The d-rings have square holes punched for receiving carriage bolts, providing a smooth and relatively trip free surface. When in use the ring portion raises for application and lies down when not in use, thus creating a flush surface. It is plenty strong and is able to receive multiple tie-down straps simultaneously. **RATED CAPACITY: Also known as "assembly capacity" or "break strength" is the minimum load a complete assembly can withstand before failure in a laboratory pull test when the product is NEW. WORKING LOAD LIMIT: The maximum load weight a tie-down should be subjected to during normal use. To assist in making the proper tie-down choice the Federal D.O.T. requires the W.L.L. to be 1/3 of the Rated Capacity. BACKING PLATE SPECIFICATIONS: 1) The backing plate measures 6-1/2" x 6-1/2". 2) Thickness is 11 gauge (a little thinner than 1/8). 3/8" holes. 3) 3-1/2" diameter cutout. 4) Steel backing plate is zinc coated to inhibit corrosion. INSTALLATION - these D-rings require routing, or cutting a hole in the floor. We recommend creating/cutting a template and then transferring to the desired location. If you use a 3-1/2" hole saw, the d-ring will fit without any further work. Better fit - use a 2-3/4" hole saw then jigsaw the flat edge and then bevel the edge with a wood hasp to create a snug fit.