Answer :
Here are the detailed answers for the questions based on the Pratt bridge diagram and provided information:
### Question 1: Determine the value of \( A_1 \) with reasons.
In a Pratt truss, the top and bottom chords are parallel with vertical members and diagonals. Assuming \( A_1 \) is the force in one of the members, it is typically calculated using the method of joints or sections.
Since \( A_1 \) is the force in the vertical member at joint A, and considering symmetry and equal lengths:
- From Symmetry: If \( A_1 \) stands for a vertical reaction or vertical member force, given equal spacing, all vertical reactions and vertical forces should be the same by symmetry and load distribution.
- Assume Uniform Load: The force in vertical members of symmetric trusses under uniform distributed loads is often zero if only horizontal loads act, or other specific forces balance each joint.
However, the exact forces will depend on the exact load distribution and points of application; generally, it seems \( A_1 \) might be zero (if no vertical loads are present directly on it).
### Question 2: Determine the value of \( F_3 \) with reasons.
In the context of a Pratt truss, \( F_3 \) could be the axial force in a diagonal member:
- From symmetry and uniform load, considering the top chords and bottom chords are parallel and spaced equally, each panel in a Pratt truss carries similar load properties if uniformly distributed.
Thus, by symmetry, if point loads or uniform loads are equally distributed:
- \( F_3 \) as a diagonal force, where each diagonal member in the truss will carry a consistent tension or compression due to equal load distribution.
### Conclusion:
Without the precise loading points and complete diagram with notation conventions (i.e., where exactly \( A_1 \) and \( F_3 \) go):
1. **\( A_1 \)**: Given symmetry reasoning, typically might be zero due to symmetrical vertical forces if no specific vertical load.
2. **\( F_3 \)**: Analyzing diagonal members with known equal sections under simple symmetric loads can estimate their equal load division, would need more context to quantify explicit values.
For exact values, a completed diagram and load details would help to clarify with method of joints or cutting sections approach.
