Design of Bolted Joints and Fasteners

Concept

Bolted joints use threaded fasteners to compress multiple parts together. The structural integrity of a bolted joint depends heavily on the initial tension applied to the bolt during assembly, known as the preload. A properly preloaded bolt stretches slightly, acting like a very stiff spring, while the clamped parts compress. When an external tensile load is applied to the joint, the bolt takes on only a portion of the external load, while the clamped parts relieve some of their initial compression. Understanding the stiffness (spring rate) of both the bolt and the clamped members is crucial to ensure the joint remains tightly clamped and the bolt does not yield or fail from fatigue.

Formula & Method

The stiffness of the bolt ($k_b$) and the stiffness of the clamped members ($k_m$) determine how the external tensile load ($P$) is shared. The proportion of the load taken by the bolt ($P_b$) and the proportion taken by the members ($P_m$) can be determined by the joint constant ($C$):

The resultant total force on the bolt ($F_b$) and the remaining compressive force in the members ($F_m$) after an external load $P$ is applied and with an initial preload $F_i$ are:

To prevent the joint from separating, $F_m$ must be greater than zero. To prevent the bolt from yielding, $F_b$ must safely remain below the proof load of the bolt.

Worked Example

A detailed worked example for designing a bolted joint with an interactive system is available at Calculation Example: Bolted Joint Design.

Engineering Meaning

The joint constant $C$ demonstrates why applying preload is highly effective. Because the clamped members ($k_m$) are usually significantly stiffer than the bolt ($k_b$), the value of $C$ is small (often between 0.2 and 0.3). Therefore, when an external force is applied, the bolt "feels" only a fraction of that force. The majority of the external load is instead spent relieving the compression in the clamped members. This is extremely important for preventing fatigue failure in bolts subjected to fluctuating loads.

Engineering Check

The preload $F_i$ must be carefully controlled during assembly, typically done through specifying a tightening torque. If the preload is too low, the joint may separate under load, leading to rapid bolt failure or leakage. If the preload is too high, the bolt may yield during tightening. The stiffness $k_m$ can be difficult to calculate and is often estimated using a "frustum of a cone" concept extending through the clamped plates.

Explicit Exclusions

This fundamental article does not cover detailed methods for calculating the stiffness of clamped members (the frustum cone method), analysis of thread stripping shear failure, and the design of welded joints.