STRUCTURAL STEEL
The structural steel is the steel used for the manufacture of rolled structural steel
sections, fastenings and other elements for use in structural steel works. Steel is
an alloy of iron, carbon and other elements in varying percentages. The strength,
hardness and brittleness of steel increases and ductility of steel decreases with
the increase of percentage of carbon. Depending on the chemical composition,
the different type of steel are classified as mild steel, medium carbon steel, high
carbon steel, low alloy steel and high alloy steel. The mild steel, medium carbon
steel and low alloy steel are generally used for steel structures. The copper bearing quality of steel contains small percentage of copper contents. The corrosive resistance of such steel is increased.
Mild steel is used for the manufacture of rolled structural steel sections, rivets
and bolts. The following operations can be done easily on mild steel 1.Cutting,
2. Punching, 3.Drilling, 4. Machining, 5. Welding and 6. Forging when heated.
All structural steels used in general construction, coming within the purview of
IS:800-84 shall, before fabrication, comply with one of the following Indian
Standard specifications
1. IS : 226-1975 structural steel (standard quality)
2. IS : 1977-1975 structural steel (ordinary quality)
3. IS : 2062-1984 weldable structural steel
4. IS : 961-1975 structural steel (high tensile)
5. IS : 8500-1977 weldable structural steel (medium and high strength
qualities)
IS : 226-1975 structural steel (standard quality).
The mild steel is designated as St 44-S for use in structural work. This steel is
also available in copper bearing quality in which case it designated as St 44-SC.
The copper content is between 0.20 and 0.35 per cent. The physical properties
of structural steel are given below:
1. Unit weight of steel 78.430 to 79.000 kN/m3
2. Young’s modulus of elasticity, E=2.04 to 2.18 x 105
N/mm2
3. Modulus of rigidity, G=0.84 to 0.98 x 105
N/mm2
4. Coefficient of thermal expansion (or contraction) α=12 x 10-6/˚C or
6.7 x 10-6/˚F
The tensile strength, yield stress and percentage elongation for IS : 226-1975
structural steel standard quality, determined in accordance with IS : 1608-1960.
The steel confirming to IS : 226 is suitable for all types of steel structures
subjected to static, dynamic and repeated cycles of loadings. It is also suitable
for welding up to 20 mm thickness. When the thickness of element is more than
20 mm, it needs special precautions while welding.
IS : 1977-1975 structural steel (ordinary quality).
The steel which did not comply with IS : 226, was formerly called as steel of
untested quality. The standards for such steel have been laid down in IS : 1977-
75 (ordinary quality). There are two grades in this standard which are designated
as St 44.0 and St 32.0. The steel St 44.0 is intended to be used for structures not
subjected to dynamic loading other than wind loads e.g., platform roofs, office
buildings, foot over bridge. The copper bearing quality is designated as St
44.0C.
The steel confirming to IS : 1977 is not suitable for welding and for the
structures subjected to high seismic forces (earth quake forces). The steel
structures using steel confirming to IS : 1977 must not be analyzed and designed
by plastic theory.
IS : 2062-1984 weldable structural steel.
This structural steel intended to be used for members in structures subjected to
dynamic loading where welding is employed for fabrication and where fatigue
and great restraint are involved e.g., crane gantry girder, road and rail bridges
etc,. it is designated as St 42-W and copper bearing quality is designated as St
42-WC. It is suitable for welding the elements of thickness between 28 mm and
50 mm. when the thickness of elements is less than 28 mm; it may be welded
provided the limiting maximum carbon content is 0.22 per cent.
IS : 961-1975 structural steel (high tensile).
The high tensile steel forms a specific class of steel in which enhanced
mechanical properties and in most of the cases increased resistance to
atmospheric corrosion are obtained by the incorporation of low proportions of
one or more alloying elements, besides carbon. These steels are generally
intended for application where saving in weight can be effected by reason of
their greater strength and atmospheric corrosion resistance. Standards of high
tensile steel have been given in IS : 961-1975. It has been classified into two
grades designated as St 58-HT and St 55-HTW. St 58-HT is intended for use in
structures where fabrication is done by methods other than welding. St 55-HTW
is intended for use in structures where welding is employed for fabrication. The
high tensile steel is also available in copper bearing quality and two grades are
designated as St 58-HTC and St 55-HTWC. The steel conforming to IS : 961 is
suitable for bridges and general building construction.
IS : 8500-1977 weldable structural steel (medium and high strength
qualities)
Various medium and high strength qualities of weldable structural steel are, Fe
440 (HT1 and HT2) Fe 540 (HT, HTA and HTB), Fe 570 HT, Fe 590 HT and
Fe 640 HT.
PRODUCTION OF STEEL
The steel is produced in the form of ingots and converted to different shapes. In
our country, Tata Iron and Steel Company, Indian Iron and Steel Company,
Mysore Iron and Steel Company and Hindustan Steel produce steel at their
plants
RECENT DEVELOPMENTS IN MATERIAL
A number of developments in material such as steel have been made recently.
The weldable qualities of steel (IS : 2062) designated as St 42-W and IS : 961
designated as St-55-HTW are developed with the large scale use of welding. IS :
961 has been developed with high tensile strength and there is saving in weight
due to enhanced mechanical properties. Its weldable quality is advantageous for
composite construction.
RIVETED CONNECTIONS
INTRODUCTION
In engineering practice it is often required that two sheets or plates are joined together and
carry the load in such ways that the joint is loaded. Many times such joints are required to
be leak proof so that gas contained inside is not allowed to escape. A riveted joint is easily
conceived between two plates overlapping at edges, making holes through thickness of both,
passing the stem of rivet through holes and creating the head at the end of the stem on the
other side. A number of rivets may pass through the row of holes, which are uniformly
distributed along the edges of the plate. With such a joint having been created between two
plates, they cannot be pulled apart. If force at each of the free edges is applied for pulling the
plate apart the tensile stress in the plate along the row of rivet hole and shearing stress in
rivets will create resisting force. Such joints have been used in structures, boilers and ships.
The following are the usual applications for connection.
1. Screws ,
2. Pins and bolts,
3. Cotters and Gibs,
4. Rivets,
5. Welds.
Of these screws, pins, bolts, cotters and gibs are used as temporary fastening i.e., the
components connected can be separated easily. Rivets and welds are used as permanent
fastenings i.e., the components connected are not likely to require separation.
RIVETS
Rivet is a round rod which holds two metal pieces together permanently. Rivets are made
from mild steel bars with yield strength ranges from 220 N/mm2
to 250 N/mm2
. A rivet
consists of a head and a body as shown in Fig 1. The body of rivet is termed as shank. The
head of rivet is formed by heating the rivet rod and upsetting one end of the rod by running it
into the rivet machine. The rivets are manufactured in different lengths to suit different
purposes. The size of rivet is expressed by the diameter of the shank.
Holes are drilled in the plates to be connected at the appropriate places. For driving the
rivets, they are heated till they become red hot and are then placed in the hole. Keeping the
rivets pressed from one side, a number of blows are applied and a head at the other end is
formed. When the hot rivet so fitted cools it shrinks and presses the plates together. These
rivets are known as hot driven rivets. The hot driven rivets of 16 mm, 18 mm, 20 mm and 22
mm diameter are used for the structural steel works.
Some rivets are driven at atmospheric temperature. These rivets are known as cold driven
rivets. The cold driven rivets need larger pressure to form the head and complete the driving.
The small size rivets ranging from 12 mm to 22 mm in diameter may be cold driven rivets.
The strength of rivet increases in the cold driving. The use of cold driven rivets is limited
because of equipment necessary and inconvenience caused in the field.
The diameter of rivet to suit the thickness of plate may be determined from the following
formulae:
1. Unwins’s formula d = 6.05 √tmin
2. The French formula d =1.5 t + 4
3. The German formula d = ට(50 t – 2)
Where d= nominal diameter of rivet in mm and t= thickness of plate in mm.
RIVET HEADS
The various types of rivet heads employed for different works are shown in Fig 2. The
proportions of various shapes of rivet heads have been expressed in terms of diameter ‘D’ of
the shank of rivet. The snap head is also termed as round head and button head. The snap
heads are used for rivets connecting structural members. Sometimes it becomes necessary to
flatten the rivet heads so as to provide sufficient clearance. A rivet head which has the form
of a truncated cone is called a countersunk head. When a smooth flat surface is required, it is
necessary to have rivets countersunk and chipped.
RIVET HOLES
The rivet holes are made in the plates or structural members by punching or drilling. When
the holes are made by punching, the holes are not perfect, but taper. A punch damages the
material around the hole. The operation known as reaming is done in the hole made by
punching. When the hole are made by drilling, the holes are perfect and provide good
alignment for driving the rivets. The diameter of a rivet hole is made larger than the nominal
diameter of the rivet by 1.5 mm of rivets less than or equal to 25 mm diameter and by 2 mm
for diameter exceeding 25 mm.
DEFINITIONS OF TERMS USED IN RIVETING
1) Nominal diameter of rivet (d):
The nominal diameter of a rivet means the diameter of the cold shank before driving.
2) Gross diameter of rivet (D):
The diameter of the hole is slightly greater than the diameter of the rivet shank. As the rivet
is heated and driven, the rivet fills the hole fully. The gross or effective diameter of a rivet
means the diameter of the hole or closed rivet. Strengths of rivet are based on gross diameter.
3) Pitch of rivet (p):
The pitch of rivet is the distance between two consecutive rivets measured parallel to the
direction of the force in the structural member, lying on the same rivet line. Minimum pitch
should not be less than 2.5 times the nominal diameter of the rivet. As a thumb rule pitch
equal to 3 times the nominal diameter of the rivet is adopted. Maximum pitch shall not
exceed 32 times the thickness of the thinner outside plate or 300 mm whichever is less.
4) Gauge distance of rivets (g):
The gauge distance is the transverse distance between two consecutive rivets of adjacent
chains (parallel adjacent lines of fasteners) and is measured at right angles to the direction of
the force in the structural member.
5) Gross area of rivet:
The gross area of rivet is the cross sectional area of a rivet calculated from the gross diameter
of the rivet.
6) Rivet line:
The rivet line is also known as scrieve line or back line or gauge line. The rivet line is the
imaginary line along which rivets are placed. The rolled steel sections have been assigned
standard positions of the rivet lines. The standard position of rivet lines for the various
sections may be noted from ISI Handbook No.1 for the respective sections. These standard
positions of rivet lines are conformed to whenever possible. The departure from standard
position of the rivet lines may be done if necessary. The dimensions of rivet lines should be
shown irrespective of whether the standard positions have been followed or not.
7) Staggered pitch:
The staggered pitch is also known as alternate pitch or reeled pitch. The staggered pitch is
defined as the distance measured along one rivet line from the centre of a rivet on it to the
centre of the adjoining rivet on the adjacent parallel rivet line. One or both the legs of an
angle section may have double rivet lines. The staggered pitch occurs between the double
rivet lines.
TYPES OF JOINTS
Riveted joints are mainly of two types, namely, Lap joints and Butt joints.
Lap Joint: Two plates are said to be connected by a lap joint when the connected ends of the
plates lie in parallel planes. Lap joints may be further classified according to number of
rivets used and the arrangement of rivets adopted. Following are the different types of lap
joints.
1. Single riveted lap joint,
2. Double riveted lap joint:
a). Chain riveted lap joint (Fig 4) b). Zig-Zag riveted lap joint (Fig 5)
Butt Joint:
In a butt joint the connected ends of the plates lie in the same plane. The abutting ends of the
plates are covered by one or two cover plates or strap plates. Butt joints may also be
classified into single cover but joint, double cover butt joints. In single cover butt joint, cover
plate is provided on one side of main plate (Fig. 6). In case of double cover butt joint, cover
plates are provided on either side of the main plate (Fig. 7). Butt joints are also further
classified according to the number of rivets used and the arrangement of rivets adopted.
1. Double cover single riveted but joint
2. Double cover chain riveted butt joint
3. Double cover zig-zag riveted butt joint
FAILURE OF A RIVETED JOINT
Failure of a riveted joint may take place in any of the following ways
1. Shear failure of rivets
2. Bearing failure of rivets
3. Tearing failure of plates
4. Shear failure of plates
5. Bearing failure of plates
6. Splitting/cracking failure of plates at the edges
Shear failure of rivets :
Plates riveted together and subjected to tensile loads may result in the shear of the rivets.
Rivets are sheared across their sectional areas. Single shear occurring in a lap joint and
double shear occurring in but joint (Fig. 8)
Bearing failure of rivets:
Bearing failure of a rivet occurs when the rivet is crushed by the plate (Fig. 9)
Tearing failure of plates :
When plates riveted together are carrying tensile load, tearing failure of plate may occur.
When strength of the plate is less than that of rivets, tearing failure occurs at the net sectional
area of plate (Fig. 10)
Shear failure of plates:
A plate may fail in shear along two lines as shown in Fig. 11. This may occur when
minimum proper edge distance is not provided.
Bearing failure of plates:
Bearing failure of a plate may occur because of insufficient edge distance in the riveted joint.
Crushing of plate against the bearing of rivet take place in such failure (Fig. 12)
Splitting/cracking failure of plates at the edges:
This failure occurs because of insufficient edge distance in the riveted joint. Splitting
(cracking) of plate as shown in Fig. 13 takes place in such failure.
Shearing, bearing and splitting failure of plates may be avoided by providing adequate
proper edge distance. To safeguard a riveted joint against other modes of failure, the joint
should be designed properly.
STRENGTH OF RIVETED JOINT
The strength of a riveted joint is determined by computing the following strengths:
1. Strength of a riveted joint against shearing - Ps
2. Strength of a riveted joint against bearing - Pb
3. Strength of plate in tearing - Pt
The strength of a riveted joint is the least strength of the above three strength.
Strength of a riveted joint against shearing of the rivets:
The strength of a riveted joint against the shearing of rivets is equal to the product of strength
of one rivet in shear and the number of rivets on each side of the joint. It is given by
Ps = strength of a rivet in shearing x number of rivets on each side of joint
When the rivets are subjected to single shear, then the strength of one rivet in single shear
Therefore, the strength of a riveted joint against shearing of rivets =
Where N=Number of rivets on each side of the joint; D=Gross diameter of the rivet;
ps=Maximum permissible shear stress in the rivet (100 N/mm2
).
When the rivets are subjected to double shear, then the strength of one rivet in double shear
= Therefore, the strength of a riveted joint against double shearing of rivets,
When the strength of riveted joint against the shearing of the rivets is determined per gauge
width of the plate, then the number of rivets ‘n’ per gauge is taken in to consideration.
Therefore,
Strength of riveted joint against the bearing of the rivets:
The strength of a riveted joint against the bearing of the rivets is equal to the product of
strength of one rivet in bearing and the number of rivets on each side of the joint. It is given
by,
Pb=Strength of a rivet in bearing x Number of rivets on each side of the joint
In case of lap joint, the strength of one rivet in bearing = D x t x pb
Where D= Gross diameter of the rivet; t=thickness of the thinnest plate; pb= maximum
permissible stress in the bearing for the rivet (300 N/mm2
). In case of butt joint, the total
thickness of both cover plates or thickness of main plate whichever is less is considered for
determining the strength of a rivet in the bearing.
The strength of a riveted joint against the bearing of rivets Pb = N x D x t x pb
When the strength of riveted joint against the bearing of rivets per gauge widh of the plate is
taken into consideration, then, the number of rivets ‘n’ is also adopted per gauge. Therefore,
Pb1 = n x D x t x pb
Strength of plate in tearing
The strength of plate in tearing depends upon the resisting section of the plate. The strength
of plate in tearing is given by Pt = Resisting section x pt
Where pt is the maximum permissible stress in the tearing of plate (150 N/mm2
). When the
strength of plate in tearing per pitch width of the plate is Pt1 = (p-D) x t x pt
The strength of a riveted joint is the least of Ps, Pb, Pt. The strength of riveted joint per gauge
width of plate is the least of Ps1, Pb1, Pt1.
STRENGTH OF LAP AND BUTT JOINT
The strength of riveted lap and butt joint given in the Fig. 14 is summarized as follows:
Strength of lap joint:
Strength of butt joint:
EFFICIENCY OR PERCENTAGE OF STRENGTH OF RIVETED JOINT
The efficiency of a joint is defined as the ratio of least strength of a riveted joint to the
strength of solid plate. It is known as percentage strength of riveted joint as it is expressed in
percentage.
Efficiency of riveted joint
Where P is the strength of solid plate = b x t x pt
Efficiency per pitch width
RIVET VALUE
The strength of a rivet in shearing and in bearing is computed and the lesser is called the
rivet value (R).