EVENTS LEADING TO THE TEST PROGRAM
Not much is known about the history of construction casting design. For many years past, manufacturers quietly produced castings in accordance with owners' specifications without much more apparent involvement than that. But with the massive road building program of the 50's and 60's, and the accelerated infrastructure development during the same period, the relationships between the specifier and the casting manufacturer began to change.
The first indication of change appeared when highway designed inlet grates used on city streets caused hazards for the then new narrow tired bicycle wheels. Engineers demanded bicycle safe replacement grates with equal or better hydraulic capacity. Neenah responded by initiating a hydraulic testing program to determine true capacities for the inlet grates offered in the catalog line. This led to other discoveries and the introduction of the hydraulically efficient Vane Grate.
With design changes, engineers became concerned whether their new choices would have the structural integrity to meet modern day roadway loads. Again the response was to furnish load capacity information through the use of strength of material equations. But there were some basic assumptions in the strength of material equations that conflicted with the characteristics of Gray Iron. Some of the assumptions include the following:
1. The material has equal tensile and compressive strengths.
2. The tensile and compressive strengths will not vary throughout the object.
3. The neutral axis is at the physical center of the object.
These assumptions may be invalid because:
1) Gray iron is similar to a steel that has been super-saturated with carbon. Its microstructure readily shows the excess carbon as graphite flakes, the balance of the carbon occurring in a manner similar to steel. The compressive strength of gray iron will be from 3 to 4 times its tensile strength because of the unique character of the graphite flakes present.
2) As described above, gray iron microstructures demonstrate the heterogeneity of the material. Gray iron, like all carbon rich ferrous metals, possesses a property known as section sensitivity. These two characteristics result in the fact that a casting's strength and hardness may differ within that casting if it varies widely in section size or if the cooling rate is not uniform in all parts of the casting.
Actual test results indicate ASTM Class 35B Gray Iron solidifying in a section 2" thick can have a tensile strength below 30,000 psi. The same iron solidifying in a section 1/4" thick can exceed 45,000 psi. Gray iron tensile and compression strengths can differ within castings of varying section sizes.
3) The assumption of the neutral axis in the center of the material is based again on the premise that the tensile and compressive strengths are equal. Research on simple cast iron beams of uniform cross section reveals the neutral axis shifts to the compressive side, and results in strengths 68% greater than calculated.
Due to these inconsistencies, another avenue of determining casting strengths was approached at Neenah.
THE TEST PROGRAM
By now the level of research and technology had become so involved at Neenah a separate department of trained professional engineers was established to continue this important work. One of the first major projects was the Proof Load Test Program, based on AASHTO-M306, which prescribes loading a 9" x 9" contact area critically located on the casting to be tested. ASTM A48 describes test bars and the tensile strength of the iron specimen. To conduct the load tests, we acquired a 200,000-pound Baldwin compression testing machine constructed to our specifications.
Specifically, a casting is selected and then produced in the foundry under the watchful eye of a technician. As the casting progresses through the foundry, the variables that affect the casting load bearing qualities are observed and recorded. Those variables include the flask size, mold hardness, time of molding, metal temperature, duration of pouring, time of casting, carbon, carbon equivalent, chill from base and ladle, inoculant and shake out time and temperature. Although there are many factors affecting the strength during production, it was determined that these variables were most important.
While the casting production data is being gathered, ASTM "B" test bars are cast from the same ladle used to pour the casting. The test bar is then machined according to ASTM and pulled in a calibrated testing machine. The tensile strength is then recorded and the metallurgy samples are removed from the test bar.
One of our seven metallurgists examines the test bars for percent carbon, silicon, manganese, phosphorus, and sulfur. These chemical ingredients most commonly effect the Gray Iron physically. After evaluating these results it can be noted, in general terms, the static load capacity of a Gray Iron casting is dependent upon the physical configuration of the casting, the load placement and area on the casting as tested on the Baldwin compression machine, and the tensile strength of the iron going into the casting.
The load bearing values for the castings are based on the tensile strength as described in ASTM A48 and the loading contact area, with the realization there is a linear relationship between the Gray Iron tensile strength and the load bearing capacity of the casting.
The data presented in this manual is a consolidation of the entire test program. The columns are as follows:
| Catalog Number | Corresponds to listings in the Neenah Catalog. i.e. R-1642 |
| Casting Description | Describes in one or two words the type of casting tested. This can include types not listed in the general catalog, yet available with the standard frame. |
| Ultimate Load | Load capacity based on Neenah Foundry Class 35B tensile strength Gray Iron |
| Deflection | Measured deflection at ultimate load in inches |
| Energy Absorption | Foot lbs. of energy capable of being absorbed before failure. |
| Design Efficiency | Lbs. of load per lbs. of casting. |
EXPLANATION OF DATA
In the Neenah test program, the ultimate load is based on ASTM A48 Class 35B Gray Iron. In other words, the accompanying ASTM "B" test bar would have a tensile strength of 35,000 psi. For castings having ASTM "B" test bar strengths less than 35,000 psi, the ultimate load for that casting is the product of the ultimate load shown in this manual and the applicable conversion factor from the table below.
| Tensile Strength | ASTM Class | Conversion Factor |
| 35,000 psi | 35 | 1.00 |
| 30,000 psi | 30 | 0.857 |
| 25,000 psi | 25 | 0.714 |
| 20,000 psi | 20 | 0.571 |
| 15,000 psi | n/a | 0.429 |
| 10,000 psi | n/a | 0.286 |
Although ASTM A48 does not have a classification for 15,000 psi or 10,000-psi tensile strength iron, we have tested the castings of many domestic and foreign suppliers and find that 10,000-psi and 15,000-psi tensile strengths are commonplace. It follows that these low tensile strength castings would require much heavier metal sections to meet the ultimate load achieved by 35,000-psi tensile strength castings.
Deflection - Cast Iron, like every other solid, has elastic properties. If cast iron was not elastic the material would shatter with the slightest impact. The greater the deflection the greater the ability to absorb energy. Occasionally specifications require deflections to be minimized by making them a function of the span length. With the casting sizes involved and accounting for what will cause deflections (be it a car, plane, semi-tractor trailer or other vehicle), most vehicles easily travel over the casting without problems. Knowing how increased deflection improves impact resistance, minimizing deflections in specifications can be counter productive and detrimental because it increases the potential for impact failure.
Energy values should give indications of how this casting will perform under field conditions. A casting with a low energy rating will be more prone to failure in duty. Energy is a combination of deflection and load capacity. If either of the values is low the result will be reflected in the energy value.
Design Efficiency is a rating of how effectively the material is being utilized.
BREAKTHROUGHS IN CASTING DESIGN
One of the significant developments coming out of the load test program is the development of the Platen Lid. The Platen Lid is an improved manhole cover. The advantages includes an increase in ultimate load, an increase in deflection, an increase in energy absorption, an increase in design efficiency, and a decrease in the weight of the casting. Typically weight is a direct indicator of relative cost. The lighter the lid the less expensive it will be.
An example of the Platen Lid attributes can be illustrated thus:
Let's look into why the R-1740-B platen lid is a stronger casting than the ribbed lid. Through the strength of materials equations, one can determine that the moment of inertia for the ribbed lid is (13.9 inches 4) and for the platen lit is it (2.64 inches 4). When determining the section modulus, which is accomplished by dividing the distance from the neutral axis to the extreme fiber in tension, one finds that the distance is much greater for the ribbed lid than for the platen lid. This results in a section modulus that's approximately the same for the platen lid (5.31 inches 3) as those of the ribbed lid (5.43 inches 3). Part of this analysis corresponds with the load test results, i.e. the deflection characteristic of the platen lid is considerable greater because it has a much lower moment of inertia. But, the load capacity of the platen is higher because shifting of the neutral axis has a more pronounced impact on its geometric configuration. The combination of these two characteristics provides the platen lid with its most significant advantage, impact resistance. Impact resistance is a function of both deflection and load capacity, and is one of the significant factors in construction casting design.
Actual tests indicate the R-1740-B Platen Lid has a load capacity 30% greater than the standard. The Platen Lid weighs approximately 90% of the standard and has an impact resistance 400% greater than the standard lid. It can be noted that the ribs on the underside of the R-1740-B make it weaker than if they were not there at all.
The Platen Lid is less expensive than the standard lid and is a superior casting design. Many municipalities and governmental bodies have adopted the Platen Lid design and now are specifying it on their plans.
INTERPRETATION OF RESULTS
In the production of any ferrous material, there is some variance in the quality of the material produced. Statistical analysis of the tensile strengths of the iron produced daily at Neenah indicates one can expect a normal distribution to exist. We produce an average tensile strength of approximately 43,000-psi. Our iron exceeds ASTM A48 Class 35B 98.6% of the time (our advertised tensile strength). Neenah's Quality Assurance team does not allow material below Class 35 to leave the foundry. Because many foundries are less stringent in quality assurance, it's advisable to specify ASTM test bars of the proper size from them on any job regardless of size.
Neenah recommends a safety factor of at least 2-1/2 for all of our castings listed. The 2-1/2 takes into account fatigue loading of Gray Iron only. Other considerations that should be taken into account are impact loading and wheel contact area. When making comparisons of different castings for normal traffic, the energy absorption and load capacity figures should have nearly equal significance. If two castings have the same load capacity, choose the casting with the larger impact value - it's a superior casting.
The observant engineer will be able to utilize the design efficiency columns and determine which designs are superior.
Special note on airport castings: for most airport work we recommend Ductile Iron castings. Ductile Iron can be supplied for most of the castings in our catalog. Check with Neenah for your airport requirements.
The Neenah Foundry Company invites you to use our test information to develop or improve the standards in your area. The professional engineers in our Specification Department can help guide you in your design.