Question

Which structural design philosophy considers both elastic and ultimate strength conditions?

• A. Limit State Design
• B. Elastic Design (Working Stress Design)
• C. Ultimate Load Design (Ultimate Strength Design)
• D. Plastic Design

Hint:

\textbf{Limit State Design (LSD):} Considers multiple limit states:
Ultimate Limit States (ULS): Safety against collapse (strength, stability). Involves ultimate strength of materials.
Serviceability Limit States (SLS): Performance under normal use (deflection, cracking, vibration). Often involves elastic behavior. LSD aims for a balance of safety and serviceability.
\textbf{Working Stress Design (Elastic Design):} Stresses under service loads < allowable stresses (fraction of yield strength). Focus on elastic behavior.
\textbf{Ultimate Load Design:} Structure's ultimate strength $\ge$ factored loads. Focus on collapse condition.
\textbf{Plastic Design:} Uses plastic theory for ultimate strength of steel structures.

Answer 1

The Correct Option is A

Structural design philosophies dictate how structures are analyzed and designed to ensure safety and serviceability. Let's consider the philosophies:
(a) Limit State Design (LSD): This is a modern structural design philosophy that aims to ensure a structure will not become unfit for its intended use during its design life. It considers various "limit states" beyond which the structure would no longer satisfy design requirements. These include:
Ultimate Limit States (ULS): Related to safety and collapse (e.g., strength, stability, overturning, fracture due to ultimate load). This involves considering the ultimate strength of materials and sections.
Serviceability Limit States (SLS): Related to performance and occupant comfort under normal service loads (e.g., deflection, vibration, cracking). This often involves considering behavior in the elastic range. LSD uses partial safety factors for loads and material strengths. It considers both ultimate strength conditions (for safety) and behavior under service loads (often elastic, for serviceability).
(b) Elastic Design (Working Stress Design / Allowable Stress Design): This is an older design philosophy. It assumes that materials behave elastically under service loads. Stresses in structural members are calculated under working (service) loads and are limited to a certain fraction of the material's yield strength (allowable stress). It primarily focuses on behavior in the elastic range and does not explicitly consider the ultimate strength or failure mechanisms of the structure.
(c) Ultimate Load Design (Ultimate Strength Design / Load Factor Design): This philosophy focuses on the ultimate load-carrying capacity of the structure. Loads are multiplied by load factors, and the structure is designed such that its ultimate strength (based on yield or ultimate strength of materials) is greater than or equal to these factored loads. It primarily considers the ultimate strength condition (collapse) but might not explicitly address serviceability criteria like deflections as comprehensively as LSD.
(d) Plastic Design: This is a specific method used mainly for steel structures, based on the ability of steel to undergo plastic deformation (yielding) and redistribute stresses. It considers the formation of plastic hinges and the plastic moment capacity of sections to determine the ultimate load-carrying capacity (collapse load). It focuses on the ultimate strength condition by allowing for plastic behavior. The design philosophy that explicitly considers both conditions related to ultimate strength (for safety against collapse) and conditions related to behavior under service loads (often elastic, for serviceability like deflection and cracking) is Limit State Design (LSD). \[ \boxed{\text{Limit State Design}} \]