Dovetail (ari) and gooseneck (kama)

Dovetail (ari) and gooseneck (kama) are shapes in which a protrusion cut on the end of one member is fitted into a socket in the other, so that the members resist being pulled apart along their length. This article covers what each shape does in the frame, in this order: the shapes, their relative strength, where they are used, why a koshikake (a protrusion on the lower piece that carries the upper piece) is added, and their names.

Shape: a protrusion that stops pulling out

The Ari-tsugi (dovetail scarf joint) is a splice in which one member has a dovetail tenon and the other a dovetail socket, and it is classed as a joint that resists tension^[15][16]. The dovetail tenon widens toward its end and is likened to the plectrum of a shamisen^[15]. A description of the Koshikake-ari-tsugi (housed dovetail scarf joint) likewise says that a male piece flaring like the tail of a pigeon is fitted into the female piece^[8].

The kama-tsugi (gooseneck splice) is a splice in which a hook-like protrusion, with neck and head in one piece, is fitted into a mortise^[2]. The upper part of the end face of one member is cut out in the shape of a raised hook and fitted into the other member^[16]. Like the dovetail, it is classed as a joint that resists tension^[16].

The blog of Mikao Kenchikukan explains that the dovetail head and the gooseneck shape are used to make the member hard to pull out^[13].

Strength: dovetail and gooseneck compared

The gooseneck is stronger in tension than the dovetail, and needs a longer splice in return^[7][9][14]. No source was found that compares the two numerically under the same conditions.

The dovetail itself is described as a weak form. A dovetail splice without a seat (koshikake) is relatively weak, suits places where little force acts, and must be combined with other joints for heavy members^[1][12]. For the koshikake-ari-tsugi as well, one source says it is not structurally strong against tension or bending in an earthquake, and that only the dovetail part can be relied on^[8]. According to Shimoyama, even with metal reinforcement added to a koshikake-ari-tsugi the stress is not transmitted, and under bending the joint comes apart more easily than a kama-tsugi^[11].

Where they are used

The kama-tsugi has been used for horizontal members that resist tension, such as sills, girders and purlins (moya)^[16][18]. The Koshikake-kama-tsugi (housed gooseneck scarf joint) is described as common for horizontal members such as dozashi, floor beams, nokigeta and purlins^[9]. The koshikake-ari-tsugi is used mainly for sills^[8]. Noguchi explains that sills are bolted to the foundation, so a simple joint such as the koshikake-ari-tsugi or koshikake-kama-tsugi is sufficient^[10].

Stronger splices are used for beams and girders. JAANUS says the dovetail shape is mainly used to join beams of the roof structure, such as roof beams^[5]; the comparison here concerns splices that join horizontal members end to end. Noguchi states that for beams, girders and dozashi the okkake-daisen-tsugi (sloped scarf splice with square pegs) is stronger than the koshikake-ari-tsugi and koshikake-kama-tsugi^[10]. Shimoyama says that kama-tsugi and ari-tsugi came to be used for beams only after construction methods began to put weight on load capacity alone, and that they were originally splices for sills and purlins^[11]. How widely the ari-tsugi is used for sills and oobiki (floor sleepers)^[19] could not be confirmed.

The dovetail shape is used not only in splices but also in shiguchi that hang a member in a T. The arikake is a shiguchi in which a dovetail tenon is cut on the end of one member and fitted into a dovetail groove dug in the side of the other, and it is used for connecting a beam to joists, among other places^[17]. In ooire (housing), a shallow recess is cut in the receiving member and the end of the beam is set into it, sometimes reinforced with a dovetail tenon or a square peg^[6].

Koshikake: carrying the upper piece on the lower

A koshikake is a form in which the upper piece rests on a protrusion cut on the lower piece. The koshikake-ari-tsugi combines a half-lap joint with a dovetail splice^[3], and the koshikake-kama-tsugi combines a half-lap joint with a kama-tsugi^[4]. In the koshikake-ari-tsugi, the male piece is fitted onto a protrusion on the lower half of the female piece, as if sitting on it^[8].

As the reason for adding a koshikake, Fukushima Kinoie Net says it is used as reinforcement, for example when a dovetail or gooseneck splice is made outside the bearing position^[7]. For sills, one explanation is that fastening the upper piece with anchor bolts holds the lower piece down^[13][8]. Araki Komuten also says that the anchor bolts hold the female piece down against the male piece and keep it from lifting in an earthquake^[8].

Shimoyama notes that because the koshikake-ari-tsugi has no sao (a long rod-shaped neck)^[20], it is hard to keep the two spliced pieces in a straight line. In the koshikake-kama-tsugi, the longer the combined length of the sao and the kama part, the greater the effect of keeping the line straight^[12].

Names

Ari corresponds to "dovetail" in English^[1][5], and in Japanese too it is likened to the tail of a pigeon^[8]. Sources differ on the origin of the word ari, and it could not be confirmed. Kama comes from the resemblance of the head of the male piece to the raised head of a snake^[9]. In English it corresponds to "gooseneck"^[2]. JAANUS says the kama-tsugi was already in use in the 7th century^[2]. Fukushima Kinoie Net calls it an old form that was also used in the Kondo (Golden Hall) of Horyuji^[7].

See them in 3D

• Ari-tsugi: Ari-tsugi and Koshikake-ari-tsugi
• Kama-tsugi: Koshikake-kama-tsugi and Shiho-kama-tsugi (a splice for the upper and lower parts of a post)
• Dovetail shiguchi: Ooire-ari-kake, Kabuto-ari-kake and Yose-ari
• All joints