Category WOOD JOINTS. IN CLASSICAL JAPANESE. ARCHITECTURE

Several systems exist for designating joints. The terminology is not standardized. As pointed out in the authors’ comments, this book aims at describing a wide variety of splices and joints, describing them in detail. However, no pretense is made of giving an accurate designation for all of the joints, nor do we try to explain the origin of the appellations selected.

The following terminology’ should provide some help in understanding the technical terms employed.

Beam (hari): horizontal structural element which receives loading from a roof or a floor and transmits it to the columns.

Girder (keta): horizontal structural element within the framework of the external wall perpendicular to the tie-beam.

Girt (dosashi): horizontal structural element within the framework of the external wall intersecting the second floor beam perpendicularly.

Eave socket (udegi): Bracket anchoring the cave rafters onto a beam or a girder. When the cave rafters arc not braced perpendicularly, the cave socket must be designed to carry a moment (cantilever cave rafter).

Hanging post (tsurizuka): Hanging posts are used to suspend the lintel from a beam or girder when column height exceeds 2.7m.

Tie (nuki): Bracing element within the internal framework of a wall running between columns.

Male-female: The positive and negative part of a splice or joint.

Upper wood-lower wood: In connection with joints, the upper and lower wood refer to two identical ends of a splice or a joint. When assembling the joint, the part which is joined onto the end already in place is called upper wood. The other end is the lower wood.

Example: Rabbeted oblique scarf splices and oblique scarf stub tenon

Stub tenon (daimochi): When the lower wood is meant to be exposed to the vertical load from the upper wood over the entire section of the joint, the assembly is called stub tenon.

Example: Blind stub tenon and tic stub tenon

Stepped joint (koshikake): When the male end of a joint is stepped to transmit a vertical load onto the female, or vice versa, the joint is said to be stepped.

Example: Stepped dovetail splice and stepped gooseneck splice

Dovetail (ari): The part of a joint shaped like the tail of a dove, narrow at the girth then flaring out.

Example: Dovetailed splice, housed dovetail splice and halved dovetail Tenon (hozo): A projection at the end of the male part of a joint.

Example: True tenon and mortise, rabbeted tenon, dovetailed tenon, blind wedged tenon

Gooseneck (kama): Refers to the ribbed end of a long tenon.

Example: Stepped gooseneck with tenon and mortise and square stepped gooseneck with tenon and mortise

Tongue and groove/tongue (mcchiirc-mechihozo): Refers to the joining of two elements. A long and narrow tenon (tongue) covers the length of the end surface of the male; an equivalent groove is carved on the end surface of the female.

Blind joint (hako): Refers to an encased tenon shaped like the letter “L” or the Japanese капа “э” (ко).

Example: Blind tenon and mortise, blind stub tenon, blind pin and blind splice Crossbilled or halved joint (isuka): Refers to a joint where the male and female arc shaped like the beak of a crossbill finch.

Example: Halved rabbeted oblique scarf splice, triple-faced and quadruple-faced rabbeted oblique scarf splice, Miyajima splice Miter joint (tome): When an inclined seam is located at the intersection of two members, the seam is called miter.

Drawpin (komisen): Two types of drawpin are used to tighten a joint. Some pins work in shear (type A) the other type withstand crushing pressure (type B)

Example:

A) rabbeted oblique scarf splice, double-faced plug, triple-faced plug

B) Mortise rabbeted oblique scarf splice, housed rabbeted oblique scarf splice, blind

pin

Key (shachi): Locking element inserted into a key hole through the sections under shear.

Example: triple-faced halved joint, Miyajima, blind stub tenon, corridor girder, etc. Wedge (kusabi): Tapered triangular element pounded between two surfaces, driving them apart from each other.

Example: wedged through halved dovetail, blind wedging joint, wedging joint, etc. Dowel (dabo): Encased element inserted into a cavity passing through two joining surfaces. Example: stub tenon on tics

The batter posts arc used on frames on which lines arc taut as reference level. The post is tapered to a point at one end to make it easy to drive into the ground. The other end is crossbillcd shaped to facilitate checking of vertical displacements of the posts.

Traditional hand level. Longer levels give more accurate readings. The level is set on top of the surface to be measured. Water is poured into the middle cavity. The hori­zontal line is found when an equal quantity of water flows on either side down the slot. In general, the length of the level is two ken (six feet) and the cross section is 2X4 sun (one sun is one tenth of a foot). This hand level has been commonly used in conjugation with batter posts to set level references around an area.

• MISCELLANEOUS 03 Ф

The gable board is used ю cover the ends of purlins and girders. The joint con­necting the two sloping board is called “ogami”, which means praying with hands clapped together. Three ways of making an
“ogami” arc shown. The gable boards arc not fixed vertically to the ends of the pur­lins. Acsthctical considerations determine the angle at which they arc fixed. The “ogami” joint is the weakest part of the gable board, consequently when exposed to loading a gap may open at the bottom. ‘Го avoid this occurence methods 1) and 2) arc preferred.

Crossbrace

• MISCELLANEOUS 02 Ф

(Engowo no keta)

This is a more complex joint than the previous hip corner joints. The structure is composed of two log girders, joined in a Tee-shape and of a log girder nose. The girder nose (В 1) gives the assembly a balanced look. The shorter girder (B-2) seems to form a single continuous member with the girder nose. To accentuate this effect the nose (B-l) and the short girder (B-2) arc cut out of the same piece of wood. This assemblage is extremely complex. First the nose (В 1) is inserted into girder A. After the tenon has gone through, the nose is rotated at 90 degrees and pushed toward A in its final position. Afterward, the short girder В 2 is assembled onto girders A and B-l.

© The girder nose 1В 1 ) is rotated at 90 degrees from its previous position. |

* ‘ ‘ -| О

Top: corner column. Bottom from left to right: Girder nose В 1 , short girder В 2. long girder A, hip rafter

• Roof utter ptdi(5 sin pfcfi)

■ Kp rafter ptch

‘ Hdi roof ofter pricW2 5 su> pttf)

В 1 : Girder nose

• MISCELLANEOUS 01 Ф

(Yosemune no sumi)

Three kinds of hip rafter joint with five sun pitch will be introduced (one sun is one tenth of a foot). All of these joints have common characteristics. The roof rafters (including corner’s cave brackets) arc normal to each other and all of them have the same pitch.

(1) Tee-shaped girder joint

Precisely manufactured tenon and mortise (goya hozo sashi) arc necessary to
make this joint. The hip rafter (sumigi) sits on top of the longer girder. An eccentric tenon and mortise serves to assemble the cave girders.

Layout (1)

A carpenter square is used to layout the dimensions on site. One side of the carpenter square has a 1:1 scale. The other side has a 1:^2 scale.

Figure a) shows how to layout the dimensions for the rafter sockets on the cave girders. On girder I. a line is extended down from the “togc” (point A) with the same pitch as the rafters’ (5 sun pitch). This line intersects girder I on the top face and outer face at point В and point C respectively. Two lines, parallel to the center line of the girder, arc layed out from point В and point C to the intersection with girder II at point G and point I). Line CD is extremely important. It is called the “Kuchiwaki” line. From point D another line is layed out with the same pitch as the roof rafter’s. This line intersects the end face of girder II at point E. Line EF is drawn across the face, parallel to the top surfaces of the girders. The planes STUV and VVXYZ represent the bottom faces of the grooves destined to receive the rafters.

The layout for the hip rafter is displayed on figure b). We have just explained how point В and C were created from the “togc" (point A) on girder I. In the same fashion, point B’ and C’ arc established on girder II. The Kuchiwaki line from point C’ and its counterpart on the top surface from point B’> are drawn along girder II. Line B’ • intersects the inner face of girder I at point II (line B’H is parallel to girder’s II center line RQjuet as line BG is parallel to girder’s I center line PQ).

Л new line, GH, is layed out on top of the intersection of the two girders. Two more points, J and К arc set one half of the hip rafter width apart from the center point H and point G. The sides of the hip rafter are projected down on the top surfaces to form lines JJ’ and line KK On those lines, point J’ and K’ arc extended to the intersection with the outer face of girder II at point 1. Line IN is drawn at one half the roof rafter’s pitch (2.5 suns pitch) on the outer face of girder II, sloping down from point I. The extension of line JJ’ and line KK’ give us point L and point M, located on the outer face of girder II. From point L and M, two vertical lines arc drawn down the face of girder II. The intersection of these lines with line IN produces point N and point О respectively. We now have all of the lines necessary to cut out the groove destined to receive the hip rafter. The bottom surface of the groove is included within J’K’ON.(J’K’:width of hip rafter)

LN=TL*WZ’ mo=km*To72′ ТГ=Т6Ї2

Figure c) demonstrates how to layout the lateral pitch on the top surface of the hip rafter. The plane BCD on figure c) is parallel to 02B, G in figure b). The lateral pitch of the top surface is given by the ratio of EF/BF on figure c).

Assume AC = h

CB=2h. AB—/5h, CD=2/2h, AD=3h, BD=2h. AB*-BE*=AE BD*-BE* «ED1, AE+ED=AD

AB2-BD,=AE2-ED,= (AD-ED)*-ED*=9ha-6h-ED ED=yh AE=AD-ED=3h-yh=-|h. EF=ED*

FD=ED* 23^-«i/2h .*• BE*-AB*-AE*»^h*

zFF’D=90®, ZFDF*=45® FF*FD * -^=h( = F’D) BF’ BD-FD=2h-h «h

BF=/BF1+FF^»v/2h л BFa+EF*=^h*=BE1

consequently ZBFE=90® =^-

Figurc d) demonstrates how to layout the cutting plane for the intersection of a roof rafter (corner cave brackets included) with the hip rafter. HIJ on figure c) is made parallel to 0,B, G in figure b). G1J on figure c) is made parallel to O^BjG in figure b). The surfaces GHJ and GKN are on the side of the rafter (corner cave brackets included). Their pitch arc GI/1J and KN/KL respectively.

ZGIJ = ZHIJ==90* ZGHI= ZGHJ=90® ZIIIJ – ZIJI 1=45®

Assume IJ – a

lll=a Ш=У2а tan (zGJH) = 1/2/2 GH=ya GF=GH*+HIa=ja*

consequently

GI/IJ=^

ZKLI=ZKLG=90e. z KNJ – Z KNG — 90°, ZHMG ZHMJ=90* GH=-|a GM = |a* *a

Assume KL=h

GL="2 h GK-^h GN=^=^|h /. KN-^h*2/2=-^h consequently

KN/KL=-^

Finally, figure c) shows the layout of the bottom of the hip rafter where it contacts girder. XYZ in figure e) is made parallel to 0,B, G in figure b). ZYD and XYD contact face of the girder. The pitch is FX/FD. Let the hip rafter width be W.

CY=EY=CX = EZ" AD«DB-XF FZ –

лг vn-не-w * 1 W

ac=yd=be-t*^-^

AX=BZ=f ™ A*

FX/FD-|w/(i|)=M

(e) Layout of the bottom of the hip rafter at the intersection with the girder

(2) Cross-shaped girder joint

In practice the girders join together in a Tee-shaped assembly. Afterward, a “nose” is added to the shorter girder, giving the joint the appearance of a cross.

——— —0————- Root ratter p! cti(5 ал pitrti)

——— +———— Ир rafter p«tcti

——— £————– Kilf f00? r*tter pitcW2 5 slt рщ

(3) Bevelled halving (nejigumi)

The cave girders cross on top of each other in a formation called hip corner. To balance the strength along both axis, the girder sections are carved out by an equal amount. This concept of joining is called bevelled halving. The girders overlap at

their intersection. The stepped tenon of the corner column slides through the intersec­tion of the girders and extend beyond it, ready to receive the hip rafter. The hip rafter joins onto a rafter column set back from the corner column.

© Assembling the eave girders (the stepped tenon of the corner column extends beyond the girders to the top surface of the hip rafter)

• CONNECTING JOINT ІЗ Ф

(Koya daimochi)

Sometimes tic beams must be spliced and tied to an interna! roof beam network. The unique characteristic of this joint is that neither the lower tie beam nor the
upper tic beam suffer a reduction in section at their splicing point. Thc diagram displays the arrangement of such a joint with provision for a purlin post on top of the assembly. Generally purlin posts arc evenly distributed. They are not always located at the connection of tie beams and roof beams. The dowels used to position the tic beams over each other arc usually 30mm wide and arc always drilled in vertically instead of normal to the internal faces of the joint.

• CONNECTING JOINT 12 Ф

In this case, the rafters’ tie beams sit directly on top of the columns and the cave girders run on top of the tie beams. The stepped tenon of the column is notched a few millimeters shorter than shown in the diagrams to avoid having the girder acci­dentally snagged by it. The Orioku system results in a lower ceiling height than the Kyoro. The Kyoro system is more ЛсхіЬІе because rafters and tic beams do not have to be supported at the same location as the columns.

© The tie beam is set onto the column
(column head: stepped tenon).

• CONNECTING JOINT II •

This roofing system joins very special­ized elements. The cave girders run on top of the columns on the outer walls. The rafters arc connected to tic beams called the rainbow beams. The rainbow beams are supported by the cave girders. The support­ing point of the rainbow beams is deter­mined with a tool called the rainbow board (hikari ita). The basis for the roof support system lies on the saddle point (toge). The saddle point is at the intersection of the bottom edge of the rafters and of a vertical line perpendicular to the center line of the cave girders. Often this point is located above the eavc girder in order to avoid weakening of the same. The rainbow beam has a dovetail shape at the end which connects onto the eavc girder. A groove as wide as the rainbow beam is also cut into the eavc girder to give full support to the rainbow beam. This system has to be made strong enough to support the roofing load and also roofers, who frequently hang onto the tie beams during erection or use them to lift loads. The full width groove protects against splitting of the tie beams. Another groove is sometimes cut through the top of the rainbow beam to receive the rafters.

• CONNECTING JOINT 10 Ф

(Dodai shiguchi)

(1) Housed dovetail (ari otoshi)

A dovetail is carved on half of the depth of one member. A rectangular mor­tise runs through the depth of the second member immediately behind the mortise for the dovetail. A column with a simple tenon may complete the assembly. A draw pin secures the column to the groundsill. The length of the pole tenon on the column equals the depth of the groundsill. Thus, even if the groundsill rots, the column will stand firm. This often happen since the groundsill is usually made of softwood and the column made of hardwood.

(2) Rabbeted tenon and mortise (konc hozo sashi)

This joint is useful to assemble corner groundsills. The male sill (B) has an eccen­tric tenon at the end. The female (A) has a mortise cut throughout. After assembly, a wedge is pounded through a slot in the tenon, locking the members together. If there is enough room left at the end of member A (distance a — H/2), a column can be connected over the joint using a stub tenon.

Rabbeted tonon and mortise (left): After the male is assem­bled, a wedge is pounded in. lock­ing the joint (no column shown Housed dovetail (right); the male slides into the female. The column is assembled and the joint is com­pleted by driving in a draw pin.

(3) Corner miter tenon (sumitome hozo sashi)

This joint also connects corner ground­sills but is more attractive than the previous one. A tongue and groove on the inside ensure rigidity while a panel on the outside gives the joint a cleaner look. The outside scam is located at the corner making the
assembly appear to have been made out of a single piece of wood. The tapered panel at the end of member В (male) reduces the distance (a) left at the end of member A (female). For thus reason, a trapezoidal stub tenon is preferred to connect a column over the corner joint.

• CONNECTING JOINT 09 •

This assembly connects three beams on three faces of a column. The two oppo­site beams arc spliced through the column. The first beam to be connected (beam 1, short male) is perpendicular to the other two beams. A dowel secures it to the col­umn. The second part of the assemblage
proceeds in the same fashion as for the preceding joint. The lower part of the tenons of beam 2 (male) and beam 3 (female) is shorter titan on the double plug due to the presence of the extra beam. A very tight joint works better. In order to achieve this, the long projecting tenon of beam 2 (male) is made a few millimeters shorter than the dimensions quoted in the figure. The triple plug gives the appearance of continuity when seen from the inside. Seen from the outside, a scam can be seen where the two opposite beams meet.

Beam 2 (male) Beam 1 (short male)

Beam 3 (female) slides over the project­ing end of the tenon of beam 2 (male).

• CONNECTING JOINT 08 Ф