RCC-Design.github.io

METHODOLOGY

✅ Design Methodologies in Structural Engineering

1. Working Stress Method (WSM)

Also known as:

• Elastic Method
• Modulus Ratio Method
• Factor of Safety Method

🔹 Key Assumptions & Principles

• Structural materials behave elastically under working (service) loads.
• Stress remains within permissible limits using a factor of safety.
• Cross-sections are designed such that:
  Developed Stress < Permissible Stress
• Allowable stress lies in the elastic range, below the yield stress.

🔹 Design Parameters

• Allowable stress = 0.6 f_y
• Design load (RCC) = Working/Service load
• Permissible stress:
  [ \text{Permissible Stress} = \frac{\text{Characteristic Strength}}{\text{Factor of Safety}} ]

Example (M20 Concrete):

• ( f_{ck} = 20 \, \text{MPa} )

🔹 Factors of Safety

Material	Factor of Safety
Concrete	3.0
Steel	1.78

🔹 Additional Notes

• Based on uniqueness theory (experience-based).
• Considers the lower bound theorem.
• Serviceability is not addressed in this method.

---

2. Ultimate Load Method (ULM)

Also called:

• Plastic Design Method
• Ultimate Load Method

🔹 Design Load in RCC

[ \text{Design Load} = \text{Working/Service Load} \times \text{Load Factor} ]

• Load factor depends on working conditions.

🔹 Design Stress

[ \text{Design Stress} = \frac{\text{Characteristic Strength}}{\text{Factor of Safety}} ]

🔹 Material Factors of Safety (Beams)

Material	Factor of Safety
Concrete	1.5
Steel	1.15

🔹 Theoretical Basis

• Upper Bound Theorem → used for determining collapse load
• Lower Bound Theorem → used for material strength
• Serviceability not considered, which reduced usage

---

3. Limit State Method (LSM)

A modern and advanced philosophy that considers both safety and serviceability.

🔹 Comparison with Other Methods

Method	Basis of Design Load
Working Stress Method	Working (Service) Load
Ultimate Load Method	Ultimate/Collapse Load
Limit State Method	Working Load (for Safety & Serviceability)

🔹 What is a Limit State?

A limit state is the acceptable threshold beyond which a structure no longer satisfies safety or serviceability requirements.

---

✅ Types of Limit States

🛡️ A. Limit State of Strength

Ensures safety against structural failure and loss of life/property under worst load combinations.

Major Types:

• Loss of equilibrium
• Loss of stability
• Excessive deformation or rupture
• Fatigue fracture
• Brittle fracture

---

🏠 B. Limit State of Serviceability

Ensures structure performs satisfactorily during its use without discomfort or damage.

🔧 Major Serviceability Criteria

Type	Description
Deflection	Causes cracks in plaster, damage to finishes, machinery misalignment, reduced strength
Fatigue	Repeated loading affects bridges, cranes, vibrating structures
Vibration	Significant in light structures and long spans (e.g., halls, platforms)
Fire Resistance	Depends on material, geometry, support, and fire protection
Durability	Ability to function throughout lifespan under expected exposure conditions

---

🌱 Durability & Sustainability Factors

• Environmental conditions
• Degree of exposure
• Shape of members
• Structural detailing
• Ease of maintenance

Difference between Working Stress Method and Limit State Method

Working Stress Method	Limit State Method
It is published in IS: 456 - 1978, 800 – 1984.	It is published in IS: 456 - 2000, 800 – 2007.
It is called an Allowable Stress Method (ASM) or Elastic Design Method.	It is also called a Plastic Design Method.
Allowable stress is within its range of elastic limit.	Partial Safety Factor (γmo) is used for Yield Stress and Partial Safety Factor (γm1) is used for Ultimate Stress.
Analysis of the structure is done by the working load.	Working load is factorized by partial safety factor (YF). Analysis of structure is done by factored load.
Working load does not yield or buckle the material.	The behaviour of the material after yield plays an important role in determining the capacity of the material.
Deformation is calculated from working load.	Deformation is calculated from the working load.
Serviceability and economy not taken into consideration during design.	Serviceability and economy are taken into consideration.
It gives heavy design.	It offers lighter sections.
Fatigue and fire resistance not taken in calculation.	Fatigue and fire resistance are taken into account.