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Connections
• A connection may be defined as the componentparts used to join together elements or membersin a structure.
• The function of a connection is to transmit co-existent forces and moments between membersat the joints.
• Connections may involve the use of bolts orweld, or a combination of both.
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Types of Connections
1. Simple Connections
are made using angle cleat or welded endplate connections without stiffening and joiningwith ordinary bolts. Significantly cheaper tofabricate.
2. Rigid Connections
can be achieved with appropriate stiffening byeither an all welded or a high strength frictiongrip bolted connection.
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Welding
A weld consists of deposited metal compatiblewith the base metal to join two plates or a plateand a solid part, generally in one of thefollowing five types:
• butt joint
• corner joint
• tee joint
• lap joint
• edge joint
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Weld Joint Types
Types of Welded Joints
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Weld Classification
The following are the common classes of welds that
are in use:
• Fillet Welds
• Groove Welds
• Plug & Slot Welds
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Groove Welds
• Groove welds are often referred to as “Butt
Welds” because one of the parts abuts on to the
other, as happens in a butt, corner or tee joint.
• The weld metal is simply an extension of one or
both of the plates connected, depending on
whether the two plates join perpendicular to
each other or along the same plane.
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Fillet Welds
• The weld metal is external to the connected
parts.
• Fillet welds are suitable for corner, tee and lap
joints.
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Plug & Slot Welds
• The weld metal is deposited into round holes or
round-ended slots cut in one of the plates, and
attached flat to the connected part.
• Plug or slot welds may be used for lap or edge
joints.
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Groove & Fillet Welds
• Groove and Fillet Welds constitute most of
the practical welding.
• Fillet welds are the most common of the
weld types because
- they involve less preparation
- are simple to implement, and
- easy to inspect and repair
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Advantages of Welding
• Welding provides possibility of visually clean
connection.
• It gives a smooth finish and good aesthetic
values.
• In a rigid frame structure, a savings in weight of
material can be realized because of the
continuity between structure members.
• Reduction in steel weight.
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Disadvantages of Welding
• Welding is an expensive and labour intensive
process.
• The biggest disadvantage is that weld often
contain defects.
• Welding creates a permanent joint, not desirable
if parts need to be fixed or replaced later.
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Standard Terminology
• There are standard terms to refer to various
parts of a weld.
• Weld metal usually extends beyond the
theoretical weld profile.
• Therefore the actual weld profile is usually
bigger than the theoretical weld profile.
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General Terms
• Actual Profile
• Fusion Zone
• Heat Affected Zone
• Incomplete Joint Penetration
• Reinforcement
• Root
• Root Opening
• Root Penetration
• Theoretical Profile
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Convexity and Concavity
• Convexity and Concavity are the curved
surface deviations of the exposed weld
face above and below the theoretical
plane connecting the two edges of the
weld at the joined pieces.
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Throat
• Throat is the general term applied to the
narrowest part of the weld through which the
stresses must pass from one of the connected
parts to the other. Thickness of weld is called
throat.
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Welding Processes
There are numerous welding processes forjoining different steels namely:
- Arc Welding
- Oxyfuel Gas Welding
- Solid State Welding
- Resistance Welding
- Soldering
- Brazing
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Metal Arc Welding
• In structural steelwork, metal arc welding is used
almost exclusively.
• In metal arc welding process, the heat is
generated by an electric arc formed between a
steel electrode and the steel parts to be welded.
• The arc heat melts the base metal and the
electrode simultaneously.
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Common Feature
• In all processes, it is due to the intense heat of
the arc that the electrode deposits molten filler
metal from the electrode on to the base metal.
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Quality of Welded Joints
Depends on the following factors:
– Base Metal
– Weld Metal
– Welding Action
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Good Workmanship of Welds
• Factors which determine good workmanship are:
– The right size and chemical composition of electrode
– The sequence and number of passes in welding
– Proper speed, voltage and current for the particular
weld
– Proper preparation of the connection before welding
To secure good quality of welding connections
requires the teamwork of competent engineers,
supervisors and welders.
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Common Defects
• Welding is susceptible to various defects or
flaws such as follows:
– Cracking
– Porosity or Voids
– Slag Inclusions
Porosity and Slag Inclusions are tolerable to a
certain extent but cracking is not to be
allowed.
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Cracks
• Most cracks are initiated from stress
concentrations.
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Causes of Defects
1. Adverse or incompatible base metal and / or
filler metal properties
2. Unsuitable process characteristics
3. Harsh environmental conditions
4. Improper joint design, preparation, alignment
or fit-up
5. Inefficient workmanship
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Distortion
• Non-uniform expansion from heating and
contraction due to cooling of the weld metal and
base metal during and after welding causes
distortion
• There are various types of distortion and
dimensional change including longitudinal,
traverse, angular and bending. Two or more
types of distortion may occur at the same time.
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Inspection
• Inspection is more vital for every welded
joint than for other types of joint.
• The general methodology for inspection:
– Pre-inspection check list
– Non-destructive testing
– Follow-up procedures
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Structural Welding Quality Handbook
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General Provisions for Inspection
The Welding Inspector shall ascertain that:
1. All fabrication and erection by welding is
performed in accordance with the
requirements of the contract documents
2. The size, length and location of all welds
conform to the requirements
3. No unspecified welds have been added
without approval.
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Testing
• We test welds to establish minimum levels of
mechanical properties and soundness of the
welded joint.
• Which properties?
– Hardness
– Toughness
– Tensile strength
– Ductility
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Non-Destructive Testing
• Non Destructive Testing (NDT) yields valid anduseful results leaving the tested component andfully serviceable.
• Many NDT methods are available:– Visual Inspection
– Penetrant Testing
– Magnetic Particle Testing (MPI)
– Radiographic Testing
– Ultrasonic Testing
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Safety in Welding
• Some of the dangers in welding are:
– Poisonous gases & fumes emitted in various
process
– Extremely high light levels during welding,
with potential permanent sight damage
– High sound levels during welding with
potential hearing damage
– Fire, explosion, electric shock
– Combustion due to misuse of oxygen
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Measures for Safe Welding Practices
1. Management Support
2. Safe Working Environment
3. Individual Protection
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Bolt Connection
• A bolt is a metal pin with a head formed at one
end and shank threaded at the other in order to
receive a nut.
• Bolts are used for joining together pieces of
metal by inserting them through holes in the
metal and tightening the nut at the threaded
ends.
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Bolt Grades
Three grades of structural bolts are commonly
used:
1. Grade 4.6 bolts, often called ‘ordinary’ or
‘black’ bolts with grade 4 nuts
2. Grade 8.8 bolts with grade 8 nuts
3. Grade 10.9 bolts
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Bolt Grade Number
• The bolt grade number is given by the ISO
method as follows:
– The first number is a one-tenth of the ultimate tensile
strength
– The second number is the ratio of yield strength to the
ultimate tensile strength x 10
– The two figures are separated by a decimal point
– The product of the two numbers indicates the yield
strength of the bolt material
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Example of Bolt Grade
• 4.6 Grade Bolts
– Ultimate Strength = 40kgf/mm2
– Yield Strength = 4 x 6 = 24 kgf/mm2
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Choice of Bolts
• The choice of bolt type depends on the selected
connection and the forces / moments to be
resisted.
• The commonly available sizes of bolts are M12,
M16, M20, M24 and M30.
• Bolts are normally used in clearance holes 2mm
greater than the nominal bolt size.
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Failure Modes for Bolted Connection
1. Tearing at the net section of either plate
2. End failure of plate
3. Shearing of bolt itself
4. Bearing of plate immediately behind the bolt
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High Strength Friction Grip
• High Strength Friction Grip (HSFG) bolts are
made from high tensile steel and are tightened
sufficiently with special torque wrenches.
• Recognized by their large diameter head and
the additional identifying marks (eg. HSFG)
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The Steel Industry
• What can happen?
– understand what, why
• What is achievable?
– understand how
• What to look out for?
– understand tell-tale signs
• What else is new?
– Learn everyday