We constructed the shelf in a very similar fashion using Simpson ties at all four corners. We flipped the table upside-down to slide the ties on, and then positioned them 7″ up the leg. The location is flexible, but they all should be kept consistent. One difference between the lower shelf and upper shelf is that we didn’t use the iron or the extra support member on the lower shelf. This shelf will be plenty strong, and we weren’t concerned with a little bit of deflection here.
