Dr. Mahalingam College of Engineering & Technology
Pollachi- 642003
CONTINUOUS ASSESSMENT TEST β III - KEY
Class & Branch
Sub. Code & Name
Semester
: III B.E. - CIVIL
: STRUCTURAL ANALYSIS - I
:V
Max. Marks
Time
Date
: 50
:
: 29/10/2011
PART β A (10 x 2 = 20 Marks)
Answer all questions :
1. What is moment distribution method?
The moment distribution method is a structural analysis method for statically
indeterminate beams and frames developed by Hardy Cross in 1930. In the moment
distribution method, every joint of the structure to be analysed is considered as fixed
so as to develop the fixed-end moments due to loads. Then after release the known
moments at joints, each fixed joints having more than one member are balanced for
unbalanced moment. The balance moments are distributed according to the
distribution factors of members and moments are carried over to other end. The
iteration is repeated until equilibrium is achieved.
2. What is the difference between absolute and relative stiffness?
Absolute stiffness is represented in terms of E (youngβs modulus), I (Moment
of Inertia) and L (Length of member) to indicate the moment/force required
4πΈπΌ
such as πΏ to produce unit slope/deflection/deformation. Whereas relative
stiffness is represented in terms of I and L, omitting constant E (when
structural elements connected to joints are made of same material).
3. Briefly explain stiffness factor and distribution factor.
Stiffness factor
Stiffness factor is the moment required to produce an unit rotation at that end without
producing translation at the far end.
Distribution factor
When several members meet at a joint and a moment is applied at the joint to
produce rotation without translation of the members, the moment is distributed
among all the members meeting at that joint proportionate to their stiffness.
stiffness of the member
Distribution factor =
Sum of stiffness of members at the joint
4. What is carry over moment and carry over factor?
Carry over moment
It is defined as the moment induced at the fixed end of the beam by the action of a
moment applied at the other end, which is hinged. Carry over moment is the same
nature of the applied moment.
Carry over factor ( C.O.F)
A moment applied at the hinged end B β carries overβ to the fixed end A, a moment
equal to half the amount of applied moment and of the same rotational sense.
C.O =0.5
5. Explain term balancing in the moment distribution method.
Algebraic sum of clockwise and counter-clockwise moments at any internal joint
should be zero for equilibrium. When this sum is not zero, equivalent and opposite
moment is applied at that joint to achieve the equilibrium. This step is called
balancing.
6. Three members meeting at a rigid joint carry moments 5 kNm(clockwise), 10kNm
(clockwise) and 9kNm (counter-clockwise). What moment should be applied to
balance?
6kNm (Counter-clockwise)
7. Distribute an applied balancing moment of 12kNm at a joint among three members
whose relative stiffnesses are 1/6, 1/3 and 1/2.
2kNm, 4kNm and 6kNm respectively for the members having relative stiffnesses of
1/6, 1/3 and 1/2.
8. An udl of 50kNm acting on one the span of two span continuous beam which is
symmetrical and each span length is l. Sketch the equivalent load interms of
symmetric and anti-symmetric loads.
25 kN/m
A
25 kN/m
25 kN/m
B
l
C
A
l
l
C
B
l
25 kN/m
Symmetrical loads
Anti-symmetrical loads
9. Sketch the reduced structure with support condition for the two span symmetric portal
whose columns are fixed at the ground.
B
B
C
C
For symmetrical
loading
For Anti-symmetrical
loading
A
A
10. What is Naylorβs method?
In the naylors method (also called cantilever moment distribution method), for
symmetrical frames, columns are assumed as one end is free to rotate and translate
and other end as fixed (similar to cantilever). The end moments are calculated based
on this assumption and then the balancing and carry over is iteratively done by
suitably modifying the member stiffness. In this method, sway analysis is completely
avoided.
PART β B ( 3 x 10 = 30 Marks)
Answer any three of the following
11. Analyse the portal frame shown in fig. 1 using slope deflection method and
sketch the bending moment and shear force diagram.
(10)
Solution Outline
48 kN
1. Find fixed end moments due to loads
2. Write slope-deflection equations
B
( Io )
3. Obtain joint equilibrium equations
2m
2m
6 kN/m
4. Obtain shear equation.
4m
5. Solve joint equilibrium equations and shear
equation to get the slopes and deflections
(Io)
6m
6. Find final end moments by substituting slopes
and deflections in slope-deflection equations.
A
7. Draw the bending moment diagram
Fig. 1
8. Draw the shear force diagram
C
10 kN/m
(2Io )
D
DETAIL SOLUTION
Step 1: Find fixed end moments due to loads
ππΉπ΄π΅ = β
ππΉπ΅πΆ = β
π€π2
12
ππ
8
=β
=β
6×42
12
48×4
8
= β8 πππ
= β24 πππ ;
;
ππΉπ΅π΄ = +
ππΉπΆπ΅ = +
π€π2
12
ππ
8
=+
6×42
12
= +8 πππ
= +24πππ
ππΉπΆπ· = β
π€π2
12
=β
10×62
12
= β30πππ;
ππΉπ·πΆ =
π€π2
12
=
10×62
12
= 30πππ
Step 2: Slope-deflection equations
In this problem
4πΈπΌπ ππ΄ 2πΈπΌπ ππ΅ 6πΈπΌπ πΏ
EπΌπ ππ΅ 3EπΌπ Ξ΄
+
β 2 = β8 +
β
ππ΄ = 0 πππ
π
π
π
2
8
ππ· = 0
2πΈπΌπ ππ΄ 4πΈπΌπ ππ΅ 6πΈπΌπ πΏ
3EπΌπ Ξ΄
ππ΅π΄ = ππΉπ΅π΄ +
+
β 2 = 8 + EπΌπ ππ΅ β
π
π
π
8
4πΈπΌπ ππ΅ 2πΈπΌπ ππΆ
πΈπΌπ ππΆ
ππ΅πΆ = ππΉπ΅πΆ +
+
= β24 + πΈπΌπ ππ΅ +
π
π
2
2πΈπΌπ ππ΅ 4πΈπΌπ ππΆ
πΈπΌπ ππ΅
ππΆπ΅ = ππΉπΆπ΅ +
+
= 24 +
+ πΈπΌπ ππΆ
π
π
2
4πΈ(2πΌπ )ππΆ 2πΈ(2πΌπ )ππ· 6πΈ(2πΌπ )πΏ
4πΈπΌπ ππΆ πΈπΌπ πΏ
ππΆπ· = ππΉπΆπ· +
+
β
= β30 +
β
2
π
π
π
3
3
2πΈ(2πΌπ )ππΆ 4πΈ(2πΌπ )ππ· 6πΈ(2πΌπ )πΏ
2πΈπΌπ ππΆ πΈπΌπ πΏ
ππ·πΆ = ππΉπ·πΆ +
+
β
=
30
+
β
π
π
π2
3
3
Step 3: joint equilibrium equations
ππ΄π΅ = ππΉπ΄π΅ +
Joint B
ππ΅π΄ + ππ΅πΆ = 0 ο β16 + 2πΈπΌπ ππ΅ +
πΈπΌπ ππΆ
2
β
3πΈπΌπ πΏ
8
=0
Joint C
ππΆπ΅ + ππΆπ· = 0 ο β6 +
πΈπΌπ ππ΅
2
+
7πΈπΌπ ππΆ
3
β
πΈπΌπ πΏ
3
=0
Step 4: Shear equation
π»π΄ + π»π· + 24 β 60 = 0
HA =
MAB +MBA
4
β
24×2
4
and HD =
MCD +MDC
6
+
60×3
6
Substituting π»π΄ πππ π»π· and Substituting moments interms of slope and deflection, the
shear equation is obtained as follows:
3πΈπΌπ ππ΅ πΈπΌπ ππΆ 43πΈπΌπ πΏ
+
β
β 18 = 0
8
3
144
Step 5: Solve joint equilibrium equations along with shear equation to get the slopes
and deflections
1523
5421
14103
πΈπΌπ ππ΅ = β 386 , πΈπΌπ ππΆ = β 772 and πΈπΌπ πΏ = β 193 are obtained by solving joint
equilibrium equations and shear equation.
Step 6: Final Fixd end moments
Substituting πΈπΌπ ππ΅ = β3.946, πΈπΌπ ππΆ = β7.022 and πΈπΌπ πΏ = β73.073 in slopedeflection equations, we get,
ππ΄π΅ = 17.43πππ ; ππ΅π΄ = 31.46πππ
ππ΅πΆ = β31.46πππ; ππΆπ΅ = +15.0πππ
ππΆπ· = β15.0πππ; ππ·πΆ = +49.68πππ
Step 7: Draw the bending moment diagram
Step 8: Draw the shear force diagram
17.43+31.46
24×2
HA =
β 4 = 0.22kN
4
HD =
β15.0 + 49.68 60 × 3
+
= 35.78kN
6
6
Find RA and RD, then draw shear force diagram
12. Analyse the structure loaded as in fig. 2. by the moment distribution method and
sketch the bending moment and shear force diagram
(10)
Solution Outline
20kN/m
1. Find fixed end moments due to loads
80 kN
2. Determine distribution factors for members
B
A
at each internal joint
(2I)
(I)
3m
2m
2m
3. Tabulate initial fixed end moments, release
or apply known moments
4. Balance the unbalanced moment and carry over
(2I) 4m
Fig. 2
the moments.
5. If the carry over moments unbalances the
D
joint equilibrium, do step 4 and 5 repeatedly
till unbalanced moment is negligible.
6. Find out the final end moment by summing all the initial, balanced moments and
carry over moments for each end of the span.
7. Draw the bending moment diagram
8. Draw the shear force diagram
DETAIL SOLUTION
Step 1: Find fixed end moments due to loads
ππΉπ΄π΅ = β
ππΉπ΅πΆ = β
π€ππ 2
π2
π€π2
12
=β
=β
ππΉπ΅π· = 0πππ;
80×3×22
52
20×22
12
= β38.4πππ;
= β6.67πππ ;
π€π2 π
ππΉπ΅π΄ =
π2
π€π2
ππΉπΆπ΅ =
12
=
=
80×32 ×2
52
20×22
12
= 57.6πππ
= 6.67πππ
ππΉπ·π΅ = 0πππ
Step 2: Determine distribution factors for members at each internal joint
Joint
Member
BA
B
BC
BD
Stiffness
Total Stiffness
3πΈπΌ 3πΈ 2πΌ
=
= 1.2πΈπΌ
π
5
4πΈπΌ 4πΈ πΌ
=
= 2πΈπΌ
π
2
4πΈπΌ 4πΈ 2πΌ
=
= 2πΈπΌ
π
4
Distribution Factor
0.2308
5.2πΈπΌ
0.3846
0.3846
Steps 3 to 6
Joint
A
Member
C.O.F
AB
BA
BD
0
0.5
0.5
D.F
1
0.2308
0.3846
FEM
Release
C/O
-38.4
Initial
Moment
B
57.6
C
D
BC
CB
DB
0.5
0.5
0.5
0.3846
1
1
0
-6.7
6.7
0
0
-6.7
6.7
0
-27
-27
+38.4
19.2
0
76.8
Balance 1
C/O 1
Final End
0
moment
-16.2
-13.5
60.6
-27
-33.7
-6.8
-13.5
-13.5
C
Step 7: Draw the bending moment diagram
Step 8: Draw the shear force diagram
Find π
π΄ , π
π΅ , π
πΆ πππ π»π·
Taking moment about B, considering left side.
π
π΄ × 5 β 80 × 2 = β60.6 ο π
π΄ = 19.88ππ
Taking moment about B, considering right side.
20×22
π
πΆ × 2 β 2 + 6.8 = β33.7 ο π
πΆ = β0.25ππ
From β V = 0
π
π΅ = 120 β 19.88 + 0.25 = 100.37ππ
Taking moment about B, considering forces and moments below.
π»π· × 4 + 13.5 = β27πππ ο π»π· = β10.25ππ (ie) 10.25kN(ο)
13. Analyse the structure loaded as in fig. 3. by the moment distribution method and
sketch the bending moment and shear force diagram
(10)
50 kN/m
C
B
(I)
50 kN/m
(I)
3m
E
3m
Fig. 3
3m ( I )
(2I)
(2I)
4m
4m
D
F
A
Solution Outline
1. Considering symmetric structure, analyzing half of the structure is enough to find all
the necessary design parameters (BM and SF). So, Consider half of the structure
with fixed end at C, since there is support at C (Modification of stiffness is not
necessary).
2. Find fixed end moments due to loads
3. Determine distribution factors for members at each internal joint
4. Tabulate initial fixed end moments
5. Balance the unbalanced moment and carry over the moments.
6. If the carry over moments unbalances the joint equilibrium, do step 4 and 5
repeatedly till unbalanced moment is negligible.
7. Find out the final end moment by summing all the initial, balanced moments and
carry over moments for each end of the span.
8. Draw the bending moment diagram
50 kN/m
9. Draw the shear force diagram
B
STEP 1
C
DETAIL SOLUTION
(I)
3m
Step 1: Consider half of the structure with fixed end at C
Step 2: Find fixed end moments due to loads
(2I)
ππΉπ΄π΅ = 0πππ;
4m
ππΉπ΅π΄ = 0πππ
ππΉπ΅πΆ = β
ππΉπΆπ΅ =
π€π2
12
π€π2
12
=
=β
50×32
50×32
12
12
= β37.5πππ ;
A
= 37.5πππ
Step 3: Determine distribution factors for members at each internal joint
Joint
B
Member
BA
BC
Stiffness
4πΈπΌ 4πΈ 2πΌ
=
= 2πΈπΌ
π
4
4πΈπΌ 4πΈ πΌ
=
= 1.333πΈπΌ
π
3
Total Stiffness
Distribution Factor
0.6
3.333πΈπΌ
0.4
Steps 4 to 7
Joint
A
Member
AB
BA
C.O.F
D.F
0.5
1
B
Initial FEM 0
Balance 1
C/O 1
11.25
Final End
11.25
moment
C
C
E
F
BC
CB
CE
EC
0.5
0.5
0.5
0.6
0.4
1
0
-37.5
37.5
22.5
15
-45
22.5 -22.5
EF
CB
7.5
22.5
-22.5
45
Step 8: Draw the bending moment diagram
Step 9: Draw the Shear Force diagram
Find π
π΄ , π
πΆ πππ π»π΄
Taking moment about C, considering left side forces and moments.
50×32
π
π΅ × 3 β 22.5 β 2 = β45 ο π
π΅ = 67.5ππ. Because of symmetry, π
πΈ = 67.5ππ
From β V = 0
π
πΆ = 300 β 67.5 β 67.5 = 165ππ
Taking moment about B, considering forces and moments below.
π»π΄ × 4 + 11.25 = β22.5πππ ο π»π΄ = β8.44ππ (ie) 8.84kN(ο)
-11.25
14. Analyse the portal frame shown in fig. 4 using Naylorβs method and sketch the
bending moment and shear force diagram.
(10)
By modifying the stiffness of member in the symmetric line,
100 kN
number of iterations can be reduced. Hence, advantage of the
50 kN/m
symmetrical structure is also used in the solution.
(I)
B
C
Solution Outline
2m
1. Equivalent loads in terms of symmetric and
3m
anti-symmetric loading
Fig. 4
2. Analysis for symmetric loading (Moment Distribution method)
4πΈπΌ
3.
4.
5.
6.
3πΈπΌ
a. Find stiffnesses (for fixed end
, for hinged
) and
4m
(2I)
(2I)
π
π
4m
modify the stiffness of member which is at symmetric
line if no support (multiply with 0.5 for symmetrical loads)
b. Determine distribution factors for members and
c. carry over factors (0.5 for fixed end, 0 for hinged)
D
d. Find fixed end moments due to symmetrical loads
A
e. Tabulate initial fixed end moments
f. Balance the moment and carry over the moments.
g. If the carry over moments unbalances significantly the
joint equilibrium, repeat balancing process.
h. Find out the total end moments for symmetrical loads by
summing all the initial, balanced moments and carry
over moments for each end of the span.
i. For other half of the structure, take the opposite moment at the corresponding ends.
Naylor method for anti-symmetric loading
a. Find stiffnesses (treat vertical members as cantilever β Stiffness for the cantilever is
πΈπΌ
4πΈπΌ
3πΈπΌ
, for fixed end
, for hinged
and modify the stiffness of member which is at
π
π
π
symmetric line if not support (multiply with 1.5 for anti-symmetrical loads)
b. Determine distribution factors for members
c. carry over factors (0.5 for fixed end, 0 for hinged and -1 for cantilever)
d. Find fixed end moments due to anti-symmetrical loads
e. Tabulate initial fixed end moments
f. Balance the moment and carry over the moments.
g. If the carry over moments unbalances significantly the joint equilibrium, repeat
balancing process.
h. Find out the total end moments for anti-symmetrical loads
i. For other half of the structure, take the same moment at the corresponding ends.
Obtain final end moment by adding moments from step 2 and step 3
Draw the bending moment diagram
Draw the shear force diagram
Step 1 : Equivalent symmetric and anti-symmetric loading for a given load
50 kN
50 kN
50 kN
50 kN/m
C
B
B
50 kN
3m
(I) 2m
3m
(2I)
A
4m
C
(I)
(2I)
D
Symmetrical Loading
4m
(2I)
A
4m
(2I)
4m
D
Anti-Symmetrical Loading
Step 2 : Analysis for symmetric loading (Moment Distribution method)
4πΈπΌ
3πΈπΌ
3a. Find stiffnesses (for fixed end π , for hinged π ) and modify the stiffness of member
which is at symmetric line if no support (multiply with 0.5 for symmetrical loads)
3b. Determine distribution factors for members
Joint
B
Member
BA
Stiffness
Total Stiffness
4πΈπΌ 4πΈ 2πΌ
=
= 2πΈπΌ
π
4
4πΈπΌ 4πΈ πΌ
BC
=
(Unmodified)
π
3
4πΈ πΌ
BC*
β 0.5 = 0.667πΈπΌ
(Modified)
3
Distribution Factor
0.75
2.667πΈπΌ
0.25
3c. carry over factors (0.5 for fixed end, 0 for hinged)
3d. Find fixed end moments due to symmetrical loads
ππΉπ΄π΅ = 0πππ; ππΉπ΅π΄ = 0πππ
ππΉπ΅πΆ = β
π€π2
12
β
π1 π1 π12
π2
β
π2 π2 π22
π2
=β
50×32
12
β
50×1×22
32
β
50×2×12
32
= β70.83πππ
(Moment for one-half the structure is enough when advantage of symmetric structure is
considered)
Steps 2e to 2i
Joint
A
Member
AB
BA
BC
C.O.F
D.F
0.5
0.5
0.5
1
0.75
0.25
0
-70.83
53.12
17.71
B
Initial FEM 0
Balance 1
C/O 1
26.56
Total End
26.56
moment
53.12
C
CB
-53.12
53.12
D
CD
CB
D
-53.12
22.5 -26.56
Step 3 : Naylor method for anti-symmetric loading
3a. Find stiffnesses (treat vertical members as cantilever β Stiffness for the cantilever is
4πΈπΌ
3πΈπΌ
πΈπΌ
,
π
for fixed end π , for hinged π and modify the stiffness of member which is at symmetric
line if not support (multiply with 1.5 for anti-symmetrical loads)
3b. Determine distribution factors for members
Joint
Member
Stiffness
Total Stiffness Distribution Factor
πΈπΌ πΈ 2πΌ
BA
0.2
=
= 0.5πΈπΌ
π
4
4πΈπΌ 4πΈ πΌ
BC
B
=
(Unmodified)
π
3
4πΈ πΌ
BC*
β 1.5 = 2.0πΈπΌ
(Modified)
3
3c. carry over factors (0.5 for fixed end, 0 for hinged)
3d. Find fixed end moments due to symmetrical loads
2.5πΈπΌ
0.8
ππΉπ΄π΅ = 0πππ; ππΉπ΅π΄ = 0πππ
ππΉπ΅πΆ = β
π1 π1 π12
π2
+
π2 π2 π22
π2
=β
50×1×22
32
+
50×2×12
32
= β11.11πππ
(Moment for one-half the structure is enough when advantage of symmetric structure is
considered)
Steps 3e to 3i
Joint
A
Member
AB
BA
BC
C.O.F
-1
-1
0.5
D.F
1
0.2
0.8
0
-11.11
2.22
8.89
B
Initial FEM 0
Balance 1
C/O 1
-2.22
Total End
-2.22
moment
2.22
C
CB
-2.22
-2.22
D
CD
CB
D
2.22 22.5 -2.22
Step 4: Obtain final end moment by adding moments from step 2 and step 3
A
Joint
Member
B
AB
BA
C
BC
CB
Moment for
symmetrical load
26.56
53.12
-53.12
53.12
Moment for Antisymmetrical load
-2.22
2.22
-2.22
-2.22
55.34
-55.34
50.90
Final End moment
for a given load
24.34
D
CD
CB
D
-53.12
22.5 -26.56
2.22
22.5
-2.22
-50.90
22.5 -28.78
Step 5: Draw the bending moment diagram
No loads at Vertical members. 2 loads are acting at beam, hence, sagging moments due to
load at salient points in the beam are as shown below:
Considering simply supported beam, reaction at left support is 75+66.67=141.67kN
π΅ππ₯ππππ ππππ‘
π΅ππ₯ππππ ππππ‘
50 × π₯ 2
= 141.67π₯ β
πππ π₯ < 1
2
2
50 × π₯
= 141.67π₯ β
β 100 × (π₯ β 1)πππ 1 β€ π₯ β€ 3
2
Mathematica β Code snippet to plot piecewise function
Plot[Piecewise[{{141.67π₯ β
50π₯ 2
50π₯ 2
, π₯ <= 1}, {141.67π₯ β
β 100(π₯ β 1), π₯ > 1}}], {π₯, 0,3}]
2
2
100
80
60
40
20
0.5
1.0
1.5
2.0
2.5
3.0
Step 6: Draw the Shear Force diagram
Find π
π΄ = 141.67 π
π· = 108.33
HA =
HD =
23.34+55.34
4
β50β28.78
4
= 19.67kN (ie) 19.67kN(ο )
= β19.67kN (ie) 19.67kN(ο)
---- End ----
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