A round picnic table, which can be a pleasant alternative from the standard quadrangle, presents unique challenges in construction. But you can use an X-leg construction, just like on other picnic tables, to provide a sturdy base and durable frame.
Arrange the five 2 x 12s next to each other to form a square 60 by 60 inches, and glue them together in position. To secure them, lay the 54-inch 2 x 4 across the centre of the boards, and secure it firmly with screws. It should reach to within three inches at each edge.
Find the centre of the square by drawing lines corner to corner and marking the intersection point. From this point, measure a 30-inch radius around the centre by dragging a 30-inch length of string with a pencil marking the line.
Cut around the circumference line with the jigsaw to form the top of the table.
Measure 20 inches outward along the planks from the centre 54-inch 2 x 4 and use screws to attach two of the 40-inch 2 x 4s across the planks, parallel with the centre 2 x 4. These will brace the tabletop and also provide a base for the legs.
Measure the centre of four of the remaining five 2 x 4s, 20 inches from each end, and drill one hole through the centre of each 2 x 4. Then cut off the ends of each of these four at a 45-degree angle, so that the ends are still parallel, and can rest flush on both the table top and the ground.
Line up the 2 x 4s in two X shapes, with their holes aligned, brace the final remaining 2 x 4 between them and use the 6-inch bolts to secure the leg-Xs to the 2 x 4, which will be a centre brace.
Place the legs on the table, with one leg-X at each of the 40-inch 2 x 4s attached to the table top. Secure the legs to these braces with screws. When the legs are firmly attached, flip the entire table over to stand on its legs.