# What is the physical significance of bolt elongation length?

Eurocode: design of steel connections mentions something called "bolt elogation length", defined as the grip length of the bolt plus half of the sum of bolt head height and nut height. But what is the significance of this length?

The bolt elongation length is (in all likelihood ) what is referred to as Effective length in the following image

Figure 1: Effective bolt length (source: Resonic)

The physical significance, is the part of the bolt that is in tension, and effectively generates the clamping force. Additionally that length can be used to estimate the spring constant of the bolt:

$$K = \frac{E A }{L}$$

The following image shows the axial forces (N diagram) along the axis of the bolt.

Update

There is a mistake in Figure 2, which ingenord pointed out. The distribution of the force on the nut is along the distance of the bolt (see image below)