Question

What is the primary factor considered in the elastic design of structural elements?

• A. Ultimate strength
• B. Serviceability
• C. Ductility
• D. Plasticity

Hint:

\textbf{Elastic Design (Working Stress Design):}
Assumes elastic behavior of materials under service loads.
Stresses under service loads $\le$ Allowable stresses (which are a fraction of yield/ultimate strength).
Primary focus is ensuring satisfactory performance (\textbf{serviceability}) under normal operating conditions, e.g., limiting deflections and preventing yielding.
Ultimate strength is considered in Ultimate Load Design or Limit State Design (ULS). Ductility and plasticity relate to behavior beyond the elastic limit.

Answer 1

The Correct Option is B

Elastic Design (also known as Working Stress Design or Allowable Stress Design) is a traditional structural design philosophy. Its core principles are:
Materials are assumed to behave elastically under service (working) loads. This means that stress is proportional to strain (Hooke's Law applies), and deformations are recoverable.
Stresses calculated in structural members under service loads must not exceed pre-defined allowable stresses.
Allowable stresses are typically set as a fraction of the material's yield strength (or sometimes ultimate strength for brittle materials), providing a factor of safety.
The primary focus is to ensure the structure performs adequately and safely under normal operating conditions, without undergoing permanent deformation or excessive deflections that would impair its function or appearance. Let's consider the options:
(a) Ultimate strength: Ultimate strength refers to the maximum load a structure or member can carry before failure (collapse or fracture). This is the primary consideration in Ultimate Load Design or Limit State Design (for Ultimate Limit States), not directly in elastic design which focuses on behavior well below this point.
(b) Serviceability: Serviceability refers to the performance of a structure under normal service loads, related to aspects like deflection, vibration, cracking, and appearance. Elastic design, by ensuring stresses remain within the elastic range and below allowable limits, inherently aims to satisfy serviceability requirements (e.g., limiting deflections by controlling stress/strain). This is a primary concern of elastic design.
(c) Ductility: Ductility is the ability of a material to undergo significant plastic deformation before fracturing. While important for overall structural safety (especially in seismic design), and a property of the material, elastic design itself operates *within* the elastic range, before significant plastic deformation (ductile behavior) occurs.
(d) Plasticity: Plasticity is the property of a material to undergo permanent deformation. Elastic design explicitly tries to *avoid* plastic deformation under service loads by keeping stresses below the yield point. Plastic design methods, on the other hand, utilize plasticity. The primary factor or concern in elastic design is to ensure that the structure behaves well under normal service conditions, remaining within its elastic limits. This is fundamentally about ensuring serviceability (e.g., avoiding excessive deflection, ensuring comfort, maintaining appearance) and preventing yielding under service loads. Safety is addressed through the factor of safety incorporated in allowable stresses. While ultimate strength is important for overall safety, elastic design's calculations are based on service loads and elastic material behavior. \[ \boxed{\text{Serviceability}} \]