The method of steel frame design that achieves the greatest overall rigidity with good economy in weight is commonly referred to as fully rigid design.

Introduction / Context:
In steel frame systems, connection behavior (moment-resisting vs. pin) governs stiffness, drift, and member forces. Choosing an appropriate design philosophy balances rigidity, strength, ductility, and economy in tonnage.

Given Data / Assumptions:

• Primary comparison among simply (pin) connected, semi-rigid, and fully rigid (moment) frames.
• Objective: achieve high rigidity while maintaining reasonable weight.

Concept / Approach:
Fully rigid (moment-resisting) connections provide rotational restraint, creating continuous frames with higher lateral and overall stiffness. Greater rigidity reduces lateral drift and may allow smaller bracing demands. While moment connections can add fabrication cost, they often lead to efficient distribution of moments and reduced deflections for a given weight target.

Step-by-Step Solution:
Compare connection types: pin vs. semi-rigid vs. rigidRecognize that rigid connections transfer significant moments, increasing frame stiffnessInfer that high rigidity for serviceability implies use of fully rigid design

Verification / Alternative check:
Frame analysis shows that moment-resisting frames have smaller story drifts than pin-jointed frames under the same lateral loads, confirming the rigidity advantage.

Why Other Options Are Wrong:

• Simply design: pin connections give minimal rotational restraint and the least rigidity.
• Semi-rigid design: intermediate stiffness; typically less rigid than fully rigid frames.
• None of these: incorrect because fully rigid design meets the stated objective.

Common Pitfalls:

• Assuming maximum rigidity always increases weight; efficient distribution of forces can offset connection weight.
• Ignoring serviceability (drift) limits when choosing connection philosophy.

Final Answer:
Fully rigid design