HR (British/French) bolt
The British/French approach following BS EN 14399-3 and BS 4395 is to use thick nuts and long thread lengths in the bolt assembly to obtain ductility predominantly by plastic elongation of the bolt.
The longer thread length is necessary to ensure that the induced strain is not localised.
These bolts are relatively insensitive to overtightening during preloading although suite control is still important.
Furthermore if severely over-tightened during preloading the ductile failure mode of the bolt assembly is predominantly by bolt breakage, which is readily detectable.
As per EN 14399-1:2015 pt. 4.2.1:
- Type HR (systems HR with or without direct tension indicators, if provided, and HRC) designed to obtain ductility predominantly by plastic elongation of the bolt (minimum nut height ≥ 0,9 D and thread length of the bolt according to ISO 888);
HV (German) bolt
The German approach following BS EN 14399-4 and DIN 6914 is to use thinner nuts and shorter thread lengths to obtain the required ductility by plastic deformation of the threads within the nut.
In Germany, the HV bolt assembly is used in both preloaded and non-preloaded applications and it can be argued that in the event of failure by thread plastic deformation the assembly still acts as a non-preloaded assembly.
These assemblies are more sensitive to over-tightening during preloading and therefore require more site control.
If severely overtightened during preloading the mode of failure by plastic deformation of the engaged thread of the bolt assembly offers little indication of impending failure.
As per EN 14399-1:2015 pt. 4.2.1:
- Type HV (system HV with or without direct tension indicators, if provided) designed to obtain ductility predominantly by plastic deformation of the engaged threads (nut height at approximately 0,8 D and bolt with short thread length).
Read more: Steel Industry Guidance Notes: Standard for Preloadable Bolts
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