CHAPTER 8-Lecture 7

7/30/2019 CHAPTER 8-Lecture 7

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CHAPTER 8

THE JOINTS OF WOOD ELEMENT

8.1. Joints Classification

The timber elements joints appears to be necessary due to limited sizes of wood products.

According to their role, the joinings might be:

- prolongation joint, when the length continuity of timber members is ensured (see figure

8.1, a);

- stiffening joint, the joining stops the relative lateral displacement of the elements (figure

8.2, b);

- at elements intersection (figure 8.1, c).

a b c

Figure 8.1. Joint types:

a prolongation joint with splices and bolts for a tensioned timber element;

b prolongation joint through direct contact of elements for a compressed timber element;

c - prolongation joint through direct contact of elements having an angle between them.

According to execution mode, the joining could be: dismounting or non-dismounting joints.

According to used joinning tools, the joints could be made by:

- notched joints, bolted joints, wedge joints and glued joints.

One can high-light the following basic principles:

- the joinings must be made so as to avoid the unfavorable effects due to shrinkage or

swelling and to not permit the occurrence of biodegradation phenomena;

- the weakenings of cross-section should be minimum;

- the joining type must be suitable to the material and to the member efforts;

- easy to make and maintain;- the execution must be proper because a wrong achievement of the joints causes

suplementary efforts of some pieces and their failure.

8.1. Cilyndrical rod joints

Cylindrical rods (figure 8.2) are metallic or wooden pieces introduced in the joining

elements and located transversally to the direction of efforts transmitted from one element to

another. The initial deformations are small for this kind of joints (due to almost perfect contact

between effort transmitting surfaces) and the final deformations are greater due to drying of timber.

Rods are introduced as follows:

- mandrels, bolts, nails with d 6 mm and screws with d 4 mm, in processed holes, thehole diameter being less than nail diameter;

- nails with d< 6 mm and screws with d< 4 mm by direct beating or screwing.


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Figure 8.2. Cylindrical rods:

a nails; b mandrel; c screw with rectangle head; d screw with round head; e bolt;1 head; 2 rod; 3 screwnut.

Function of loading type, rod joints can be symmetrical or not (figure 8.3).

There are one or more shear section joints function of number of planes in which

displacements of joint elements can be produced.

I

Figure 8.3. Rod joining

schemes:

I symmetrical joints;

II non-symetrical joints;

a with two shearing

sections;

b with four shearing

sections;

c with one shearing

section;

d with three shearing

sections;II

The efforts are transmitted from one element to another through: rod bending, crushing ofrod hole, element shearing between rods (figure 8.4).

Figure 8.4. Effort transfer in a

rod joint:

a elevation;

b plane section;

c cross-section;

1 timber shearing;

2 timber crushing;

3 rod bending;4 shear sections.

The collapse of rod joints is a result of same complex phenomena, as follows: rod bending

combined with irregular crushing of wooden joined elements (or of the rod if it is made of wood)

and shearing or splitting of joined elements.

Safety bolts are provided (almost 25% from total number of joined elements) in order to

tighten the planks during works development and to prevent the joint destruction due to plank

swelling. The cylindrical rods usually used for this kind of joints are: nails, bolts, mandrels and

screws.

8.1.1. Nails those used in timber construction are given according to table 8.1.

The nail length can be equal to package width, longer or shorter (figure 8.5). On a length of

1,5d, the nail point doesnt transmit efforts. In order to take into account the shearing sections, the

nail must enter 3,5din the element.


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Nails for construction

Table 8.1

Diameter

[mm]

Total length

[mm]

Diameter

[mm]

Total length

[mm]

Diameter

[mm]

Total length

[mm]

1,80 30; 40 3,00 60; 70; 80 5,60 160

2,00 40; 45 3,55 70; 80; 90 6,00 180

2,24 50; 60 4,00 100; 110 7,10 200; 225

2,50 40; 50; 60 4,50 120; 130 8,00 2502,80 50; 60 5,00 120; 150 - -

Figure 8.5. Typical nail penetration through

the package:

I on the whole section of the package;

II exceeding the package thickness;

III not exceeding the package thickness.

When establishing the nail length, the thickness of joined elements, the joints between

elements, the nail point and the minimum penetration depth are taken into account:

( ) d5,1d5,300,21ncnLn

+++= ,

when n is the number of joined elements and c is the elements thickness. The minumum thickness

of the thickest element in the joint must be at least 4din order to avoid cracking ot the pieces when

nails are beaten.

The bolts are made up of reinforcement bars (OB37) with head and screwnut having

reinforcement steel diameters (12, 14, 16, 18, , 30 mm). The bolts dimeter is chosen as usual

(1/301/40) s, where s is the package thickness but not less than 12 mm.

The screws used in timber construction joint can be: notched, headed, burried notched head,

hexagonal headed or rectangular headed.

When detailing rod joint, the following design requirement must be fulfilled:- the bolts, screws and mandrels located on an even number of longitudinal rows in order

to avoid the location of rods in the weak central zone of the timber web;

- the nails must be located on even or odd number of rows as in figure 8.6;

Figure 8.6. Nail location:

a, b, c in a prolongation joint in straight rows, zig-zag or diagonal;

d in plane truss joints.

- the minimum location distances for rods (established by experimental tests preventing

the posibility of timber splitting between rods) are given in table 8.2 and 8.3, where:


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s1 is the distance between nail axis along the grains of timber elements in which

the nails are beaten (along the effort direction);

s2 distance between nail and element edge along the longitudinal grains;

s3 distance between nails normal to grains;

s4 distance between last rod row and element edge, nornal to grains;

- for plain trusses with nailed joints it is compulsory to respect the rules of nails

distribution for each element (figure 8.6, d);

- in the case of both-sided nail beating, when their heads penetrate in the central piecefrom both sides on a length less than 2c/3 (figure 8.7, a), the distance between nails is

independently established; if the length is greater than 2c/3, the distances are established

taking into account the nails on both sides of the packages (figure 8.7, b);

- from prolongation joints of elements made of boards or planks, the splices thickness is

equal with the thickness of joined elements. For the prolongation joint of simple section

beams the splices thickness is recommended to be at least 0,6 from the beam thickness;

- for bolted prolongation joints, for economical reasons, part of the rods can be replaced

by mandrels, but at least 25% must be bolts;

- for nailed prolongation joints at least four nails are necessary in order to joint two

elements;

- in nailed joints, at least one safety bolt must be provided (minimum 12 mm in diameter);- the prolongation joints for tensioned elements must not be joined with nails.

Minimum location distances for bolts, steel or timber mandrels

Table 8.2

Joints with:Bolts, screw and

steel mandrelsOak mandrels

Package thickness lp > 10d 10d > 10d 10d

Minimum distances

along grains

between rod axis s1 7d 6d 5d 4dform rod to element edge s2 7d 6d 5d 4d

Minimum distancesnormal to grainsbetween rod axis s3 3,5d 3d 3d 2,5dform rod to element edge s4 3d 2,5d 2,5d 2,5d

Minimum distances for nails location

Table 8.3

Nails beaten without previously pierced

Nails beaten in:longitudinal or

zig-zag rowsOblique rows

The thickness of thickest element c 10d =4d 10d =4d

Minimum distances

along grains

between rod axis s1 15d 25d 15d 20dform rod to element edge s2 15d 15d

Minimum distances

normal to grains

between rod axis s3 4d 3dform rod to element edge s4 4d 6d(5d)

Nails beaten with previously pierced

Minimum distances

along grains

between rod axis s1 10d 15d 10d 15dform rod to element edge s2 10d 10d

Minimum distances

normal to grains

between rod axis s3 4d 3dform rod to element edge s4 4d 6d(5d)

Remark: for c values between 4dand 10d, s1 will be computed by interpolation.

In case of elastic cylindrical rod joints, crushing efforts occur in the joined element andbending effort in the rod, due to loads acting on it; no shear efforts will occur due to metal strength,

and neighter the possibility of rotation of the rod in the joint.


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a b

Figure 8.7. The nail location distances in case of cross beating:

a if the penetration depth in the central element is less than 2c/3;

b if the penetration depth is greater than 2c/3.

So, the computation of the cylindrical rod joints implies the strength checking both for rods

(for bending, if they are made in steel, or of bending and crushing normal to grains if they are made

of timber) and for timber elements in the joint for crushing, shearing, splitting and failure in the

weakest section.

In order to avoid shear and splitting of the timber elements between rods, the minimumdistances between rods and between element edges and rods must be respected.

The exact calculus of cylindrical rod joints is a complex task, practically impossible. This is

the reason why in order to accomplish the computation, a series of aproximate methods based on

several simplificative hypothesis have been developed.

The relation for establishing the capacity in a rod is ( inccapmcapccapmin t L,L,LminL = .In table 8.4 are given the capable efforts for central and edge elements crushing and for rod

in bending.

Capable efforts in rods

Table 8.4

Joint type Calculus conditions

Capable effort, N, for a shear section

joint with:

NailsScrews and

steel mandrelsOak mandrels

Symmetrical jointsCentral element crushing,Lcap c 4cd 4cd 2cd

Edge elements crushing,Lcap m 5ad 5ad 3ad

Non-symmetrical jointsCentral element crushing,Lcap c 3cd 3cd 2cd

Edge elements crushing,Lcap m 5ad 5ad 3ad

Symmetrical and non-

symetrical jointsRod bending,Lcap inc 30d2 20d2 5d2

c the thickness of the thickest central element in mm;

a - the thickness of the thickest edge element in mm;d rod diameter in mm.

The cylindrical rod joint computation consists of:

- establishing the rod capable effort;

- establishing the no. of rods;

- rods location.

The capacity of a cylindrical rod joint is given by relation:

ruTftmin ticapmmmnnLL =

where: is a safety coefficient;

nt number of cylindrical rods in the joint;

nf number shear section for one rod;

mT treatment coefficient;

mu working coefficient which take into account the humidity;

mr coefficient taking into account the inequal effort distribution in rods.


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Nails used in buildings:

Diameter

[mm]

Length

[mm]

1.8 30;40

2.0 40;45

2.24 50;602.5 40;50;60

2.8 50;60

3.0 60;70;80

3.55 70;80;90

4.0 100;110

4.5 120;130

5.0 120;150

5.6 160

6.0 180

7.1 200;2258.0 250

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CHAPTER 8-Lecture 7