When proportionate loads act on a structural system whose members all lie in a single plane, what is the correct classification of the structure?

Introduction / Context:
Structural systems are categorized by the geometry of their members and the dimensionality of load transfer. Distinguishing between plane frames, space frames, grids, and trusses is foundational for analysis and design in civil and structural engineering.

Given Data / Assumptions:

• Members lie in one plane.
• Loads are applied proportionately (i.e., all load components act within that plane).

Concept / Approach:
Plane frame: members and loads confined to a single plane; members resist axial force, shear, and bending.Space frame: 3-D arrangement; forces and stiffness in all three directions.Grid frame: orthogonal members primarily bending in their own planes to carry out-of-plane loads (e.g., floor or roof grids).Truss frame: members ideally carry axial forces only (pin-jointed assumption) with negligible bending.

Step-by-Step Solution:
Identify dimensionality → single plane → not space frame.Members resist bending under proportionate loads → not a pure truss.Not a grid slab system → therefore it is a plane frame.

Verification / Alternative check:
Matrix stiffness classification: dofs limited to in-plane translations and rotations confirm plane frame behavior.

Why Other Options Are Wrong:

• Grid frame: typically orthogonal beams for slab systems with torsion/out-of-plane behavior.
• Space frame: requires 3-D member layout and 3-D load transfer.
• Truss frame: would preclude member bending as a primary action.

Common Pitfalls:

• Calling any 2-D truss a plane frame; trusses and frames differ in internal force characteristics.

Final Answer:
plane frame