Gratings are made from metal bars that cross each other or metal bars with filler strips welded to the metal bars. Metal grating can be assembled with cross-pieces fitted into joints of the other members or welded together at the intersections. The outer edge of the grate is enclosed by a metal frame. Bearing bars are the stronger bars used in the grating. They cover the entire span or length of the grate. Filler strip or cross-bars range from equal size to much smaller than the bearing bars.
Metal grates are defined by several dimensions. The bars are specified by length, width, and depth. The entire grate itself has a length or span, width and depth. The grid of bars is also specified by the space between the bars and location of the filler or cross-bars.
The National Association of Architectural Metal Manufacturers sets standards on steel grating. The "Metal Bar Grating Manual 531" is the standard for the stresses and loads metal grates can handle. Those standards are shared by the American National Standards Institute.
Bearing Bar Sizes
The smallest standard size of metal grates referenced in the "Metal Bar Grating Manual 531" is from bars that are 3/4 inch in depth and thickness. The largest standard bars are 2 1/2 inches in depth and thickness. Smaller bar sizes such as 3/4 inch by 1/8 inch and 3/4 inch by 3/16 inch cannot support as much weight as thicker steel grating. Bearing bars with equal width and depth are called I bar. In all other cases, bar depth is greater than its thickness.
The minimum allowable bar size for steel grating depends on how it is connected. "Metal Bar Grating Manual 531" defines different minimum bearing and cross bar sizes for welded, and pressure locked and riveted steel grating. Welded bars can have a minimum thickness of 1/8 inch for bearing bars and 1 1/2 inch depth. Riveted bars cannot be smaller than 3/4 inch for the bearing bar depth for a 1/8 inch thickness. The manual bases steel grating tolerances on American Society for Testing and Materials standard A749. Standard A749 is the general specification for strip, carbon and high-strength steel. Steel grating thicknesses have a tolerance of plus and minus 0.009 inches. The allowable tolerances for steel grating increase with depth.