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Structural Analysis - II
Plastic Analysis
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Module IV
Plastic Theory • Introduction-Plastic hinge concept-plastic section modulus-shape
factor-redistribution of moments-collapse mechanism- • Theorems of plastic analysis - Static/lower bound theorem; Kinematic/upper bound theorem-Plastic analysis of beams and
portal frames by equilibrium and mechanism methods.
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Plastic Analysis -Why? What?
• Behaviour beyond elastic limit? • Plastic deformation - collapse load • Safe load – load factor
• Design based on collapse (ultimate) load – limit design
• Economic - Optimum use of material
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Materials
• Elastic
•Elastic-Perfectly plastic
σ σ
Elastic limit
ε
Elastic limit
ε
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Upper yield
point A
Plastic
range
B
C Lower yield
point
O
strain
Idealised stress-strain curve of mild steel
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O
strain
Idealised stress-strain curve in plastic theory
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• Elastic analysis - Material is in the elastic state
- Performance of structures under service loads - Deformation increases with increasing load
• Plastic analysis - Material is in the plastic state
- Performance of structures under
ultimate/collapse loads - Deformation/Curvature increases without an
increase in load.
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Assumptions
• Plane sections remain plane in plastic
condition
• Stress-strain relation is identical both in
compression and tension
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Process of yielding of a section
• Let M at a cross-section increases gradually. • Within elastic limit, M = σ.Z • Z is section modulus, I/y
• Elastic limit - yield stresses reached My = σy.Z
• When moment is increased, yield spreads into inner fibres. Remaining portion still elastic
• Finally, the entire cross-section yields
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σ σ
y
σy
σ
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Change in stress distribution during
yielding
σ
σ
σ
σ
y
y
σ
σ
y
y
σ
σ
y
y
Rectangular cross section
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σy
σy σy σy
σy
Inverted T section
σy
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Plastic hinge
• When the section is completely yielded, the section is fully plastic • A fully plastic section behaves like a hinge - Plastic hinge
Plastic hinge is defined as an yielded zone due
to bending in a structural member, at which
large rotations can occur at a section at constant plastic moment, MP
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Mechanical hinge
Plastic hinge Reality
Concept Resists zero
Resists a constant moment
moment MP
Mechanical Hinge
Plastic Hinge with MP= 0
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• M - Moment corresponding to working load
• My - Moment at which the section yields
• MP - Moment at which entire section is under yield stress
MP
T
σ
σ
y
y
C
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Plastic moment • Moment at which the entire section is
under yield stress C =T
Aσ = Aσ ⇒A
A
=A=
c y t y c t 2
•NA divides cross-section into 2 equal parts
A
C=T = σ 2
y
16 16
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σ
yc
y
C=Aσ 2
y c
A
T = σ 2
y
σ
yt
y
A
Similar to σ
y Z
⇒ σ
(
y +y
)
=σ Z
•Couple due to
Aσ y
c
2
Z
p
t y p y
2
Plastic modulus
Z
(>1) is the shape factor
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Shape factor for various cross-sections b
Rectangular cross-section: d
Section modulus
Z =
I
y
=
3 (bd 12)
(d2) A
=
2 bd
6 bd ⎛d d
2 ⎞ bd
Plastic modulus
Z = (y +
y)= + =
Shape factor
Z Z
p
p
= 1.5
2
c t 2
⎜ ⎝4 4
18
⎟ ⎠ 4
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Circular section
Z =
A
(y +y)
p
d =
2 ⎛πd
⎜
c
2
t
⎞⎛2d
⎟⎜
+
2d⎞
⎟
=
3
d
4
8
⎝ ⎠
3
⎝3π 3π⎠ 6
(πd 64) πd
Z= = d 2 32
Z S =
p
=1.7 Z
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Triangular section
⎛ bh
⎜
3
⎞ ⎟ 2
Z=
⎝
36
2h
⎠
bh =
24
2h 3 CG axis h
3
Z =
A
(y +y) b
p 2
c t
Equal area axis
yc
y
t S = 2.346
b
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I section
20mm
250mm
10mm
200 mm
S = 1.132
20mm
Mp = 259.6 kNm
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Load factor
Load factor =
collapse load M
=
P
σ
=
Z
y P
working load M
σZ
Rectangular cross-section:
M
P
=σ
Z
y P
=σ
y
2 bd
4
bd
M =σZ=σ
2
=
σ
y
2
bd
M
P
2 ⎛ bd ⎞ ⎛σ
6 1.5 6 2
bd ⎞
y ∴LF= =
M
⎜ ⎝
σ
y 4
⎟ ⎠
÷ ⎜ ⎝
1.5 6
=2.25
⎟ ⎠
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Factor of safety
Factor of Safety =
Yield Load W
=
y
Working Load W
Yield Stress σ = =
y
WorkingStress σ
σ =
y
=1.5 (σ
y
/1.5)
Elastic Analysis - Factor of Safety
Plastic Analysis - Load Factor
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Mechanisms of failure
• A statically determinate beam will collapse if one plastic
hinge is developed
• Consider a simply supported beam with constant cross
section loaded with a point load P at midspan
• If P is increased until a plastic hinge is developed at the point of maximum moment (just underneath P) an unstable
structure will be created.
• Any further increase in load will cause collapse
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• For a statically indeterminate beam to collapse, more than one
plastic hinge should be developed
• The plastic hinge will act as real hinge for further increase of load (until sufficient plastic hinges are developed for
collapse.)
• As the load is increased, there is a redistribution of moment, as the plastic hinge cannot carry any additional moment.
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Beam mechanisms
Determinate beams
& frames: Collapse after first plastic
hinge
Simple beam
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Indeterminate beams &
frames: More than one
plastic hinge to develop mechanism Fixed beam
Plastic hinges develop at the ends first
Beam becomes a simple beam
Plastic hinge develops at the centre
Beam collapses
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Indeterminate beam: More than one plastic
hinge to develop
mechanism Propped cantilever
Plastic hinge develops at the fixed support first
Beam becomes a simple beam
Plastic hinge develops at the centre
Beam collapses
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Panel mechanism/sway mechanism
W
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Gable Mechanism
W
Composite (combined) Mechanism
- Combination of the above
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Methods of Plastic Analysis
• Static method or Equilibrium method - Lower bound: A load computed on the basis of an assumed
equilibrium BM diagram in which the moments are not greater than
MP is always less than (or at the worst equal to) the true ultimate
load.
• Kinematic method or Mechanism method or Virtual work
method - Work performed by the external loads is equated to the internal
work absorbed by plastic hinges
- Upper bound: A load computed on the basis of an assumed mechanism is always greater than (or at the best equal to) the true
ultimate load.
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• Collapse load (Wc): Minimum load at which
collapse will occur - Least value
• Fully plastic moment (MP): Maximum moment capacity for design - Highest value
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Determination of collapse load
1. Simple beam
Equilibrium method:
M
MP
M
P
W .l
= 4
4M
u
P
∴W
=
u l
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Virtual work method: W
=W
E
W
u
⎛l
⎜
I
θ⎟=M
P
.2θ
W
⎝2 ⎠
u 4M
∴W =
P
l 2
2θ θ
u l
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2. Fixed beam with UDL M
CENTRE
wl =
2
, 24
M
ENDS
wl =
2
>M
CENTRE 12
Hence plastic hinges will develop at the ends first.
MC1 C2
M1 P
P
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Equilibrium: 2 w
w l
u
2M =
P
u 8 θ θ
2θ
16M ∴w =
P u
Virtual work:
l
2
W =W
⎛
2 w
l
⎞
⎛
⎜
0+
l
2
θ
E I
⎞
⎟ =M (θ+2θ+θ
∴w =
) u
16M 2
P ⎜ u
⎝ 2
⎟⎜ ⎠⎜
2
⎟ P
⎟
l
⎝
⎠
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3. Fixed beam with point load
W
θ
Virtual work:
l
u
MP
θ
2θ
Equilibrium:
MP
Wu ⎜ ⎟θ =M
⎝2⎠
P
(θ+2θ +θ
)
2M
P
=W
u
l 4
∴W
u
8M =
l
P ∴W
u
8M =
l
P
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4. Fixed beam with eccentric point load
W
a
u
b Equilibrium:
2M =W
ab P u
MP 2M ∴W =
l l
P u
MP
ab
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Virtual work:
θ1
W
u
a
b
θ
2
aθ =bθ
1 2
b
⇒θ = θ 1 2
1
a
θ +θ
2
W(aθ)=M
⎡θ⎣ +(θ +θ)+θ⎦
u 1 P
1 1 2 2
⎡ b ⎤
W
u (bθ
2 )=M
P
2 ⎣
θ a
2
+2θ
2 ⎦
2M l
∴W
u
M =
bθ
P
2
⎡ 2 ⎣
b θ
a
2
+2θ
2
⎤ 2M =
⎦
P
(a+b) ab
W
u
P =
ab
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5. Propped cantilever with point load at midspan
MP B1
MPC1
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W
u
2θ
Virtual work: Equilibrium:
W =W
E
I
⎛
W l l M +0.5M =u
⎞ P P (W
u ) ⎜ ⎝
θ =M ⎟ 2⎠
P
(θ+2θ)
∴W
4
6MP
= ∴W
u
6M =
l
P
u l
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6. Propped cantilever with UDL
2 wl
8
MP
MP
Maximum positive BM
x1
At collapse E
x2
Required to locate E
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wlx wx
2 ⎛x ⎞
M
E
u 2 =
2
−
u 2 2
−M
P
2 ⎜
⎝l
⎟ ⎠
=M
(1) P
For maximum, dM dx
wl
E
2
=0
M
u −wx −
P =0
( 2
) 2
From (1) and (2),
u 2
x
l
= 0.414l
2
From (2), w
M =11.656
P u
l
2
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Problem 1: For the beam, determine the design plastic moment capacity.
50 kN
75 kN
1.5 m
1.5 m
7.5 m
• Degree of Indeterminacy, N = 3 - 2 = 1 • No. of hinges, n = 3 • No. of independent mechanisms ,r = n - N = 2
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50 kN
50 kN
1.5 m
θ
75 kN
75 kN
4.5 m
θ + θ1
1.5 m
1.5m
θ1
Mechanism 1
1.5
1.5θ
=
6θ
⇒θ = θ
50(1.5θ)+75⎜1.5×
⎝
1.5
6
θ
⎞
⎟ ⎠
=M
p
⎛
⎜ ⎝
θ +θ+
1
1.5 6
θ
1
⎞
⎟ ⎠
6
∴M =4
p
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50
1.5 m
θ
Mechanism 2
75
4.5 m
1.5 m
θ1
θ + θ1
6θ= 1.5θ
1.5
1
⇒θ = θ1
6
50⎜1.5×
1.5
θ
⎞
+75(1.5θ)=
⎛1.5
M
θ +
1.5
θ +θ
⎞
⎝ 6
1 ⎟ ⎠
1 p
⎜ ⎝ 6
1 6
1 1
⎟ ⎠
∴M
p
=87.5kNm
Design plastic moment (Highest of the above)
= 87.5 kNm
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Problem 2: A beam of span 6 m is to be designed for an ultimate UDL
of 25 kN/m. The beam is simply supported at the ends. Design a
suitable I section using plastic theory, assuming σy= 250 MPa.
25 kN/m
6 m
• Degree of Indeterminacy, N = 2 - 2 = 0 • No. of hinges, n = 1 • No. of independent mechanisms, r = n-N = 1
25 kN/m
Mechanism
θ
2θ
3 m
θ
3 m
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Internal work done
W =
I
0+M ×2θ+0=
p
2M θ
p
External work done
⎛
W =2×25×
3×
0+3θ⎞
=225θ E
⎜
⎟
W =W ⇒
I E
∴M
2M θ =225θ
p
p
⎝ 2 ⎠
=112.5kNm
6
Plastic modulus Z
P
M =
σ
P
y
112.5×10 =
250
5
5 3 = 4.5×10 mm
Z Section modulus Z=
4.5 ×10 P =
1.15
5 = 3.913×10
3 mm
S
Assuming shape factor S = 1.15
Adopt ISLB 275@330 N/m (from Steel Tables - SP 6)
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Problem 3: Find the collapse load for the frame shown.
B A/2
W
A/2 C
F
W/2
A/2 Mp
E 2Mp
A/2
A
2Mp
D
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• Degree of Indeterminacy, N = 5 - 3 = 2
• No. of hinges, n = 5 (at A, B, C, E & F)
• No. of independent mechanisms ,r = n - N = 3
• Beam Mechanisms for members AB & BC
• Panel Mechanism
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Beam Mechanism for AB
B M
p W = M θ+2M (2θ)+Mθ= Mθ
A/2 θ
2θ
I
W =
p
WA
p p p
θ E
W/2
E
2Mp
p
W
22
28M
=W ⇒W =
p
A/2
θ
2Mp
E I c A
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Beam Mechanism for BC W
B A/2
θ
Mp
F A 2
θ
A/2
θ
Mp
2θ
C
Mp
Mp
W
I
=M
θ+M
p p
A
(2θ)+M
θ =
p
4M
θ
p
W
=W θ
E
W =W
2
8M
⇒W =
p
E I c A
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Panel Mechanism W =2Mθ+M
θ+Mθ =4Mθ
I
W
p p
W
p
WA =
p
θ
Mp
θ
A/2
Aθ
A/2 A/2
F
Mp
W
E θ
E 22
16M =W ⇒W =
I c A
p
W/2
2
E
A/2
2Mp
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Combined Mechanism W
W
=
2M
(θ) +
M
(2θ)
A/
θ
2 A/
A
θ 2
I Mp
2 θ
p p
+M (θ +θ)
p
=6Mθ
W/2
A/2
A 2 E
θ 2θ Mp
θ
θ
W =
WA
p
A 3 θ+W θ = WAθ
E
A/2
22 2 4
8Mp
2Mp
W E
=W
I
⇒W =
c A
TrueCollapseLoad,(
8M Lowest of the above,)W=
p c
A
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Problem 4: A portal frame is loaded upto collapse. Find
the plastic moment capacity required if the frame is of uniform section throughout.
10 kN/m B
C 25 kN
8m
Mp
4m
Mp
Mp
A
D
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• Degree of Indeterminacy, N = 4 - 3 = 1
• No. of possible plastic hinges, n = 3 (at B, C and between B&C)
• No. of independent mechanisms ,r = n - N = 2
• Beam Mechanism for BC
•Panel Mechanism
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10 kN/m Beam Mechanism for BC
⎛ 0+4θ
B
Mp
⎞
θ θ
4θ
2θ
C Mp
W =2×10×
4× =160θ
E
W =M
⎜ ⎝ 2
(θ+2θ+θ)=
⎟ ⎠
4Mθ
Mp
I
∴M
p
p
p
=40kNm
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Panel Mechanism
W =W
25 kN
4θ
Mp
θ
Mp
θ
⇒M
p
E I
(θ+θ)=25×4θ
⇒M
p
=50kNm
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Combined Mechanism
It is required to locate the
plastic hinge between B & C
Assume plastic hinge is
formed at x from B
xθ=(8−x)θ
25kN
4m
4θ
θ
10kN/m
x 8−x θ xθ
θ+ θ1
Mp
θ1
Mp
θ
1
⎛
xθ
⎞
⎛
(8−
x
)θ
⎞
W =25×4θ
+10x× +10×
1 (8−x)×
E
⎜ ⎝2
⎟ ⎜ ⎠ ⎝
⎟ 2 ⎠
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⎡
x
⎤
W =M
⎣(
θ +θ)+θ +θ⎦⎤=
2M θ +θ
I p 1
1 p
⎢ ⎥
W =W ⇒
5(5+2x M =
⎣8−x ⎦
)(8−x) E I p
dM
For maximum, dx
P
4
=0
⇒ x = 2.75 m
∴M
p
5(5+2x)(8−x) =
4
=68.91kNm
Design plastic moment of resistance,
(largest of the above),M
p
=68.91kNm
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Problem 5: Determine the Collapse load of the continuous beam. P
P
A
A/2 B
D A
A/2 C E A
SI = 4 − 2 = 2
A collapse can happen in two ways:
1. Due to hinges developing at A, B and D
2. Due to hinges developing at B and E
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Equilibrium:
Hinges at A, B and D
M
Mp
P
p
A
Mp >M
E
p
u 4
PA 8M
u
PA 4
u 4
=M
p
+M
p
⇒P=
u
p
A
Moment at E is greater than Mp. Hence this mechanism is not possible.
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Hinges at B and E
M
M pM
p
p
PA
PA
u
PA
u
u 4
M
p
6M
4
p
=M +
⇒P = 4
p 2
u A
6M
p True Collapse Load,
P
=
u A
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Virtual work: A
P
B A/2 D
A
P
C E A/2
A
SI = 4 − 2 = 2 θ
θ
2θ
Hinges at A, B and D ⎛A ⎞
θ θ
2θ
8Mp
P u
⎜ ⎝2
θ⎟=M ⎠
p
(θ+2θ +θ)
⇒P
u
= A
Hinges at B and E
P
u
⎛A
⎜ ⎝2
⎞
θ⎟=M ⎠
p
(θ+2θ)
⇒P
u
6Mp
= A
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Problem 6: For the cantilever, determine the collapse load.
W
A
L/2
2 Mp
L/2 C
B Mp
• Degree of Indeterminacy, N = 0
• No. of possible plastic hinges, n = 2 (at A&B)
• No. of independent mechanisms ,r = n - N = 2
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Wu L/2
Mp
L
L/2 θ
2M
Mechanism 1 Lθ/2
p
W × θ=M θ
∴W
=
u
p
u
2
L
θ 2Mp
L
Wu
Lθ
2M
Mechanism 2
W
u
×Lθ=2M
θ
p
∴W =
u
p
L 2M
p TrueCollapse Load,(
Lowest of the above,)W =
c L
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Problem 7: A beam of rectangular section b x d is subjected to a
bending moment of 0.9 Mp. Find out the depth of elastic core.
Let the elastic core be of depth 2y0
External bending moment must be
resisted by the internal couple.
Distance of CG from NA,
σ
y
2y
0
σ
y
b
⎛d
−y
⎞
×σ ×
⎡
y +
1⎛d
−y
⎞⎤
+by
σy2
y
⎜ ⎝
0⎟ y 2 ⎠
⎢ 0 ⎜ 0
⎣ 2⎝2
⎟⎥ 0
⎠⎦ 2 3
0 2 2
3d −4y y′=
⎛
d ⎞ σ
= y
0
12(d − y) b −y σ
+by 0
⎜ ⎝2
0 ⎟ ⎠
y 0 2
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Internal couple (moment of resistance)
⎧ ⎛d ⎞
2 2
σ ⎫ 3d −4y =2× b
−y σ +by
y 0 ×
⎨
⎜ ⎝2
0 ⎟ ⎠
y 0 2
⎬ 12(d − y)
⎩
⎭
0
2
2
3d =
−4y 12
bσ
y
bd
2
External bending moment = 0.9M
p
=0.9×Z
σ
p y
=0.9× σ 4
y
3d
2
−4y
2
bd
2
0 Equating the above,
12
bσ
y
=0.9× σ 4
y
⇒ y
= 0.274d
0
Hence, depth of elastic core
=2y = 0.548d
0
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Summary
Plastic Theory • Introduction-Plastic hinge concept-plastic section modulus-shape
factor-redistribution of moments-collapse mechanism- • Theorems of plastic analysis - Static/lower bound theorem; Kinematic/upper bound theorem-Plastic analysis of beams and
portal frames by equilibrium and mechanism methods.
69