analysis of statically indeterminate structures · 2019. 7. 14. · 2- statically indeterminate...
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THEORY OF STRUCTURES -------------------- BY THAAR AL-GASHAM
1 WASSIT UNIVERSITY – ENGINEERING COLLEGE- CIVIL ENGINEERING DEPARTMENT
Analysis of statically indeterminate structures
Structure of any type is classified as statically indeterminate when the number of
unknown reactions or internal forces exceeds the number of equations available for its
analysis.
Advantages of statically indeterminate structures
1- For a given loading the maximum stress and deflection of an indeterminate
structure are generally smaller than those of its statically determinate
counterpart.
2- Statically indeterminate structures can redistribute its load to its redundant
supports in cases where faulty design or overloading occurs.
Although from these advantages of selecting statically indeterminate structures, also;
there are some disadvantages such as deformations caused by relative support
displacement, or changes in member length caused by temperature or fabrication errors
will introduce additional stresses in the structure, which must be considered when
designing indeterminate structures.
Methods of analysis
When analyzing any indeterminate structures, it is necessary to satisfy equilibrium,
compatibility, and force-displacement requirements for the structure. Equilibrium is
satisfied when the reactive forces hold the structure at rest, and compatibility is satisfied
when the various segments of the structures fit together without intentional breaks or
overlaps. the force-displacements requirements depends upon the way of material
responds. Generally, there are two different methods to satisfy these requirements: the
force or flexibility method, and the displacement or stiffness method.
In this stage, the following methods will be studied:
1- Consistent method
2- Slope-deflection method
3- Moment distribution method
4- Stiffness matrix method
THEORY OF STRUCTURES -------------------- BY THAAR AL-GASHAM
2 WASSIT UNIVERSITY – ENGINEERING COLLEGE- CIVIL ENGINEERING DEPARTMENT
1-consistent method
1- Determine the degree of indeterminacy of structures.
The above structure is statically indeterminate to third degree.
2- Remove redundant to convert structure from statically indeterminate to stable
statically determinate structure , this structure is named as primary structure.
Primary structure
3- Determine internal moment at each part of primary structure (M)
4- Apply unit load in position of removed redundant, then internal moment at each
part of structure is calculated as shown in figures.
m1
1
m2
1
m3
1
X1 X2
X3
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5- Determine redundant by solving the following simultaneous equations
∆10 + 𝑠11𝑥1 + 𝑠12𝑥2 + 𝑠13𝑥3 = 𝑜 𝑜𝑟 𝑠𝑒𝑡𝑡𝑙𝑚𝑒𝑛𝑡 𝑎𝑡 𝑠𝑢𝑝𝑝𝑜𝑟𝑡 1-----1
∆20 + 𝑠21𝑥1 + 𝑠22𝑥2 + 𝑠23𝑥3 = 𝑜 𝑜𝑟 𝑟𝑜𝑡𝑎𝑡𝑖𝑜𝑛𝑎𝑙 𝑎𝑡 𝑠𝑢𝑝𝑝𝑜𝑟𝑡 2---2
∆30 + 𝑠31𝑥1 + 𝑠32𝑥2 + 𝑠33𝑥3 = 𝑜 𝑜𝑟 𝑠𝑒𝑡𝑡𝑙𝑚𝑒𝑛𝑡 𝑎𝑡 𝑠𝑢𝑝𝑝𝑜𝑟𝑡 3---3
Where
∆10= 𝑀𝑚1 𝑑𝑥
𝐸𝐼
∆20= 𝑀𝑚2 𝑑𝑥
𝐸𝐼
∆30= 𝑀𝑚3 𝑑𝑥
𝐸𝐼
𝑠11 = 𝑚1. 𝑚1 𝑑𝑥
𝐸𝐼
𝑠12 = 𝑚1. 𝑚2 𝑑𝑥
𝐸𝐼
𝑠13 = 𝑚1. 𝑚3 𝑑𝑥
𝐸𝐼
𝑠22 = 𝑚2. 𝑚2 𝑑𝑥
𝐸𝐼
𝑠33 = 𝑚3. 𝑚3𝑑𝑥
𝐸𝐼
S12=S21 , S31=S13, S32=S23
Applications of consistent method
1-beams
Example(1):- determine the reaction at the supports
A B L
W
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4 WASSIT UNIVERSITY – ENGINEERING COLLEGE- CIVIL ENGINEERING DEPARTMENT
Solution
The beam is statically indeterminate to first degree, the roller is considered as redundant
and is removed
𝑙𝑜𝑎𝑑 𝑎𝑡 𝑑𝑖𝑠𝑡𝑎𝑛𝑐𝑒 𝑥 𝑓𝑟𝑜𝑚 𝑏 =𝑊𝑋
𝐿
𝑀 = −𝑋𝑊𝑋
𝐿.1
2.𝑋
3= −
𝑊𝑋3
6𝐿
Apply unit load at position of removed support
𝑚1 = −𝑋
∆10 + 𝑠11𝑥1 = 𝑜
∆10= 𝑀𝑚1 𝑑𝑥
𝐸𝐼=
−𝑤𝑥3
6𝑙
𝑙
𝑜
∗−𝑥𝑑𝑥
𝐸𝐼=
𝑤𝑙4
30𝐸𝐼
𝑠11 = 𝑚1. 𝑚1 𝑑𝑥
𝐸𝐼= −𝑥
𝑙
0
−𝑥𝑑𝑥
𝐸𝐼=
𝑙3
3𝐸𝐼
𝑤𝑙4
30𝐸𝐼+
𝑙3
3𝐸𝐼𝑥1 = 𝑜
A L
W
X
M
A L
X
m1 1
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5 WASSIT UNIVERSITY – ENGINEERING COLLEGE- CIVIL ENGINEERING DEPARTMENT
𝑥1 = −𝑤𝑙
10=
𝑤𝑙
10↑
After the reaction at roller is known , remaining reactions can be determined by applying
equilibrium equations.
𝑅𝑦𝐴 =𝑤𝑙
2−
𝑤𝑙
10=
2
5𝑤𝑙 ↑
𝑀𝐴 =𝑤𝑙
10𝑙 −
𝑤𝑙
2∗
1
3= −
𝑤𝑙2
15=
𝑤𝑙2
15 𝑐𝑜𝑢𝑛𝑡𝑒𝑟𝑐𝑙𝑜𝑐𝑘 𝑤𝑖𝑠𝑒
H.W: repeat previous example and choose end moment at A as redundant
Example (2): analysis the following beam
The beam is statically indeterminate to first degree due to symmetry
𝑀 =𝑊𝐿
2𝑋 −
𝑊𝑋2
2=
𝑊
2 𝐿𝑋 − 𝑋2
A L B
w
A L B
w M
X
X
A L B
m 1 1
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6 WASSIT UNIVERSITY – ENGINEERING COLLEGE- CIVIL ENGINEERING DEPARTMENT
m=1
∆10= 𝑀𝑚1 𝑑𝑥
𝐸𝐼=
𝑤
2𝐸𝐼 𝑙𝑥 − 𝑥2
𝑙
0
𝑑𝑥 =𝑤𝑙3
12𝐸𝐼
𝑠11 = 𝑚1. 𝑚1 𝑑𝑥
𝐸𝐼=
𝑑𝑥
𝐸𝐼
𝑙
0
=𝑙
𝐸𝐼
∆10 + 𝑠11𝑥1 = 𝑜
𝑤𝑙3
12𝐸𝐼+
𝑙
𝐸𝐼𝑥1 = 𝑜
𝑥1 = −𝑤𝑙2
12
Example (3):- determine the internal moments acting in the beam at support B and c .
the wall at A moves upward 30mm. take E=200GPa,I=90(106)mm4.
The structure is statically indeterminate to first degree.
MAB=0
MBC=-20X
MCD=-40X
10 m 10 m 5 m
40 kN A B C D
M M M
60
10 m 10 m 5 m
A B C D 40 kN
X X X
20
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mAB=-X
mBC=-(10+X)+2X=-10+X
mCD=0
∆10= 𝑀𝑚1 𝑑𝑥
𝐸𝐼=
1
𝐸𝐼 −20𝑥 −10 + 𝑥 𝑑𝑥
10
0
=10000
3𝐸𝐼𝑘𝑁2. 𝑚3
𝑠11 = 𝑚12 𝑑𝑥
𝐸𝐼=
1
𝐸𝐼 𝑥2𝑑𝑥 +
10
0
−10 + 𝑥 2𝑑𝑥10
0
=2000
3𝐸𝐼𝑘𝑁2. 𝑚3
∆10 + 𝑠11𝑥1 = 𝑠𝑒𝑡𝑡𝑙𝑒𝑚𝑒𝑛𝑡
10000
3 ∗ 200 ∗ 90+
2000
3 ∗ 200 ∗ 90𝑥1 = −30 ∗ 10−3
𝑥1 = −5.81 𝑘𝑁
RA=5.81 KN upward
RB=-20+2*-5.81=-31.62=31.62 downward
RC=60-1*5.81=65.81 upward
MB=5.81*10=58.1 kN.m
MC=40*5=200 kN.m
10 m 10 m 5 m
A B C D
1
X X X
2 1
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2-frame
Example(4):- Analysis the frame shown. There is rotational slip of 0.003 rad counter
clockwise at support B. take EI=104 kN.m2
The structure above is statically indeterminate to 2nd degree, there is part of structure is
statically determinate
4kN/m
3m
4m
4m 2m 1m 1m
2kN
4kN/m
3m
4m
4m
1kN
2kN.m
THEORY OF STRUCTURES -------------------- BY THAAR AL-GASHAM
9 WASSIT UNIVERSITY – ENGINEERING COLLEGE- CIVIL ENGINEERING DEPARTMENT
Primary structure
∆10 + 𝑠11𝑥1 + 𝑠12𝑥2 = 𝑜
∆20 + 𝑠21𝑥1 + 𝑠22𝑥2 = 0.003
∆10= 𝑀𝑚1 𝑑𝑥
𝐸𝐼=
1
𝐸𝐼 7.6𝑋 − 1.6𝑋2 (0.8𝑋)𝑑𝑥
5
0
=53.33
𝐸𝐼
4kN/m 3m
4m
4m
1kN
2kN.m
11.5
9.5
0
X
M
M=0
X
M
M=9.5(0.8x)-
4x(0.4x)=7.6x-
1.6x2
3m
4m
4m 1.75
1.75
1
X
M
M=X
X
M
M=1.75(0.8X)-
1(0.6X)=0.8X
1
3m
4m
4m 0.25
0.25
1 X
M
M=-1
X
M
M=-0.25(0.8X)=-
0.2X
0
THEORY OF STRUCTURES -------------------- BY THAAR AL-GASHAM
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∆20= 𝑀𝑚1 𝑑𝑥
𝐸𝐼=
1
𝐸𝐼 7.6𝑋 − 1.6𝑋2 (−0.2𝑋)𝑑𝑥
5
0
=−13.33
𝐸𝐼
𝑠11 = 𝑚12 𝑑𝑥
𝐸𝐼=
1
𝐸𝐼 (0.8𝑋)2𝑑𝑥 +
5
0
𝑥 2𝑑𝑥4
0
=48
𝐸𝐼
𝑠12 = 𝑚1. 𝑚2 𝑑𝑥
𝐸𝐼=
1
𝐸𝐼( 0.8𝑋 −0.2𝑋 𝑑𝑥 + −𝑋𝑑𝑥) =
4
0
5
0
−14.67
𝐸𝐼
𝑠22 = 𝑚22 𝑑𝑥
𝐸𝐼=
1
𝐸𝐼 (−0.2𝑋)2𝑑𝑥 +
5
0
−1 2𝑑𝑥4
0
=5.67
𝐸𝐼
53.33
𝐸𝐼+
48
𝐸𝐼𝑋1 −
14.67
𝐸𝐼𝑋2 = 0
48𝑋1 − 14.67𝑋2 = −53.33 − − − − − 1
−13.33
𝐸𝐼−
14.67
𝐸𝐼𝑋1 +
5.67
𝐸𝐼𝑋2 = 0.003
−14.67𝑋1 + 5.67𝑋2 = 43.33 − − − − − 2
Solving Eqs 1&2,gives
X1=5.85kN , X2=22.78 kN.m
Final result
4kN/m
3m
4m
4m 2m 1m 1m
2kN
1
6.96
14.04
5.85
5.85
22.78
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Example(5):- using method of consistent deformation, analysis the frame shown
EI=104 kN.m2, settlement at support A=0.002m ↓and 0.003m← , rotational slip at
A=0.003 rad counter clock wise , settlement at B=0.004m ↓
The structure is statically indeterminate to 1st degree, the deformations of support
A must be included by using virtual work
3m
4m
4m
A
B
3m
4m
4m
A
B
1
1
4
X
m=0.8X
X m=4
∆equ
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1𝑥∆𝑒𝑞 = 4𝑥0.003 − 1𝑥0.002 + 0𝑥0.003 = 0.01
∆𝑡𝑜𝑡𝑎𝑙 = 0.01 + 0.004 = 0.014𝑚
∆10 + 𝑠11𝑥1 = 0.014
∆10= 𝑜 𝑛𝑜 𝑙𝑜𝑎𝑑 𝑎𝑝𝑝𝑙𝑖𝑒𝑑
𝑠11 = 𝑚12 𝑑𝑥
𝐸𝐼=
1
𝐸𝐼( 16 𝑑𝑥 + 0.8𝑥 2𝑑𝑥) =
90.67
𝐸𝐼
5
0
4
0
90.67
𝐸𝐼𝑥1 = 0.014
𝑥1 = 1.54 ↓ 𝑘𝑁
3-Arch
Example(6):- Analysis the semi-circular arch shown in figure by the method of consistent
deformation. Radius=R, EI constant
3m
4m
4m
A
B
1.54
1.54
6.16
P
THEORY OF STRUCTURES -------------------- BY THAAR AL-GASHAM
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The arch is statically indeterminate to 1st degree
∆10 + 𝑠11𝑥1 = 0
0.5P
P
0.5P
M
θ
𝑀 =𝑃
2𝑅(1 − 𝑐𝑜𝑠𝜃)
m
θ
𝑚 = −𝑅𝑠𝑖𝑛𝜃
1 1