Understanding Permissible Tensile Stress in Column Base Bolts

The question asks about the permissible tensile stress for bolts specifically used in column base connections. In structural design, it is crucial to ensure that components like bolts do not fail under the expected loads. Design codes specify permissible stresses for different materials and loading conditions to guarantee safety and serviceability.

Permissible Stress for Bolts in Steel Structures

Steel bolts are subject to various stresses when used in structural connections, including tension and shear. The permissible stress values are generally lower than the material's yield or ultimate strength to provide a factor of safety. These values are determined based on extensive research, testing, and the requirements of relevant design standards, such as IS 800 (the Indian standard for steel structures).

For bolts in tension, the permissible tensile stress depends on the grade of the bolt and the specific application. Column bases often involve anchor bolts that connect the column base plate to the concrete foundation, and these bolts can experience significant tensile forces, especially under overturning moments or uplift conditions.

Analyzing the Options

Let's consider the typical range of permissible stresses for bolts in steel structures:

• Tensile Stress: Permissible tensile stress for bolts is usually in the range of 100 to 200 N/mm², depending on the bolt grade (e.g., 4.6, 8.8) and the specific code provisions.
• Shear Stress: Permissible shear stress is also specified but is different from tensile stress.
• Bearing Stress: Permissible bearing stress on the connected parts is also relevant but not directly about the bolt material's internal stress.

Looking at the given options:

• 120 N/mm²
• 150 N/mm²
• 0.4 N/mm²
• 0.6 N/mm²

The values 0.4 N/mm² and 0.6 N/mm² are extremely low for permissible stress in steel bolts under tension. These values are significantly less than even the yield strength of mild steel ($~250 N/mm^2$), let alone high-strength steel used for bolts, and much lower than typical permissible stresses. Permissible stresses are usually a fraction of the yield strength (e.g., 0.6 $\times$ Yield Strength or 0.5 $\times$ Ultimate Strength), but the numbers 0.4 and 0.6 are too small to represent this fraction of a material strength measured in N/mm².

The values 120 N/mm² and 150 N/mm² fall within the expected range for permissible tensile stress in steel bolts. Specifically, for common structural bolts (like grade 4.6), the permissible tensile stress is often around 120 N/mm² as per design standards for certain conditions, or it might be related to the proof load or yield strength divided by a safety factor.

Based on standard design practice and code provisions for bolts in tension, including those used as anchor bolts in column bases, a value like 120 N/mm² is a commonly cited permissible tensile stress for certain bolt grades and applications.

Conclusion on Permissible Tensile Stress

Considering the typical values specified in structural design codes for the permissible tensile stress in steel bolts, especially those used for critical connections like column bases, the value of 120 N/mm² aligns with standard practice for certain bolt grades and design scenarios.

Therefore, 120 N/mm² represents a permissible tensile stress value commonly associated with bolts used in column bases.

Revision Table: Key Concepts in Bolt Stress

Additional Information on Bolt Design and Column Bases

Designing column bases involves considering various load combinations, including axial load, shear, and bending moment. The anchor bolts must be designed to resist the resulting tension and shear forces transferred from the base plate to the foundation. The permissible stresses used in design calculations are derived from material properties and safety factors specified in standards like IS 800.

The actual permissible tensile stress can vary based on:

• Bolt Grade: Higher grade bolts (e.g., 8.8, 10.9) have higher material strengths and thus higher permissible stresses than lower grade bolts (e.g., 4.6).
• Loading Conditions: Static vs. dynamic loads.
• Code Provisions: Specific requirements and safety factors mandated by the applicable design code.

For anchor bolts in tension, checks are performed to ensure the bolt itself is adequate (based on permissible tensile stress), the anchorage length in concrete is sufficient, and the concrete doesn't cone out. The permissible tensile stress of 120 N/mm² is a common value for grade 4.6 bolts under axial tension as per some standards.