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Chapter 4

Q. 4.12

A wall bracket with a rectangular cross-section is shown in Fig. 4.39. The depth of the cross-section is twice of the width. The force P acting on the bracket at 600 to the vertical is 5 kN. The material of the bracket is grey cast iron FG 200 and the factor of safety is 3.5. Determine the dimensions of the cross-section of the bracket.
Assume maximum normal stress theory of failure.


Verified Solution

Given P = 5 kN

S_{u t}=200 N / mm ^{2} \quad(f s)=3.5 \quad d / w=2 .

Step I Calculation of permissible stress

\sigma_{\max }=\frac{S_{u t}}{(f s)}=\frac{200}{3.5}=57.14 N / mm ^{2}          (i).

Step II Calculation of direct and bending tensile stresses
The stress is maximum at the point A in the section XX. The point is subjected to combined bending and direct tensile stresses. The force P is resolved into two components—horizontal component P_h and vertical component P_v .

P_{h}=P \sin 60^{\circ}=5000 \sin 60^{\circ}=4330.13 N .

P_{v}=P \cos 60^{\circ}=5000 \cos 60^{\circ}=2500 N .

The bending moment at the section XX is given by

M_{b}=P_{h} \times 150+P_{v} \times 300 .

=4330.13 \times 150+2500 \times 300 .

=1399.52 \times 10^{3} N – mm .

\sigma_{b}=\frac{M_{b} y}{I} .

=\frac{\left(1399.52 \times 10^{3}\right)(t)}{\left[\frac{1}{12}(t)(2 t)^{3}\right]}=\frac{2099.28 \times 10^{3}}{t^{3}} N / mm ^{2} .

The direct tensile stress due to component P_h is given by,

\sigma_{t}=\frac{P_{h}}{A}=\frac{4330.13}{2 t^{2}}=\frac{2165.07}{t^{2}} N / mm ^{2} .

The vertical component P_v induces shear stress at the section XX. It is however small and neglected.

Step III Calculation of dimensions of cross-section
The resultant tensile stress \sigma_{\max } . at the point A is given by,

\sigma_{\max }=\sigma_{b}+\sigma_{t}=\frac{2099.28 \times 10^{3}}{t^{3}}+\frac{2165.07}{t^{2}}               (ii).

Equating (i) and (ii),

\frac{2099.28 \times 10^{3}}{t^{3}}+\frac{2165.07}{t^{2}}=57.14 .


t^{3}-37.89 t-36739.24=0 .

Solving the above cubic equation by trial and error method,

t=33.65 mm \cong 35 mm .

The dimensions of the cross-section are 35 × 70 mm.