Tekla Structures uses values from the bolt catalog and the bolt assembly catalog when it calculates the bolt length. If the bolt catalog does not contain bolts with the length that you need, you must add them to the bolt catalog.

These Assembly settings in the Bolt properties dialog affect the bolt length calculation.

(1) If this checkbox is not selected, only a hole is created.

(2) First washer

(3) Second washer

(4) Additional washers

(5) First nut

(6) Second nut

If the checkbox for a bolt element is selected, the bolt element is used in the bolt assembly and in the bolt length calculation.

The chart and the steps below explain the process of bolt length calculation.

1. Tekla Structures calculates the minimum possible length of the bolt:

   first washer thickness
   + material thickness
   + second washer thickness
   + additional washer thickness
   + first nut thickness
   + second nut thickness
   + extra length

2. Tekla Structures searches for the closest match in the bolt catalog.

3. Tekla Structures calculates the number of washers required (must not exceed 10) so that the length of the shaft is less than:

   first nut thickness 
   + material thickness 
   + second nut thickness 
   + first washer thickness 
   + second washer thickness 
   + (number of washers * additional washer thickness)

4. Tekla Structures checks that the bolt found in step 2 is longer than:

   extra length 
   + first nut thickness 
   + material thickness 
   + second nut thickness 
   + add. dist (from the bolt catalog) 
   + first washer thickness 
   + second washer thickness 
   + (number of washers * additional washer thickness)

5. If the selected bolt does not fulfill the criteria in step 4, Tekla Structures returns to step 2. Otherwise, it continues to step 6.

6. Tekla Structures checks that the selected bolt fulfills all the following conditions.

   These calculations refer to the following parts of the bolt:

   

   (1) Total shaft length

   (2) Grip length: the length of the unthreaded shank, excluding the runout

   (3) Thread length: the length of the thread, excluding the runout

   (4) Runout: the part of the bolt between the unthreaded shank and the thread

   • Can the thread be inside the material to be connected?

     Even if the thread cannot be inside the material, the calculation always allows 3 or 4 mm of thread to be inside the material, depending on the bolt diameter. If the bolt diameter is ≥ 24 mm, it allows 4 mm, otherwise it allows 3 mm.

   • Grip length must be more than:

     material thickness
     + extra length
     + first washer thickness
     - maximum allowed thread in material (OR 3 mm or 4 mm if thread in material = no)

   • Grip length is calculated as:

     total shaft length
     - thread length
     - runout

   • The length of the runout is calculated as follows:

     Diameter of bolt (mm)	Runout (mm)
     >33.0	10.0
     >27.0	8.0
     >22.0	7.0
     >16.0	6.0
     >12.0	5.0
     >7.0	4.0
     >4.0	2.5
     ≤4	1.5

7. If the selected bolt does not fulfill all the above conditions, Tekla Structures returns to step 2 and tries the next longest bolt.

8. If the advanced option XS_​BOLT_​LENGTH_​EPSILON is set, the epsilon thickness is added to or subtracted from the material thickness to avoid inaccurate bolt length calculation.

   For example, if this value is not taken into account and the calculated length is 38.001 mm, a 39 mm bolt might be selected.