RIVETS
The sections used in
steel structures are connected to each other by riveting or bolting. Welding is
another alternative method of connecting the structural sections. The rivet is
rendered soft by heating and then it is placed in the rivet hole. The shank
extends out of the prepared rivet hole and the extended part of the shank is
given the shape of a second head by riveting machine.
The usual form of
rivet head which is cup shaped in appearance is termed as snap head. Thus snap
head always extends out from the face of the riveted plate. Incase, it is
desired to have the second head flush with the plate or member to be riveted,
countersunk head is employed. Depending upon the nature of connection, the
rivets may be subjected to single shear or double shear. In single shear, one
cross sectional plane of the rivet is subjected to shearing action while in
double shear, two cross sectional planes of the rivet are subjected to shearing
action.
Following important rules are observed in designing the
riveted connections:
· The center to center distance between
the rivets should never be less than three times the nominal diameter of the
rivet.
·
In
case of members subjected to tension: 10 times the thickness of the thinnest
outside plate or angle or 20 cm whichever is less.
·
In
case of members subjected to compression: 16 times the thickness of the
thinnest outside plate or angle or 15 cm whichever is less.
· The distance between the edge of a
rivet and edge of the pair etc. should never be less than the diameter of the
rivet.
BEAMS
I-sections or rolled
steel joists are commonly used as simple beams. They are suitable for moderate
loading conditions and spans. But for heavy loads and longer spans, compound
beams are used. They are fabricated from one or more rolled steel joists or
riveted with plates and angles. In some cases, only plate and angle are riveted
together to form a compound beam. Plate girder and box girder are two examples
of compound beams.
Internally, beams subjected
to loads that do not induce torsion or axial loading experience compressive,
tensile and shear stresses as a result of the loads applied to them. Typically,
under gravity loads, the original length of the beam is slightly reduced to
enclose a smaller radius arc at the top of the beam, resulting in compression,
while the same original beam length at the bottom of the beam is slightly
stretched to enclose a larger radius arc, and so is under tension. Modes of
deformation where the top face of the beam is in compression, as under a
vertical load, are known as hogging. The same original length of the middle of
the beam, generally halfway between the top and bottom, is the same as the
radial arc of bending and so it is under neither compression nor tension and
defines the neutral axis.
COMPRESSION MEMBERS
A compression member
is one, which is subjected to compressive stresses in a direction parallel to
its principal axis. The compression stresses tend to buckle the member or
shorten it. Hence, in the design of a compression member, its shape, area of
cross section and its length area of vital importance. Stanchions and columns
are the examples of compression members.
COLUMNS AND STANCHIONS
Column is a vertical
member which is circular in cross section. It may be hollow or solid in section. Vertical
compression member may be termed as stanchion. Depending upon the height of
stanchions, and the loading conditions; they may be fabricated in various
shapes. Long column tends to fail in bending and as such from consideration of
economy, the column should be so shaped that the variation between the maximum
and minimum radii about the two principal axis and the value of its area/least
radius of gyration should be as small as possible. For columns required to
carry heavy loads two or more sections of angle channels, rolled steel joists
etc. are joined to form a compound section.
There are several
types of columns which are used in different parts of structures. Column is a
vertical member that carry loads mainly in compression. It might transfer loads
from a ceiling, floor slab, roof slab, or from a beam, to a floor or
foundations.
Commonly columns also
carry bending moments about one or both of the cross section axes.
FRAMING LONG SPANS
For auditoriums,
theatres, hangers, large industrial buildings where greater clear distance is
necessary, girders, trusses, arches etc. fabricated from steel section are used
for roofing. Girders are used in situations where depth of framing is limited
to 1 m. when the designed depth of the beam works out to be more, plate girders
are employed. But in places where depth of framing is not restricted from
architectural consideration, trusses are used.
TRUSSES
A trusses consists of
an assembly of rigid but elastic members jointed in the form of triangles to
act as a beam. The safe working tensile stress of mild steel is about 20 times
that of structural timber. Thus steel trusses work out to be economical, especially
for bigger spans. Out of the various shapes of steel sections, angles are
considered most suitable for steel roof truss. This is on account of the fact
that angles and resist both compressive and tensile stresses effectively. In
additional angles can be produced economically and can be joined easily.
A truss gives a stable
form of supporting considerable external load over a large span with the
component parts stressed primarily in axial tension or compression. The individual
pieces intersect at truss joints, or panel points.
Advantages of steel roof truss over timber truss given below:
· Steel trusses are stronger than
timber trusses.
·
Steel
section forming the truss are light in weight and can be fabricated in any
desired pattern to suit the architectural requirement.
·
There
is no danger of the material being eaten away by white ants or other insects.
·
Steel
trusses are more fire resisting than timber trusses.
·
They
do not have span restrictions and as such steel trusses can be used for
industrial buildings and other such structures where large areas are required
to be covered without obstructions due to columns etc.
·
The
sections forming a steel truss are easy in transportation.
·
The
sections can be obtained in any desired form or length to suite the
requirements and there is not much wastage of the material in cutting etc.
· On account of their easy erection
techniques, the progress of roofing work with steel trusses is fast.
STANCHION BASE
Stanchions are riveted
to specially built bases. The base of a stanchion consists of an arrangement of
base plate, guest plates, angle cleats, stiffeners, fastenings etc. The
stanchion is fully connected to the base plate with the help of the above
fixtures and finally the base plate is fixed to the foundation of stanchion
through rag bolts. The width of the base plate varies from 2 to 4 times the
width of the stanchion and the height of the gusset plates vary from 1 ½ to 2
times the width of the stanchion. To avoid shear and bending the base plate
should not project more than 8 times its thickness beyond the stanchion face.
WELDING
It is a process of
jointing or more structural members by introducing fused metal into fillets
between them or by raising the temperature of their surfaces to the fusion
temperature and then applying pressure. It may thus be said to be an
alternative method of connecting structural steel sections. With the
development of welding it has become possible to render architectural beauty to
steel structures also. Structural members of the desired pattern can thus be
joined flush to a smooth and pleasing surface.
· Rapid execution of work.
·
Noise
produced during the process of riveting is eliminated in welding.
·
It
is economical when used on a large scale.
·
There
is a great saving of material as the angle cleats which are necessary in
riveted connection, are eliminated in welding.
· In welded connection the entire
section is considered effective for taking up tensile stresses as there is no
reduction in sectional areas on account of hole etc.
There are two
different methods of welding commonly adopted.
1.Electric arc welding
2. Oxy acetylene
Welding.
Electric arc welding
is economical and is commonly used in places, where electricity is easily
available. In case of oxy-acetylene welding, a flame of high temperature is
produced by burning a mixture of acetylene and oxygen.
TYPES OF WELDING JOINTS
There are three
different types of welds, namely 1. Butt welds 2. Fillet welds 3. Lap welds.
Butt welds: In this type, the edge of one plate is brought in line with
the edge of a second plate and the joint is filled with welding metal or the
two edges are heated to their fusion temperatures and pressed together.
A butt joint is the
most common type of joint employed in the fabrication of welded pipe systems. A
butt joint is the most universally used method of joining pipe to itself,
fittings, flanges, valves and other equipment. This welding technique is widely
applied in situations where a quality weld desired, and the weld by V-ray
technically should be investigated.
ASME B16.25 covers the
preparation of butt-welding ends of piping components to be joined into a
piping system by welding. It includes requirements for welding bevels, for
external and internal shaping of heavy wall components, and for preparation of
internal ends.
Fillet welds: In this type, the edge of one plate is brought against the surface of another and welding metal is fused in the corner between the two plates. Thus the joint can be welded on one or both the sides.
A fillet weld symbol can be used with an arrow side other side significance or on both sides. When a fillet weld is required on both sides of the reference line it is called a double fillet weld. The vertical leg of the symbol will always be placed to the left regardless of which way the arrow is pointing.
Fillet welds may have
a size associated with them. This size is called out on the left side of the symbol
before the vertical side. The size is indicating the leg length of the weld. If
a single size is called out this is specifying that weld should have equal leg
sizes.
There are times when
an unequal leg fillet weld is called out. In this situation, the part must be
dimensioned in order to apply the correct leg size to the right member being welded.
If there were no indication of which leg is which the part could be welded
incorrectly.
Lap weld: In this type, the edges of plates are lapped over the other and the edges of one plate are welded of one plate are welded to the surface of the other.
Procedure:
· The given work pieces are thoroughly
cleaned, i.e. rust, scales are removed and the edges are filed.
·
The electrode is held in an electrode holder
and ground clamp is clamped to the welding plates and the power is supplied.
·
The
work pieces are positioned on the table to form a ‘Lab Joint’.
·
The
tag weld is done on the both the ends of joining plates to avoid the movement
of work pieces during welding.
·
The
welding is carried throughout the length of the work pieces on both sides by maintaining
3 mm gap between plates and the welding rod.
·
The
welded plates are allowed for air cooling after the slags are removed.
· The weld joint portions are cleaned by
wire brush.
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