Prying Force in Structural Connections

In the context of structural engineering, especially concerning bolted connections, prying force is a critical concept. Prying force refers to an additional tensile force that develops in fasteners (like bolts) due to the flexibility of the connected elements, such as plates or brackets. When an external tension force is applied to a connection, it can cause the connected plates to deform or bend. This bending action, often referred to as a "lever arm" effect, induces an extra pulling force on the bolts, beyond the initial directly applied tension.

Understanding Additional Tension Force Due to Prying

The phenomenon of prying occurs because the flexible elements in a connection tend to flatten out when subjected to a tensile load. This straightening action creates a lever effect that pulls the bolts further, thereby imposing an additional tension force on them. This added stress can significantly increase the total load that the bolts must withstand.

• Mechanism: Consider a T-stub or an end plate connected by bolts. When a tensile load is applied, the flanges of the T-stub or the edges of the end plate may bend outwards, acting as levers. This lever action exerts an increased pulling force on the bolts.
• Importance in Design: Ignoring prying forces can lead to underestimation of the actual tension experienced by bolts, potentially resulting in premature failure of the connection. Structural engineers must account for this additional tension to ensure the safety and reliability of the structure.

Analysis of the Given Options for Prying Force

Let's evaluate each option to determine the correct description of prying force:

• Additional tension force: This option accurately describes prying force. As discussed, the lever action created by the flexibility of connected components under tensile load directly results in an increased pulling, or tensile, force on the bolts. This makes the fasteners work harder in tension.
• Additional bending moment: While bending of the plate or connected element is the cause of prying action, the prying force itself is a force applied to the bolt, not a bending moment on the bolt. The bending moment occurs within the plate, which then translates into an additional tensile force on the bolt.
• Additional compressive force: Compressive forces act to push components together. Prying force, by its nature, acts in the opposite direction, increasing the separation tendency and thus adding to the tension in the fasteners. Therefore, it is not a compressive force.
• Additional torsional force: Torsional forces are twisting forces. Prying force arises from bending and direct tension, not from twisting actions. Hence, it does not represent an additional torsional force on the connection.

Conclusion on Prying Force Definition

Based on the mechanics and effects observed in structural connections, prying force is fundamentally an additional tension force exerted on the fasteners due to the lever action of flexible components. Proper consideration of this additional tension is vital for the safe and efficient design of bolted connections in any structure.