Bolt length calculation: thread inside material

Question:
In bolt length calculation algorithm description, it says that “even if thread in material is not allowed, the calculation always allows 3 or 4 mm of thread to be inside the material, depending on the bolt diameter.”
But I actually never saw it happen? Why? 

Answer:
Bolt length calculation algorithm consists of six basic steps. You can see all of them here.

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If the option “Thread in material” is set to  “No” in bolt properties, then the bolt needs to pass the final check at step 6 of the bolt length calculation algorithm (“Yes” means that step 6 is omitted). Algorithm checks there if length of the shaft (unthreaded part of bolt) is big enough. It can allow for 3 or 4 mm of material to be inside the part, but for that to happen bolt needs to pass the test:

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The main “problem” is that thread end (“run out”) is taken into consideration. Run out is a part of the bolt between the shaft and the thread. It’s value varies depending on bolt diameter. For most bolts (<12 mm diameter) it is more than 5 mm.

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Even if Tekla Structures allows for 3-4 mm of thread to be inside the material, “run out” value is still bigger (5 mm is more than 3 or 4 mm). In most cases this causes for the bolt to fail the final check and longer bolt is chosen.

Only for bolts that are smaller than 4 mm runout is small enough to allow the thread to be inside the material. See below pictures for explanation. On those pictures differently colored parts are used to visualize and dimension different “zones” of bolt and aforementioned values used by the algorithm.

Here is an example for bolt M16*80 and M3*80 used on the same situation (same material thickness). They both use the same washers and nut thickness. The only difference between those bolts is diameter.
The basic idea is that the red length (calculated shaft length) must be longer than purple length (material thickness, minus 3 or 4 mm) for the bolt to be selected as long enough.

M16*80:
(click here to see full size picture)

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M3*80:

(click here to see the full size picture)

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Bolt M16*80 would not be selected because calculated shaft length is too small (52 mm against 55 mm). Algorithm will choose next bolt from catalog and run the same tests.

Bolt M3*80 would be selected as the correct one because its calculated shaft length is big enough (55.5 mm against 55 mm).