When an external tensile load tries to separate a preloaded joint, the entire load is not normally added directly to the bolt force while the joint remains clamped.

The bolt and the compressed members act like coupled elastic springs.

Joint stiffness ratio

If the bolt stiffness is $k_b$ and the effective member stiffness is $k_m$, a simple linear joint model assigns the fraction

$$C=\frac{k_b}{k_b+k_m}$$

of an external separating load $P$ to increased bolt tension.

The bolt force increases approximately by

$$\Delta F_b=CP,$$

while the member compression decreases by approximately

$$\Delta F_m=(1-C)P.$$

Joint separation

As external load rises, member compression can eventually fall to zero. Beyond joint separation, the simple clamped-spring load-sharing model changes and the bolt can receive much more of the external load.

Why stiffness matters

A relatively flexible bolt combined with stiff clamped members can reduce the cyclic change in bolt load caused by a fluctuating external force.

This is one reason a carefully preloaded joint can give a bolt better fatigue conditions than an apparently looser connection.