Jawaharlal Nehru Technological University Hyderabad B.Tech III DESIGN OF STEEL STRUCTURES Previous Question Paper (August - 2022 - R16)

Welded joints offer several structural and economic advantages over traditional riveted joints, though they also present specific challenges regarding quality control and material behavior.

Advantages:

• Efficiency and Weight: Welded connections are more efficient as they do not require holes for fasteners, meaning the full cross-sectional area of the member is available to resist loads. This leads to a reduction in the weight of the structure.
• Aesthetics and Smoothness: Welded joints provide a smooth, clean appearance, which is advantageous for architectural structures and reduces areas where corrosion can initiate.
• Speed and Economy: Welding is generally faster than riveting, requiring fewer components (such as gusset plates or bolts), which reduces labor and material handling costs.
• Rigidity: Welded joints provide high structural rigidity and continuity, which is beneficial for frame design and load distribution.
• Noise Reduction: Welding is a silent process compared to the noise generated by pneumatic riveting hammers.

Disadvantages:

• Quality Control: The integrity of a weld is highly dependent on the skill of the welder. Defects such as porosity, slag inclusions, or lack of fusion are difficult to detect without non-destructive testing (NDT).
• Residual Stresses: The intense local heating and subsequent cooling during welding induce significant residual stresses and potential distortion in the parent metal.
• Brittle Fracture: Welded joints are more susceptible to brittle fracture, particularly at low temperatures, due to the metallurgical changes in the Heat Affected Zone (HAZ).
• Inspection Difficulty: Unlike rivets, which can be visually inspected for looseness, internal weld defects require specialized equipment like ultrasonic or radiographic testing.

Given: Factored load (P) = 250 kN = 250,000 N; Steel grade = Fe410 (f_u = 410 MPa); Shop welding (γ_mw = 1.25).

1. Size of Weld (s): For plates of 10 mm thickness, the minimum size of the fillet weld as per IS 800:2007 is 3 mm. Providing a size of 6 mm is practical and standard for this thickness.

2. Throat Thickness (t_t):
t_t = k × s = 0.7 × 6 = 4.2 mm.

3. Design Strength of Weld per mm length (f_wd):
f_wd = (f_u / (√3 × γ_mw)) × t_t
f_wd = (410 / (√3 × 1.25)) × 4.2 = 189.37 × 4.2 = 795.35 N/mm.

4. Required Length of Weld (L):
L = P / f_wd = 250,000 / 795.35 = 314.33 mm.

Adding 2 times the weld size for end returns (2 × 6 = 12 mm), the total length required is approximately 326 mm. Providing two side fillet welds, the length per side = 326 / 2 = 163 mm.

Result: Provide a 6 mm fillet weld with a total effective length of 326 mm (163 mm on each side).