An introduction to preloaded bolting
NSC reports on bolted connections, and the regulations, standards and current
practice for preloaded bolting.
10 NSC
Nov/Dec 18
Bolted connections in structural steel
can be separated into two categories.
Where a small amount of joint slip
has no consequence for the structure, nonpreloaded
bolting is satisfactory. However,
if connections are subject to vibration,
load reversal, fatigue or where slip must
be avoided, preloaded bolting assemblies
should be used.
Standards
Preloaded bolting assemblies are covered by
the BS EN 14399 series European standard
which comprises 10 parts; of particular note
are:
Part 1: General requirements (this is
the harmonised or CE Marking
standard)
Part 2: Suitability test for preloading
Part 3: System HR – Hexagon bolt and nut
assemblies
Part 4: System HV – Hexagon bolt and nut
assemblies
Part 7: System HR – Countersunk head
bolt and nut assemblies
Part 9: System HR or HV – Direct
tension indicators for bolt and nut
assemblies
Part 10: System HRC – Bolt and nut
assemblies with calibrated preload
Direct Tension Indicators (DTI) are also
known as ‘load indicating washers’ and
system HRC is the same as Tension Control
Bolts (TCBs)
Current practice
Currently only system HR (High Resistance)
and HRC (High Resistance Calibrated)
assemblies are used in the UK. System
HV (Hochfest Vorgespannte Verbindung)
assemblies have a short bolt thread and use
a thinner nut to obtain ductility by plastic
deformation of the threads within the nut.
Both HR and HRC assemblies achieve the
necessary ductility primarily by plastic
deformation of the bolt threads. These
systems of bolting are less susceptible to
overtightening during preloading. However,
if these assemblies are overtightened, the
ductile failure mode is by yielding and
eventually fracture of the bolt. This type of
failure is easily detectable. The use of both
HR/HRC and HV systems on the same site
should always be avoided because of the risk
of confusion and misuse.
System HR assemblies are available in
M12 to M36 diameter in property class 8.8
or 10.9, whilst HRC (or TCBs) are available
from M12 to M36 in property class 10.9
only. Both HR and HRC are usually supplied
in the UK as k-class K0 only (no specific k
factor or torque value required).
Countersunk head preloaded bolts are
available as system HR and HRC (or TCBs)
and are used where a flush finish is essential
for functional reasons.
Weathering grade bolting assemblies
are not specifically covered in the BS EN
14399 series. The execution standard EN
1090-2 section 5.6.6 gives advice in regard
to suitable fasteners, but there is limited
availability of such bolting assemblies
in the UK. System HR weathering grade
assemblies are available in property class 8.8,
M24 diameter only. System HRC (or TCBs)
are available in property class 10.9, M24
and M30 diameter; availability of any stock
should always be sought from suppliers.
Installation and tightening of bolts is a
major site activity and the designer should
always consider the access for operatives
and equipment. If insufficient care is taken,
the result can be components that cannot be
fitted or bolts that cannot be tightened with
standard equipment. CAD blocks for both
bolting assemblies and associated tooling
(e.g. shear wrenches to install TCBs) for
use in modelling are available from most
reputable suppliers/manufacturers.
Sector Focus: Structural Fasteners
Sponsors
Structural Fasteners
Bronze: Tension Control Bolts Ltd
A typical preloaded
bolted assembly
/Fatigue_design_of_bridges#The_mechanism_of_fatigue
/Preloaded_bolting
/CE_marking
/Preloaded_bolting#Direct_tension_indicator_.28DTI.29_method
/Preloaded_bolting#HRC_method
/Weathering_steel#Preloaded_bolts
/Preloaded_bolting#Installation_of_preloaded_bolts