1. The Pratt truss has diagonal members that alternate between tension and compression, reducing the need for oversized members.
2. The Pratt truss has shorter compression members and diagonal members that remain in tension, minimizing buckling concerns.
3. The Warren truss distributes forces more evenly, making it superior in all load conditions.
4. The Warren truss has longer diagonal members that only experience tension, making it more efficient for bridges.

Explanation

Explanation: A Pratt truss is more material-efficient than a Warren truss in bridge structures because its diagonal members are always in tension, while vertical members handle compression. Since tension members do not buckle, they can be designed with smaller cross-sections, making the structure lighter and more efficient. In contrast, a Warren truss's diagonal members alternate between tension and compression depending on load position, requiring them to be designed for both failure modes—particularly buckling under compression. This often leads to oversized members compared to a Pratt truss, making it less optimized for moving loads. Therefore, Pratt trusses are commonly preferred for bridges with significant live loads, as they allow for a more predictable and efficient force distribution.

1. 1,890 kN
2. 2,250 kN
3. 2,520 kN
4. 2,813 kN

Explanation

Solution: 

1. Factored uniformly distributed load = (1.25 × 30) + (1.5 × 25) = 37.5 + 37.5 = 75 kN/m
2. Total factored load over span = 75 kN/m × 60m = 4,500 kN
3. Maximum shear force at support = 4,500 kN ÷ 2 = 2,250 kN
4. In the first panel, the diagonal carries the panel shear force
5. Panel width = 6m, truss depth = 8m
6. Diagonal length = √(6² + 8²) = √100 = 10m
7. Angle of diagonal with horizontal = tan⁻¹(8/6) = 53.13°
8. Axial force in diagonal = Shear force ÷ sin(53.13°) = 2,250 ÷ 0.8 = 2,812.5 kN

Explanation: In a steel truss bridge, diagonal members play a critical role in transferring shear forces through the structure. The axial force in a diagonal member depends on the shear force at that location and the geometry of the truss (particularly the panel dimensions and truss depth). For a uniformly loaded truss, the maximum shear occurs at the supports and decreases toward the center. The most heavily loaded diagonal is typically located in the end panel, where it must resist the maximum shear force. The diagonal's orientation affects the axial force it experiences - the more vertical the diagonal, the lower the axial force required to resist a given shear. This calculation demonstrates why truss depth is a crucial design parameter: a deeper truss creates steeper diagonals, reducing axial forces but increasing overall height. In practice, designers must balance these considerations alongside factors such as material costs, fabrication complexity, and transportation constraints when determining optimal truss geometry.