McNIChols Bar Grating Load Tables: A Specifier's Field Guide

McNIChols bar grating load tables are not universal strength charts. They are configuration-specific lookup tools that only give safe span and load values when you match the load type, bearing condition, and deflection limit. A load value without those three conditions is not a capacity; it is an answer to a question that has not been asked. Read carefully, because the difference between a safe pick and a failed one usually hides in a footnote.

What Load Tables Actually Tell You

Most first-time specifiers expect a load table to behave like a simple strength chart: find your product, read the largest number, and you are done. That instinct is exactly what leads to over-specification or, worse, a grating that flexes dangerously underfoot. The table does not give you a material's capacity in the abstract; it gives you an allowable load for a specific set of assumptions. The same bar, at a different span, may carry half the load or double the deflection. The numbers are not conservative constants—they are answers to a question that starts with three parameters. If you do not know those parameters, the table cannot answer you. So before looking at any number, locate the span column, the load row, and the support condition note. Those three define everything that follows. The catalog is not hiding a single capacity from you; it is forcing you to specify the conditions before it will commit to a value.

Open a McNIChols load table and you will find a grid with span lengths along one axis and load values in the body. The columns typically represent span, usually in feet or inches, while rows correspond to either the load type or the bearing condition, depending on the series. Footnotes carry the supporting details: whether the bar is assumed to be continuously supported, simply supported, or cantilevered, and what deflection limit was used to compute the number. That last footnote is easy to miss and often the most important. The table is not a single matrix; it is a family of matrices, each one valid for one support arrangement and one deflection criterion. When a catalog lists several tables for the same product, the difference is not the metal; it is the assumption about how the bar sits on its supports. Read the wrong matrix and you are selecting against a condition your project does not have.

Why do two tables for the same physical profile look so different when the material changes? The evidence is in the alloy. Aluminum's density is about one third that of steel, copper, or brass, and some aluminum alloys can match or exceed the strength of common structural steel. That is why a 6061-T6 aluminum bar can appear in a table with a usable load that surprises anyone used to steel. The alloy family matters as much as the base metal: 3003 offers general-purpose formability, 5052 adds corrosion resistance, and 6061 is the structural workhorse. These differences do not just change the numbers; they change which table you should enter in the first place. You cannot take a steel load, multiply by a factor, and call it aluminum. The table for each material is built from its own modulus and yield strength, so the safe span for a given load shifts accordingly. Always check that you are using the table for the exact alloy and temper you plan to specify.

The cost of misunderstanding is not just a conservative over-spec. A specifier who reads the largest load on a table and writes it into a design for a longer span has created a safety hazard. The table's largest number is valid only at the shortest listed span; at a longer span, the same bar may be at 60 percent of that capacity. Without checking the support condition, the real allowable load could be half of what you assumed. In practice, this shows up as excessive deflection—a grating that bounces under foot traffic, or a trench cover that sounds hollow and insecure. Over-specification wastes money on heavier bars and thicker materials, but under-specification sends people back to the drawing board and, in the worst case, creates a liability. The table is designed to prevent both, but only if you treat it as a contract between you and the manufacturer: meet the conditions, and the number is yours.

The Three Numbers That Really Matter

When you are matching a spacing to a project, the span is the first number you fix. The span is the distance between the supports that the grating actually sits on—not the overall length of the panel, and not the width of the pit. Manufacturers typically list spans in the final increments between bearing seats. The load table tells you the maximum uniformly distributed load that a given bar can support at that span without exceeding the deflection limit. Shorten the span and the allowable load rises, often dramatically. Double the span and you have roughly quadrupled the deflection, which is why a table that looks generous at four feet can look stingy at eight. That relationship is not linear, and it is the reason you cannot extrapolate a number from one span to another by simple division. Always look up the exact span in your layout, or use the next longer span in the table to stay conservative.

Now the question that trips up more specifiers than any other: how is the grating actually supported? The load table assumes an ideal bearing condition—usually continuous support along the ends, with the bars fully seated on the steel frame. In the field, bearing is often less ideal: a bar sitting on an angle with only partial contact, or a frame that has been cut short, or a support that flexes under load. When the bearing condition is worse than the table assumes, the allowable load drops. How much? A change from continuous support to simple support can cut the allowable load by half, and a cantilevered condition is a different problem entirely. The table does not hide this; it states the assumption in a footnote. But a busy specifier can easily skip that line. Treat the support condition as a hard input. If your frame does not provide full bearing, either derate the table or contact the manufacturer for the correct table variant.

The third number is the load type. Grating tables typically distinguish between a uniform load, expressed in pounds per square foot (psf) or per linear foot, and a concentrated load, sometimes with a specific wheel or point load. A uniform load spreads the force across the panel; a concentrated load hits a narrow area and creates a much higher local stress. If the project calls for a forklift wheel or a heavy piece of equipment, you need the concentrated-load table, not the uniform-load one. Some products list both, and the governing case is whichever gives the lower allowable load. Many tables also include a deflection limit—often L/120 or L/100—and the load values are the ones that meet that limit. Do not assume that the larger uniform number is the one you need just because it looks safer. The real world applies both types, and the one that fits your traffic pattern determines the design.

Here is the condition that turns a reasonable selection into a failed one: deflection limits often govern before ultimate strength is reached. A bar can be strong enough to carry a load without yielding, yet deflect enough in the middle of a span to feel springy, to crack a floor finish above it, or to fail an L/120 serviceability check. The table shows high loads at short spans, but as the span grows, the deflection equation grows faster than the stress equation. The result is that a table may list a respectable load at, say, six feet, but that same load at eight feet exceeds the deflection limit even though the material has not come close to failure. When you see two numbers in a table—one for strength and one for deflection—the lower one is the real capacity. This is why the standard advice is to check deflection first on long spans, and to let the deflection limit set the maximum span.

How to Work Through a Selection

The working method is straightforward once you accept the table for what it is. Start by writing down three values: the clear span between supports, the design load with its type, and the bearing condition you can actually guarantee. Next, pick the material family. If corrosion is a concern near the coast or in a chemical plant, stainless steel moves up the list; if weight is critical, aluminum is your starting point. Match those inputs to the appropriate McNIChols table, then read across the span to the allowable load. If the value meets or exceeds your design load, the product is acceptable for that condition. If it does not, you have three options: shorten the span, change the bar spacing or size, or move to a stronger material column. Keep the table with your drawings and note the assumptions, because every one of those inputs is a decision you should be able to defend.

What happens when your span falls between two rows in the table? The conservative answer is to use the next longer span, and that is what most engineers do. Interpolating between two spans is mathematically possible, and some tables are marked as allowing it, but interpolation assumes the relationship is linear, and deflection is not linear with span. A small error in interpolation can move the result past the real limit. If you must interpolate, do it with the deflection column, not the load column, and only within the same table with the same support condition. When a project sits between two standard sizes, the safer route is to choose the product that works at the longer span, even if it costs more. That extra cost is usually small compared with the cost of a field failure. If the gap is large, ask the manufacturer for a specific calculation—many will provide a non-catalog value for your exact span.

Choosing the table for the wrong material can do more than change a number; it can change the product's life. The evidence from materials selection is blunt: 316 stainless steel contains 2 to 3 percent molybdenum, which is what gives it resistance to chloride corrosion, and it costs 30 to 40 percent more than 304. A specifier who saves that money in a marine environment gets a lesson in failure. In one documented case, a chemical plant installed 304 stainless headers for a seawater heat exchanger to save about 12,000 in material costs, and the pipe walls failed from chloride corrosion. Bar grating in the same environment will follow the same logic: the cheaper stainless alloy may look identical in the table, but the table's load values do not include a corrosion rate. If chlorides are present, the 304 column is not interchangeable with the 316 column, no matter what the numbers say at the same span.

You do not need a structural engineer for every grating selection, but there are clear boundary cases. If the design load includes dynamic forces—forklift traffic on a regular path, vibrating equipment, or pedestrian crowds—a static table value may not cover the fatigue and impact. If the bearing condition is uncertain, such as existing steel where you cannot verify the support width, an engineer should confirm the assumption. If the span is long enough that deflection controls, the table may still give a number, but the in-service performance depends on the structure around it. In those situations, a professional review is worth the fee because the table's assumptions are not your field conditions. Use the table for the routine 90 percent of jobs; bring in a reviewer for anything with unusual loads, irregular supports, or a public-safety consequence. The best specifiers know exactly where their authority ends and the engineer's begins.

The Rule I Give Every New Specifier

The rule that ties all of this together is simple to say and hard to forget: always verify the bearing before you trust the strength, and let deflection set the limit on long spans. The reason the rule works comes from the same logic that shapes materials selection. In the copper-alloy world, the wrong choice for a component—brass when you need bronze for friction, or copper when you need strength—fails in ways that look nothing like a load table. Electrical connectors overheat because of poor conductivity; marine fittings seize from saltwater corrosion. The lesson for grating is that the material column is not a decoration; it is a set of mechanical and chemical properties that determine how the product behaves in your specific service. When you switch material families, you are not just changing a number; you are changing the failure mode. The table helps you pick a size, but only you can pick the right context for that size.

If you take away one thing from this article, make it this: a McNIChols load table is a lookup tool with three inputs and one output, and the output is only as valid as the inputs. The three inputs are span, load type, and support condition. When you read a number, you are implicitly accepting the table's assumptions about all three. The moment one of those assumptions changes—the span extends, the load concentrates, or the supports become partial—the number is no longer yours. This is not a flaw in the table; it is the table doing its job. The catalog gives you a precise answer, but only after you give it a precise question. The next time you open the page, write down the three inputs, find the matching row and column, and let the footnote tell you whether the answer fits your project.

Use this decision rule on every selection. First, fix the span and the bearing condition exactly as they will be built, not as you hope they will be. Second, identify the governing load type—uniform, concentrated, or a combination—and write it down. Third, go to the correct material and alloy table; if chlorides or marine exposure are present, verify that the alloy is the corrosion-resistant one, not just the cheaper one. Fourth, read the allowable load for the span, and then check the deflection footnote. If the table shows separate strength and deflection limits, the lower number is your cap. If the allowed load meets your design load, the product works; if it does not, increase the bar size, shorten the span, or change to a stronger material column. Never assume, interpolate casually, or carry a number from one span to the next. That rule prevents both over-specification and unsafe under-specification, and it takes about two minutes per selection.

What about the cases the table does not cover? A grating carrying a point load from a machine leg, a mezzanine edge with a partial bearing seat, or a trench cover in an area with a heavy forklift but no continuous support—these are the moments a specifier either calls the manufacturer for a custom calculation or brings in an engineer. The table is a starting point, not a ceiling. When the load is unusual or the support is compromised, the right move is to ask: what is the table not telling me? That question, more than any single number, is the one that separates a careful specifier from a careless one. The catalog is a tool; the judgment is yours.

Remember: verify bearing before strength, check deflection before span, and let the table answer only the question you actually asked.