A long strut AB of length L is of uniform section throughout. A thrust P is applied at the ends eccentrically, on the same side of the centre line with eccentricity at the end B twice than that at the end A. Show that the maximum bending moment occurs at a distance x from end A, where

$\tan \lambda x=\frac{2-\cos \lambda L}{\sin \lambda L} \text { and } \lambda=\sqrt{\frac{P}{E I}}$

Question

A long strut AB of length L is of uniform section throughout. A thrust P is applied at the ends eccentrically, on the same side of the centre line with eccentricity at the end B twice than that at the end A. Show that the maximum bending moment occurs at a distance x from end A, where

$\tan \lambda x=\frac{2-\cos \lambda L}{\sin \lambda L} \text { and } \lambda=\sqrt{\frac{P}{E I}}$

[latex] an \lambda x=\frac{2-\cos \lambda L}{\sin \lambda L} ext { and } \lambda=\sqrt{\frac{P}{E I}} [/latex]

Accepted Answer

Assuming the column to be fundamental one (i.e., pinned at both ends) with eccentric load at both ends, we show the schematic structural diagram of the column in Figure 8.21(a).
In Figure 8.21(a), we represent the eccentricity at ends A and B as ε and 2ε, respectively. In Figure 8.21(b), we represent the free-body diagram of the entire column while in Figure 8.21(c), we show the free-body diagram of the column of a section cut out at a distance x (0 ≤ x ≤ L) from end A. In Figure 8.21(c), axial force [latex] N_{x} [/latex], shear force [latex] V_{x} [/latex] and bending moment [latex] M_{x} [/latex] are shown in their positive senses.

Now, from Figure 8.21(b), for equilibrium [latex] \left( ext { note } \Sigma F_x=0 ext { and } \Sigma F_y=0 ight. [/latex] have automatically been satisfied), we must have:

[latex] \sum M_z=0 \Rightarrow Q L=P \varepsilon [/latex]

or [latex] Q=\frac{P \varepsilon}{L} [/latex] (1)

Again from Figure 8.21(c):

[latex] \sum M_z=0 \Rightarrow M_x=P \varepsilon+P y+Q x [/latex]

where y is the depression of the column at a distance x from end A [as shown in Figure 8.21(b) above].
Therefore, from flexure equation:

[latex] (E I) \frac{ d ^2 y}{ d x^2}=-M_x=-P y-Q x-P \varepsilon [/latex]

or [latex] \frac{ d ^2 y}{ d x^2}=\left\lgroup -\frac{P}{E I} ight group y-\left\lgroup \frac{Q}{E I} ight group x-\left(\frac{P}{E I} ight) \varepsilon [/latex]

or [latex] \frac{ d ^2 y}{ d x^2}+\lambda^2 y=-\lambda^2 \varepsilon-\lambda^2 \frac{Q}{P} x \quad\left( ext { where } \lambda^2=P / E I ight) [/latex]

or [latex] \frac{ d ^2 y}{ d x^2}+\lambda^2 y=-\lambda^2\left\lgroup \frac{Q}{P} x+\varepsilon ight group [/latex] (2)

Clearly, the above is a second-order non-homogeneous linear differential equation and will have complementary function and particular integral as its solution components. Therefore,

[latex] y=\left(A_1 \cos \lambda x+A_2 \sin \lambda x ight)-\left(\frac{Q}{P} x+\varepsilon ight) [/latex] (3)

where [latex] A_1 ext { and } A_2 [/latex] are the two arbitrary constants to be determined from the geometric boundary conditions of the problem. Obviously, we have at x = 0, y = 0 and at x = L, y = 0. Thus, from Eq. (3), we get

[latex] A_1-\varepsilon=0 \Rightarrow A_1=\varepsilon [/latex] (4)

and [latex] \varepsilon \cos \lambda L+A_2 \sin \lambda L-\frac{Q L}{P}-\varepsilon=0 [/latex]

But from Eq. (1), QL/P = ε. Thus, the above expression becomes

[latex] \varepsilon \cos \lambda L+A_2 \sin \lambda L-2 \varepsilon=0 [/latex]

[latex] A_2=\frac{\varepsilon(2-\cos \lambda L)}{\sin \lambda L} [/latex] (5)

Now, from Eq. (3), we obtain

[latex] y=\varepsilon\left[\cos \lambda x+\left\lgroup \frac{2-\cos \lambda L}{\sin \lambda L} ight group \sin \lambda x-\frac{x}{L}-1 ight] [/latex] (6)

This is the equation of the elastic line of the column. Now, bending moment at any distance x is

[latex] M_x=P y+P \varepsilon+Q x [/latex]

or [latex] M_x=P\left[y+\varepsilon+\frac{\varepsilon x}{L} ight] [/latex] [as QL = Pε by Eq. (1)]

Putting expression of y from Eq. (6) above, we get

[latex] M_x=P \varepsilon\left[\cos \lambda x+\left\lgroup\frac{2-\cos \lambda L}{\sin \lambda L} ight group \sin \lambda x ight] [/latex] (7)

For [latex] M_x [/latex] to be maximum, clearly [latex]\cosλ x +[(2 - \cos λL)/ \sinλL] \sin λ x[/latex] is to be maximum, that is, when

[latex] \frac{ d }{ d x}\left\{\cos \lambda x+\left(\frac{2-\cos \lambda L}{\sin \lambda L} ight) \sin \lambda x ight\}=0 [/latex]

or [latex] -\sin \lambda x+\left(\frac{2-\cos \lambda L}{\sin \lambda L} ight) \cos \lambda x=0 [/latex]

or [latex] an \lambda x=\left\lgroup \frac{2-\cos \lambda L}{\sin \lambda L} ight group \quad ext { (where } \lambda^2=P / E I ext { ) } [/latex]

Alternative Solution: Let (2 - cosλL)/sinλL = p. Now,

[latex] \begin{aligned} \cos \lambda x+\left\lgroup \frac{2-\cos \lambda L}{\sin \lambda L} ight group \sin \lambda x & =\cos \lambda x+p \sin \lambda x \ & =\sqrt{1+p^2} \sin (\lambda x+ heta) \end{aligned} [/latex]

where tan θ= 1/p. Therefore, cosλ x + p sinλ x is maximum, that is, 1 when

[latex] \lambda x+ heta=\frac{\pi}{2} [/latex]

or [latex] \lambda x=\frac{\pi}{2}- heta [/latex]

or [latex] an \lambda x=\cot heta=p=\frac{2-\cos \lambda L}{\sin \lambda L} \quad ext { (as before) } [/latex]

Question 8.8: A long strut AB of length L is of uniform section throughout...

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