m4l28 lesson 28 the direct stiffness method: beams (continued)
DESCRIPTION
Online lecture notes – module 1 (energy methods); module 2 (force method); module 3 (displacement method); module 4 (direct stiffness method); module 5 (cables and arches); module 6 (approximate methods); module 7 (influence lines)- Authorship: Modules 1 – 6 written by Prof. L. S. Ramachandra, Module 7 written by Prof. S. K. Barai, both of IIT Kharagpur - available at http://www.facweb.iitkgp.ernet.in/~baidurya/CE21004/online_lecture_notes/online_lecture_modules.htmlTRANSCRIPT
Module 4
Analysis of Statically Indeterminate
Structures by the Direct Stiffness Method
Version 2 CE IIT, Kharagpur
Lesson 28
The Direct Stiffness Method: Beams
(Continued)
Version 2 CE IIT, Kharagpur
Instructional Objectives After reading this chapter the student will be able to 1. Derive member stiffness matrix of a beam element. 2. Assemble member stiffness matrices to obtain the global stiffness matrix for a
beam. 3. Write the global load-displacement relation for the beam. 4. Impose boundary conditions on the load-displacement relation of the beam. 5. Analyse continuous beams by the direct stiffness method. 28.1 Introduction In the last lesson, the procedure to analyse beams by direct stiffness method has been discussed. No numerical problems are given in that lesson. In this lesson, few continuous beam problems are solved numerically by direct stiffness method. Example 28.1
Analyse the continuous beam shown in Fig. 28.1a. Assume that the supports are unyielding. Also assume that EI is constant for all members.
The numbering of joints and members are shown in Fig. 28.1b. The possible global degrees of freedom are shown in the figure. Numbers are put for the unconstrained degrees of freedom first and then that for constrained displacements.
Version 2 CE IIT, Kharagpur
The given continuous beam is divided into three beam elements Two degrees of freedom (one translation and one rotation) are considered at each end of the member. In the above figure, double headed arrows denote rotations and single headed arrow represents translations. In the given problem some displacements are zero, i.e., 0876543 ====== uuuuuu from support conditions. In the case of beams, it is not required to transform member stiffness matrix from local co-ordinate system to global co-ordinate system, as the two co-ordinate system are parallel to each other.
First construct the member stiffness matrix for each member. This may be done from the fundamentals. However, one could use directly the equation (27.1) given in the previous lesson and reproduced below for the sake convenience.
Version 2 CE IIT, Kharagpur
[ ]
⎥⎥⎥⎥⎥⎥⎥⎥
⎦
⎤
⎢⎢⎢⎢⎢⎢⎢⎢
⎣
⎡
−
−−−
−
−
=
LEI
LEI
LEI
LEI
LEI
LEI
LEI
LEI
LEI
LEI
LEI
LEI
LEI
LEI
LEI
LEI
k
zzzz
zzzz
zzzz
zzzz
4626
612612
2646
612612
22
2323
22
2323
(1)
The degrees of freedom of a typical beam member are shown in Fig. 28.1c. Here equation (1) is used to generate element stiffness matrix. Member 1: , node points 1-2. mL 4= The member stiffness matrix for all the members are the same, as the length and flexural rigidity of all members is the same.
[ ]
1
3
5
6
0.1375.05.0375.0
375.01875.0375.01875.0
5.0375.00.1375.0
375.01875.0375.01875.0
'
1356..
⎥⎥⎥⎥⎥⎥
⎦
⎤
⎢⎢⎢⎢⎢⎢
⎣
⎡
−
−−−
−
−
= zzEIk
fodGlobal
(2)
On the member stiffness matrix, the corresponding global degrees of freedom are indicated to facilitate assembling. Member 2: , node points 2-3. mL 4=
[ ]
2
4
1
3
0.1375.05.0375.0
375.01875.0375.01875.0
5.0375.00.1375.0
375.01875.0375.01875.02413..
2
⎥⎥⎥⎥⎥⎥
⎦
⎤
⎢⎢⎢⎢⎢⎢
⎣
⎡
−
−−−
−
−
= zzEIk
fodGlobal
(3)
Member 3: , node points 3-4. mL 4=
Version 2 CE IIT, Kharagpur
[ ]
7
8
2
4
0.1375.05.0375.0
375.01875.0375.01875.0
5.0375.00.1375.0
375.01875.0375.01875.07824..
3
⎥⎥⎥⎥⎥⎥
⎦
⎤
⎢⎢⎢⎢⎢⎢
⎣
⎡
−
−−−
−
−
= zzEIk
fodGlobal
(4)
The assembled global stiffness matrix of the continuous beam is of the order . The assembled global stiffness matrix may be written as, 88×
[ ]
⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥
⎦
⎤
⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢
⎣
⎡
−−−
−
−
−
−−−
−−−
−
−
=
1875.0375.0001875.00375.00
375.00.100375.005.00
001875.0375.001875.00375.0
00375.00.10375.005.0
1875.0375.000375.01875.00375.0
001875.0375.01875.0375.0375.00
375.05.0000375.00.25.0
00375.05.0375.00.05.00.2
zzEIK
(5) Now it is required to replace the given members loads by equivalent joint loads. The equivalent loads for the present case is shown in Fig. 28.1d. The displacement degrees of freedom are also shown in Fig. 28.1d.
Version 2 CE IIT, Kharagpur
Thus the global load vector corresponding to unconstrained degree of freedom is,
{ }⎪⎭
⎪⎬⎫
⎪⎩
⎪⎨⎧ −
=⎪⎭
⎪⎬⎫
⎪⎩
⎪⎨⎧
=33.2
5
2
1
p
ppk (6)
Writing the load displacement relation for the entire continuous beam,
⎪⎪⎪⎪⎪⎪⎪
⎭
⎪⎪⎪⎪⎪⎪⎪
⎬
⎫
⎪⎪⎪⎪⎪⎪⎪
⎩
⎪⎪⎪⎪⎪⎪⎪
⎨
⎧
⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥
⎦
⎤
⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢
⎣
⎡
−−−
−
−
−
−−−
−−−
−
−
=
⎪⎪⎪⎪⎪⎪⎪
⎭
⎪⎪⎪⎪⎪⎪⎪
⎬
⎫
⎪⎪⎪⎪⎪⎪⎪
⎩
⎪⎪⎪⎪⎪⎪⎪
⎨
⎧ −
8
7
6
5
4
3
2
1
8
7
6
5
4
3
18703750001870037500
3750010037500500
00187037500187003750
0037500103750050
18703750003750187003750
00187037501870375037500
37505000037500250
003750503750005002
332
5
u
u
u
u
u
u
u
u
....
....
....
....
.....
.....
.....
......
EI
p
p
p
p
p
p
.
zz
(7) where is the joint load vector, { }p { }u is displacement vector.
Version 2 CE IIT, Kharagpur
We know that 0876543 ====== uuuuuu . Thus solving for unknowns and , yields
1u
2u
⎪⎭
⎪⎬⎫
⎪⎩
⎪⎨⎧
⎥⎥⎦
⎤
⎢⎢⎣
⎡=
⎪⎭
⎪⎬⎫
⎪⎩
⎪⎨⎧ −
2
1
0.25.0
5.00.2
33.2
5
u
uEI zz (8)
⎪⎭
⎪⎬⎫
⎪⎩
⎪⎨⎧ −
⎥⎥⎦
⎤
⎢⎢⎣
⎡
−
−=
⎪⎭
⎪⎬⎫
⎪⎩
⎪⎨⎧
333.2
5
0.25.0
5.00.2
75.31
2
1
zzEIu
u (9)
⎪⎭
⎪⎬⎫
⎪⎩
⎪⎨⎧−
=909.1
977.21
zzEI
Thus displacements are,
zzzz EIu
EIu 909.1 and 977.2
21 =−
= (10)
The unknown joint loads are given by,
⎪⎭
⎪⎬⎫
⎪⎩
⎪⎨⎧−
⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥
⎦
⎤
⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢
⎣
⎡
−
−
=
⎪⎪⎪⎪⎪
⎭
⎪⎪⎪⎪⎪
⎬
⎫
⎪⎪⎪⎪⎪
⎩
⎪⎪⎪⎪⎪
⎨
⎧
909.1
977.21
375.00
5.00
0375.0
05.0
0375.0
375.00
8
7
6
5
4
3
zzzz EI
EI
p
p
p
p
p
p
(11)
⎪⎪⎪⎪⎪
⎭
⎪⎪⎪⎪⎪
⎬
⎫
⎪⎪⎪⎪⎪
⎩
⎪⎪⎪⎪⎪
⎨
⎧
−
−
−=
715.0
955.0
116.1
488.1
116.1
715.0
Version 2 CE IIT, Kharagpur
The actual reactions at the supports are calculated as,
⎪⎪⎪⎪⎪
⎭
⎪⎪⎪⎪⎪
⎬
⎫
⎪⎪⎪⎪⎪
⎩
⎪⎪⎪⎪⎪
⎨
⎧
−
−
−=
⎪⎪⎪⎪⎪
⎭
⎪⎪⎪⎪⎪
⎬
⎫
⎪⎪⎪⎪⎪
⎩
⎪⎪⎪⎪⎪
⎨
⎧
−
−
−+
⎪⎪⎪⎪⎪
⎭
⎪⎪⎪⎪⎪
⎬
⎫
⎪⎪⎪⎪⎪
⎩
⎪⎪⎪⎪⎪
⎨
⎧
−
=
⎪⎪⎪⎪⎪
⎭
⎪⎪⎪⎪⎪
⎬
⎫
⎪⎪⎪⎪⎪
⎩
⎪⎪⎪⎪⎪
⎨
⎧
+
⎪⎪⎪⎪⎪
⎭
⎪⎪⎪⎪⎪
⎬
⎫
⎪⎪⎪⎪⎪
⎩
⎪⎪⎪⎪⎪
⎨
⎧
=
⎪⎪⎪⎪⎪
⎭
⎪⎪⎪⎪⎪
⎬
⎫
⎪⎪⎪⎪⎪
⎩
⎪⎪⎪⎪⎪
⎨
⎧
284.3
715.1
116.1
489.1
116.10
716.5
715.0
955.0
116.1
488.1
116.1
715.0
4
67.2
0
0
9
5
8
7
6
5
4
3
8
7
6
5
4
3
8
7
6
5
4
3
p
p
p
p
p
p
p
p
p
p
p
p
R
R
R
R
R
R
F
F
F
F
F
F
(12)
Member end actions for element 1
⎪⎪⎭
⎪⎪⎬
⎪⎨=
⎫
⎪⎪⎩
⎪⎧
−
−−
⎪⎭
⎪⎪⎬
⎫
⎪⎩
⎪⎪⎨
⎧
−
⎥⎥⎥
⎦
⎤
⎢⎢⎢
⎣
⎡
−−−−
−−
+
⎪⎪⎭
⎪⎪⎬
⎫
⎪⎩
⎪⎪⎨
⎧
=
⎪⎪⎭
⎪⎪⎬
⎫
⎪⎩
⎪⎪⎨
⎧
977.2116.1488.1116.1
977.2
00
1
0.1375.05.0375.0375.01875.375.01875.05.0375.00.1375.0
375.01875.0375.01875.0
0000
4
3
2
1
zzzzEI
qqqq
⎪⎪⎥⎢⎪⎪ 00 EI (13) Member end actions for element 2
⎪⎪⎭
⎪⎪⎬
⎫
⎪⎪⎩
⎪⎪⎨
⎧−
⎥⎥⎥⎥
⎦
⎤
⎢⎢⎢⎢
⎣
⎡
−−−−
−−
+=
⎪⎪⎭
⎪⎪⎬
⎫
⎪⎪⎩
⎪⎪⎨
⎧
909.10977.20
1
0.1375.05.0375.0375.01875.0375.01875.05.0375.00.1375.0
375.01875.0375.01875.0
4
3
2
1
zzzz EI
EI
qqqq
⎪⎪⎭
⎪⎪⎬
⎫
⎪⎪⎩
⎪⎪⎨
⎧
−
=
58.44.598.26.4
(14)
Version 2 CE IIT, Kharagpur
Member end actions for element 3
⎪⎪⎭
⎪⎪⎬
⎫
⎪⎪⎩
⎪⎪⎨
⎧
⎥⎥⎥⎥
⎦
⎤
⎢⎢⎢⎢
⎣
⎡
−−−−
−−
+
⎪⎪⎭
⎪⎪⎬
⎫
⎪⎪⎩
⎪⎪⎨
⎧
−
=
⎪⎪⎭
⎪⎪⎬
⎫
⎪⎪⎩
⎪⎪⎨
⎧
00909.10
1
0.1375.05.0375.0375.01875.0375.01875.05.0375.00.1375.0
375.01875.0375.01875.0
67.20.4
67.20.4
4
3
2
1
zzzz EI
EI
qqqq
⎪⎪⎭
⎪⎪⎬
⎫
⎪⎪⎩
⎪⎪⎨
⎧
−
=
72.128.358.472.4
(15)
Example 28.2 Analyse the continuous beam shown in Fig. 28.2a. Assume that the supports are unyielding. Assume EI to be constant for all members.
The numbering of joints and members are shown in Fig. 28.2b. The global degrees of freedom are also shown in the figure. The given continuous beam is divided into two beam elements. Two degrees of freedom (one translation and one rotation) are considered at each end of the member. In the above figure, double headed arrows denote rotations and single headed arrow represents translations. Also it is observed that displacements
from support conditions. 06543 ==== uuuuFirst construct the member stiffness matrix for each member. Member 1: , node points 1-2. mL 4=
Version 2 CE IIT, Kharagpur
The member stiffness matrix for all the members are the same, as the length and flexural rigidity of all members is the same.
(1) [ ]
1
3
5
6
0.1375.05.0375.0
375.01875.0375.01875.0
5.0375.00.1375.0
375.01875.0375.01875.0
'
1356..
⎥⎥⎥⎥⎥⎥
⎦
⎤
⎢⎢⎢⎢⎢⎢
⎣
⎡
−
−−−
−
−
= zzEIk
fodGlobal
On the member stiffness matrix, the corresponding global degrees of freedom are indicated to facilitate assembling. Member 2: , node points 2-3. mL 4=
[ ]
2
4
1
3
0.1375.05.0375.0
375.01875.0375.01875.0
5.0375.00.1375.0
375.01875.0375.01875.02413..
2
⎥⎥⎥⎥⎥⎥
⎦
⎤
⎢⎢⎢⎢⎢⎢
⎣
⎡
−
−−−
−
−
= zzEIk
fodGlobal
(2)
The assembled global stiffness matrix of the continuous beam is of order 66× . The assembled global stiffness matrix may be written as,
[ ]
⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥
⎦
⎤
⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢
⎣
⎡
−
−
−−−
−−−
−
−
=
1875.0375.001875.00375.0
375.00.10375.005.0
001875.01875.0375.0375.0
1875.0375.01875.0375.0375.00
00375.0375.00.15.0
375.05.0375.005.02
zzEIK (3)
Now it is required to replace the given members loads by equivalent joint loads. The equivalent loads for the present case is shown in Fig. 28.2c. The displacement degrees of freedom are also shown in figure.
Version 2 CE IIT, Kharagpur
Thus the global load vector corresponding to unconstrained degree of freedom is,
{ }⎪⎭
⎪⎬⎫
⎪⎩
⎪⎨⎧
=⎪⎭
⎪⎬⎫
⎪⎩
⎪⎨⎧
=67.6
0
2
1
p
ppk (4)
Writing the load displacement relation for the entire continuous beam,
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(5)
⎪⎪⎪⎪⎪
⎭
⎪⎪⎪⎪⎪
⎬
⎫
⎪⎪⎪⎪⎪
⎩
⎪⎪⎪⎪⎪
⎨
⎧
⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥
⎦
⎤
⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢
⎣
⎡
−
−
−−−
−−−
−
−
=
⎪⎪⎪⎪⎪
⎭
⎪⎪⎪⎪⎪
⎬
⎫
⎪⎪⎪⎪⎪
⎩
⎪⎪⎪⎪⎪
⎨
⎧
6
5
4
3
2
1
6
5
4
3
1875.0375.001875.00375.0
375.00.10375.005.0
001875.01875.0375.0375.0
1875.0375.01875.0375.0375.00
00375.0375.00.15.0
375.05.0375.005.02
67.6
0
u
u
u
u
u
u
EI
p
p
p
pzz
We know that 06543 ==== uuuu . Thus solving for unknowns and , yields 1u 2u
⎪⎭
⎪⎬⎫
⎪⎩
⎪⎨⎧
⎥⎥⎦
⎤
⎢⎢⎣
⎡=
⎪⎭
⎪⎬⎫
⎪⎩
⎪⎨⎧
2
1
0.15.0
5.00.2
67.6
0
u
uEI zz (6)
⎪⎭
⎪⎬⎫
⎪⎩
⎪⎨⎧
⎥⎥⎦
⎤
⎢⎢⎣
⎡
−
−=
⎪⎭
⎪⎬⎫
⎪⎩
⎪⎨⎧
67.6
0
0.25.0
5.00.1
75.11
2
1
zzEIu
u
⎪⎭
⎪⎬⎫
⎪⎩
⎪⎨⎧−
=627
90511
.
.
EI zz
(7)
Thus displacements are,
zzzz EIu
EIu 62.7 and 905.1
21 =−
=
The unknown joint loads are given by,
⎪⎭
⎪⎬⎫
⎪⎩
⎪⎨⎧−
⎥⎥⎥⎥⎥⎥
⎦
⎤
⎢⎢⎢⎢⎢⎢
⎣
⎡
−−=
⎪⎪⎪
⎭
⎪⎪⎪
⎬
⎫
⎪⎪⎪
⎩
⎪⎪⎪
⎨
⎧
62.7
905.11
0375.0
05.0
375.0375.0
375.00
6
5
4
3
zzzz EI
EI
p
p
p
p
Version 2 CE IIT, Kharagpur
⎪⎪⎪
⎭
⎪⎪⎪
⎬
⎫
⎪⎪⎪
⎩
⎪⎪⎪
⎨
⎧
−
−
−=
714.0
95.0
14.2
857.2
(8)
The actual support reactions are,
⎪⎪⎪
⎭
⎪⎪⎪
⎬
⎫
⎪⎪⎪
⎩
⎪⎪⎪
⎨
⎧
−=
⎪⎪⎪
⎭
⎪⎪⎪
⎬
⎫
⎪⎪⎪
⎩
⎪⎪⎪
⎨
⎧
−
−
−+
⎪⎪⎪
⎭
⎪⎪⎪
⎬
⎫
⎪⎪⎪
⎩
⎪⎪⎪
⎨
⎧
−=
⎪⎪⎪
⎭
⎪⎪⎪
⎬
⎫
⎪⎪⎪
⎩
⎪⎪⎪
⎨
⎧
286.9
62.7
86.7
857.22
714.0
95.0
14.2
857.2
10
67.6
10
20
6
5
4
3
R
R
R
R
(9)
Member end actions for element 1
⎪⎪⎭
⎪⎪⎬
⎫
⎪⎪⎩
⎪⎪⎨
⎧
−⎥⎥⎥⎥
⎦
⎤
⎢⎢⎢⎢
⎣
⎡
−−−−
−−
+
⎪⎪⎭
⎪⎪⎬
⎫
⎪⎪⎩
⎪⎪⎨
⎧
−
=
⎪⎪⎭
⎪⎪⎬
⎫
⎪⎪⎩
⎪⎪⎨
⎧
905.1000
1
0.1375.05.0375.0375.01875.0375.01875.05.0375.00.1375.0
375.01875.0375.01875.0
66.61066.6
10
4
3
2
1
zzzz EI
EI
qqqq
⎪⎪⎭
⎪⎪⎬
⎫
⎪⎪⎩
⎪⎪⎨
⎧
−
=
565.8714.10707.5285.9
(10)
Member end actions for element 2
⎪⎪⎭
⎪⎪⎬
⎫
⎪⎪⎩
⎪⎪⎨
⎧−
⎥⎥⎥⎥
⎦
⎤
⎢⎢⎢⎢
⎣
⎡
−−−−
−−
+
⎪⎪⎭
⎪⎪⎬
⎫
⎪⎪⎩
⎪⎪⎨
⎧
−
=
⎪⎪⎭
⎪⎪⎬
⎫
⎪⎪⎩
⎪⎪⎨
⎧
62.70905.10
1
0.1375.05.0375.0375.01875.0375.01875.05.0375.00.1375.0
375.01875.0375.01875.0
66.61066.6
10
4
3
2
1
zzzz EI
EI
qqqq
⎪⎪⎭
⎪⎪⎬
⎫
⎪⎪⎩
⎪⎪⎨
⎧
=
0856.7565.814.12
Version 2 CE IIT, Kharagpur
Summary In the previous lesson the beam element stiffness matrix is derived from fundamentals. Assembling member stiffness matrices, the global stiffness matrix is generated. The procedure to impose boundary conditions on the load-displacement relation is discussed. In this lesson, a few continuous beam problems are analysed by the direct stiffness method.
Version 2 CE IIT, Kharagpur