Page 1
Design of Tension Members
1.0 Introduction
The Tension member considered for the design is a linear member which
carries an axial pull. The members undergo extension due to this axial pull. This
is one of the common types of force transmitted in the structural system.
Tension members are very efficient since the entire cross section carries
uniform stress unlike flexural members. The tension members do not buckle
even when stressed beyond the elastic limit. Hence the design is not effected
by the type of section used i.e., Plastic, Compact or Semi-compact. Some of the
common examples of tension members in structures are; Bottom chord of pin
jointed roof trusses, bridges, transmission line and communication towers,
wind bracing system in multi-storey buildings, etc.
The objective of this exercise is to determine the tensile strength of a given
member having a specified end connection. The strength of these members is
influenced by several factors such as the length of connection, type of
connection (by bolts or welds), connection eccentricity, size and shape of
fasteners, net area of cross-section and shear lag at the end connection.
2.0 Types of Tension Members
The tension members may be made of single structural shapes. The standard
structural shapes of typical tension members are:
• Angle section • Tee section
• Channel section • Box section
• I section • Tubular section
Page 2
Design of Tension Members
1.0 Introduction
The Tension member considered for the design is a linear member which
carries an axial pull. The members undergo extension due to this axial pull. This
is one of the common types of force transmitted in the structural system.
Tension members are very efficient since the entire cross section carries
uniform stress unlike flexural members. The tension members do not buckle
even when stressed beyond the elastic limit. Hence the design is not effected
by the type of section used i.e., Plastic, Compact or Semi-compact. Some of the
common examples of tension members in structures are; Bottom chord of pin
jointed roof trusses, bridges, transmission line and communication towers,
wind bracing system in multi-storey buildings, etc.
The objective of this exercise is to determine the tensile strength of a given
member having a specified end connection. The strength of these members is
influenced by several factors such as the length of connection, type of
connection (by bolts or welds), connection eccentricity, size and shape of
fasteners, net area of cross-section and shear lag at the end connection.
2.0 Types of Tension Members
The tension members may be made of single structural shapes. The standard
structural shapes of typical tension members are:
• Angle section • Tee section
• Channel section • Box section
• I section • Tubular section
The sections can also be built up using a number of the above structural
shapes.
Single angle members are economical but the connection produces eccentric
force in the member. These are generally used in towers and in trusses. Double
angle members are more rigid than single angle members. They are used in
roof trusses. Since there exists a gap of about 6 to 10 mm between the two
members (which depends on the thickness of the gusset plate), they are
generally interconnected at regular intervals so that they act as one integral
member. In the members of bridge trusses the tensile forces developed are
very large and hence require more rigid members. In these structures single
channel, single I-section, built-up channels, or built-up I-sections will be
generally used.
3.0 Behaviour of Tension Members
The load-deformation behavior of members subjected to uniform tensile stress
is similar to the load-deflection behavior of the corresponding basic material.
The typical stress-strain behavior of mild steel under axial tensile load is shown
in Fig. 1. The upper yield point is merged with the lower yield point for
convenience. The material shows a linear elastic behavior in the initial region
(O to A). The material undergoes sufficient yielding in portion A to B. Further
deformation leads to an increase in resistance, where the material strain
hardens (from B to C). The material reaches its ultimate stress at point C. The
stress decreases with increase in further deformation and breaks at D. The high
strength steel members do not exhibit the well defined yield point and the
yield region (Fig. 1). For such materials, the 0.2 percent proof stress is usually
taken as the yield stress (E).
Page 3
Design of Tension Members
1.0 Introduction
The Tension member considered for the design is a linear member which
carries an axial pull. The members undergo extension due to this axial pull. This
is one of the common types of force transmitted in the structural system.
Tension members are very efficient since the entire cross section carries
uniform stress unlike flexural members. The tension members do not buckle
even when stressed beyond the elastic limit. Hence the design is not effected
by the type of section used i.e., Plastic, Compact or Semi-compact. Some of the
common examples of tension members in structures are; Bottom chord of pin
jointed roof trusses, bridges, transmission line and communication towers,
wind bracing system in multi-storey buildings, etc.
The objective of this exercise is to determine the tensile strength of a given
member having a specified end connection. The strength of these members is
influenced by several factors such as the length of connection, type of
connection (by bolts or welds), connection eccentricity, size and shape of
fasteners, net area of cross-section and shear lag at the end connection.
2.0 Types of Tension Members
The tension members may be made of single structural shapes. The standard
structural shapes of typical tension members are:
• Angle section • Tee section
• Channel section • Box section
• I section • Tubular section
The sections can also be built up using a number of the above structural
shapes.
Single angle members are economical but the connection produces eccentric
force in the member. These are generally used in towers and in trusses. Double
angle members are more rigid than single angle members. They are used in
roof trusses. Since there exists a gap of about 6 to 10 mm between the two
members (which depends on the thickness of the gusset plate), they are
generally interconnected at regular intervals so that they act as one integral
member. In the members of bridge trusses the tensile forces developed are
very large and hence require more rigid members. In these structures single
channel, single I-section, built-up channels, or built-up I-sections will be
generally used.
3.0 Behaviour of Tension Members
The load-deformation behavior of members subjected to uniform tensile stress
is similar to the load-deflection behavior of the corresponding basic material.
The typical stress-strain behavior of mild steel under axial tensile load is shown
in Fig. 1. The upper yield point is merged with the lower yield point for
convenience. The material shows a linear elastic behavior in the initial region
(O to A). The material undergoes sufficient yielding in portion A to B. Further
deformation leads to an increase in resistance, where the material strain
hardens (from B to C). The material reaches its ultimate stress at point C. The
stress decreases with increase in further deformation and breaks at D. The high
strength steel members do not exhibit the well defined yield point and the
yield region (Fig. 1). For such materials, the 0.2 percent proof stress is usually
taken as the yield stress (E).
Fig. 1 Typical stress-strain diagram for mild steel and high strength steel
4.0 Slenderness Ratio
Apart from strength requirement, the tension members have to be checked for
minimum stiffness by stipulating the limiting maximum slenderness ratio of the
member. This is required to prevent undesirable lateral movement or
excessive vibration. The slenderness limits specified in IS: 800-2007 for tension
members are given in Table 1.
Table 1 Maximum values of effective slenderness ratio as per IS: 800-2007
Member
Maximum effective
slenderness ratio (l/r)
A tension member in which a reversal of direct stress
occurs due to loads other than wind or seismic forces
180
A member subjected to compressive forces resulting
only from a combination of wind/earthquake actions,
provided the deformation of such a member does not
adversely affect the stresses in any part of the
structure
250
A member normally acting as a tie in a roof truss or a
bracing member, which is not considered effective
when subject to reversal of stress resulting from the
action of wind or earthquake forces
350
Page 4
Design of Tension Members
1.0 Introduction
The Tension member considered for the design is a linear member which
carries an axial pull. The members undergo extension due to this axial pull. This
is one of the common types of force transmitted in the structural system.
Tension members are very efficient since the entire cross section carries
uniform stress unlike flexural members. The tension members do not buckle
even when stressed beyond the elastic limit. Hence the design is not effected
by the type of section used i.e., Plastic, Compact or Semi-compact. Some of the
common examples of tension members in structures are; Bottom chord of pin
jointed roof trusses, bridges, transmission line and communication towers,
wind bracing system in multi-storey buildings, etc.
The objective of this exercise is to determine the tensile strength of a given
member having a specified end connection. The strength of these members is
influenced by several factors such as the length of connection, type of
connection (by bolts or welds), connection eccentricity, size and shape of
fasteners, net area of cross-section and shear lag at the end connection.
2.0 Types of Tension Members
The tension members may be made of single structural shapes. The standard
structural shapes of typical tension members are:
• Angle section • Tee section
• Channel section • Box section
• I section • Tubular section
The sections can also be built up using a number of the above structural
shapes.
Single angle members are economical but the connection produces eccentric
force in the member. These are generally used in towers and in trusses. Double
angle members are more rigid than single angle members. They are used in
roof trusses. Since there exists a gap of about 6 to 10 mm between the two
members (which depends on the thickness of the gusset plate), they are
generally interconnected at regular intervals so that they act as one integral
member. In the members of bridge trusses the tensile forces developed are
very large and hence require more rigid members. In these structures single
channel, single I-section, built-up channels, or built-up I-sections will be
generally used.
3.0 Behaviour of Tension Members
The load-deformation behavior of members subjected to uniform tensile stress
is similar to the load-deflection behavior of the corresponding basic material.
The typical stress-strain behavior of mild steel under axial tensile load is shown
in Fig. 1. The upper yield point is merged with the lower yield point for
convenience. The material shows a linear elastic behavior in the initial region
(O to A). The material undergoes sufficient yielding in portion A to B. Further
deformation leads to an increase in resistance, where the material strain
hardens (from B to C). The material reaches its ultimate stress at point C. The
stress decreases with increase in further deformation and breaks at D. The high
strength steel members do not exhibit the well defined yield point and the
yield region (Fig. 1). For such materials, the 0.2 percent proof stress is usually
taken as the yield stress (E).
Fig. 1 Typical stress-strain diagram for mild steel and high strength steel
4.0 Slenderness Ratio
Apart from strength requirement, the tension members have to be checked for
minimum stiffness by stipulating the limiting maximum slenderness ratio of the
member. This is required to prevent undesirable lateral movement or
excessive vibration. The slenderness limits specified in IS: 800-2007 for tension
members are given in Table 1.
Table 1 Maximum values of effective slenderness ratio as per IS: 800-2007
Member
Maximum effective
slenderness ratio (l/r)
A tension member in which a reversal of direct stress
occurs due to loads other than wind or seismic forces
180
A member subjected to compressive forces resulting
only from a combination of wind/earthquake actions,
provided the deformation of such a member does not
adversely affect the stresses in any part of the
structure
250
A member normally acting as a tie in a roof truss or a
bracing member, which is not considered effective
when subject to reversal of stress resulting from the
action of wind or earthquake forces
350
Members always in tension (other than pre-tensioned
members)
400
5.0 Shear Lag
The tensile force to a tension member is transferred by a gusset plate or by the
adjacent member connected to one of the legs either by bolting or welding.
This force which is transferred to one leg by the end connection locally gets
transferred as tensile stress over the entire cross section by shear. Hence, the
distribution of tensile stress on the section from the first bolt hole to the last
bolt hole will not be uniform. Hence, the connected leg will have higher
stresses at failure while the stresses in the outstanding leg will be relatively
lower. However, at sections far away from the end connection, the stress
distribution becomes more uniform. Here the stress transfer mechanism, i.e.,
the internal transfer of forces from one leg to the other (or flange to web, or
from one part to the other), will be by shear and because one part ‘lags’
behind the other, the phenomenon is referred to as ‘shear lag’.
The shear lag reduces the effectiveness of the component plates of a tension
member that are not connected directly to a gusset plate. The efficiency of a
tension member can be increased by reducing the area of such components
which are not directly connected at the ends. The shear lag effect reduces with
increase in the connection length.
6.0 Modes of Failure
The different modes of failure in tension members are
1. Gross section yielding
2. Net section rupture
3. Block shear failure
Page 5
Design of Tension Members
1.0 Introduction
The Tension member considered for the design is a linear member which
carries an axial pull. The members undergo extension due to this axial pull. This
is one of the common types of force transmitted in the structural system.
Tension members are very efficient since the entire cross section carries
uniform stress unlike flexural members. The tension members do not buckle
even when stressed beyond the elastic limit. Hence the design is not effected
by the type of section used i.e., Plastic, Compact or Semi-compact. Some of the
common examples of tension members in structures are; Bottom chord of pin
jointed roof trusses, bridges, transmission line and communication towers,
wind bracing system in multi-storey buildings, etc.
The objective of this exercise is to determine the tensile strength of a given
member having a specified end connection. The strength of these members is
influenced by several factors such as the length of connection, type of
connection (by bolts or welds), connection eccentricity, size and shape of
fasteners, net area of cross-section and shear lag at the end connection.
2.0 Types of Tension Members
The tension members may be made of single structural shapes. The standard
structural shapes of typical tension members are:
• Angle section • Tee section
• Channel section • Box section
• I section • Tubular section
The sections can also be built up using a number of the above structural
shapes.
Single angle members are economical but the connection produces eccentric
force in the member. These are generally used in towers and in trusses. Double
angle members are more rigid than single angle members. They are used in
roof trusses. Since there exists a gap of about 6 to 10 mm between the two
members (which depends on the thickness of the gusset plate), they are
generally interconnected at regular intervals so that they act as one integral
member. In the members of bridge trusses the tensile forces developed are
very large and hence require more rigid members. In these structures single
channel, single I-section, built-up channels, or built-up I-sections will be
generally used.
3.0 Behaviour of Tension Members
The load-deformation behavior of members subjected to uniform tensile stress
is similar to the load-deflection behavior of the corresponding basic material.
The typical stress-strain behavior of mild steel under axial tensile load is shown
in Fig. 1. The upper yield point is merged with the lower yield point for
convenience. The material shows a linear elastic behavior in the initial region
(O to A). The material undergoes sufficient yielding in portion A to B. Further
deformation leads to an increase in resistance, where the material strain
hardens (from B to C). The material reaches its ultimate stress at point C. The
stress decreases with increase in further deformation and breaks at D. The high
strength steel members do not exhibit the well defined yield point and the
yield region (Fig. 1). For such materials, the 0.2 percent proof stress is usually
taken as the yield stress (E).
Fig. 1 Typical stress-strain diagram for mild steel and high strength steel
4.0 Slenderness Ratio
Apart from strength requirement, the tension members have to be checked for
minimum stiffness by stipulating the limiting maximum slenderness ratio of the
member. This is required to prevent undesirable lateral movement or
excessive vibration. The slenderness limits specified in IS: 800-2007 for tension
members are given in Table 1.
Table 1 Maximum values of effective slenderness ratio as per IS: 800-2007
Member
Maximum effective
slenderness ratio (l/r)
A tension member in which a reversal of direct stress
occurs due to loads other than wind or seismic forces
180
A member subjected to compressive forces resulting
only from a combination of wind/earthquake actions,
provided the deformation of such a member does not
adversely affect the stresses in any part of the
structure
250
A member normally acting as a tie in a roof truss or a
bracing member, which is not considered effective
when subject to reversal of stress resulting from the
action of wind or earthquake forces
350
Members always in tension (other than pre-tensioned
members)
400
5.0 Shear Lag
The tensile force to a tension member is transferred by a gusset plate or by the
adjacent member connected to one of the legs either by bolting or welding.
This force which is transferred to one leg by the end connection locally gets
transferred as tensile stress over the entire cross section by shear. Hence, the
distribution of tensile stress on the section from the first bolt hole to the last
bolt hole will not be uniform. Hence, the connected leg will have higher
stresses at failure while the stresses in the outstanding leg will be relatively
lower. However, at sections far away from the end connection, the stress
distribution becomes more uniform. Here the stress transfer mechanism, i.e.,
the internal transfer of forces from one leg to the other (or flange to web, or
from one part to the other), will be by shear and because one part ‘lags’
behind the other, the phenomenon is referred to as ‘shear lag’.
The shear lag reduces the effectiveness of the component plates of a tension
member that are not connected directly to a gusset plate. The efficiency of a
tension member can be increased by reducing the area of such components
which are not directly connected at the ends. The shear lag effect reduces with
increase in the connection length.
6.0 Modes of Failure
The different modes of failure in tension members are
1. Gross section yielding
2. Net section rupture
3. Block shear failure
The strength of tension members under the different modes are failure, i.e.,
design strength due to yielding of gross section, T
dg
, rupture of critical section,
T
dn
and block shear T
db
are first determined. The design strength of a member
under axial tension, T
d
, is the lowest of the above three values.
6.1 Gross section yielding
Steel members (plates, angles, etc.) without bolt holes can sustain loads up to
the ultimate load without failure. However, the members will elongate
considerably (10 to 15 % of its original length) at this load, and hence make the
structure unserviceable. Hence the design strength T
dg
is limited to the yielding
of gross cross section which is given by
T
dg
= f
y
A
g
/?
m0
where
f
y
= yield strength of the material in MPa
A
g
= gross area of cross section in mm
2
?
m0
= 1.10 = partial safety factor for failure at yielding
6.2 Net section rupture
This occurs when the tension member is connected to the main or other
members by bolts. The holes made in members for bolts will reduce the cross
section, and hence net area will govern the failure in this case. Holes in
members cause stress concentration at service loads. From the theory of
elasticity, the tensile stress adjacent to a hole will be about two to three times
the average stress on the net area (Fig. 2a). This depends on the ratio of the
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