Bolted connections

BOLTED CONNECTIONS

1 Dr. N. Subramanian

Why Connection Failure
Should be Avoided?
 A connection failure may be lead to a
catastrophic failure of the whole structure
 Normally, a connection failure is not as
ductile as that of a steel member failure
 For achieving an economical design, it is
important that connectors develop full or
a little extra strength of the members, it
is joining.
Connection failure may be avoided by
adopting a higher safety factor for the
joints than the members.


Classification of Connections
 Method of fastening: rivets, bolts and
welding.
 Connection rigidity: simple, rigid or
semi-rigid.
 Joint resistance: Bearing connections
and friction connections
 Fabrication location: Shop or field
connections.
 Joint location: Beam-column, beam-to
beam, column to foundation


Classification of Connections
(cont.)
 Connection geometry: Single web
angle, single plate, double web angle,
top and seat angles (with and without
stiffeners), end plates, or header plate,
welded connections using plates and
angles, etc.
 Type of force transferred across the
structural connection: Shear
connections, shear and moment
connection or simply moment
connection, tension or compression,
tension or compression with shear.


Classification Based on
Joint Rigidity
 Rigid: That develop the full moment capacity
of connecting members and retain the
original angle between the members under
any joint rotation. Rotational movement of the
joint will be very small
 Simple: No moment transfer is assumed
between the connected parts and hence
assumed as hinged (pinned). Rotational
movement of the joint will be large.
 Semi-Rigid: May not have sufficient rigidity to
hold the original angles between the members
and develop less than the full moment capacity
of the connected members. In reality all the
connections will be semi-rigid only.


Examples of Rigid
Connections


Examples of Pinned Connections


Rivets and Riveted Connections
Riveting not used now due to:

The necessity of preheating
the rivets prior to driving

Labour costs associated with
large riveting crews.

Cost involved in careful
inspection and removal of
poorly installed rivets

High level of noise associated
with driving rivets


Types of Bolts
 Unfinished bolts or black bolts or C
Grade bolts (IS: 1363-1992)-bearing type
connections
 Turned bolts - Expensive & used in Spl. jobs
 Precision (A-Grade)& Semi-precision
(B-Grade) bolts (IS: 1364-1992) -They
are used when no slippage is permitted
 Ribbed bolts (Rarely used in ordinary steel
structures)
 High strength bolts (IS: 3757-1985 and
IS:4000 - 1992)-Friction type connections


Black or Ordinary Bolt and Nut

Source: AISC

Hexagonal Head Black
Bolt and Nut (IS 1363)

Figures in brackets are for High-strength Bolts & Nuts

Black bolts are inserted in clearance holes of about 1.5mm to
2mm more than the bolt diameter and then tightened through
the nuts.

Tensile Properties of Fasteners

For grade 4.6 bolts, nuts of grade 4 are used and for grade 8.8, nuts of grade 8 or 10 are used.

In property class 4.6, the number 4 indicates 1/100th the
nominal ultimate tensile strength in N/mm2 and the number 6
indicates the ratio of yield stress to ultimate stress, expressed
as a percentage. Thus the ultimate tensile strength of class 4.6
bolt is 400 N/mm2 and yield strength is 0.6 times 400, which
is 240 N/mm2

Dimensions of Grade 4.6-
Hexagon Head Bolts (IS 1364)

Sizes in Brackets not preferred.


High-Strength Bolts (IS 3757)

Made from bars of medium carbon steel.
Bolts of property class 8.8 and 10.9 are commonly used.


High-Strength Bolts (cont.)
 The material of the bolts do not have a
well defined yield point.
 Instead of using yield stress, a so-
called proof load is used.
 The proof load is the load obtained by
multiplying the tensile stress area
(approximately equal to 0.8 times the
shank area of bolt) by the proof stress.
 In IS:800 the proof stress is taken as
0.7 times the ultimate tensile stress of
the bolt.


High-Strength Bolts (cont.)
 They are identified
by manufacturer’s
identification symbol
and property class
identification symbol
8 S or 8.8 S or 10 S
or 10.9 S which will
be embossed on the
Source: heads of these bolts.
www.nichiasteel.co.jp

Class 10.9 bolts should not be hot-dip galvanized

High-Strength Friction Grip
(HSFG) Bolts
 Special techniques are used for tightening
the nuts to induce a specified initial
tension in the bolt (called the proof-load),
which causes sufficient friction between
the faying faces.
 Such bolts are called High-Strength
Friction Grip bolts (HSFG).
 Due to this friction, the slip in the joint is
eliminated; joints with HSFG bolts are
called non-slip connections or friction
type connections


Bolt Tightening Techniques
 When slip resistant connections are not
required, high strength bolts are
tightened to a ‘snug-tight’ using an
ordinary spud wrench.
 When slip resistant connections are
desired with HDFC bolts, three methods
are used:
 Turn-of-the-nut tightening (part–turn
method) –Cheap, more reliable, and
common method.
 Direct tension indicator tightening,
 Calibrated wrench tightening (torque
control method).


Bolt tightening using impact
wrench

Source: AISC


Turn-of-the nut Tightening

In the American practice, 1/3 turn of the nut is prescribed
for bolt length less than 4 d, 1/2 turn of the nut for bolt
lengths from 4 to 8 d or 200 mm and 2/3 turn of the nut for
bolt lengths greater than 8 d or 200 mm, where d is the
diameter of bolt (Salmon and Johnson, 1996). The bolts are
tensioned using 1/8 turn increments.

Behaviour of bolt-Turn-of-
the-nut Method

21 Dr. N. Subramanian In this method the bolt deformation is
a critical factor

Direct Tension Indicator
Tightening
 There are two types of proprietary load – indication
devices.
 The first type of device indicates the load by producing a
measurable change in gap between the nut and the gripped
material.


Direct Tension Indicator
Tightening (cont)
 In the second type, the bolt is tightened by
turning a nut, which has a protruding nib; the
tightening is continued till the nib shears off.


Calibrated Wrench tightening
 Wrenches are calibrated by
tightening, in a hydraulic
tension-measuring device,
using a minimum of three bolts
of the same diameter.
 Impact wrenches are set to
stall when the prescribed bolt
tension is reached. A click
sound can be heard and felt
when the set torque is
reached.
 Manual torque wrenches have
a torque indicating device,
using which the torque
required to produce the initial
tension is measured.


Advantages of
Bolted connections
 Bolted connections offer the following
advantages over riveted or welded
connections:
 Use of unskilled labour and simple tools
 Noiseless and quick fabrication
 No special equipment/process needed for
installation
 Fast progress of work
 Accommodates minor discrepancies in
dimensions
 The connection supports loads as soon as the
bolts are tightened (in welds and rivets,
cooling period is involved).
 Main drawback of black bolt is the slip
of the joint when subjected to loading


Load-Deformation Behaviour of
Different Types of Fasteners


Advantages of HSFG Bolts
 HSFG bolts do not allow any slip between the
elements connected, especially in close tolerance
holes, Thus they provide rigid connections.
 Because of the clamping action, load is
transmitted by friction only and the bolts are not
subjected to shear and bearing.
 Due to the smaller number of bolts, the gusset
plate sizes are reduced.
 Deformation is minimized.
 Holes larger than usual can be provided to ease
erection and take care of lack-of-fit. However
note that the type of hole will govern the strength
of the connection.
 Noiseless fabrication, since the bolts are
tightened with wrenches.

Advantages of HSFG Bolts
 The possibility of failure at the net section under
the working loads is eliminated.
 Since the loads causing fatigue will be within
proof load, the nuts are prevented from loosening
and the fatigue strength of the joint will be
greater than in welded/connections.
 Since the load is transferred by friction, there is no
stress concentration in the holes.
 Unlike riveted joints, few persons are required for
making the connections.
 No heating is required and no danger of tossing of
bolt. Thus safety of the workers is enhanced.
 Alterations, if any (e.g. replacement of defective
bolt) is done easily than in welded connections.


Drawbacks of HDFC Bolts
 Bolting usually involves a significant
fabrication effort to produce the bolt holes
and associated plates or cleats.
 Special procedures are required to ensure
that the clamping actions required for
preloaded friction-grip joints are
achieved.
 The connections with HSFG bolts may not
be as rigid as a welded connection.
 HSFG bolts are about 50% higher than
black bolts
 The percentage elongation at failure is
12% only.


Bolt Holes
 Bolt holes are usually drilled.
 IS: 800 allows punched holes only in
materials whose yield stress (fy) does not
exceed 360 MPa and where thickness
does not exceed (5600/fy) mm.
 Bolt holes are made larger than the bolt
diameter to facilitate erection.
 Oversize holes should not exceed 1.25d or
(d+8) mm in diameter, where d is the
nominal bolt diameter in mm.
 Slotted hole [provided to accommodate
movements) should not exceed 1.33d in
length (for short slotted hole) and 2.5 d in
length (for long slotted hole).


Pitch, Staggered holes &
Gauge
The edge
distance
should be
sufficient for
bearing
capacity and
to provide
space for bolt
head, washer
and nut.

A minimum spacing of 2.5 times the nominal diameter of
the fastener is specified in the code to ensure that there
is sufficient space to tighten the bolts, to prevent
overlapping of the washers and to provide adequate
resistance to tear-out of the bolts.


Bolt Dia, Pitch & Edge
Distances as per IS 800


Gauge Distances
for bolts as per SP-1


Note on IS Rolled Sections

Bolting is often poorly executed:
 Shank gets bent due to tapered flange
 To avoid it use
Tapered washers
(IS 5372/IS 5374)


THANK YOU!


Bolted connections

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