Understanding Limit State Design in Steel Structures per IS 800:2007

Limit State Design is the primary method used in IS 800:2007 for designing steel structures. This approach considers various potential failure modes or conditions that a structure might experience throughout its lifespan and ensures the structure remains safe and serviceable. These potential failure conditions are categorised into different "limit states".

According to IS 800:2007, limit states are broadly classified into two main categories:

• Limit State of Strength
• Limit State of Serviceability

Limit State of Strength Explained (IS 800:2007)

The Limit State of Strength deals with the safety of the structure against collapse or failure under ultimate loads. It focuses on ensuring that the structure can safely resist the maximum expected loads without failure. IS 800:2007 lists several phenomena that fall under the Limit State of Strength. These typically include:

• Loss of equilibrium of the structure or its components (e.g., overturning).
• Loss of stability of the structure or its components (e.g., buckling).
• Rupture of critical sections or components.
• Plastic collapse.
• Progressive collapse.
• Fracture due to fatigue.
• Brittle fracture.

These are conditions where the structural integrity is compromised, potentially leading to a catastrophic failure.

Limit State of Serviceability Explained (IS 800:2007)

The Limit State of Serviceability, on the other hand, deals with the performance of the structure under normal service loads. It focuses on ensuring that the structure remains functional and comfortable for occupancy during its design life. Phenomena considered under the Limit State of Serviceability include:

• Deflection (excessive deformation).
• Vibration (unacceptable oscillations).
• Durability (resistance to environmental effects like corrosion).
• Fire resistance.

Failure in the Limit State of Serviceability does not usually imply collapse but can affect the usability or appearance of the structure.

Analyzing the Given Options based on IS 800:2007

Let's examine each option in the context of the Limit State of Strength as defined by IS 800:2007:

1. corrosion and durability: As discussed, durability, which includes resistance to corrosion, is related to the long-term performance and resistance to environmental degradation. IS 800:2007 classifies this under the Limit State of Serviceability, not the Limit State of Strength.
2. rupture of the structure: Rupture involves the actual breaking or tearing apart of a structural element, which is a direct failure mechanism. This is explicitly included under the Limit State of Strength in IS 800:2007.
3. stability against overturning: Overturning is a form of loss of equilibrium or stability of the entire structure or a part of it. Ensuring stability against overturning is a critical aspect of preventing structural collapse and is thus included in the Limit State of Strength as per IS 800:2007.
4. fracture due to fatigue: Fatigue fracture occurs when a material fails under repeated or cyclic loading, even if the stresses are below the yield strength. This is a failure mode that can lead to structural collapse over time and is considered under the Limit State of Strength in IS 800:2007.

Based on the provisions of IS 800:2007 regarding limit state design, corrosion and durability are aspects of the Limit State of Serviceability, while rupture, stability against overturning, and fracture due to fatigue are aspects of the Limit State of Strength.

Conclusion on IS 800:2007 Limit States

Therefore, according to IS 800:2007, the Limit State of Strength does NOT include corrosion and durability. These are covered under the Limit State of Serviceability.

Revision Table: IS 800:2007 Limit States for Steel Design

Additional Information on IS 800:2007 Design Philosophy

IS 800:2007 provides the standard provisions for the design of steel structures in India. The Limit State Method is the preferred design philosophy in this code, offering a more rational and economic approach compared to the older Working Stress Method.

The design process using the Limit State Method involves:

1. Identifying all potential limit states relevant to the structure.
2. Calculating the design loads by applying appropriate partial safety factors to the characteristic loads.
3. Calculating the design strength or resistance of the structural elements by applying appropriate partial safety factors to the characteristic material strengths.
4. Ensuring that for all relevant limit states, the design action effect (due to design loads) does not exceed the design resistance/capacity.

For the Limit State of Strength, the focus is on ultimate load-carrying capacity and stability. For the Limit State of Serviceability, the focus is on performance under normal service loads. Both limit states must be satisfied for a safe and functional design according to IS 800:2007.