CONNCETIONS: -
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Various elements on
a Steel Structure like tension and compression & flexural of members are connected
fasteners Connectors.
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The need of
designing Connections are: -
1)
To connect plates, angles,
channel, I-Sections etc;
2)
To connect different members at
the ends.
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The connections are
generally provided in the following cases:
1)Unless
the requirement to account for the heavy load and long span is met, then the
built-up parts must be given. In this case, to get a good section this section
should be connected together.
2)
The length of the standard section needs to be connected to another section in
case of longer duration. In this case it is necessary to attach the proper
design of multiple parts of the connections.
3)
At the end the different members need to be linked (e.g. secondary beams to be
linked to primary beams, columns, footings, etc.)
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Design of
connections are very important because the failure of joints is Sudden and catastrophic. Normally, a connection
failure is not as ductile as that of a Steel member failure. Connections may be
classified according to the following:
a) Method of
Fastening (rivets, bolts, welding)
b) Connection Rigidity (rigid, Simple,
Semi-rigid)
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According to IS
code, based on rigidity connection, the joint can be defined as:
1) Rigid
2) Simple
3) Semi-rigid
Semi-Rigid: -
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Semi-rigid
connections have sufficient rigidity to hold the original angles between
members.
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In reality all the connections
will be Semi-Rigid.
Types of Connections: -
The
following three types of connections may be made in steel connections:
1) Riveted Connections
2) Bolted Connections
3)
Welded Connections
Bolted Connection: -
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“Bolt” a metal pin
with a head formed at one end and stank threaded at the other in order to
receive a nut.
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Bolts are used for
joining together pieces of metals by inverting them through holes in the metal
and tightening the nut at the threaded ends. This process /technique are called
“Bolting”.
Types of Bolts according to type of
Shanks: -
a) Unfinished/black
bolts: -
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It is made from
mild Steel rod with Square or hexagonal head. The Shank is left unfinished i.e.
rough as rolled. As Shank of black
bolts are unfinished, the bolt may not establish contact with structural member
at entire zone of contact surface. So, joints remain quite loose resulting into
large deflections.These bolts are
Used for light structures, temporary Connections (trusses etc ;).These bolts are not
Used for - impact, fatigue, or dynamic loading Subjected to structure.Bolt of property class 4.6 means, Ultimate
Strength is 400 N/mm2 and yield Strength is 400 x 0.6 = 246 N/mm2.If a bolt is
designated as m16, m20, m24,……, it means Shank dia. of 16mm, 20mm, 24mm,….. (as
per IS 1364).
b) Finished/turned Bolts: -
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These bolts are
also made from mild Steel, but they are formed from hexagonal rods, which are
finished by turning to circular shape.Actual dimension is larger than the nominal
dia. (1.2 mm to 1.3 mm).Bolt hole is 1.5 mm
larger than the nominal diameter of bolt.IS 3640 covers the
specification.
Uses Finished bolts: -
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special jobs like connecting special parts
subjected to dynamic loading.
c) HSFG:
-
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Made from buyers of
medium Carbon Steel.Less ductile than black bolts.Special techniques are used for tightening the
nuts to induce specified initial tension in bolts. These bolts with included
initial tension are called “HSFG Bolts”.Due to friction the
sleep in the joint is eliminated hence the connection is called “non slip /
friction type connection”. Induced initial tension is called “Proof
Load”.Coefficient of friction is called “Slip
Factor”. Forces are transferred by friction only not
subjected to shear of bearing.
Uses of HSGF:-
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Material Cost is
High.
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Special attention
to be given to workmanship.
According to material and strength: -
(i) Ordinary Structure Bolt.
(ii) High Strength Steel Bolt.
According to pitch and fit of thread: -
(i) Standard pitch bolt.
(ii) Fine pitch bolt.
(iii) Coarse pitch bolt.
According to Shape of head and nut:
-
(i) Square bolt
(ii)
Hexagonal Bolt
Types of Bolts based on Load transfer: -
(i)
Bearing type – (black, turned)
(ii) Friction type – (HSFG bolts)
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Black & turned
bolts in bearing type since they transfer Shear force from one member to other
member by bearing, whereas HSFG bolts belongs to friction grip type Since they
transfer shear by friction.
Advantages of Bolted Connection:
-
1)
Making joints noise
less.
2)
Do not need skilled
labour.
3)
Structure can be
put to use immediately.
4)
Working area
required in the field is less.
5)
Connections can be
made quickly.
6)
Bolting is a cold
process.
Disadvantages of Bolted Connection: -
Disadvantages of Bolted Connection: -
1) Tensile strength is reduced considerably due to
reduction of area at the root of
thread.
2) Rigidity of joint is reduced due to loose fit.
3) Due to vibration nuts are likely to loosen.
4) Unfinished bolts have lesser strength because of
non-uniform diameter.
TERMINOLOGY IN BOLTED CONNECTION: -
1)
PITCH (P): -
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It is Center to Center spacing of bolts in a
row measured in the direction of Load.
2)
GAUGE (G): -
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It is the distance
b/w two consecutive bolts of adjacent rows and is measured at right
angle
to direction of load.
3)
END DISTANCE (e’): -
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Distance of the
nearest bolt hole from the end of the plate in the direction of load (e')
4)
EDGE DISTANCE (e): -
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Distance of the
center of bolt from the adjacent edge of the plate (e) measured right
angles
to the direction of the load.
5) STAGGERED DISTANCE: -
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It is the centre to
centre distance of staggered bolt measure in the direction of load (Ps).
I.S 800-2007 Specifications: -
Pitch: -
i)
Shall not be less than “2.5d’, Where d= Nominal Diameter.
ii)
Shall not be more than
a)
16t or 200 mm (whichever is less) for Tension members.
b) 12t or 200mm (whichever is less) for Compression
members.
iii)
In case of staggered pitch, pitch may be increased by 50% of values Specified
above provided Gauge distance is <75mm.
iv) In case of bolt joint, max pitch will be
“4.5d”.
Gauge Length: -
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The gauge length
should to be more than 100+4(t) or 200mm whichever is less.
Min Edge & End Distance: -
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min edge & end
distance Shall not be
a) Less than 1.7 x hole diameter in case of sheared
(or) hand frame out edges.
b) Less than 1.5 x hole diameter in case of
rolled, machine frame cut, sawn and planed edges.
Max. Edge Distance: -
(i)
12+ε, ε =√250/Fy, t= Thickness of outer-plate
(ii) Not
greater than 40mm+ 4t, t= thickness of thinner connected plate.
(iii)
Area of bolt at root of the thread is less than that at Shank of the bolt is
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Anb= 0.78 x Asb
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Fasteners. Max. Edge Distance: -
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If value of gauge length exceeds after
providing design fasteners at max. edge distances.
i)
32t (or) 300 (lesser value) – plates are not exposed to weather.
ii)
16t (or) 200mm (lesser value) - plates are exposed to weather.
TYPES OF BOLTED JOINTS: -
(i) Depending upon the arrangement of
bolts and plates: -
a) Lap joint: -
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It is the Simplest
type of joints:
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Plates to be
connected overlap one another.
b) Butt joint: -
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Two main pales butt
against each other and the Connection is made by providing a Single cover plate
connected to main plate or by double cover plates one on either Side connected
to main plates.
(ii) Depending upon the arrangement of
modes of load transmission: -
(iii) Depending upon nature and location
of load: -
TYPES OF ACTIONS ON FASTENERS: -
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Depending upon the
types of Connections and loads, loads are subjected to the following types of
actions.
Failures of Bolted joints: -
1)
Shear failure of bolts/pirates
2)
Bearing failure of bolts/plates
3)
Tensile failure of bolts/pirates
4)
Block Shear failure in tension.
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Plates in a joint
made with a bearing of bolts may fall under tensional forces due to these 3
Causes:
a)
Shearing of edges
b)
Crushing of plates
c) Rupture of plates
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Block Shear failure
is a Combination of yielding & rupture. It occurs along path involving tension
on one plate and Shear on a perpendicular along the fasteners.
