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$6–34$. The Howe truss is subjected to the loading shown. Determine the force in members $GH$, $BC$, and $BG$ of the truss and state if the members are in tension or compression.

$6–34$. The Howe truss is subjected to the loading shown. Determine the force in members $GH$, $BC$, and $BG$ of the truss and state if the members are in tension or compression.

$6–34$. The Howe truss is subjected to the loading shown. Determine the force in members $GH$, $BC$, and $BG$ of the truss and state if the members are in tension or compression.

6–34. The Howe truss is subjected to the loading shown. Determine the force in members GH, BC, and BG of the truss and state if the members are in tension or compression.

​$ \text{Step by Step Solution​} $

$ \text{Step 1: Given Data } $

The truss is subjected to the following vertical loads:
\begin{array}{c}
W_A = 2\,{\rm{kN}} \quad \text{(at point A)} \\
W_H = 5\,{\rm{kN}} \quad \text{(at point H)} \\
W_G = 5\,{\rm{kN}} \quad \text{(at point G)} \\
W_F = 5\,{\rm{kN}} \quad \text{(at point F)} \\
W_E = 2\,{\rm{kN}} \quad \text{(at point E)}
\end{array}

The truss has the following dimensions:
\begin{array}{c}
AB = 2\,{\rm{m}} \quad \text{(horizontal member)} \\
BC = 2\,{\rm{m}} \quad \text{(horizontal member)} \\
CD = 2\,{\rm{m}} \quad \text{(horizontal member)} \\
DE = 2\,{\rm{m}} \quad \text{(horizontal member)} \\
CG = 3\,{\rm{m}} \quad \text{(vertical member)} \\
AC = 4\,{\rm{m}} \quad \text{(distance from A to C)} \\
BE = 6\,{\rm{m}} \quad \text{(distance from B to E)} \\
AE = 8\,{\rm{m}} \quad \text{(distance from A to E)}
\end{array}

$ \text{Step 2: Determine Support Reactions} $

First, we analyze the entire truss to find the reaction forces:

\begin{array}{c}
\text{Horizontal equilibrium:} \\
\sum F_x = 0 \Rightarrow A_x = 0\,{\rm{kN}} \\
\text{(No horizontal loads are applied, so } A_x \text{ must be zero)}
\end{array}

\begin{array}{c}
\text{Moment equilibrium about point E:} \\
\sum M_E = 0 \Rightarrow -A_y \times AE + W_A \times AE + W_H \times BE + W_G \times CE + W_F \times DE = 0 \\
\text{Where:} \\
CE = AC = 4\,{\rm{m}} \quad \text{(since C is midpoint between A and E)} \\
DE = 2\,{\rm{m}} \quad \text{(given)} \\
\text{Substituting values:} \\
-A_y \times 8 + 2 \times 8 + 5 \times 6 + 5 \times 4 + 5 \times 2 = 0 \\
-8A_y + 16 + 30 + 20 + 10 = 0 \\
\hline
A_y = \frac{16 + 30 + 20 + 10}{8} = \frac{76}{8} = 9.5\,{\rm{kN}}
\end{array}

$ \text{Step 3: Calculate Angles} $

We need to determine the angles for force resolution:

free body diagram of the truss part ABH

\begin{array}{c}
\text{Angle } \alpha \text{ between members GC and BC:} \\
\tan \alpha = \frac{GC}{BC} = \frac{3\,{\rm{m}}}{2\,{\rm{m}}} = 1.5 \\
\alpha = \tan^{-1}(1.5) \approx 56.3^\circ \\
\text{(This angle is needed to resolve forces at joint B)}
\end{array}

\begin{array}{c}
\text{Angle } \theta \text{ between members GC and AC:} \\
\tan \theta = \frac{GC}{AC} = \frac{3\,{\rm{m}}}{4\,{\rm{m}}} = 0.75 \\
\theta = \tan^{-1}(0.75) \approx 37^\circ \\
\text{(This angle is needed to resolve forces at joint G)}
\end{array}

$ \text{Step 4: Analyze Section ABH} $

We isolate section ABH to find member forces:

\begin{array}{c}
\text{Moment equilibrium about point G to find } F_{BC}: \\
\sum M_G = 0 \Rightarrow W_A \times AC + W_H \times BC + F_{BC} \times GC – A_y \times AC = 0 \\
\text{Where:} \\
AC = 4\,{\rm{m}} \text{ is the horizontal distance} \\
GC = 3\,{\rm{m}} \text{ is the vertical moment arm} \\
\text{Substituting values:} \\
2 \times 4 + 5 \times 2 + F_{BC} \times 3 – 9.5 \times 4 = 0 \\
8 + 10 + 3F_{BC} – 38 = 0 \\
\hline
F_{BC} = \frac{38 – 8 – 10}{3} = \frac{20}{3} \approx 6.67\,{\rm{kN}}\,(\rm{T}) \\
\text{(Positive value indicates tension)}
\end{array}

$ \text{Step 5: Determine Length HB} $

Using geometric similarity of triangles:

\begin{array}{c}
\frac{GC}{HB} = \frac{AC}{BC} \\
\frac{3\,{\rm{m}}}{HB} = \frac{4\,{\rm{m}}}{2\,{\rm{m}}} \\
\hline
HB = \frac{3 \times 2}{4} = 1.5\,{\rm{m}} \\
\text{(This length is needed for moment calculations about point B)}
\end{array}

$ \text{Step 6: Calculate Force in GH} $

\begin{array}{c}
\text{Moment equilibrium about point B:} \\
\sum M_B = 0 \Rightarrow W_A \times AB – A_y \times AB – F_{GH} \times \cos\theta \times HB = 0 \\
\text{Where:} \\
\cos\theta = \cos37^\circ \approx 0.799 \\
HB = 1.5\,{\rm{m}} \text{ (from Step 5)} \\
\text{Substituting values:} \\
2 \times 2 – 9.5 \times 2 – F_{GH} \times 0.799 \times 1.5 = 0 \\
4 – 19 – 1.197F_{GH} = 0 \\
\hline
F_{GH} = \frac{-15}{1.197} \approx -12.5\,{\rm{kN}}\,(\rm{C}) \\
\text{(Negative value indicates compression)}
\end{array}

$ \text{Step 7: Calculate Force in BG} $

\begin{array}{c}
\text{Horizontal force equilibrium at joint B:} \\
\sum F_x = 0 \Rightarrow F_{BC}\cos\alpha + F_{BG} + F_{GH}\cos\theta = 0 \\
\text{Where:} \\
\cos\alpha = \cos56.3^\circ \approx 0.554 \\
\cos\theta = \cos37^\circ \approx 0.799 \\
\text{Substituting values:} \\
6.67 \times 0.554 + F_{BG} – 12.5 \times 0.799 = 0 \\
3.69 + F_{BG} – 9.99 = 0 \\
\hline
F_{BG} = 9.99 – 3.69 = 6.3\,{\rm{kN}}\,(\rm{T}) \\
\text{(Positive value indicates tension)}
\end{array}

$ \text{Step 8: Final Results} $

\begin{array}{c}
\text{Member BC: } 6.67\,{\rm{kN}}\,(\rm{T}) \\
\text{Member GH: } 12.5\,{\rm{kN}}\,(\rm{C}) \\
\text{Member BG: } 6.3\,{\rm{kN}}\,(\rm{T})
\end{array}

$ \text{Step 9: Verification} $

\begin{array}{c}
\text{Moment equilibrium about E: } \sum M_E = 0 \quad \checkmark \\
\text{Horizontal force equilibrium: } \sum F_x = 0 \quad \checkmark \\
\text{Vertical force equilibrium: } \sum F_y = 0 \quad \checkmark \\
\hline
\text{All equilibrium conditions are satisfied}
\end{array}

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