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Bolted Connections

Professor Dr. Jahangir Alam

CEO and Founder of Qlearn
Former Professor of Civil Engineering, BUET, Dhaka

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BOLT INSTALLATION
3 basic joint types:
• Snug tight
• Pretensioned
• Slip-critical

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Snug tight
• A snug-tight condition occurs when the bolts are in
direct bearing and the plies of a connection are in
firm contact.
• This can be accomplished by the full effort of a
worker using a spud wrench
• Used for simple shear connections and tension-only
connections
• not permitted for connections supporting non-static
loads, nor are they permitted with A490 bolts loaded
in tension.

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Pretensioned
• A pretensioned joint has a greater amount of clamping
force than the snug-tight condition and therefore
provides a greater degree of slip-resistance in the
joint.
• Pretensioned joints are used for joints that are subject
to cyclical loads or fatigue loads. They are also
required for joints with A490 bolts in tension.

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Applications of Pretensioned Bolt

• Column splices in buildings

• Moment connections
• Connections within the load path of the lateral force
resisting system
• Connections supporting impact or cyclical loads such
as cranes or machinery

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Pretensioned
• Design strength: Snug tight = Pretensioned
• In a pretensioned joint, slip is prevented
until the friction force is exceeded. Once
the friction force is exceeded, the bolts slip
into direct bearing and the pretension or
clamping force is essentially zero (i.e.,
equivalent to a snug-tight condition).
• For both snug tight and pretensioned bolts,
the faying surface is permitted to be
uncoated, painted, or galvanized, but must
be free of dirt and other foreign material.

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Installation of Pretensioned Bolts

The AISC specification stipulates that the minimum required
clamping force should be at least 70% of the nominal tensile
strength, Rn, of the fastener.

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Methods of Installation of Pretensioned Bolts

• Turn of the Nut: When a nut is advanced along the length of a bolt, each
turn corresponds to a certain amount of tensile force in the bolt. Therefore,
there is a known relationship between the number of turns and the amount
of tension in the bolt.
• Calibrated Wrench Tightening: For this method, calibrated wrenches are
used so that a minimum torque is obtained, which corresponds to a specific
tensile force in the bolt. On any given project, the calibration has to be
done daily for each size and grade of bolt.
• Twist-off-type Tension-control Bolts: As discussed in Section 9.1, these
bolts conform to ASTM F1852 and are equivalent to ASTM A325 for
strength and design. These bolts have a splined end that breaks off when the
bolt is tightened with a special wrench (see Figure 9-1).
• Direct tension indicator: Washers that conform to ASTM F959 have ribbed
protrusions on the bearing surface that compress in a controlled manner
such that it is proportional to the tension in the bolt (see Figure 9-2). The
deformation in the ribs is measured to determine whether the proper tension
has been achieved.

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Twist-off type tension control bolts

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Direct tension indicator washer

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Slip Critical Joints

• Failure is assumed to occur when the applied load is
greater than the friction force and thus slip does not
occur between the faying surfaces.
• Used for joints subjected to cyclical loads or fatigue
loads.
• Used in connections that have slotted holes parallel to
the direction of the load or in connections that use a
combination of welds and bolts along the same faying
surface.

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Slip Critical Joints

• The amount of pretension or clamping force for a slip-
critical bolt is the same that was used for pretensioned
joints (see Table 9-1).
• The design strength is generally lower than that of a
bearing-type connection since the friction resistance is
usually lower than any other failure mode for a bolt
(such as direct shear or bearing).

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Minimum Bolt Pretension in Slip Critical Joints

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Hole Types and Spacing

Requirements
• Standard,
• Oversized
• Short-slotted
• Long-slotted

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Nominal Hole Dimensions

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Bolt hole types

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Maximum Hole Sizes In Column Base Plates

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Edge and Spacing Requirements For Bolts

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STRENGTH OF BOLTS
Basic failure modes:
• Bearing
• Shear
• Tension
• Slip

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BEARING STRENGTH OF BOLTS

Bearing strength for holes •  = 0.75,
slots parallel to the direction • Rn = Nominal bearing
of the load. strength, kips,
• Lc = Clear distance
between the edge of the
hole and the edge of an
adjacent hole, or the edge
Bearing strength for holes of the connected member
slots perpendicular to the in the direction of the
load (see Figure 9-6),
direction of the load.
• t = Thickness of the
connected material, in.,
• d = Bolt diameter, in.,
and
• Fu = Minimum tensile
strength of the connected
member.
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SHEAR STRENGTH OF BOLTS

The design shear strength of a snug-tight or pretensioned
bolt is,

• φ = 0.75,
• Rn = Nominal shear strength
• Fn = Nominal shear strength (Fnv) (see Table 9-3)
• Ab = Nominal unthreaded body area of bolt

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TENSION STRENGTH OF BOLTS

The design tension strength of a snug-tight or
pretensioned bolt is,

• φ = 0.75,
• Rn = Nominal tension strength
• Fn = Nominal tension strength (Fnt) (see Table 9-3)
• Ab = Nominal unthreaded body area of bolt

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STRENGTH OF BOLTS

X = threads will be excluded from the shear plane

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STRENGTH OF BOLTS

Nominal
shear
strength

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STRENGTH OF BOLTS

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• Du = 1.13 (the constant value

SLIP STRENGTH OF BOLTS that represents the ratio
The design slip resistance of a between the mean installed
slip-critical bolted connection bolt pretension and the
minimum required bolt
pretension; alternate values
can be used if it is verified), F=0.35*Tb
•  = 1.0 if prevention of slip is a
serviceability limit state • hsc = Hole size factor
•  = 0.85 if prevention of slip is = 1.0 for standard holes (STD)
at the required strength level, = 0.85 for oversized and short-
• Rn = Nominal shear strength, slotted holes (OVS and SSL) Tb
kips, = 0.70 for long-slotted holes
• μ = Mean slip coefficient (LSL)
= 0.35 for Class A surfaces • Ns = Number of slip planes
= 0.50 for Class B surfaces • Tb = Minimum bolt
= 0.35 for Class C surfaces pretension (see Table 9-1).

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IMPORTANT NOTES ON SLIP-CRITICAL BOLTS

• When designing slip-critical connections, a Class A
surface is usually assumed, which is conservative.
• Steel with a Class B surface would require blast
cleaning, which adds labor, time, and cost.
• It is also generally good practice to use standard holes
since oversized and slotted holes are typically not
necessary.
• When a connection requires slip-critical bolts, they
should be designated as A325SC or A490SC.

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SHEAR AND TENSION COMBINED IN BOLTS

When fasteners are loaded such that there exists shear
and tension components (see Figure 9-7), an ALL BOLT TYPES
interaction equation is required for design. Research has
indicated that the interaction curve is,

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Interaction curves for combined loading

AISC EQN. J3-3a ALL BOLT TYPES

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REDUCTION OF SLIP RESISTANCE DUE TO COMBINED LOADING

SLIP CRITICAL BOLTS

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EXAMPLE-9.1

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EXAMPLE-9.1

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EXAMPLE-9.2

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EXAMPLE-9.2

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EXAMPLE-9.2
Slip resistance is
the lowest

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EXAMPLE 9.3

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EXAMPLE 9.3

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EXAMPLE 9.3

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EXAMPLE 9.3

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ECCENTRICALLY LOADED BOLTS: SHEAR

Analytical procedure:
• The instantaneous center (IC), of rotation method
• the elastic method

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ECCENTRICALLY LOADED BOLTS: SHEAR

• In both cases, the connection is designed to resist the
applied shear, P, and the additional shear generated from
the moment due to the applied shear acting at an
eccentricity, Pe.
• Both methods are relatively complex and are generally
not used in practice without the aid of computers.
• The IC method is more accurate, but requires an
iterative solution.
• The elastic method is less accurate and more
conservative in that the ductility of the bolt group and
redundancy (i.e., load distribution) are both ignored.
• Tables 7-7 through 7-14 in the AISCM, which use the
IC method, are design aids for these types of
connections and are more commonly used in practice
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The Instantaneous Center (IC),

of Rotation Method
• Induces both a translation and a rotation
• The location of the IC is a function of the
geometry and the direction and
orientation of the load.
• The location of the IC (see Figure 9-13)
requires an iterative solution.

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The Instantaneous Center (IC),

of Rotation Method

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The Instantaneous Center (IC),

of Rotation Method

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The Instantaneous Center (IC), of Rotation Method

The equilibrium equations,

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The Instantaneous Center (IC), of Rotation Method

The equilibrium equations, •
P = Applied load, with
components Px, Py,
• Rn = Shear in fastener, n,
with components (Rx)n,
(Ry)n,
• rn = Distance from the
fastener to the IC, with
components rx, ry, r0 =
Distance from the IC to the
centroid of the bolt group,
• e = Load eccentricity;
distance from the load to the
centroid of the bolt group
with components ex, ey,
• n = Subscript of the
individual fastener, and
• m = Total number of
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Elastic Method
Each bolt resists an
equal proportion of the
applied load, P, and a
portion of the shear
induced by the moment,
Pe, proportional to its
distance from the
centroid of the bolt
group. Shear in each
bolt due to applied load,
• rp = Force in each bolt due
to applied load with
components rpx, rpy,
• P = Applied load with
components Px, Py, and
• n = Number of bolts.
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Elastic Method
The shear in the bolt
most remote from the • rm = Force in each bolt due to applied
moment with components rmx, rmy,
centroid of the bolt
• M = Resulting moment due to
group due to the applied eccentrically applied load
moment, = Pxey + Pyex,
• c = Radial distance from the centroid of
the bolt group with components cx, cy,
• e = Load eccentricity; distance from the
load to the centroid of the bolt group with
components ex, ey, and
• Ip = Polar moment of inertia of the bolt
group
=(Ix + Iy), where I = Ad2
= (cx + cy ) for bolts with the same
cross-sectional area within a bolt group.

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Elastic Method
The critical fasteners force,

The value of r above is used to determine the

required strength of each fastener in the bolt
group.

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EXAMPLE 9-6

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EXAMPLE 9-6

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EXAMPLE 9-6

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EXAMPLE 9-6

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EXAMPLE 9-6

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EXAMPLE 9-6

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EXAMPLE 9-6

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EXAMPLE 9-6

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EXAMPLE 9-6

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EXAMPLE 9-6

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EXAMPLE 9-6

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Eccentrically Loaded Bolts: Bolts In Shear

& Tension
Design approach:
• Case-1: neutral axis is not necessarily
at the centroid of the bolt group and is
generally below the centroid of the
bolt group.
• Case-2: more simplified and
conservative in that the neutral axis is
assumed to be at the centroid of the
bolt group and only the bolts above
the neutral axis resist the tension due
to the eccentric load.

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Eccentrically Loaded Bolts: Bolts In Shear

& Tension

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Eccentrically Loaded
• beff = Effective width of the compression
Bolts: Bolts In Shear zone,
& Tension • tf = Connecting element thickness (use the
average flange thickness of the connecting
For Case I, a trial position for element where the flange thickness is not
the neutral axis has to be constant), and
assumed. A value of one-sixth of • bf = Width of the connecting element.
the depth of the connecting
element is recommended as a
baseline value (see Figure 9-19).
The area below the neutral axis
is in compression, but only for a
certain width. The width of the
compression zone is defined as
valid for connecting elements of W-shapes, S-shapes, plates,
beff =8tf ≤bf
and angles.
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Eccentrically Loaded Bolts: Bolts In

Shear & Tension

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Eccentrically Loaded Bolts: Bolts In

Shear & Tension
The location assumed for the neutral axis is verified by
summing moments about the neutral axis in that the
moment of the bolt area above the neutral axis is
compared with the moment due to the compression stress
block below the neutral axis.

• Ab = Sum of the bolt areas above the neutral axis,

• y = Distance from the centroid of the bolt group above the
neutral axis to the neutral axis,
• beff = Effective width of the compression zone, and
• d = Depth of the compression stress block.
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Eccentrically Loaded Bolts: Bolts

In Shear & Tension
Once the exact location of the neutral axis is
determined, the tensile force in each bolt,

• rt = Force in each • c = Distance from the neutral

bolt due to the axis to the most remote bolt in
applied moment, the tension group,
• M = Resulting • e = Load eccentricity, and
moment due to the • Ix = Combined moment of
eccentrically applied inertia of the bolt group and
load, Pe, compression block about the
• Ab = Bolt area neutral axis.
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Eccentrically Loaded Bolts: Bolts

In Shear & Tension
For Case II, the neutral axis is • rt = Force in each bolt due to
assumed to be at the centroid of the applied moment,
the bolt group (see Figure 9- • M = Resulting moment due to
20). Therefore, the bolts above the eccentrically applied load,
the neutral axis are in tension Pe,
and the bolts below the neutral • n' = Number of bolts above
the neutral axis,
axis are assumed to be in
• dm = Moment arm between
compression. The tensile force
the centroid of the tension
in each bolt is then defined as
group and centroid of the
compression group, and
• e = load eccentricity.

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Eccentrically Loaded Bolts: Bolts

In Shear & Tension

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EXA.-9.7

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EXA.-9.7

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EXA.-9.7

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EXA.-9.7

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EXA.-9.7

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EXA.-9.7

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Prying Action: Bolts in Tension

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Prying Action: Bolts in Tension

• T = Applied tensile force in the bolt, k,

𝑑
• 𝑏 ′ = 𝑏 − 2𝑏
• b = Distance from the bolt centerline to
the face of the tee stem for a T-shape,
in.
= Distance from the bolt centerline to the
angle-leg centerline for an L-shape, in.,
• db = Bolt diameter, in.,
• p = Tributary length of the bolts
(should be less than g (see Figure 9-
26)), and
• Fu = Minimum tensile strength of the
connecting element.
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Prying Action: Bolts in

Tension
A more complex and less
conservative approach
assumes a value of q that is
greater than zero, thus
magnifying the tensile force in
the bolt. A preliminary
connecting element thickness
must first be selected as
follows:

Once a preliminary thickness

has been selected, then the
required thickness is
determined as follows:

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EXA-9.8

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EXA-9.8

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EXA-9.8

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EXA-9.8

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FRAMED BEAM CONNECTIONS

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FRAMED BEAM CONNECTIONS

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FRAMED BEAM CONNECTIONS

All-Bolted Double-Angle Connections

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EXA 9.9

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EXA 9.9

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EXA 9.9

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EXA 9.9

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EXA 9.9

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Bolted/Welded Shear End-Plate

Connections

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EXA 9.10

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EXA 9.10

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Bolted/Welded Shear End-Plate

Connections
Single-Plate Shear Connections

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Bolted/Welded Shear End-Plate

Connections

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Bolted/Welded Shear End-Plate

Connections

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Bolted/Welded Shear End-Plate

Connections

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END

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