The top surface has much to do with the structure of a beam. If you have tusk tenon mortices coming into both faces and a kerf running down the top center you have destroyed the I-beam effect. Width of the top cord does make a difference. In effect the mortices cut most or all the way through the web in each half, the structure is nearly gone. I wouldn't kerf a beam as I have described, for that matter I don't kerf timber at all.
How is that for laying it on the chopping block. I hope the axe is sharp.
This reminds me of the time my wife and daughter had had enough of the rooster, the boys were selected to take the head off via the block and axe. When I came home that evening I had to finish the old bird off, Lincoln was his name, had flinched, resulting in in his comb being the only thing removed, he was very bloody and still strutting the yard.
Tim
