Download - chapter 6 Beam Stresses
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AER 520Chapter 6 Bending
1.Shear and moment diagram2.Graphic method3.Bending deformation 4.Flexure formula5.Unsymmetric bending
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6.1 Shear and Moment DiagramsBeam: slender, support loading are perpendicular to the longitudinal axis
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Determine the internal forces (normal force, shear force and bending moment) on a section passing
through the beam at point C.
Using balance equations: Force balance and moment balance two free body diagrams
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Sign Convention
• Shear ~ positive direction is denoted by an internal force that causes a clockwise rotation on which it acts.
• Moment ~ positive direction is denoted by and internal moment that causes a compression or pushing on the upper part of the member
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Shear and Moment Diagrams
• For beams we can draw the shear and moment diagrams as a function of position along the longitudinal axis of the beam.
• Internal normal force will not be considered as most loadings are vertical on a beam and are primarily concerned about shear and bending failures.
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Support reaction forces: Symmetric structure
Internal shear force and moment: first half
02
02
y
A
PF V
PM M x
Example 6.1
Sign convention: V: clockwise rotation M: Compression on the upper part
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Internal shear force and moment: the left segment including region BC
0 / 2 0
0 02
y
A
F V P P
LM M Vx P
/ 2
2
V P
PM L x
Example 6.1
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Support reaction forces:
0
0
0 0
0 0
A C
C A
M F L M
M F L M
Example 6.2
0
0
C
A
MF
LM
FL
Shear and moment in region AB:
0
0
0 0
0 0
y
MF V
LM
M M xL
0
0
MV
LM
M xL
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Example 6.2
Shear and moment in region BC:
0
00
0 0
0 0
y
MF V
LM
M M x ML
0
0 1
MV
Lx
M ML
A up jump at the middle
dMV
dx
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Problem 6-12
Point B: pin joint, only vertical force
Support reaction forces: AB
0 6*4 *6 0
0 6 0
A B
y B A
M F
F F F
4kip
10kipB
A
F
F
Support reaction forces: BC
0 8 0y B CF F F 4kipCF
6 ?kip
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Problem 6-12Free-body diagram:
V
M
6kip
Shear and moment functions:
6kip
6 kip.ft
V
M x
dMV
dx
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Example 6.5
FA FC
0 *18 36*9 36*12 0
0 *18 36*9 36*6 0
A C
C A
M F
M F
42kip
30kipC
A
F
F
Shear and moment functions:2
30 29
xV x
2
2 * * 30* 02 9 3
x x xM x x
3230
27
xM x x
Support reaction forces:
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EXAMPLE: Differences
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AER 520
Chapter 6 Bending
6.2 Graphical Method for shear and moment diagrams
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Regions of distributed load
0 0
0 0
y
O
F V V w x x V
M M M w x x k x M V x
2
V w x x
M V x w x k x
slope of = -distributed
the shear load intensity
dVw x
dx
slope of = the shear
the moment
dMV
dx
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Regions of concentrated force and moment
0 0yF V V F V
For concentrated force: (downward)
V F
For concentrated moment: (clockwise)
00 0OM M M M M
0M M
Same direction of F
0same sign of M
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Shear and moment diagram
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Procedure for analysis
• Support reaction forces and moments• Shear diagram:
/ or - ( )dV dx w x V w x dx V F
• moment diagram:
/ ( )dM dx V or M V x dx 0M M
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Support reaction forces:
0
0
0 0
0 02
y O
O
F F w L
LM M w L
Example 6.9
0
20
2
O
O
F w L
w LM
Shear diagram:
0
0
; 0
/O eF w L F
dV dx w
Moment diagram:2
0 ; 02O e
w LM M
M Vdx
V is positive and decrease to zero;shear diagram: a straight linemoment diagram: parabolic
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Support reaction forces:
4kipA BF F
Problem 6-4
Shear diagram:
(0) 4; (20) 4
/ 0
V V
dV dx
(0) 0; (20) 0
( )
M M
M V x dx
Moment diagram:
There are four jumps (downward at –2kip)
A B
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Support reaction forces:
0 *45 2*45 / 2*45 / 3 0
0 *45 2*45 / 2*2*45 / 3 0
E O
O E
M F
M F
Example 6.11
15
30O
E
F
F
Shear diagram:
0
15; 30
/O EF F
dV dx w
V: slope from 0 to –2
To find the cross point where V=0
115 2 0 26.0
2 45
xx x ft
O E
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Example 6.11
0; 0
/O EM M
dM dx V
Moment diagram:
x=0, slope=15;x=45, slope=-30
The maximum M is at x=26 (V=0)
max
1 26 2615*26 2 26 260
2 45 3M
M Vdx
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Support reaction forces:
1 68 6 2*6* 2*4*2 /10 4.4kip
2 3
1 68 4 2*6* 6 4 2*4*12 /10 17.6kip
2 3
A
C
F
F
Example 6.13
Shear diagram:
4.4; 0;A DV V
B: downward jump 8kip (FA)
BC: further downward at a slope -w
C: upward jump 17.6kip (FC)
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Example 6.13
0; 0
( )
A DM M
M V x dx
Moment diagram:
Cross point of the moment:
AB: linear increase at slope 4.4
BC: decrease in the slope of V
dMV
dx
B and C: turning points
B CM M
parabolic
parabolic
cubic
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Support reaction forces:
2*8*5 /10 8kip
2*8*5 /10 8kip
A
B
F
F
Problem 6-24
4kip/ft upward2
BB
Fw
Shear diagram:
8; 0;
/A BV V
dV dx w
Moment diagram:
0; 0;
/
( )
A BM M
dM dx V
or M V x dx
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Support reaction forces:
2*8*5 /10 8kip
2*8*5 /10 8kip
A
B
F
F
Problem 6-24
4kip/ft upward2
BB
Fw
Shear diagram:
8; 0;
/A BV V
dV dx w
Moment diagram:
0; 0;
/
( )
A BM M
dM dx V
or M V x dx
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6.3 Bending Deformation
Symmetric cross-section
Homogeneous material
Subject to a bending moment
Top compress; bottom stretch
Neutral surface: there is no change in length
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Bending Deformation
Neutral surfaceCompress
Stretch
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0
0
lim
lim
s
s
s s
sy
y
Normal strain calculation
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Maximum Strain
max
c
y
max y
c
max
y
c
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6.4 The Flexure Formula
Applying Hooke’s Law– σ=Eε
max
c
y
max
c
yNormal strain relation:
A linear variation in strain results in a linear variation in stress.
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From the force equilibrium:
0
0
x
A
F
ydA
It means that the neutral axis is the horizontal centroidal axis
ZM M
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From the force equilibrium:
0xF
It means that the neutral axis is the horizontal centroidal axis
ZM M
0A
dA max
c
y
0A
ydA
A
M ydF max
A A
yy dA y dA
c
2max
A
M y dAc
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Flexure Formula
I
Mcmax
2
A
I y dA
I: the moment of inertia of the cross sectional area about the neutral axis
My
I
T
J
J: the polar moment of inertia of the cross sectional area about the center
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Moment of Inertia Review
Centroid ydA2Moment of Inertia y dA
2Polar Moment of Inertia dA
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Parallel Axis Theorem
2'
2'
x x y
y y x
I I Ad
I I Ad
x
y
x
y
yd
xd(m 4)
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Determine the area moment of inertia for the rectangle shown about the axis x’ and about the axis xb.
22 2
2
33 2
2
1
3 12
h
xh
h
h
I y dA y bdy
bhby
2 2
0
33
0
1
3 3
b
h
x
h
I y dA y bdy
bhby
23 32
' 12 2 3bx x y
bh h bhI I Ad bh
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Composite Shapes
2( )x x yI I Ad
2'
33
2
( )
0.25 6 12[ 6 0.25
12 12
0.25*6 3.125 ]
x x yI I Ad
x
Take away two small blocks
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Example 6.15Internal moment:
15*3 5 3 *1.5 22.5kNMM
Section property:
2'
3 2
3
( )
12 0.25 0.02 0.25 0.02 0.16
12
10.02 0.3
12
x x yI I Ad
Bending stress:
3
max 6
22.5 10 *0.1712.7MPa
301.3 10
Mc
I
max 12.7MPaD
max
0.1512.7 11.2MPa
0.17B
B
y
c
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Example 6.16Neutral axis NA:
0.01 0.25*0.02 2 0.1 0.2*0.015
0.25*0.02 2* 0.2*0.015
0.05909
i i
i
y Ay
A
m
Section property:
2'
3 2
3 2
6 4
( )
10.25 0.02 0.25 0.02 0.04909
12
12 0.015 0.2 0.015 0.2 0.04091
12
42.26 10 m
x x yI I Ad
Internal moment about the NA:
2.4*2 1*0.05909 4.859kNMM
Maximum bending stress:
3
max 6
4.859 10 0.140916.2MPa
42.26 10
Mc
I
?
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Determine the maximum tensile and compressive bending stress in the part
Neutral axis NA:
i i
i
y Ay
A
Section property:
2'( )x x yI I Ad
Bending stress:
Tensile bending stress at the bottom;Compressive bending stress at the top.
My
I
0.0175m
6 40.3633 10 m
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Flexure Formula1. Internal moment reaction support force shear force moment diagram2. Section property neutral axis parallel axis theorem3. Bending stress coordinate of the position
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AER 520
Stress Analysis
6.5 Unsymmetric bending
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Symmetric cross-section
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Unsymmetric BendingPrincipal axes: y and z
z
My
I
The moments of inertia are the maximumor minimum; see A.4 for detail
Moment applied to the principal axis:
Y axis: y
Mz
I
Z axis:
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Unsymmetric Bending
cos
sinz
y
M M
M M
Moment arbitrarily applied:
1. Moment divided to two moments:
2. Normal stress calculation:
yz
z y
M zM y
I I
3. Orientation of the neutral axis NA:
0yz
z y
M zM y
I I
tan tanz
y
Iy
z I
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Example 6.181. Moment divided to two moments
312* 7.2kNM
54
12* 9.6kNM5
z
y
M
M
2. Section properties:
3 3 410.2 0.4 1.067 10 m
12zI
3 3 410.4 0.2 0.2667 10 m
12yI
3. Bending stress:
yz
z y
M zM y
I I
3 3
3 3
7.2 10 0.2 9.6 10 0.12.25MPa
1.067 10 0.2667 10B
3 3
3 3
7.2 10 0.2 9.6 10 0.14.95MPa
1.067 10 0.2667 10C
Z axis is the NA
y axis is the NA
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Example 6.18
2.25
4.95D
E
MPa
MPa
4. Orientation of neutral axis:
tan tanz
y
I
I
79.4
306.9 ; 53.1
From +z toward +y
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Calculate the bending stress at A and B
1. The centroid location:
376*12 *6 2 150*12 *75
36.6mm376*12 2 150*12
z
2. Section properties:
800cos( 60 ) 400Nm
800sin( 60 ) 692.82Nmz
y
M
M
3. Bending stress:
yz
z y
M zM y
I I
3 6
692.82 0.15 0.0366400*0.24.38MPa
0.18869 10 16.3374 10A
3 6
400 0.2 692.82 0.03661.13MPa
0.18869 10 16.3374 10B
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Example 6.191. Internal moment components:
15cos 60 7.5kN M
15sin 60 12.99kN Mz
y
M
M
2. Section properties:
0.05 0.1 0.04 0.115 0.03 0.2
0.1 0.04 0.03 0.2
.089
zAz
A
m
3 3
6 4
1 10.1 0.04 0.03 0.2
12 12
20.53 10 m
zI
3 2
3 2
6 4
10.04 0.1 0.04 0.1 0.89 0.5
121
0.3 0.03 0.3 0.03 0.115 0.8912
13.92 10 m
yI
Z axis is the NA
Y axis is the NA
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Example 6.193. Bending stress:
yz
z y
M zM y
I I
4. Orientation of neutral axis:
tan tan ; 60z
y
I
I
6 6
7.5 0.1 12.99 0.04174.8
20.53 10 13.92 10B MPa
6 6
7.5 0.02 12.99 0.08990.4
20.53 10 13.92 10C MPa
68.6
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Determine the force P so that max
180A MPa
1. Internal moment components
0; 2 cos30 0z zM M P 0; 2 sin 30 0y yM M P
1.732
1.0z
y
M P
M P
2. Section properties:
3
3 6 4
12 0.01 0.2
12
10.015 0.01 13.3458 10
12
yI
m
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2. Section properties:
3 3
6 4
1 10.2 0.17 0.19 0.15
12 12
28.4458 10
zI
m
3. Maximum bending stress:
yz
z y
M zM y
I I
1.732
1.0z
y
M P
M P
Choose y positive and z negative: point A
6
6 6
1.732 0.085 1.0 0.1180 10
28.4458 10 13.3458 10
P P
14208 14.2P N kN
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Determine the maximum bending stress in the shaft with 30mm diameter.