CONNECTIONS 2 5

Total force below neutral axis is also equal to F and acts in the opposite direction. These two forces forms a couple and resist the applied moment. Let M be the factored applied moment. Then

M = Force x Lever arm I V

M = n

M =

n =

2 P l n - 1 VPn3

6(n -1 ) 6M n - : VP n

nP 2 x nP 2 3

This approximation is on safer side. If there are two vertical lines of bolts, a value of 2V is used and n obtained is the number pf bolts required in each vertical line. After arranging the bolts, the connection is checked for its safety.

DESIGN OF BEARING BOLTS SUBJECTED TO ECCENTRIC LOADING CAUSING MOMENT IN THE PLANE PERPENDICULAR TO THE PLANE OF GROUP OF BOLTS The tensile force m a bolt Tbi xS proportional to its distance yi from the line of rotation. i.e. Tb = Kv, where K is constant

Total moment of resistance M' provided by bolts in tension. Tu; y>

T b l = 2>? .-. Total tensile force in bolts

= ET t

For equilibrium, Total tensile force = Total compressive force

T - C

Taking moment about Neutral axis (N-A)

= m%Yi i o

z ^ -

M '2> i Z y f

2 h M = M' + C x - x -3 7

M 'Ty j 2 , M = M'+ ' xh

I > f 2 1

M = M'

M'

1 + 2h X Z l 2i y ?

M

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2 6 S T E E L L I M I T S T A T E S

This type of connection is shown below.

e^t P be factored load at an eccentricity 'e\ Then the section is subjected to a direct shear force P and moment M = P x e. If there are n' number of bolts in the connection, direct design shear force on each bolt is given by

The moment causes tension in top portion of the connection and compression in the bottom portion. On tension side, only bolts resist load but on compression side entire contact zone between the columns and the connecting angle resist the load. Hence the neutral axis will be much below in these connections.

It is assumed to lie at a height of ~ t l j of the depth of the bracket measured from the bottom edge of

the angle. Hence M' is found, now tensile force Tb in extreme bolt can be found. Then the interaction equation of shear and tension is checked

i.e. VTdh )

CONNECTIONS 2 7

Parallel shank HSFG bolts are designed for no-slip at serviceability loads. Hence they slip at higher loads and slip into bearing at ultimate load. Hence such bolts should be checked for their bearing strength at ultimate load. Waisted shank HSFG bolts are designed for no slip even at ultimate load and hence there is no need to check for their bearing strength. IS 800-2007 recommends use of the following expression for finding nominal shear capacity of HSFG (Parallel shank or waisted shank) bolts :

V n s f = ^ l f n e K h F 0 .

where, \i{ = Coefficient of friction (called slip factor) given in table below ne = Number of effective interfaces offering frictional resistance to the slip [Note : ne = 1 for Lap joints and 2 for double cover bult joints]

K^ = 1.0 for fasteners in clearance holes = 0.85 for fasteners in oversized and short slotted holes and for long slotted holes loaded

perpendicular to the slot. = 0.70 for fasteners in long slotted holes loaded parallel to the slot.

F0 = Minimum bolt tension at installation and may be taken as Anbf0

A * = Net area of the bolt at threads (= 0.78 - d2).

Proof stress = 0.70 f ub Typical Average Values for Coefficient of Friction (mf)

si No,

CM

i)

m

ni)

iv)

v)

Vi> vii)

vm)

ix)

X )

xi ) xti)

Treatment o f Sur face

(2) Surfaces not treated Surfaces blasted with short or grit with any loose rust removed, no pitting Surfaces blasted with shoe or grit and hot-dip galvanized Surfaces biasced with shot or grit and spray-roeteUized with zinc (thickness 50-70 jum) Surfaces blasted with shot or grit and painted with ethylene silicate mm (thickness 30-60 f$n ) Sand blasted surface, after light rusting Surfaces blasted with shot or grit and painted with eihyfzinc silicate coat (thickness 60-80 /um) Surfaces blasted with shot or grit md painted with a lea I? zinc mlxmw coal (thickness 60-80 / im) Surface blasted with shot or grit and spray .metallized with aluminium (thickness > 5 0 p m ) Clean mill scale Sand blasted surface Red lead painted surface

Coefficient of Friction,

Mt (3 )

0.20 0.50

OJO

0.30 0.52 0.30

0.30

0.50

0.33 0.48 0J

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2 8 S T E E L L I M I T S T A T E S

The slip resistance should be taken as

V sf YmF

where ymf = 1.10, if the slip resistance is designed at service load (Parallel shank HSFG) = 1.25, if the slip resistance is designed at ultimate load (waisted shank HSFG)

It may be noted that the reduction factors for capacity of bolts in case of long joints, large grip length, packing plate of more than 6 mm thickness, specified for bearing type of bolts also applies to HSFG bolts.

For commonly used HSFG bolts (Grade 8.8) yield stress f b = 640 MPa arid ultimate stress fub = 800 MPa.

S The expression for tensile strength of HSFG bolt is same as that for bearing type bolts i.e.,

rp __ Minimum 0.9fubAn fybAsb Y m b < m /

where, An = Area of bolt at root of thread ^ 0.78 nd2

71 , 9 Asb = Shank area = dw N Ymb = 1 - 2 5 , Y m = 1.1 fub for bolts of grade 8.8 is 800 MPa and fyb = 640 MPa

INTERACTION FORMULA FOR COMBINED SHEAR AND TENSIO

If bolts are under combined action of shear and axial tension, the interaction formula to be satisfied is

V sf V. df ) VTdf J

CONNECTIONS 29

where Q = Prying force 2Te = Total applied tensile force.

lv - distance from the bolt centre line to the toe of the fillet weld or to half the root radius for a rolled section.

I - distance between prying force and bolt centreline and is the minimum of either the end distance or the value given by:

I = Lit

p = 2 for non pre-tensioned bolt and 1 for pretensioned bolts.

r| = 1.5

bg = effective width of flange per pair of bolts fjj = proof stress in consistent units t = thickness of the end plate

Note : that prying forces do not develop in case of ordinary bolts, since when bolt failure takes place contact between the two connecting plates is lost

Grade and properties of bolts

TSiHi"

4.6

4.8

5.6

5.8

8.8

210 MPa

320 MPa

300 MPa

400 MPa

640 MPa

X T

400 MPa

420 MPa

500 MPa

520 MPa

800 MPa

EFFICIENCY OF A JOINT

It is defined as the ratio of strength of joint and strength of solid plate in tension. It is usually expressed in percentage. Thus,

Efficienc = S t r e n g t h f j o i n t -100 iciency r\ Strength of solid plate

/Strength of solid plate: /is in general governed( >by yielding of plate

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3 0 STEEL LIMIT STATES

Welding consists of joining two pieces of metal by establishing a metallurgical bond between them. The elements to be connected are brought closer and the metal is melted by means of electric arc or oxyacetylene flame alongwith weld rod which adds metal to the joint. After cooling the bond is established between the two elements.

There are three types of welded joints : 1. Butt weld 2. Fillet weld 3. Slot weld and Plug weld

1. Butt Weld

Butt weld is also known as groove weld. Depending upon the shape of groove made for welding butt welds.

Types of Butt Welds

(i) Square Butt weld on one side

(ii) Square butt weld on both side

(iii) Single V butt joint

(iv) Double V-butt joint

(v) Single U-butt joint

(vi) Single bevel butt joint

(vii) Single J-butt joint

2. Fillet Weld

Fillet weld is a weld of approximately triangular cross-section joining two surfaces approximately at right angles to each other in lap joint, tee joint or corner joint.

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CONNECTIONS 31

- r - 7K\ 1 yToe

^ i M Leg size

X Leg ( Toe ^ i M Leg

size T / ^ i

M Leg size Root' kl

^ i M Leg size

Leg size When the cross-section of fillet weld is isoceles triangle with face at 45. It is known as standard fillet weld. In special circumstances 60 and 30 angle are also used. A fillet weld is known as concave fillet weld, convex fillet weld or as mitre fillet weld depending upon the shape of weld face.

v s - | s K | s N- |

Concave Convex Mitre

3, Slot We!d and Plug Weld

Figure below shows a typical slot weld in which a plate with circular hole is kept with another plate to be joined and then fillet welding is made along the periphery of the hole.

Fillet

'MM

Slot weld

Figure below shows typical plug welds in which small holes are made in one plate and is kept over another plate to be connected and then the entire hole is filled with filler material.

Plug welds

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32 STEEL LIMIT STATES

PECIFiCATIONS FOR WELDING

Important specifications regarding butt weld, fillet weld and plug and slot weld as per IS 800-2007 are:

Butt Weld

1. The size of butt weld shall be specified by the effective throat thickness. In case of a complete penetration butt weld it shall be taken as thickness of the thinner part joined. Double U, double V, double J and double level butt welds may be generally regarded as complete pentration butt welds. The effective throat thickness in case of incomplete penetration butt weld shall be taken as the minimum thickness of the weld metal common to the parts joined excluding .reinforcement. In the absence of actual data it may be taken as 5/8th of thickness of thinner material.

2. The effective length of butt weld shall be taken as the length of full size weld. 3. The minimum length of butt weld shall be four times the size of the weld. 4. If intermittent butt welding is used, it shall have an effective length of not less than four times the

weld size and space between the two welds shall not be more than 16 times the thickness of the thinner part joined.

Angle Between Fusion Faces 6()-90 91-1006 101-106 107-113 114-120

Constant, K 0.70 0.65 0.60 0.55 0.50

1. Size of Fillet Weld

(a) The size of normal fillet weld shall be taken as the minimum weld leg size. (b) For deep penetration welds with penetration not less than 2.4 mm, size of weld is minimum leg size

+ 2.4 mm. (c) For fillet welds made by semi automatic or automatic processes with deep penetration more than

2.4 mm, if purchaser and contractor agree S = minimum leg size + Actual penetration

2. Minimum size of fillet weld specified in 3 mm. To avoid the risk of cracking in the absence of preheating the minimum size specified are : For less than 10 mm thickness plate 3 mm For 10 to 20 mm thickness palte 5 mm For 20 to 32 mm thickness plate 6 mm . For 32 to 50 mm thickness plate 8 mm

3. Effective throat thickness : It shall not be less than 3 mm and shall not generally exceed 0.7 t (or t under special circumstances) where t is the thickness of the thinner plate at the elements being welded. If the face of plates being welded are inclined to each other, the effective throat thickness shall be taken as K times the fillet size where K is as given in table below :

Values of K for Different Angles Between Fusion Faces

Angle Between Fusion Faces 60o-f)0 91M006 101-106 107-113 114-120

Constant. K 0.70 0.65 0.60 0.55 0.50

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