design of bolts
DESCRIPTION
aTRANSCRIPT
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CE470 Lecture 10 Bolts
Types of Fasteners, Properties
Slip-Critical and Bearing-Type Connections
Methods of Tightening Bolts
Tension, Shear, and Bearing capacity of bolts
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RivetsMild carbon steel, Fy = 28 38 ksi
Clamping force varied
Bad rivet? Difficult & expensive to remove
Required crew of 4 skilled workers
Types of Fasteners
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Types of Fasteners
Unfinished Bolts
Low-carbon steel, ASTM A307, Fu = 60 ksi
Least expensive
Typically used in light structures and secondary members (small trusses, purlins, girts etc.)
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Types of Fasteners
High-Strength Bolts
less bolts required
More labor (washers)
Most economical
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Grip is the distance from behind the bolt head to the back of the nut or washer
Sum of the thicknesses of all the parts being joined exclusive of washers
Thread length is the threaded portion of the bolt
Bolt length is the distance from behind the bolt head to the end of the bolt
Parts of the Bolt Assembly
HeadShank
Washer
NutWasher
Face
Grip
Thread
Length
Slide courtesy of David Ruby, Ruby & Associates
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WASHER
using to tighten bolt
(head or nut)
A325
High-Strength Bolts
AISC Table 7-15 [7-80]
Standard dimensions
(F, H, W, thread length)
FH H W
Thread length
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ASTM Material Fub
A325 Medium carbon steel
105 - 120 ksi
A490 Heat-treated alloy steel
150 ksi
Common Sizes
Buildings
Bridges
AISC Table 2-5 [2-41]
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Markings
COR
A325
Material Specification
Manufacturer
(initials or abbreviation;
here
Underline if Type 3 bolt
(weathering steel)
Otherwise, Type 1 standard
(Type 2 discontinued)
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SLIP-CRITICAL
- used when slip resistance desired at service loads
(Joints subject to fatigue, bolts in combination with welds,
anytime deformation due to slip unacceptable for design)
Bolts tightened to
specified tensile
stress
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In a slip-critical joint the bolts must be fully pre-tensioned .
This force develops frictional resistance between the connected
elements
The frictional resistance allows the joint to withstand loading without
slipping into bearing, although the bolts must still be designed for
bearing
The slip critical joint faying surfaces may require preparation
Slip-Critical Joints
Slide courtesy
of David Ruby,
Ruby &
Associates
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Contact or
bearing on
plate
BEARING TYPE
- all plies in a joint are in firm contact
May be PRE-TENSIONED [AISC J1.10]
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In a bearing joint the connected elements are assumed to slip into bearing
against the body of the bolt
If the joint is designed as a bearing joint, the load is transferred through
bearing whether the bolt is installed snug-tight or pretensioned
Bearing Joints
Slide courtesy of David Ruby, Ruby & Associates
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Bolt InstallationTurn-of-the-nut
Simplest method
Calibrated wrenchManual torque wrenches
Variation +/ - 30%
Wrenches MUST be calibrated DAILY
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Turn-of-Nut Method
Slide courtesy of David Ruby, Ruby & Associates
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Turn-of-Nut Method
Installation Procedure:
Check bolts and nuts for rust and lubrication
Step 1
having all faying surfaces in tight contact
Slide courtesy of David Ruby, Ruby & Associates
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Turn-of-Nut Method
Step 3
Rotate nut specified
-of-
Step 2
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nut and base steel
(this procedure is not required
By RCSC specification)
Note: Bolt may be tightened by turning the bolt head
Slide courtesy of David Ruby, Ruby & Associates
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Turn-of-Nut Method
Check for rotated Tolerance
For 1/3 turn, +/- 30 degrees
For 1/2 turn, +/- 30 degrees
For 2/3 turn, +/- 45 degrees
Step 4
Slide courtesy of David Ruby, Ruby & Associates
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Turn-of-Nut Method
The turn-of-nut method of
installation is reliable and
produces bolt pretensions that
are consistently above the
prescribed values.
Slide courtesy of David Ruby, Ruby & Associates
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Bo
lt T
en
sio
n
Proof Load = yield stress x tensile stress area
= approx. 70 80% of tensile capacity
A325
10K
40K
55K
1/3
to
1/2 3/4 to 1 ~1-3/4
Pretension 39K
= Proof Load
for A325
Pretension = 70% of tensile capacity
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Calibrated Wrench Method
Slide courtesy of David Ruby, Ruby & Associates
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Calibrated Wrench Method
Skidmore-Wilhelm Calibrator
Portable bolt-tension calibration
-convert tool output to bolt-
tension
-Torque-Control Wrenches
-Conventional Impact Wrenches
-Turn-of-Nut Method
Slide courtesy of David Ruby, Ruby & Associates
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Bolt Installation
Alternative-design bolts
- -control bolts
Special wrench required
Spline designed to twist off at required level of torque / tension
ANIMATION http://www.tcbolts.co.uk/2_installation.html
Spline
http://www.tcbolts.co.uk/2_installation.html -
Direct Tension Indicator Bolts
ASTM F1852-08 Twist-Off Bolts
Slide courtesy of David Ruby, Ruby & Associates
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Direct Tension Indicator Bolts
Slide courtesy of David Ruby, Ruby & Associates
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Bolt InstallationDirect Tension Indicators (DTIs)
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Direct Tension Indicator Washers
Slide courtesy of David Ruby, Ruby & Associates
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Direct Tension Indicator Washers
Slide courtesy of David Ruby, Ruby & Associates
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TENSION FAILURE SHEAR FAILURE
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BEARING FAILURE
Deformation /
elongation of bolt
hole
Shear rupture /
splitting of plate
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Bolts in bearing joints are designed to meet two limit states:
1. Yielding, which is an inelastic deformation (above left)
2. Fracture, which is a failure of the joint (above left)
The material the bolt bears against is also subject to yielding or fracture
if it is undersized for the load (above right)
Bolted Joint Failure Modes
Bearing
Fracture
Bearing
Yield
Bearing
Yield
Bearing
Fracture
Slide courtesy of David Ruby, Ruby & Associates
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Resistance Factor
un PR
75.0 Use this for :-- tension capacity
-- shear capacity
-- bearing resistance
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bnn AFR
Tensile Strength
butn FFF 75.0
AISC J3.6 [16.1-108],
Table J3.2
Nominal,
unthreaded cross
section (in2)
Tensile stress
capacity
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bbubun AFmAmR )5.0(
Shear Strength bvn AFRAISC J3.6 [16.1-108], Table J3.2
Number of shear
planes
PP
P
P/2 P/2
m = 1
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Shear Strength
P/2
P/2P
m = 2
P/4P/4
P
P/4P/4
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Shear Strength
bbubun AFmAmR )5.0(
Connection length effect = 0.8
shear factor (from tests) = 0.62
0.8 x 0.62 ~ 0.5
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bbubun AFmAmR )4.0(
Shear Strength (threads included)A325X(threads excluded
from shear plane)
A325N(threads included
in shear plane)
0.5 x 0.75 ~ 0.4
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The shear plane is the
plane between two or
more pieces of steel.
The threads of a HS bolt
may or may not be
assumed to be included in
the shear plane; however,
based on the fixed length
of thread, it is highly
unlikely.
The bolt capacity is
greater with the threads
excluded from the shear
plane
The most commonly used
bolt is an ASTM A325 3/4
HS bolt with the threads
assumed to be included in
the shear plane
Threads in the Shear Plane
Threads Included In The Shear Plane
Threads Excluded From The Shear Plane
Slide courtesy of David Ruby, Ruby & Associates
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Bearing Limit State
Le
t
d
Rn = 2 t [Le- d/2] p
Rn = 3.0Fud tif Le = 2-2/3 d
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Design Bearing Resistance
uucn dtFtFLR 4.22.1
AISC J3.10 [16.1-111]
Deformation IS a design consideration
(do not want hole elongation > inch)
LcLc
Clear distance (in)
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Design Bearing Resistance
uucn dtFtFLR 4.22.1
AISC J3.10 [16.1-111]
Bolt diameter (in)
Plate / angle thickness (in)
Plate / angle tensile
stress (ksi)
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uucn dtFtFLR 0.35.1
)()( individualnboltgroupn RR
Deformation is NOT a design consideration
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Minimum Spacing
s
bolt
bolt
d
ds
3
3
22
preferred
AISC J3.3 [16.1-106]
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Minimum Edge Distances
AISC Table J3.4
Le
Bolt Diameter
for Sheared Edge
for Rolled Edge
1-
1- [d] 1-
1- [d] 1-
boltd5.1preferred
[d] permitted to be 1-1/4 in. at ends
of beam connection angles and
shear end plates