How to connect simple trusses to form a compound truss A compound truss is formed by joining two simple, individually rigid truss frames. Select the correct way to connect them so that the combined system is rigid and statically determinate.

Introduction / Context:
When two simple trusses are combined, the objective is to eliminate relative motion between them without introducing redundancies. The connection strategy must provide three independent constraints in the plane to prevent translations and rotation.

Given Data / Assumptions:

• Two planar, simple, and rigid trusses are to be connected.
• Pin-jointed members; axial forces only.
• Determinacy is desired (no redundant members).

Concept / Approach:
Two rigid bodies in a plane have three relative degrees of freedom. Therefore, three independent bars are required to constrain these motions fully and exactly, yielding a rigid and statically determinate compound truss.

Step-by-Step Solution:

Verification / Alternative check:
Structural determinacy can be checked by ensuring the three connections are not all parallel or all meeting at a single point, which would fail to constrain all motions independently.

Why Other Options Are Wrong:

• two bars: Leaves one relative DOF; mechanism persists.
• three parallel bars: Constraints are not independent; rotation may remain.
• three bars intersecting at a single point: Also not independent; translations can remain unrestrained.
• None of these: Incorrect since three appropriately placed bars are correct.

Common Pitfalls:
Placing the three bars so that they are concurrent or parallel; miscounting DOF; assuming that any three bars will suffice without checking independence.

Final Answer:
three bars