Chord Force in a Pratt Truss
Mediumtrussesmethod of sections
A simply supported Pratt truss spans 30 m in five 6 m panels with a height of 4 m. Vertical loads of 40 kN act at the two top-chord joints located 6 m from each support. Using the method of sections, the force in the bottom-chord panel at midspan is most nearly:
- A40 kN tension
- B60 kN tension
- C60 kN compression
- D90 kN tension
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Answer: B. 60 kN tension
By symmetry each reaction is (2×40)/2 = 40 kN. Cut the truss through the midspan panel and take moments about the top-chord joint directly above the cut, which eliminates the diagonal and top-chord forces. ΣM = 40(12) − 40(6) = 240 kN·m, and dividing by the truss height gives F = 240/4 = 60 kN. The bottom chord of a simply supported truss under gravity load is in tension.
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