Understanding Allowable Stress in Axial Tension

Allowable stress is a critical concept in structural engineering. It represents the maximum stress that a material or structural member can safely withstand under a specific type of loading, such as axial tension, bending, or shear. This value is typically determined by dividing the yield strength or ultimate tensile strength of the material by a factor of safety. The factor of safety accounts for uncertainties in material properties, loading conditions, design assumptions, and manufacturing processes.

Effect of Member Thickness on Allowable Stress

For members subjected to axial tension, design codes and standards often specify that the allowable stress must be reduced if the thickness of the member exceeds a certain limit. This is done to account for various factors that can affect the behavior and strength of thicker materials.

Based on common design practices and the options provided, the allowable stress in axial tension is generally kept less if the thickness of the member is more than 20 mm.

Why Allowable Stress is Reduced for Thicker Members

Several reasons contribute to the reduction in allowable stress for thicker structural members under axial tension:

• Residual Stresses: Thicker sections, especially rolled steel sections or heavily welded plates, can contain higher levels of residual stresses locked in during the manufacturing process (cooling, rolling, welding). These residual stresses can add to the stresses caused by the applied external loads, potentially causing the material to yield or fail earlier than anticipated based purely on the applied load.
• Material Properties: In some materials, particularly certain grades of steel, the mechanical properties like ductility, notch toughness, or even yield strength can vary slightly through the thickness, with the core sometimes exhibiting slightly different behavior than the surface.
• Through-Thickness Properties: Resistance to tearing or fracture in the through-thickness direction can sometimes be a concern in very thick plates under complex stress states, although axial tension is relatively simple.

By reducing the allowable stress for thicknesses exceeding 20 mm, engineers add an extra layer of safety to account for these potential issues, ensuring the structural integrity and durability of the member under axial tension.

Defining Axial Tension

Axial tension occurs when a structural member is subjected to forces that pull on it along its longitudinal axis, tending to stretch it. Examples include the forces in the bottom chord of a truss, hanger rods supporting a load, or anchor bolts pulling on a foundation.

Summary: Thickness Limit for Allowable Stress in Axial Tension

In the context of the question, the specific thickness threshold identified is 20 mm. This means that structural design calculations for axial tension members with a thickness greater than 20 mm will typically use a lower allowable stress value compared to thinner members.

Revision Table: Key Concepts in Axial Tension

Additional Information: Design Code Requirements

It is important to note that the exact thickness threshold (like 20 mm) and the specific method for reducing allowable stress for thicker sections are detailed in relevant national or international structural design codes (e.g., AISC in the US, Eurocode in Europe, IS codes in India). These codes provide precise formulas, tables, and requirements that engineers must follow. The 20 mm limit is a common value found in many standards where special considerations for thicker plates become necessary.