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Simple Guide to High Strength Bolting –
Background, Design Approaches
Presented by:
Hossein Bajehkian
Payman Hosseini
21/07/2015
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Outline
 Rivets
 Bolts
 Different methods of Pre-tenstioning
of High-strength Bolts
 Tension Behavior of Bolts and Prying Action
 Shear Behavior of Bolts
 Typical Shear Connections
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Rivets
ASTM A502
More expensive to install than
bolts
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More man power
More equipment
Problematic when slip critical
conditions are required as the
amount of pretension depend
on
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Compactness of griped material
Initial temperature
Pretension is limited to ultimate
strength which is much lower
than bolts
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Bolts
Two Types of Bolts
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Common
High-strength
High-Strength Bolts
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Common Bolts
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ASTM A307
Min tensile strength 415 MPa
Used for general applications
Good for static loads
Cheap
ASTM A325
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ASTM A490
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Min Tensile Strength
830 Mpa for dia, up to 1 inch
725 Mpa for larger dia
Min Tensile Strength 1030 Mpa
Max Tensile Strength 1168 Mpa
Main advantage is the ability to have
large magnitudes of pretension
.
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Pretension of High-Strength Bolts
Pretension is required
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Cyclic Loading
Oversized or slotted holes
Shear connections under seismic
load
Failure to achieve pretension
can lead to unwanted
displacement or fatigue-type
failure
In seismic applications
effectiveness of energy
dissipation mechanism depends
on preloading of the joint
Vibration loosening and loss of
nut is prevented by adequate
pretensioning
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Desired amount of pretension is
70% of bolt’s min specified
tensile strength (2004 CSA Std.)
Snug-tight condition is adequate
for many applications
Four methods of pretentioning
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Turn-of-nut method
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Calibrated wrench
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Use of Tension-Control bolts
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Use of direct tension indicators
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Turn-of-nut Method
Pretension is achieved by turning
the nut past the snug-tight condition
Strain controlled method
Amount of pretension depends on
the friction between the bolt threads
and nut
Amount of pretension is about 35%
higher than minimum specified
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Calibrated Wrench Method
Tension controlled method
There is relationship between the
applied torque and magnitude of
pretension
Complex relationship depends on
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Thread pitch
Thread angle
Friction coefficient between the
nut and bolt
RCSC Specifications does not
recognize the available formulas
and tables to relate the torque to
tension
Wrenches are calibrated on site
using a representative sample of
bolts that are being used
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Tension-control Bolts
Also known as Twist-off Bolts
ASTM F1852
They meet the requirements of A325
but have special features
Pretension depends on
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Bolt material strength
Diameter of annular groove
Thread conditions
Surface conditions at nut-washer
interface
Min pretension achieved is likely to
exceed the specified pretension by
13%.
Three advantages
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Single person can perform the installation
Installation is quicker
Inspection is simple as the spline end of
the bolt is twisted off
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Use of Direct Tension Indicators (DTIs)
ASTM F959
Washer-type elements that have
arch-shaped protrusion
Protrusions compress in response
to tension force in the bolt
Tension force is determined by
measuring the size of the gap
Deformation controlled method and
friction related variables do not
affect the pretension
Min pretension achieved is likely to
exceed the specified pretension by
12%.
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Tension Behavior of Single Bolt
Tensile Strength of a single bolt
depends on:
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Ultimate Tensile Strength
Effective Area
Represented in S16-09 as:
Rn  0.75b Ab Fu
Where:
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Ab is the nominal crosssectional
area of the bolt and 0.75 factor
accounts for the reduction in the
threaded section
Fu is the ultimate tensile strength of
the bolt
φb is the resistance factor for the
bolts which is equal to 0.80
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Tension Behavior of Group of Bolts
Bolts are normally pre-tensioned
to their connected elements to
provide slip resistance
The force in a pre-tensioned
bolt can be calculated assuming
equal deformation of bolt and
plate until the separation of
connected elements
The most important issue is
prying action:
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Occurs when the bolt force is
amplified due to the deformation of
connected elements
Depends on bolt deformation
capacity. flange stiffness and
location of bolts in the flange
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Shear Behavior of Single Bolts
Ultimate Shear Strength of bolts are
closely related to:
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Tensile Strength
Type of Test
Axial Load on the Fasteners (Tension
or Compression)
Factors Affecting Shear Resistance
of a Bolted Connection
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Ultimate Shear Strength ~ 60% of
Tensile Strength
Number of Shear planes and bolts
Shear Area
S16-01 Shear Resistance Equation:
Vr  0.60b nmAb Fu
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Slip Condition in Bolts Loaded in Shear
To ensure low probability of slip
during the life of structure
Joints shall not slip due to
serviceability load
Used in connections subjected
to stress reversals and
fluctuations
Slip Load depends on:
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ks = slip coefficient
m = number of slip planes
ΣT = Sum of bolt tensions
P  ks mTi
S16-09 Slip Resistance Equation:
c1 is a coefficient that relates
the specific initial tension and
mean slip to a 5% probability of
slip for bolts installed by the
turn-of-the-nut method and
depends on the type of bolt
0.53nAbFu denotes the
clamping force of the bolt in the
S16-09 equation where:

0.53 comprises of 0.70 factor which
reflects that the amount of
pretension need to be 70% of the
ultimate tensile strength and 0.75
factor accounts for the reduction in
area
Vs  0.53c1ks mnAb Fu
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Shear Splices
Plates or parts of steel section
connected to each other by
series or rows of bolts
Variations include: lap joints,
shingle joints, gusset plates
The shear plane is parallel to
the longitudinal axis of the
member
Reduction in ultimate capacity
of the bolt due to uneven
distribution of stresses of the
bolts
Higher stresses at the ends of
the plates and lower stresses in
the middle
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Gusset Plate
Used to transfer the load from
one member to the other when
the longitudinal axis of two or
more member are inclined with
respect to each other
Insignificant out of plane
bending
Classical Analysis Procedure:
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Taking sections parallel and
perpendicular to the chord of the
truss
identifying the bolt forces that had
been delivered to the gusset plate,
and utilize these forces to calculate
the shear, normal force, and
moment at the cut section using
beam theory
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Gusset Plate
Whitmore’s Analysis Approach.
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maximum normal stress can be
estimated by assuming that the
member force was distributed
uniformly over an effective area of
plate material
This area was obtained by
multiplying the thickness of the
plate by an effective length which is
obtained by constructing 30° lines
from the outer fasteners to the
bottom fasteners as shown.
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Finite Element Method Analysis
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Several models have been
developed and analyzed the gusset
plate in the elastic range and also
predicted its behavior up to its
ultimate strength
Whitmore’s conclusions were
confirmed by the results of the
analysis
Inaccuracy of the beam theory
employed in the classical approach
was also confirmed however it
appeared to be more conservative.
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Eccentrically Loaded Joints
When the applied load results in
a line of action passing outside
the center of rotation of the
fastener group.
Common examples: brackettype connections, web splices in
beams and girders, and
standard beam connections
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Eccentrically Loaded Joints
Design Procedure:
•
Three equations of equilibrium
must be employed to determine the
coordinates of the instantaneous
center of rotation and the
maximum value of the load that
results in slip of the connection.
n
 R sin 
i 1
s
0
i
n
 R cos
i 1
s
i
Design Assumptions:
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the connection rotates about an
instantaneous center of rotation.
Maximum slip resistance of all
fasteners is reached at slip load.
The slip resistance of each fastener
can be represented by a force at the
center of the bolt acting
perpendicularly to the radius of
rotation
P0
n
P(e  r0 )   ri Rs  0
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i 1
A trial centre of rotation is selected
and iterated until the three above
equations of equilibrium are
satisfied.
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Questions
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