Kanawa-tsugi and okkake-daisen-tsugi

The Kanawa-tsugi (rabbeted scarf splice with a square peg) and the Okkake-daisen-tsugi (sloped scarf splice with square pegs) are two splices of similar shape, used to extend beams, girders and posts. This article compares them under four headings: the mechigai and how each is assembled, the pegs, where each is used, and the strengths obtained in tests.

What they share: two identical pieces locked together

In both, the ends of the two pieces are cut as a half-lap with an ago, fitted together and tightened with pegs. The shape is described as common to the okkake-daisen-tsugi, the kanawa-tsugi and the shiribasami-tsugi (a splice related to the kanawa-tsugi)^[7]. The two pieces are cut identically, with no distinction between a male and a female piece^[1][3]. In the test-specimen drawings of Toda, both are drawn at the full depth of the member with half the width cut away^[8].

Mechigai and assembly

The kanawa-tsugi has a T-shaped mechigai (a small locating tongue) at the tip of each gooseneck-shaped end. Because of this, the pieces cannot be dropped in from above. The two pieces are put together and then shifted along the member by the amount of the komisen (a square peg)^[1][2]. According to Shimoyama, driving the peg makes the mechigai bite into each other and presses the two pieces tightly together^[5].

The okkake-daisen-tsugi is assembled by setting the lower piece and then dropping the upper piece onto it. The faces of the ago have a gentle sliding slope^[3][4], and according to Shimoyama the two pieces come into close contact as the upper piece is dropped, after which the pegs are driven from the thicker side^[5]. Toda's drawings show a small mechigai at each end of the joint line^[8]. The main difference between the two splices is whether the joint has a T-shaped mechigai that prevents dropping the piece in^[2].

Number of pegs and direction of driving

The okkake-daisen-tsugi is pierced from the side by pegs^[3][4]. A version without pegs is called okkake-tsugi^[3]. One source gives two pegs as standard^[9], and Toda's specimens also have two^[8].

The kanawa-tsugi specimens have one peg, which the drawing shows inserted along the depth of the member^[8]. With the member laid horizontally, the kanawa-tsugi is pegged from the top face and the okkake-daisen-tsugi from the side face^[5][8]. Shimoyama states that the kanawa-tsugi is pegged from above or below, with a joint line with mechigai visible on the side face, and that the okkake-daisen-tsugi shows a joint line and the heads of the daisen on the side face^[5]. Toda's drawings show the same directions^[8], and the drawings of Uchida et al. do the same for the kanawa-tsugi (their okkake-tsugi has no peg shown)^[6].

Where they are used

The okkake-daisen-tsugi is described as a splice for dozashi, beams, girders and moya (purlins)^[21] that carry bending^[5]. One source says it is used for large members such as nokigeta^[3], and another says it is used mainly for girders and dozashi^[11].

The kanawa-tsugi is used for netsugi (repairing the foot of a post)^[5][11][12]. One source says it is also used for sills and girders^[11]. A questionnaire asked builders and others across Japan about their experience with netsugi (15 organizations, 33 buildings). The kanawa-tsugi accounted for 73% of the answers, and the okkake-daisen-tsugi and the Juji-mechigai-tsugi for 7% each^[12]. On workability, one source says the kanawa-tsugi in its post netsugi form is relatively easy to work on site^[1]. Shimoyama states that the kanawa-tsugi is assembled from the side, so it is easier to splice on the ground before the frame is raised, and that the okkake-daisen-tsugi suits the raising stage^[5].

Strength: values by test

Shimoyama states that a member spliced with either joint can be treated as equivalent to a single piece^[5]. In tests, however, the joints do not reach the strength of a member without a joint. Kobayashi et al. report bending tests on 27 specimens of okkake-tsugi without the daisen (Douglas fir), varying member depth, ago width and splice length. The ratio of bending strength to that of a member without a joint was 0.19 to 0.32, averaged for each type of specimen^[7]. In a model test of the column splice of the main hall of Jigenji temple in the Agency for Cultural Affairs test data collection (cedar, 165 mm square, bending at the third points), the bending strength of an unreinforced kanawa-tsugi was about 1/4 of the section performance of a column without a joint about the strong axis, and 1/6 to 1/7 about the weak axis^[10].

Values for the two joints are given test by test. In Toda's tension tests (cedar, member width 120 mm), the maximum load for a member depth of 180 mm and a splice length of 360 mm was 36.8 kN for the kanawa-tsugi and 49.0 kN for the okkake-daisen-tsugi. For a depth of 300 mm and a splice length of 600 mm it was 55.6 kN and 78.6 kN. The okkake-daisen-tsugi was stronger at every size tested^[8]. In the bending tests of Uchida et al. (spruce, 10.5 cm square), the maximum load was 8,283 kg without a joint, 1,878 kg for the kanawa-tsugi and 1,443 kg for the pegless okkake-tsugi (no peg is shown in the drawing). In this test the kanawa-tsugi was stronger. A kanawa-tsugi specimen bent sideways fell to 848 kg^[6]. Because species, dimensions and type of load differ from test to test, the values cannot be compared across tests. One blog post says the okkake-daisen-tsugi broke first while the kanawa-tsugi had margin to spare, but it does not give the test conditions, so it is not used here^[15].

See them in 3D

• Kanawa-tsugi and Okkake-daisen-tsugi
• A relative of the kanawa-tsugi: Shiribasami-tsugi
• All joints