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Module
1
Energy Methods in
Structural Analysis
Version 2 CE IIT, Kharagpur

Lesson
3
Castigliano’s Theorems

Instructional Objectives
After reading this lesson, the reader will be able to;
1. State and prove first theorem of Castigliano.
2. Calculate deflections along the direction of applied load of a statically
determinate structure at the point of application of load.
3. Calculate deflections of a statically determinate structure in any direction at a
point where the load is not acting by fictious (imaginary) load method.
4. State and prove Castigliano’s second theorem.

3.1 Introduction
In the previous chapter concepts of strain energy and complementary strain
energy were discussed. Castigliano’s first theorem is being used in structural
analysis for finding deflection of an elastic structure based on strain energy of the
structure. The Castigliano’s theorem can be applied when the supports of the
structure are unyielding and the temperature of the structure is constant.

3.2 Castigliano’s First Theorem
For linearly elastic structure, where external forces only cause deformations, the
complementary energy is equal to the strain energy. For such structures, the
Castigliano’s first theorem may be stated as the first partial derivative of the
strain energy of the structure with respect to any particular force gives the
displacement of the point of application of that force in the direction of its line of
action.


Let P1 , P2 ,...., Pn be the forces acting at x1 , x 2 ,......, x n from the left end on a simply
supported beam of span L . Let u1 , u 2 ,..., u n be the displacements at the loading
points P1 , P2 ,...., Pn respectively as shown in Fig. 3.1. Now, assume that the
material obeys Hooke’s law and invoking the principle of superposition, the work
done by the external forces is given by (vide eqn. 1.8 of lesson 1)

1 1 1
W = P1u1 + P2 u 2 + .......... + Pn u n (3.1)
2 2 2


Work done by the external forces is stored in the structure as strain energy in a
conservative system. Hence, the strain energy of the structure is,

1 1 1
U= P1u1 + P2 u 2 + .......... + Pn u n (3.2)
2 2 2

Displacement u1 below point P1 is due to the action of P1 , P2 ,...., Pn acting at
distances x1 , x 2 ,......, x n respectively from left support. Hence, u1 may be expressed
as,

u1 = a11 P1 + a12 P2 + .......... + a1n Pn (3.3)

In general,

u i = ai1 P1 + ai 2 P2 + .......... + ain Pn i = 1,2,...n (3.4)

where a ij is the flexibility coefficient at i due to unit force applied at j .
Substituting the values of u1 , u 2 ,..., u n in equation (3.2) from equation (3.4), we
get,

1 1 1
U= P1 [ a11 P1 + a12 P2 + ...] + P2 [ a 21 P1 + a 22 P2 + ...] + ....... + Pn [ a n1 P1 + a n 2 P2 + ...] (3.5)
2 2 2

We know from Maxwell-Betti’s reciprocal theorem a ij = a ji . Hence, equation (3.5)
may be simplified as,

1
U= ⎡ a11 P 2 + a22 P22 + .... + ann Pn2 ⎤ + [ a12 P P2 + a13 P P3 + .... + a1n P Pn ] + ... (3.6)
2⎣ ⎦
1 1 1 1

Now, differentiating the strain energy with any force P1 gives,

∂U
= a11 P1 + a12 P2 + .......... + a1n Pn (3.7)
∂P1

It may be observed that equation (3.7) is nothing but displacement u1 at the
loading point.
In general,
∂U
= un (3.8)
∂Pn

Hence, for determinate structure within linear elastic range the partial derivative
of the total strain energy with respect to any external load is equal to the


displacement of the point of application of load in the direction of the applied
load, provided the supports are unyielding and temperature is maintained
constant. This theorem is advantageously used for calculating deflections in
elastic structure. The procedure for calculating the deflection is illustrated with
few examples.

Example 3.1
Find the displacement and slope at the tip of a cantilever beam loaded as in Fig.
3.2. Assume the flexural rigidity of the beam EI to be constant for the beam.

Moment at any section at a distance x away from the free end is given by

M = − Px (1)

L
M2
Strain energy stored in the beam due to bending is U = ∫ dx (2)
0
2 EI

Substituting the expression for bending moment M in equation (3.10), we get,

L
( Px) 2 P 2 L3
U =∫ dx = (3)
0
2 EI 6 EI


Now, according to Castigliano’s theorem, the first partial derivative of strain
energy with respect to external force P gives the deflection u A at A in the
direction of applied force. Thus,

∂U PL3
= uA = (4)
∂P 3EI

To find the slope at the free end, we need to differentiate strain energy with
respect to externally applied moment M at A . As there is no moment at A , apply
a fictitious moment M 0 at A . Now moment at any section at a distance x away
from the free end is given by

M = − Px − M 0

Now, strain energy stored in the beam may be calculated as,

( Px + M 0 ) 2
L 2
P 2 L3 M 0 PL2 M 0 L
U =∫ dx = + + (5)
0
2 EI 6 EI 2 EI 2 EI

Taking partial derivative of strain energy with respect to M 0 , we get slope at A .

∂U PL2 M 0 L
= θA = + (6)
∂M 0 2 EI EI

But actually there is no moment applied at A . Hence substitute M 0 = 0 in
equation (3.14) we get the slope at A.

PL2
θA = (7)
2 EI

Example 3.2
A cantilever beam which is curved in the shape of a quadrant of a circle is loaded
as shown in Fig. 3.3. The radius of curvature of curved beam is R , Young’s
modulus of the material is E and second moment of the area is I about an axis
perpendicular to the plane of the paper through the centroid of the cross section.
Find the vertical displacement of point A on the curved beam.


The bending moment at any section θ of the curved beam (see Fig. 3.3) is given
by

M = PR sinθ (1)

Strain energy U stored in the curved beam due to bending is,
π /2
P 2 R 2 (sin 2 θ ) Rdθ P 2 R 3 π π P 2 R 3
s
M2
U =∫ ds = ∫ = = (2)
0
2 EI 0
2 EI 2 EI 4 8 EI

Differentiating strain energy with respect to externally applied load, P we get

∂U b π PR 3
uA = = (3)
∂P 4 EI

Example 3.3
Find horizontal displacement at D of the frame shown in Fig. 3.4. Assume the
flexural rigidity of the beam EI to be constant through out the member. Neglect
strain energy due to axial deformations.


The deflection D may be obtained via. Castigliano’s theorem. The beam
segments BA and DC are subjected to bending moment Px ( 0 < x < L ) and the
beam element BC is subjected to a constant bending moment of magnitude PL .

Total strain energy stored in the frame due to bending

L L
( Px) 2 ( PL) 2
U = 2∫ dx + ∫ dx (1)
0
2 EI 0
2 EI
After simplifications,

P 2 L3 P 2 L3 5P 2 L3
U= + = (2)
3EI 2 EI 6 EI

Differentiating strain energy with respect to P we get,

∂U 5 P L3 5 P L3
= uD = 2 =
∂P 6 EI 3EI


Example 3.4
Find the vertical deflection at A of the structure shown Fig. 3.5. Assume the
flexural rigidity EI and torsional rigidity GJ to be constant for the structure.

The beam segment BC is subjected to bending moment Px ( 0 < x < a ; x is
measured from C )and the beam element AB is subjected to torsional moment of
magnitude Pa and a bending moment of Px ( 0 ≤ x ≤ b ; x is measured from B) . The
strain energy stored in the beam ABC is,

a b
M2 ( Pa) 2 b ( Px)
2
U =∫ dx + ∫ dx + ∫ dx (1)
0
2 EI 0
2GJ 0 2 EI

After simplifications,

P 2 a 3 P 2 a 2b P 2b3
U= + + (2)
6 EI 2GJ 6 EI

Vertical deflection u A at A is,

∂U Pa 3 Pa 2 b Pb 3
= uA = + + (3)
∂P 3EI GJ 3EI


Example 3.5
Find vertical deflection at C of the beam shown in Fig. 3.6. Assume the flexural
rigidity EI to be constant for the structure.

The beam segment CB is subjected to bending moment Px ( 0 < x < a ) and
beam element AB is subjected to moment of magnitude Pa .
To find the vertical deflection at C , introduce a imaginary vertical force Q at C .
Now, the strain energy stored in the structure is,

( Pa + Qy) 2
a b
( Px) 2
U =∫ dx + ∫ dy (1)
0
2 EI 0
2 EI

Differentiating strain energy with respect to Q , vertical deflection at C is obtained.

∂U 2( Pa + Qy ) y
b
= uC = ∫ dy (2)
∂Q 0
2 EI

b
1
EI ∫
uC = Pay + Qy 2 dy (3)
0


1 ⎡ Pab 2 Qb 3 ⎤
uC = ⎢ + ⎥ (4)
EI ⎣ 2 3 ⎦

But the force Q is fictitious force and hence equal to zero. Hence, vertical
deflection is,

Pab 2
uC = (5)
2 EI

3.3 Castigliano’s Second Theorem
In any elastic structure having n independent displacements u1 , u 2 ,..., u n
corresponding to external forces P1 , P2 ,...., Pn along their lines of action, if strain
energy is expressed in terms of displacements then n equilibrium equations may
be written as follows.

∂U
= Pj , j = 1, 2,..., n (3.9)
∂u j

This may be proved as follows. The strain energy of an elastic body may be
written as

1 1 1
U= P1u1 + P2 u 2 + .......... + Pn u n (3.10)
2 2 2

We know from Lesson 1 (equation 1.5) that

Pi = ki1u1 + ki 2u2 + ..... + kinun , i = 1, 2,.., n (3.11)

where kij is the stiffness coefficient and is defined as the force at i due to unit
displacement applied at j . Hence, strain energy may be written as,

1 1 1
U= u1[k11u1 + k12u2 + ...] + u2 [ k21u1 + k22u2 + ...] + ....... + un [k n1u1 + kn 2u2 + ...] (3.12)
2 2 2

We know from reciprocal theorem kij = k ji . Hence, equation (3.12) may be
simplified as,

1
U= ⎡ k11u12 + k22u2 + .... + knn un ⎤ + [ k12u1u2 + k13u1u3 + .... + k1n u1un ] + ...
⎣
2 2
⎦ (3.13)
2


Now, differentiating the strain energy with respect to any displacement u1 gives
the applied force P at that point, Hence,
1

∂U
= k11u1 + k12 u2 + ........ + k1n un (3.14)
∂u1

Or,

∂U
= Pj , j = 1, 2,..., n (3.15)
∂u j

Summary
In this lesson, Castigliano’s first theorem has been stated and proved for linearly
elastic structure with unyielding supports. The procedure to calculate deflections
of a statically determinate structure at the point of application of load is illustrated
with examples. Also, the procedure to calculate deflections in a statically
determinate structure at a point where load is applied is illustrated with examples.
The Castigliano’s second theorem is stated for elastic structure and proved in
section 3.4.


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