Splines are popular because they overcome the poor tension value in a mortise and tenon joint. The problem of course, being the shearing out of the end of the tenon by the force of the peg when in high tension.
There are many framers that rely heavily on splines to provide both tension and torsion to a joint. A hardwood spline of enough depth will allow you to tie a beam through a post to a beam on the opposite side, it increases the surface area for pegging, and its vertical stiffness will also help resist a beam's potential twisting torque on the joint.
When the spline is used in this way, then posts only need to provide vertical support and hold down capability. As a result the posts dimensions can be diminished due to the simplified joinery.
The reality is that in traditional timber frame joinery there are very few joints that stand up to significant tension. We all know how often we rely on panels and/or steel to hold these trouble spots together. While the use of splines strays from traditional methods, there are occasions where there use will solve a connection where steel may have been the only alternative.
Spline use is a popular solution to the unique loading demands in seismic areas and is popular with nortwestern timber framing companies. But they are popluar all over due to their strength, ease of implementation, and ability to use smaller posts due to joinery design issues. At S.P. Brooks & Co. we rely almost 100% on the use of splines. We pre-insert and glue our splines to our posts, simplifying erection, although complicating the loading of the truck! When tested we have found the glued spline becomes stronger than the wood (similar to steel and welds) and failure occurs in the pine first, but not until the joint has resisted some suprisingly high forces. As it turns out, the softwood shrinking around the dry hardwood spline, saturated in glue not only provides a tremendously strong tenon, but adds significant resistance to racking, working much like a small brace.
In summary, properly designed and implemented spline joinery is an economical alternative to more labor intensive traditional joinery requiring larger diameter posts. They are superior in strength to most tradtional joints and they are a good compromise for a framer that would prefer not to implement the use of steel in his or her frame but is challenged by some high tension loads at a particular connection.
