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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