Chapter 10Design of Timber Connector Joints

NBC 2016 (SP 7:2016) Volume 1

2016 · PDF page 645

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10.1 In large span structures, the members have to transmit very heavy stresses requiring stronger jointing techniques with metallic rings or wooden disc-dowels. Improvised metallic ring connector is a split circular band of steel made from mild steel pipes. This is placed in the grooves cut into the contact faces of the timber members to be joined, the assembly being held together by means of a connecting bolt (see Fig. 10).

FIG. 10 SPLIT RING CONNECTOR

10.1.1 Dimensions of Members

Variation of thickness of central (main) and side members affect the load carrying capacity of the joint. The thickness of main member shall be at least 57 mm and that of side member 38 mm with length and width of members governed by placement of connector at joint.

The metallic connector shall be so placed that the loaded edge distance is not less than the diameter of the connector and the end distance not less than 1.75 times the diameter on the loaded side.

10.1.2 Design Considerations

The primary stresses in a split ring connector joint under

tension are illustrated in Fig. 11. The shaded areas represent the part of wood in shear, compression and tension. Related formulae for the same are given in 10.1.2.1.

For fabrication of structural members, a hole of the required size of the bolt is drilled into the member and a groove is made on the contact faces of the joint.

Theoretical safe loads in design shall be corroborated with sample tests done in accordance with accepted standard [6-3A(10)].

NOTE — A pilot study on determination of strength of improvised split ring connector joint in a compression test for a specific design problem conducted using Mesua ferrea (Mesua), yielded the data which is given below, for guidance:

10.1.2.1 Strength of joint is controlled by one or another or combination of three properties:

a) Tension area (see Fig. 11A):

t _ {1} w - \left[ 2 \left(\frac {a d _ {2}}{2}\right) + b \left(t _ {1} - a\right) \right]

10.2 Wooden Disc-Dowel

10.2.1 It is a circular hardwood disc generally tapered each way from the middle so as to form a double conical frustum. Such a disc is made to fit into preformed holes (recesses), half in one member and the other half in another, the assembly being held by one mild steel bolt through the centre of the disc to act as a coupling for keeping the jointed wooden members from spreading apart.

As timber always fails by shearing or crushing in front of bolts, the function of disc-dowel incorporated in the joint is to distribute the shearing stresses on a larger area of wood so as to transmit the full amount of stresses.

10.2.2 Dimensions of Members

The thickness of dowel may vary from 25 mm to 35 mm and diameter from 50 mm to 150 mm. The diameter of dowel shall be 3.25 to 3.50 times the thickness.

The edge clearance shall range from $12\mathrm{mm}$ to $20\mathrm{mm}$ as per the size of the dowel. The end clearance shall be at least equal to the diameter of dowel for joints subjected to tension and three-fourth the diameter for

compression joints. Disc dowel shall be turned from radial (quarter sawn) planks of seasoned material.

10.2.3 Choice of Species

Wood used for making dowels shall be fairly straight grained, free from excessive liability to shrink and warp, and retain shape well after seasoning and having high shear strength. Species recommended include,

a) Babul,

b) Dhaman,

c) Irul,

d) Sissoo,

e) Rose Wood,

f) Sandal,

g) Axle-Wood,

h) Padauk, and

j) Yon.

NOTE — Data on the above species are as per Table 1.

10.2.4 Design Considerations

The forces on dowel in a lap joint and butt joint are illustrated in Fig. 12. Dowel is subjected to shearing at the mid-section, and compression along the grain at the bearing surfaces. For equal strength in both the forces, formula equations are given in the Fig. 12 to determine the size of dowel.

(2ad2)

+

b

(t1−a)

]

b) Compression area (see Fig. 11B and Fig. 11C):

2 \left(\frac {a d _ {2}}{2}\right) + b \left(t _ {1} - a\right)

c) Shear area (see Fig. 11D):

1) \quad \text {Within core:} \quad 2 \left(\frac {\pi d _ {1} ^ {2}}{4}\right)

No. and Diameter of Ring Used in a Joint		No. and Size of Bolt Used in a Joint		Side Member		Central Member		Load Direction with respect to Grains of Wood	End Distance	Intermediate Distance	Load Per Pair of Connector
No.(1)	Sizemm(2)	No.(3)	Sizemm(4)	Thicknessmm(5)	Widthmm(6)	Thicknessmm(7)	Widthmm(8)	Load Direction with respect to Grains of Wood	End Distance	Intermediate Distance	Load Per Pair of Connector
No.(1)	Sizemm(2)	No.(3)	Sizemm(4)	Thicknessmm(5)	Widthmm(6)	Thicknessmm(7)	Widthmm(8)	(9)	mm(10)	mm(11)	kgf(12)
2	63	1	$12 \times 125$	31	92	38	92	Parallel	63	—	3 930
2	63	1	$12 \times 125$	31	92	38	92	Parallel	75	—	4 185
2	63	1	$12 \times 150$	38	92	63	92	Parallel	75	—	4 010
2	63	1	$12 \times 150$	38	117	63	117	Parallel	75	—	4 450
2	63	1	$12 \times 125$	31	138	38	92	Perpendicular	69	—	2 520
2	63	1	$12 \times 125$	38	138	38	92	Perpendicular	69	—	3 515
2	100	1	$19 \times 175$	38	138	66	138	Parallel	100	—	7 075
2	100	1	$19 \times 175$	38	138	66	138	Parallel	125	—	7 370
2	100	1	$19 \times 175$	41	138	75	138	Parallel	100	—	7 220
2	100	1	$19 \times 175$	41	138	75	138	Parallel	125	—	7 645
4	100	2	$19 \times 200$	38	138	66	138	Parallel	100	150	5 655
4	100	2	$19 \times 200$	41	138	75	138	Parallel	100	150	5 925
4	100	2	$19 \times 200$	41	138	75	138	Parallel	125	200	7 135

NOTE — Symbols are exclusive to this figure.

FIG. 11 STRESS DISTRIBUTION IN A SPLIT RING CONNECTOR

PART 6 STRUCTURAL DESIGN — SECTION 3 TIMBER AND BAMBOO: 3A TIMBER

2) Below core:

2 \left[ d _ {2} e - \frac {1}{2} \left(\frac {\pi d _ {2} {} ^ {2}}{4}\right) + 2 \left(\frac {a e}{2}\right) \right]

where

$d_{1} =$ inside diameter of connector;

$d_{2} =$ outside diameter of connector;

e = end distance from centre of connector to end of member;

$a$ = depth of connector;

$b =$ diameter of bolt;

$t_{1}$ = thickness of member;

$t_2$ = thickness of metal; and

w = width of member;

The making of wooden discs may present some problems in the field, but they may be made in small workshop to the specifications of the designer. This is also economically important. Once the wood fittings are shop tailored and made, the construction process in the field is greatly simplified.

Theoretical safe loads in design shall be confirmed through sample tests.

NOTE — Some experimental studies have indicated the following safe loads, in kN, for dowels bearing parallel to the grain, which are given here for guidance only:

Species		Safe Loads for Dowels Bearing Parallel to the Grain, in kN Size of Dowel (mm)
Members	Dowels	62 × 25	75 × 25	87 × 25	100 × 31	112 × 37	125 × 37
Sal	Babul	6.671	9.810	13.342	17.805	22.269	27.566
Sal	Sissoo	5.346	7.554	10.251	13.342	16.922	20.895

10.2.4.1 Rigid specifications regarding use of particular planks for fabrication of wooden disc-dowels, and the specific alignment of grains to be in line with grains of members forming the joint, can be relaxed. Application of appropriate factors in design of such joints whatever be the angle of load and members etc, would eliminate risks involved by providing 60 percent more number of dowels at a joint. Thus, decrease the

calculated load per dowel to 60 percent.

10.2.4.2 Analysis of variance for safe load in case of dowel joints made with MS bolt, timber bolt, bamboo pin as joiners showed no significant difference. Thus the concept of ‘all-timber construction’ hold good for chemical industries where corrosion due to acid fumes is a recurring problem.