Thanks for all the ideas guys. I think I have at least solved the problem temporarily. I welded approx. a 1/4 to 3/8 inch steel strip on the bottom of the angle steel that hold the grate in place. For now the grate does not move around so much on the concrete ledge so this hopefully will stop anymore wear on the concrete until I have a permanent fix and I believe the fix will be making up a new frame to fit in the ledge tightly with the steel grate to fit inside the angle frame, which I have now. But the new one will have support on all sides to prevent anymore wear. By the way the pit is more like 50 inches not 42 and the bay is 16 ft wide not 12. The concrete is not wearing from the tires it is wearing from the cars driving over the right side of the grate (smaller cars) (which are more of average cars today)and over time the grate had worn the concrete ledge on that side only making the grate and frame flip up on the left side as the car drives over it in turn causing more wear. I probably should have made more of this clear in the original post. Again, thanks.