Grapes like to climb and hide their fruits below the leaf canopy. Makes it easier to find and harvest them if they are above our heads. Far as I can tell the difference between an arbor and a pergola is that an arbor is narrow whereas a pergola has a fuller footprint. Being a fan of curve wood my natural choice is to add an arch into the top. And why not build a floor into the design so as to benefit from the shade given by the canopy? Maybe even sleep under the canopy of grape leaves on hot summer nights to guard against thieving raccoons or skunks.
I built a simple 8' x 10' floor supported by concrete blocks at the corners. 1 1/2" x 8" joists on 20" centers support the 1 3/4" floor boards. Corner posts are 4 x 4, diagonals and collar ties are 2 x 4. For the curved truss I had some wider 2" boards with defect that I pulled a nice curve out of using a thin batten as a guide. They are joined in the middle with a support down to the collar tie. Pin the ends well to keep the truss from collapsing. Rafters are clear 2" x 2" on 16" centers spanning 8', which should do fine for supporting a load of luscious grapes. All construction wood is Alaska yellow cedar for longevity.
We chose 2 different seedless grape varieties hardy down to zone 4, suitable for our climate, one each side. They will be great for drying into raisins or eating fresh. Don't be surprised if an addition onto the structure comes in the form of a sunburst added to the back that will support additional grapes for making balsamic vinegar. Plus the sunburst design will provide charm in winter when the leaves have fallen. Grapes are fast growing and long lived. They produce fruit on first year canes so don't be bashful about pruning heavily. Some varieties are hardy down to zone 2, so what's stopping you?
