How to Read McNichols Bar Grating Load Tables Correctly

The row that matters in a McNichols bar grating load table is the one that matches your actual clear span, bearing-bar size, spacing, and material — nothing else counts. Grab a value from a shorter span or a different material column and you are designing from a number that has nothing to do with your platform. And in most installations the binding limit is not bending stress but deflection, so the deflection column in that same row is usually what forces a larger bar or a tighter spacing.

The one page that answers 'can it hold this load?'

A McNichols bar grating load table is not another page of product literature; it is the engineering source of truth for what a grating panel can carry. Every column of numbers is anchored to a specific bearing-bar size, a specific spacing between bars, a specific material, and a specific clear span. When a specifier asks whether a proposed platform can take a 100 psf uniform live load, the table is the document that settles the question—provided the row under inspection matches the real opening and the real layout. The authority runs one way: the physical test and calculation data behind that table define the allowable load, and no rule of thumb about bar width or a memory of a similar job overrides what the row says. Moreover, the table encodes the interaction between strength and stiffness, which is why the same project can pass on bending stress and still fail on deflection. Treat the table as a contract between the stated span and the stated load, not as a suggestion.

Skipping the load table is how grating failures begin. A designer who picks a bearing bar because it looks substantial—say, 2 inches deep and 1/4 inch thick—without checking the clear-span row is betting the deck against physics. The open span multiplies the effect of the load: as the unsupported distance grows, the internal bending moment grows in direct proportion at the center, while deflection grows by an even steeper power. The result is a platform that may feel rigid under a light foot traffic but flexes alarmingly under a concentrated wheel load, or permanent set over time. When a grating panel is under-rated, the consequences are not merely aesthetic: a walkway can bounce, a support angle can twist, and in an industrial setting a failed panel can drop a worker or a load through the deck. The load table exists to close that gap between perception and performance. It converts an assembly of bars, at a known spacing, with known material properties, into a load number that already accounts for the structural behavior—but only when the reader applies the row for the actual span.

With that authority established, the practical question becomes: how do you translate your open area and proposed bar into exactly the right row on the page? The table looks like a grid of numbers, but the grid is organized around four decisions that you, the specifier, control—the depth and thickness of the bearing bars, the center-to-center spacing of those bars, the material they are made of, and the clear span they must bridge. Get one of those decisions wrong, and the number you read may be for a completely different product than the one you intend to buy. The next section breaks the table into those four look-ups, starting with the span row because it is the most common source of error.

Read the row that matches your span and material

Open any McNichols bar grating load table and you are looking at a matrix of allowable uniform loads for one product family. Along the top—or in separate blocks—you will find the series designations for bearing-bar size and shape: a typical welded grating might list bars at 1-inch by 3/16-inch, 1-1/4-inch by 3/16-inch, and so on, with each bar size getting its own column of numbers. Running down the left side are the clear span rows, usually in six-inch or one-foot increments, from a short 2-foot span up to the maximum span the bar can bridge. Spacing is often fixed within a series—say 1-3/16-inch or 1-3/8-inch centers—but that spacing is part of the table's heading, not something you can silently change. Finally, the table is printed for a specific material: galvanized carbon steel, aluminum, or stainless steel, and the allowable loads are only valid for that material.

The most dangerous move in reading a load table is taking a value from a span row that does not match your opening. Because allowable load drops quickly as span increases, a short-span row may show a load comfortably above your requirement, while the next longer span row—the one that actually applies—shows a number below it. Suppose your platform has a 6-foot clear span and the table lists values at 5, 6, and 7 feet. The 5-foot row is not a safety margin; it is a different structural case. You must locate the row that corresponds to the longest unsupported length of your bearing bars, measured clear of any bearing surface. If the layout has an edge condition or a cutout that changes the effective span, use the longer effective span for the check. This is not a conservative preference; it is the only reading that represents the physical member being loaded.

The material under which the table is printed is as important as the numbers themselves. Aluminum grating, for example, is often chosen for its weight advantage: aluminum is a light metal, roughly one third the density of steel, and this makes panels far easier to lift and install, particularly on elevated platforms or in retrofit work. But that weight saving comes with a stiffness penalty that cannot be ignored. Because aluminum has a lower modulus of elasticity than steel, an aluminum bar of the same size and span will deflect more under the same load. A specifier who carries a steel load value across to an aluminum line and simply divides by some rule of thumb is reasoning outside the table. The aluminum column already reflects the material's density and stiffness; the only valid comparison is between the table lines that are labeled for the material you intend to specify.

What changes when span, bar size, or material changes

Once the correct row is in front of you, the numbers reveal a hierarchy: span dominates. For a given bearing bar, a longer clear span raises the maximum bending moment and pushes deflection disproportionately higher. The load table captures this cause-and-effect directly—as the span rows get longer, the allowable uniform load in each column gets smaller. A bar that is acceptable at 4 feet may be limited to about two-thirds of that load at 5 feet, and roughly a third at 6 feet. That is why you cannot treat the first column of values as a general capacity; capacity is always paired with a span. When the row for your span cannot reach your required load, the answer is not to accept a lower row but to choose a deeper bar or tighter spacing—changes that create a different line in the table. That new line usually comes with extra weight or cost, so the span row is always the starting point for the trade-off.

Material changes allowable load because it changes both strength and stiffness, and the alloy matters as much as the metal family. In aluminum the load tables are tied to a specific working alloy—3003, 5052, or 6061. 3003 is a general-purpose grade with moderate strength; 5052 adds corrosion resistance and strength, which is why it appears in marine environments; 6061 is a strong structural alloy that can approach mild steel on a strength-to-weight basis. A 6061-T6 bar will carry more than a 3003 bar of the same dimensions, and the table shows that difference. Because aluminum weighs about one third as much as steel, many buyers reach for it to reduce dead load—but its lower elastic modulus means a given aluminum bar deflects more than a steel bar of matching size. So the aluminum line is not a conversion of the steel line; it is a separate structural identity. Check the alloy column before you compare numbers, because identical geometry in 3003 and 6061 will not deliver the same allowable load.

Within the stainless family, the table's material label hides another cost-and-corrosion choice. The usual specification question is 304 versus 316. 304 protects against atmospheric exposure and mild chemical contact, while 316 adds 2 to 3 percent molybdenum, which improves resistance to chloride attack from saltwater and industrial process environments. That extra alloying is not free; 316 typically costs 30 to 40 percent more than 304, and the premium carries into grating. A coastal or chemical plant that specifies 304 to save money can end up paying in unplanned replacements: documented cases show 304 equipment in a seawater system failing from chloride attack after a short service life, wiping out the initial savings. For grating, match the grade to the environment rather than to the load alone. If the atmosphere is marine or chloride-laden, the 316 line is the defensible baseline; if the site is dry and sheltered, 304 can be entirely adequate.

So when the load tables show that both a steel and an aluminum option can carry your uniform load, which do you choose? The load number alone will not answer that question, because both options clear the strength bar. The real decision variables sit off the table: the environment that will attack the material, the deflection limit you are willing to live with, and the weight you have to lift into place. A steel panel might be the lower-cost first cost, but if the platform is at the coast, the long-term corrosion cost can flip the total. An aluminum panel might win on weight and corrosion in a humid interior, but its lower stiffness can force a deeper bar than steel would require for the same deflection limit. The right question is not simply 'can it carry the load?' but 'which material line can carry the load and still meet deflection, corrosion, and cost constraints together?'

Check deflection before you finalize the row

After you have found a row that clears your required uniform load, the next check is the deflection column—and for many grating installations this column, not the strength column, is the binding constraint. The strength column tells you the maximum load at which the bar approaches its allowable stress; the deflection column tells you how far the bar will sag under a specified load. Walkways and platforms are serviceability-sensitive: a grating that is structurally adequate but bounces or visibly dips under a wheel load is a grating users will reject, and one that sags enough can create a tripping hazard or cause panels to rattle and fatigue at their supports. Because aluminum has a lower modulus, its advantage in weight can turn into a disadvantage in stiffness—the same bar that squeaks past strength may show a deflection beyond your limit. That is why specifiers should compare the candidate against the deflection limit before they commit to the material and bar size.

The table's numbers are not unconditional promises; they carry a set of boundary conditions that a careful reader must respect. Typically the tabulated allowable load applies to a uniformly distributed load over a simple span, with the grating assumed to be properly supported and the bearing bars free to act. When your actual loading includes a concentrated load—a forklift wheel, a pallet jack, a point-bearing piece of equipment—the uniform-load row may no longer govern, because a point load produces a different moment and a different local stress. The same is true for dynamic or impact loads, for panels with cutouts or notches near the bearing ends, and for installations where the support does not allow simple-span behavior. Many tables state these limitations in footnotes; ignoring them converts a valid design aid into a guess. If your project carries any of those conditions, the responsible move is not to interpolate between rows in the dark but to have an engineer calculate the panel for the actual load case.

Here is the rule that ties the whole process together: select the row that matches your bearing-bar size, spacing, material, and longest clear span; confirm that the allowable uniform load is above your requirement; then verify that the deflection column is within your acceptance limit. If all three pass and your load case is uniform and static, the selection is defensible from the table alone. If the load is concentrated, dynamic, or the environment attacks the material, step outside the table to an engineering review before you buy. Use that same sequence for every panel in the project—not just the one that looks heaviest—and the risk of pulling a value from the wrong row disappears. That sequence is repeatable, and it is the difference between a grating spec that is merely plausible and one that is actually engineered.

The table gives you the row, but only the deflection column—read with the real span and the correct material—gives you the answer. A load number from a neighboring span or a borrowed material column is not a safety factor; it is an error waiting for a load to expose it. Keep the row honest, and the engineering choice follows.