
This weight relationship is confirmed in the table above. US Foundry and D&L grating have body thicknesses that are less than the other three manufacturers. The weights of their products are also less than the others. All things equal, their strengths under service should be less than those of Neenah, EJ and TDS products. However, the design of the openings and supporting “ribbed” structure is important.
To test this hypothesis, we employed a Solid Works stress analysis on digital models of each of these grating designs. A digital representation was drawn from physical samples of each product. The analysis simulated a H20 load testing, which applies a 40,000-pound load to a 9” x 9” area directly in the center of the grating. True H20 load testing has a time element involved. This model we employed shows where failure will occur but not the time to fail.
A color chart is associated with each grating sample. The color “green” is a base line of no stress. The color “yellow” indicates stress developing and “red” indicates points where failure can occur. Each grating sample is viewed from the top and bottom under a stress event. The results are presented in the following five pairs of illustrations.
USF-6118 (Top)
USF-6118 (Bottom)
Neenah-R4990-CX (Top)
Neenah-R4990-CX (Bottom)
EJ-6953-M2 (Top)
EJ-6953-M2 (Bottom)
DL-J4222-01 (Top)
DL-J4222-01(Bottom)
TDS-RG-151224-AB (Top)
TDS-RG-151224-AB (Bottom)
Results and Discussion
The products that showed the most significant stress levels were the D&L J4222-01 and the EJ 6953-M2 Grates. The USF 6118 product also showed areas of stress, but not to the extent that EJ and D&L products showed. It is important to note that each of these products use leveling pads on the bottom of the grate. USF 6118 employs 6 leveling pads while the other two have only 4 leveling pads (one on each corner).
Neenah R4990-CX and TDS RG-151224-AB showed no (or miniscule) stress levels. These two products also don’t have leveling pads as part of their design. The advantage of the RG-151224 being made with ductile iron rather than 35B gray iron doesn’t present an advantage in this testing. The design and composition of the Neenah grate is sufficient to minimize stress under H20 loading. An additional simulation using higher loading will be able to tell when the difference in material and design will separate the performance of the Neenah and TDS gratings.
Meanwhile, we can assume that weight differences in two similar grates may be an indicator of performance. Lower weighs indicate less material and weaker parts. However, the design of the grate has a large effect on the performance. In particular, the presence of leveling pads seems to adversely affect the stress levels in the grates that are only supported at the corners. More pads distributed along the length of the grating reduces inflection points and distributes stress more evenly to the supporting surface.
This point was demonstrated in the results of the USF 6118 product. USF 6118 had the lowest weight but was designed with additional leveling pads supporting the length and minimizing stress levels. D&L J4222-01 had a similar weight but an inferior design. The thinner body along with the wide span of the leveling pads lead to higher concentrated stress at the midpoint of the grate.
Conclusions and Real-Life Applications
The Solid Works Stress Test analysis is a useful tool for analyzing the relative resiliency of similar trench drain grating. Caution must be used when drawing the illustration to be analyzed as to capture all the design nuances. Missing a fillet or mis-measuring a ribbing can lead to misleading data. This tool is probably more useful in the design of a casting which will be subjected to a load.
Furthermore, it is important to note that the analysis results don’t include the time element (one minute under load) that is part of a dynamic H20 or HS20 testing. Other software is available to help simulate the results of load as a function of time. However, this method is helpful in identifying products that have incorrect load ratings.
Case in point, most grating manufacturers recognize the “Heavy Duty” rating description as able to pass the AASHTO M306 (H20) load testing, which meets a minimum proof load of 40,000 lbs. A gray area exists in the marketplace as Neenah defines “Heavy Duty” as all grates that exceed a 16,000-pound proof load. EJ (previously East Jordan Iron Works) considers proof loads between 16,000 – 40,000 pounds to be a “Medium Duty” load rating. D & L Foundry acknowledges the popular “Heavy Duty” designation but chooses not to classify the load rating of their grates in their literature.
After experiencing multiple episodes of J-4222-01 failure in service, TDS engineering evaluated the D&L grating using Solid Works stress modeling and learned that we misrepresented the load rating of this grating as “Heavy Duty”. Our cut sheets of the product were changed from the “Heavy Duty” classification to “Medium Duty”. Thereafter, additional modeling of other marketable products was performed, which helped confirm or question the load ratings advertised by the manufacturer.
About the Author – Since 2004, Michael Schroer has been the President of Trench Drain Systems. Mr. Schroer has experience in the design of ceramic products for the metal casting industry. He has also worked in international purchasing, sales of technology and the pre-cast concrete industries. He has degrees in Geology, Metallurgy and Business.
