Tenon length

I am building a 20 X 40 foot timber frame building. The bents will consist of three 8 x 8 whie pine posts supporting an 8 x 10 white pine beam. The tenons at the ends of beams are 2 x 9 x 5 inches long (I reduced the ends of the 8 x 10 beams to 9 inches). The 8 x 8 posts have a one inch deep housing on which the beam will rest. I originally cut these members last summer and since then, after becoming more educated on the subject of timber framing (through Timber Frame Construction by Sobon & Schroeder and various articles in the Guild publications), I am now wondering if these tenons are too short. Is a five inch long beam tenon entering an 8 x 8 inch post a candidate for failure? Is it worth the time and effort to lengthen these tenons to seven inches (and make through mortises)? This will also change the geometry of the braces and shorten the width of the building by four inches. I would certainly appreciate any input from people more knowledgeable on this subject than myself. Thanks for your time.

Hi Dave - if you look at the TFG publication, Timber Frame Joinery and Design Workbook, you will see several plans that have the same tenon spec that you have. Based on that and the articles in that book on pegging, I'd say you're ok. jekbear

It depends what else you are doing to resolve thrust.

Will's point is critical. If your beam needs to resist tension at the joint with the post, then you'll need a healthy end distance (relish) on the tenon. Based on our testing, you need at least 4" on EWP from the center of the peg to the end of the tenon to avoid failure of the tenon. However, if the joint does not need to resist tension, then you need enough bearing on the bottom side of the beam and tenon to carry the beam shear. If you are not sure of how the structure will carry loads, find an engineer to do an analysis. You can get a copy of the EWP test results from the members only section of the web site. Get the "long-term report" by Schmidt and Scholl.