How a Mortise-and-Tenon Joint Transfers Force
A mortise-and-tenon joint transfers force through the contact between its members and, for some loads, through its peg. Under shear, the tenon bears within the mortise or the tenoned member bears on a housing. Under tension, a peg can transfer load laterally between the tenon and mortised member when the joint meets the design conditions. The mortise removes timber from the receiving member, so its remaining section also matters. These principles help a reader annotate a reviewed detail; they do not establish dimensions or capacity for a particular frame. [1]
- For shear, TFEC requires direct bearing between the tenon and mortised member, or between the tenoned member and a mortise housing. It does not count pegs toward the tenoned member’s shear strength. [1]
- A fully housed beam bears across its full width; an unhoused beam bears across the width of the tenon. [1]
- For tension, TFEC permits lateral force transfer through pegs under stated conditions, including close contact between tenon and mortise faces and adequate peg penetration and spacing. [1]
- For tension-loaded pegged joints, TFEC 1-2019 limits peg diameter to 0.75 to 1.25 inches and to no more than two-thirds of tenon thickness; it also limits tenon thickness to one-third of the receiving face breadth. [1]
- A mortise removes timber from the receiving member. Connection detailing must account for the remaining section and splitting risk. [1]
Start with the bearing surfaces
A mortise is the opening in one member; a tenon is the projection fitted into it. The shoulder is the face where the tenoned member steps down around the tenon. If the receiving member has a housing, the tenoned member bears on that recess. The bearing surface is the contact area that transfers shear from one member to the other. [1]
The Timber Frame Engineering Council (TFEC) requires shear transfer by direct bearing between the tenon and mortised member, or between the tenoned member and a mortise housing. In an unhoused joint, bearing is across the width of the tenon. In a fully housed joint, the tenoned member bears across the beam’s full width. The surface changes with the geometry, so a drawing should show which faces are intended to meet. [1]
On a reviewed detail, mark the beam, post, tenon, mortise, shoulder, housing if present, and peg. Then trace the shear load from the beam to the post across the bearing face. A shoulder is not automatically a bearing surface: it bears only where the detail gives it a mating surface. The contact faces, fit and load direction determine the intended path. A generic sketch can explain those parts but cannot confirm a project’s capacity. [1]
The peg helps resist separation
A peg passes through the mortised member and tenon. For a tension load, TFEC permits lateral load transfer through one or more pegs, subject to conditions that include close contact between the tenon and mortise faces, load perpendicular to the peg axis, double shear, sufficient penetration into the mortise side walls, and adequate tenon net section. [1]
The peg has a different job from the bearing faces. TFEC excludes pegs from the tenoned member’s shear strength because a peg carrying shear would create tension perpendicular to the tenon’s grain, which can cause brittle failure at relatively low loads. TFEC’s commentary says the stiffer bearing path takes virtually all the joint’s shear load once the surfaces are in contact. A peg may carry some short-term load during assembly and erection before full contact is made. [1]
TFEC also says to place pegs as close as practicable to the bearing surface in shear-loaded connections. The commentary explains that this placement helps control shrinkage effects and maintain direct bearing. The drawing should make the intended bearing contact and peg location legible together, rather than treating the peg as a substitute for a missing bearing path. [1]
The mortise leaves timber to check
Cutting a mortise removes material from the receiving member. The wood left around the opening must be considered in the connection design, including the risk of splitting. For tension-loaded pegged joints, TFEC requires the tenon’s net section to resist the applied load and sets detailing rules for peg diameter, penetration, spacing and distances to edges and ends. [1]
For tension-loaded pegged joints, TFEC 1-2019 limits a peg diameter to 0.75 to 1.25 inches and to two-thirds of the tenon thickness at most. It limits tenon thickness to one-third of the breadth of the mortised member’s receiving face. Its Table 3C gives minimum edge, end and spacing distances as multiples of peg diameter D; for a loaded edge perpendicular to grain, the listed minimum is 2.5D when the geometry factor is 1.0. These are provisions within that standard, not a project design or a universal set of dimensions. [1]
Annotate a hypothetical housed joint
In a hypothetical, generic housed beam-to-post joint, first mark the housing seat and the full-width face of the beam that bears on it. Draw the illustrative shear path from the beam into that seat and onward into the post. For a separate illustrative tension direction that tends to pull the beam away from the post, trace lateral transfer through the peg and into the mortise side walls. Keep the arrows distinct: they show different load cases, not a combined capacity. [1]
On the reviewed drawing for an actual connection, check that the beam and post, tenon, mortise, housing, peg, bearing faces and load directions are identified. For a tension-loaded peg, verify that the drawing allows review of peg diameter, penetration into each mortise side wall, tenon thickness, and edge, end and spacing distances under the applicable provisions. TFEC requires the peg penetration in each side wall to be at least the tenon thickness. If the drawing omits these checks or the load direction, ask the project engineer to resolve them before cutting. Do not transfer the hypothetical sketch’s geometry to a building. [1]
Research on timber joints also shows why the load case matters. A 2010 experimental study by Toku Nishimura, Masami Goto and Yoshiyuki Suzuki reported static loading tests and described a mechanical model that accounts for contact and friction, stress along and perpendicular to the grain, compression inclined to the grain, cotter resistance and splitting. Its findings concern the tested joints and model requirements; they do not establish one force path for every pegged connection. [2]
The Timber Framers Guild’s introductory explanation describes heavy timber as joined by mortises and tenons and secured with wooden pegs. For a specific frame, however, the approved connection detail and its engineering checks govern. [3] [1]