Question

Consider the pin-jointed truss shown in the figure. Influence line is drawn for the axial force in the member G-I, when a unit load travels on the bottom chord of the truss. 
Identify the CORRECT influence line from the following options:
Note: Positive value corresponds to tension and negative value corresponds to compression in the member.

• A.
• B.
• C.
• D.

Hint:

To find influence lines for truss members, use geometry-based influence line theory and remember that bottom chord members generally carry tension under downward unit loads.

Answer 1

The Correct Option is B

We are to find the influence line for the axial force in member G–I when a unit load travels along the bottom chord of the truss.

From the geometry of the truss:
- Total span = $6 \times 6 = 36$ m
- Height of truss $h = 6$ m
- Panel point spacing = 6 m
- Distance from A to G = 2 panels = 12 m
- Distance from G to right support = 4 panels = 24 m

Now, to find the ordinate of the influence line for member G–I under G, we use the standard formula for diagonal or bottom chord members in simple trusses: $$\text{Ordinate} = \frac{a b}{l h}$$ Where:
- $a = 12$ m, - $b = 24$ m, - $l = 36$ m, - $h = 6$ m
Substitute into the formula: $$\frac{12 \times 24}{36 \times 6} = \frac{288}{216} = 1.33$$
Hence, the influence line has an ordinate of $+1.33$ under joint G, which corresponds to tension in member G–I. Therefore, the correct diagram is option (B).