JNTU B.Tech III DESIGN OF STEEL STRUCTURES Previous Question Paper (March/April - 2021 - R16)

In the limit state design of steel structures, the objective is to ensure that the structure remains fit for its intended use throughout its design life. The design process considers two primary categories of limit states: the Limit State of Strength and the Limit State of Serviceability.

The Limit State of Strength relates to the safety of the structure and its occupants. It ensures that the structure can withstand the design loads without collapsing. The key considerations include:

• Plastic collapse: Ensuring the structure does not undergo uncontrolled deformation leading to failure.
• Buckling: Preventing instability in compression members, such as columns, or in the compression flanges of beams.
• Fracture: Avoiding failure due to excessive stress in tension members or connections.
• Fatigue: Considering the effect of repeated load cycles, which can lead to crack propagation and sudden failure.
• Brittle fracture: Preventing failure in steel at low temperatures or under high strain rates.

The Limit State of Serviceability relates to the functional performance and comfort of the users under normal service conditions. It ensures the structure remains aesthetically pleasing and operational. The key considerations include:

• Deflection: Limiting vertical and horizontal displacements of beams and frames to prevent damage to non-structural elements like partitions, glazing, or ceilings.
• Vibration: Controlling the dynamic response of floors and bridges to ensure human comfort and prevent structural damage.
• Durability: Protecting the steel against corrosion and environmental degradation through protective coatings or material selection.
• Fire resistance: Ensuring the structure maintains its integrity for a specified duration during a fire event.

By designing for these limit states using partial safety factors for both loads and material strengths, the structural engineer achieves a reliable balance between safety, economy, and functionality.

To determine the length of the fillet weld for a tie member of 75 mm × 8 mm carrying a load of 90 kN, we assume the use of site welds and Fe410 grade steel with E410 electrodes.

1. Design parameters:

The effective throat thickness t_e is taken as 0.7 × s, where s is the weld size. For an 8 mm plate, the minimum weld size is 3 mm and the maximum is 6.5 mm (8 mm - 1.5 mm). We select a weld size s = 6 mm.

t_e = 0.7 × 6 = 4.2 mm.

2. Design strength of the weld:

The design stress of the weld f_wd = f_u / (√3 × γ_mw) = 410 / (1.732 × 1.5) = 157.6 N/mm².

3. Required weld length:

Total load P = 90,000 N.

Rounding up, we provide a total weld length of 136 mm.

4. Overlap calculation:

For a lap joint with two longitudinal fillet welds, the length of each weld is L/2 = 68 mm. Adding the necessary end returns (usually 2 × s = 12 mm per weld end), the required overlap length is 68 mm + 12 mm = 80 mm.