Transcript Document

TENSION MEMBERS
DESIGN OF TENSION MEMBERS
• Introduction
• Behaviour of Tension Members
– Plates Under Tension
– Threaded Rods
– Angles Under Tension
• Code Provisions
• Summary
© Dr S R Satish Kumar IIT Madras
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Tension Members in Buildings
suspenders
rafter
tie
(a) Roof Truss
purlin
Sag rod
(b) Suspended Building
Top chord
(c) Roof Purlin System
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(d) Braced frame
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INTRODUCTION
Characteristics
– Members Experience
• Axial force
• Stretching
• Uniform stress over the cross section
– Very Efficient Member
• Strength governed by the material strength
• Bolt holes affect the strength
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INTRODUCTION
Cross Sections Used for Tension Members
Channel
Angle
Double Angle
Rod
Cable
Built up sections
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BEHAVIOUR IN TENSION
Material Properties
High Strength Steel
T
d
fy
Mild Steel
Yield Plateau
b
c
e
a

0.2%
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BEHAVIOUR IN TENSION
Plates with a Hole
y
(a) Elastic
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(b) Elasto-Plastic
u
(c) Ultimate
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BEHAVIOUR IN TENSION
Plates with Holes (Cl 6.3)
1
b
d
g
2
3
(a)
(b)
Case a:
Net Area: = t (b - 2*d)
Case b:
Net Area: = t (b – d)
Case c:
t (b - 2*d)<Area< t (b –d)
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p
4
(c)
P = pitch
g = gauge
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BEHAVIOUR IN TENSION
Plates with Holes
An = [ b – n d + (p2 / 4 g)]t
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BEHAVIOUR IN TENSION
• ANGLES
Eccentrically Loaded Through Gussets
Gusset plate
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Revised IS 800:2007
SECTION 6 DESIGN OF TENSION MEMBERS
6.1
Tension Members
6.2
Design Strength due to Yielding of Gross Section
6.3
Design Strength due to Rupture of Critical Section
6.3.1 Plates
6.3.2
Threaded Rods
6.3.3
Single Angles
6.3.4 Other Sections
6.4
Design Strength due to Block Shear
6.4.1
Plates
6.4.2
Angles
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Revised IS 800:2007 CODAL PROVISIONS
6.1 Tension Members
The factored design tension T, in the members T < Td
6.2 Design Strength due to Yielding of Gross Section
The design strength of members under axial tension Tdg,
Tdg = fy Ag /m0
mo = 1.1
6.3 Design Strength due to Rupture of Critical Section
6.3.1 Plates  The design strength in tension of a plate, Tdn,
Tdn =0.9 fu An / m1 m1 = 1.25

pi2 
An = b  nd h   4 g  t
i

i 

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Cont…
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Revised IS 800:2007 CODAL PROVISIONS
6.3.2 Threaded Rods 
The design strength of threaded rods in tension, Tdn,
Tdn =0.9 fu An / m1
f < fy
fy
fu
droot
dgross
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elastic
elastic Plastic
Ultimate
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Revised IS 800:2007 CODAL PROVISIONS
6.3.3 Single Angles  The design strength, Tdn, as governed
by tearing
Tdn
= 0.9 fu Anc / m1 + Ago fy /m0
 = 1.4 – 0.076 (w/t) (fu/fy) (bs/L )
[≈ 1.4-0.52(bs/L)]
Alternatively, the tearing strength of net section may be
taken as
Tdn =  An fu /m
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Revised IS 800:2007 CODAL PROVISIONS
w
w
w1
bs=w+w1
bs=w
Fig. 6.2 Angles with end connections
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Revised IS 800:2007 CODAL PROVISIONS
6.4 Design Strength due to Block Shear
6.4.1 Plates –The block shear strength, Tdb, of connection shall be taken as the
smaller of
Tdb = ( Avg fy /(3 m0) + fu Atn /m1 )
or
Tdb = ( fu Avn /(3 m1) + fy Atg /m0 )
1
2
4
3
Fig 6.2 Block Shear Failure of Plates
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2
4
3
Fig 6.3 Block Shear Failure of Angles
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DESIGN OF TENSION MEMBERS
Efficiency
 = Pt /(Ag * fy / M0)
Design Steps
An = Pt / (fu / M1)
Ag = Pt / (fy / M0)
Choose a trial section
Analyse for its strength
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DESIGN OF TENSION MEMBERS
Stiffness Requirements
Designed for compression under
stress reversal
/r < 250
Not designed for compression under
stress reversal
/r < 350
Members under tension only /r < 400
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DESIGN OF TENSION MEMBERS
Summary
Efficient load carrying members
Efficiency may reduce due to
– bolt holes
– shear lag
– block shear
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• www.steel-insdag.org
Dr S R Satish Kumar
Asso. Professor & Head,
Structural Engineering Laboratory,
Dept. of Civil Engineering,
IIT Madras, Chennai 600 036.
[email protected]
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