2
$\begingroup$

enter image description here

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?

$\endgroup$
2
$\begingroup$

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

enter image description here

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.

enter image description here

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)

enter image description here

Figure 3: Distribution of load along threads (source sciepub František Trebuňa)

The graph shows that approximately 34% of the load is transferred on the 1st thread, about 70% by the first 3, and about 100% by all 6 threads.

$\endgroup$
3
  • $\begingroup$ The elongation length is the stretching of the bolt under correct tension, where sigma =Ft x Lb / E where Lb is thr effective length and Ft the required axial stress. $\endgroup$
    – morbo
    Jun 16 at 7:36
  • $\begingroup$ Your force diagram is incorrect. The axial force is assumed to be transferred to the nut over the entire length of the nut, which is why half the length of the nut is included. The head is more complex but probably approximated by considering it equivalent to a nut. $\endgroup$
    – ingenørd
    Jun 16 at 15:59
  • $\begingroup$ @ingenørd I stand corrected. Approximately 1/3 of the load is after the 3rd thread which is in engagement (usually a nut is about 6 threads long). Thanks for correcting me. $\endgroup$
    – NMech
    Jun 16 at 16:12

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.