Module 7 Simple Beam Theory - Massachusetts Institute of Technology

[Pages:22]Module 7 Simple Beam Theory

Learning Objectives

? Review simple beam theory ? Generalize simple beam theory to three dimensions and general cross sections ? Consider combined effects of bending, shear and torsion ? Study the case of shell beams

7.1 Review of simple beam theory

Readings: BC 5 Intro, 5.1

A beam is a structure which has one of its dimensions much larger than the other two. The importance of beam theory in structural mechanics stems from its widespread success in practical applications.

7.1.1 Kinematic assumptions

Readings: BC 5.2

Beam theory is founded on the following two key assumptions known as the EulerBernoulli assumptions:

? Cross sections of the beam do not deform in a significant manner under the application of transverse or axial loads and can be assumed as rigid

Concept Question 7.1.1. With reference to Figure 7.1,

1. what is the main implication of this assumption on the kinematic description

(overall displacement field) of the cross section?

Solution:

The cross section can only undergo a rigid-body motion in its plane, i.e. two rigid

body translations and one rotation.

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MODULE 7. SIMPLE BEAM THEORY

e3

e2 u3

e1 u2

Figure 7.1: First kinematic assumption in Euler-Bernoulli beam theory: rigid in-plane deformation of cross sections.

2. To simplify further the discussion, assume for now that there is no rotation of the

cross section around the e3 axis. Write the most general form of the cross-section

in-plane displacement components:

Solution: The cross section can only

translate rigidly in the e2 and e3 directions, i.e. the displacement components in the plane cannot depend on the position in the plane x2, x3 and:

u2(x1, x2, x3) = u?2(x1), u3(x1, x2, x3) = u?3(x1)

(7.1)

? During deformation, the cross section of the beam is assumed to remain planar and normal to the deformed axis of the beam.

Concept Question 7.1.2. With reference to Figure 7.3,

1. what is the main implication of this assumption on the kinematic description

(overall displacement field) of the cross section?

Solution:

The cross section can only translate rigidly in the axial direction, or rotate with

respect to the e2 and e3 axes by angles 2 and 3, respectively (using the angles shown in the figure), both angles are of course allowed to be a function of x1: 2 = 2(x1), 3 = 3(x1).

2. Based on these kinematic assumptions, write the most general form of the cross-

section out-of-plane displacement component:

Solution:

The rigid out-of-plane translation allows for a uniform displacement of the form:

u1(x1, x2, x3) = u?1(x1) as it would be the case for a truss element which sustains

7.1. REVIEW OF SIMPLE BEAM THEORY

e3 e2

e1

153 e3

e2 e1

(a)

e3

e2 3(x1)

e1

2(x1)

(b)

(c)

Figure 7.2: Second kinematic assumption in Euler-Bernoulli beam theory: cross sections remain planar after deformation.

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MODULE 7. SIMPLE BEAM THEORY

a uniform uni-axial strain. The rotations 2 and 3 will give contributions to the total axial displacement which are linear in the in-plane coordinates x3 and x2 respectively, see Figure 7.3. It can be easily inferred from the figure that these contributions adopt the form: x32(x1) and -x23(x1), respectively. It should be carefully noted that assuming both positive angles, this contribution indeed has a negative sign.

In summary, the out-of-plane kinematic restrictions imposed by the second Euler-

Bernoulli assumption results in the following form of the u1 displacement component:

u1(x1, x2, x3) = u?1(x1) + x32(x1) - x23(x1)

(7.2)

It should be noted that we have not defined the origin of the coordinate system but we have implicitly assumed that it corresponds to the intersection of the lines which do not exhibit stretch or contraction under out-of-plane rotations of the cross sections. Later we will define these lines as the neutral axes for bending.

3. If you noticed, we have not applied the constraint that the sections must remain normal to the deformed axis of the beam. Use this part of the assumption to establish a relation between the displacements fields u?2(x1), u?3(x1) and the angle fields 2(x1), 3(x1), respectively, see Figure ??.

2

3

e3 u3(x1)

e2 u2(x1)

+ e2

e1

e3

e1

(a)

(b)

Figure 7.3: Implications of the assumption that cross sections remain normal to the axis of the beam upon deformation.

Solution: from the figure one can see clearly that:

2

= - du?3 , dx1

3

=

du?2 dx1

(7.3)

7.1. REVIEW OF SIMPLE BEAM THEORY

155

Concept Question 7.1.3. Combine the results from all the kinematic assumptions to

obtain a final assumed form of the general 3D displacement field in terms of the three

unknowns u?1(x1), u?2(x1), u?3(x1):

Solution:

u1(x1, x2, x3) = u?1(x1) - x3u?3(x1) - x2u?2(x1) u2(x1, x2, x3) = u?2(x1) u3(x1, x2, x3) = u?3(x1)

(7.4) (7.5) (7.6)

These assumptions have been extensively confirmed for slender beams made of isotropic materials with solid cross-sections.

Concept Question 7.1.4. Use the kinematic assumptions of Euler-Bernoulli beam theory

to derive the general form of the strain field:

Solution: It follows directly from (7.4)

that:

11 = u1,1 = u?1(x1) - x3u?3(x1) - x2u?2(x1)

(7.7)

22

=

u2,2

=

u?2(x1) x2

=

0

(7.8)

33

=

u3,3

=

u?3(x1) x3

=

0

(7.9)

2

23

=

u2,3

+

u3,2

=

u?2(x1) x3

+

u?3(x1) x2

=

0

(7.10)

2 31 = u3,1 + u1,3 = u?3(x1) + x3

u?1(x1) - x3u?3(x1) - x2u?2(x2)

=

u?3(x1)

-

x3 x3

u?3(x1)

=

0

1

(7.11)

2 12 = u1,2 + u2,1 = x2

u?1(x1) - x3u?3(x1) - x2u?2(x2)

+

u?2(x1)

=

-

x2 x2

u?2(x1)

+

u?2(x1)

=

0,

1

(7.12)

Concept Question 7.1.5. It is important to reflect on the nature of the strains due to bending. Interpret the components of the axial strain 11 in Euler-Bernoulli beam theory Solution:

? The first term represents a uniform strain in the cross section just as those arising in bars subject to uni-axial stress

? The second and third terms tell us that the axial fibers of the beam stretch and contract proportionally to the distance to the neutral axis. The constant of proportionality is the second derivative of the function describing the deflections of the axis of the beam. This can be seen as a linearized version of the local value of the curvature.

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MODULE 7. SIMPLE BEAM THEORY

? There are no shear strains!!!! This is a direct consequence of assuming that the crosssection remains normal to the deformed axis of the beam.

? There are no strains in the plane. This is a direct consequence of assuming that the cross section is rigid.

One of the main conclusions of the Euler-Bernoulli assumptions is that in this particular beam theory the primary unknown variables are the three displacement functions u?1(x1), u?2(x1), u?3(x1) which are only functions of x1. The full displacement, strain and therefore stress fields do depend on the other independent variables but in a prescribed way that follows directly from the kinematic assumptions and from the equations of elasticity. The purpose of formulating a beam theory is to obtain a description of the problem expressed entirely on variables that depend on a single independent spatial variable x1 which is the coordinate along the axis of the beam.

7.1.2 Definition of stress resultants

Readings: BC 5.3

Stress resultants are equivalent force systems that represent the integral effect of the internal stresses acting on the cross section. Thus, they eliminate the need to carry over the dependency of the stresses on the spatial coordinates of the cross section x2, x3.

The axial or normal force is defined by the expression:

N1(x1) = 11(x1, x2, x3)dA

A

The transverse shearing forces are defined by the expressions:

(7.13)

V2(x1) = 12(x1, x2, x3)dA

A

V3(x1) = 13(x1, x2, x3)dA

A

The bending moments are defined by the expressions:

(7.14) (7.15) (7.16)

M2(x1) = x311(x1, x2, x3)dA

A

(7.17)

M3(x1) = - x211(x1, x2, x3)dA

A

(7.18) (7.19)

The negative sign is needed to generate a positive bending moment with respect to axis e3, see Figure 7.4

7.2. AXIAL LOADING OF BEAMS

11

13

N1

V3

M2

e3

?

e2

13

V3

11

e1

M2

N1 N1

M3 11

12

V2

dx1

157

e2

12 M3 V2

11

e3 e1

N1

dx1

Figure 7.4: Sign conventions for the sectional stress resultants

7.2 Axial loading of beams

Readings: BC 5.4

e2 p1(x1)

e1 P1

L

Figure 7.5: Beams subjected to axial loads.

Consider the case where there are no transverse loading on the beam, Figure 7.5. The only external loads possible in this case are either concentrated forces such as the load P1, or distributed forces per unit length p1(x1).

7.2.1 Kinematics

In this case, we will assume that the cross sections will not rotate upon deformation.

Concept Question 7.2.1. Based on this assumption, specialize the general displacement (7.4) and strain field (7.7) to this class of problems and comment on the implications of the possible deformations obtained in this theory Solution: Since there are no rotations, 2 = -u?3(x1) = 0, 3 = u?2(x1) = 0 and we obtain for u1(x1) = u?1(x1).

The boundary condition at x1 = 0 dictates u?2(0) = u?3(0) = 0, which combined with the previous assumption of no rotation implies that the transverse deflections are zero every-

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MODULE 7. SIMPLE BEAM THEORY

where, u?2(x1) = u?3(x1) = 0. The displacement field is then:

u1(x1, x2, x3) = u?1(x1) u2(x1, x2, x3) = 0 u3(x1, x2, x3) = 0

The strain field follows directly from this as:

11(x1, x2, x3) = u?1(x1)

and all the other strain components are zero. The assumption of allowing only rigid motions of the cross section implies that there cannot be any in-plane strains. This creates a state of uni-axial strain.

7.2.2 Constitutive law for the cross section

We will assume a linear-elastic isotropic material and that the transverse stresses 22, 33 0. By Hooke's law, the axial stress 11 is given by:

11(x1, x2, x3) = E 11(x1, x2, x3)

Replacing the strain field for this case:

11(x1, x2, x3) = Eu?1(x1) In other words, we are assuming a state of uni-axial stress.

(7.20)

Concept Question 7.2.2. This exposes an inconsistency in Euler-Bernoulli beam theory.

What is it and how can we justify it?

Solution: The inconsistency is

that we are assuming the kinematics to be uni-axial strain, and the kinetics to be uni-axial

stress. In other words one can either have:

22 = 33 = 0

(Euler-Bernoulli hypothesis) or

22 = 33 = 0

These two cannot co-exist except when the Poisson ratio is zero. However, this in general has a small effect in most problems of practical interest. The theory is developed assuming that we can ignore both these strains and stresses.

The axial force can be obtained by replacing (7.20) in (7.13):

N1(x1) =

Eu?1(x1)dA =

EdA u?1(x1)

A(x1)

A(x1)

S(x1)

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