After testing the first stair railing I built earlier this week, I have decided that the butt joints need to be strengthened with a mortise and floating tenon joint. Even though these butt joints overlap and are glued to a continuous sistered section of plywood, they are not as stiff as I would like them to be. I fabricated the tenons from 3/8″ baltic birch plywood and made the mortises with a jig I have for this purpose. I glued up a couple of 9-foot sections and will test their strength tomorrow morning. I also completed the design for a 3D printable connector that will anchor the hollow steel tube balusters to the stair treads and the handrail.