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
Types of Fasteners
Rivets




Mild carbon steel, Fy = 28 – 38 ksi
Clamping force varied
Bad rivet? Difficult & expensive to remove
Required crew of 4 skilled workers
Types of Fasteners
Unfinished Bolts




Low-carbon steel, ASTM A307, Fu = 60 ksi
“Machine”, “Common” bolts
Least expensive
Typically used in light structures and
secondary members (small trusses, purlins,
girts etc.)
Types of Fasteners
High-Strength Bolts




started use in 1950’s
less bolts required
More labor (washers)
Most economical
Parts of the Bolt Assembly
Grip
Washer
Washer
Face
Nut
Shank
Head
Thread
Length
•
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
Slide courtesy of David Ruby, Ruby & Associates
AISC Table 7-14
High-Strength Bolts
Standard dimensions
(F, H, W, thread length)
Thread length
A325
F
H
WASHER
goes under part you’re
using to tighten bolt
(head or nut)
H
W
AISC Table 2-6
ASTM
Material
Fub
A325
Medium
carbon steel
105 - 120 ksi
Heat-treated
alloy steel
150 ksi
(Group A)
A490
(Group B)
Common Sizes


Buildings  3/4” and 7/8”
Bridges  7/8” and 1”
for 0.5” to
1” diameter
Markings
Material Specification
A325
Underline if Type 3 bolt
(weathering steel)
COR
Otherwise, Type 1 – standard
(Type 2 discontinued)
Manufacturer
(initials or abbreviation;
here“Cordova Bolt”)
SLIP-CRITICAL
Bolts tightened to
specified tensile
stress
“Friction-type” – 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)
Slip-Critical Joints
Slide courtesy
of David Ruby,
Ruby &
Associates
• 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
BEARING TYPE
Contact or
bearing on
plate
Permitted to be “snug-tight” – all plies in a joint are in firm contact
May be PRE-TENSIONED [AISC J1.10]
Bearing Joints
•
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
Slide courtesy of David Ruby, Ruby & Associates
Bolt Installation
Turn-of-the-nut


Simplest method
1/3 to 1/2 turn, typically, beyond “snug
tight”
Calibrated wrench



Manual torque wrenches
Variation +/- 30%
Wrenches MUST be calibrated DAILY
Turn-of-Nut Method
Slide courtesy of David Ruby, Ruby & Associates
Turn-of-Nut Method
Installation Procedure:
Check bolts and nuts for rust and lubrication
Install nut and washer with “markings up”
Washer, if installed, must be under the “turned” element
Step 1
Tighten bolt to “snug tight” condition
having all faying surfaces in tight contact
Slide courtesy of David Ruby, Ruby & Associates
Turn-of-Nut Method
Step 2
“Match-Mark” bolt tip,
nut and base steel
(this procedure is not required
By RCSC specification)
Step 3
Rotate nut specified
“Turn-of-Nut” amount
Note: Bolt may be tightened by turning the bolt head
Slide courtesy of David Ruby, Ruby & Associates
Turn-of-Nut Method
Step 4
Check for rotated Tolerance
For 1/3 turn, +/- 30 degrees
For 1/2 turn, +/- 30 degrees
For 2/3 turn, +/- 45 degrees
Slide courtesy of David Ruby, Ruby & Associates
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
Proof Load = yield stress x tensile stress area
= approx. 70 – 80% of tensile capacity
Pretension = 70% of tensile capacity
Bolt Tension
55K
40K
10K
“Snug”
Pretension 39K
= Proof Load
for A325
A325
7/8” diameter
1/3
to
1/2
~1-3/4
3/4 to 1
Turns from “Snug”
Calibrated Wrench Method
Slide courtesy of David Ruby, Ruby & Associates
Calibrated Wrench Method
Portable bolt-tension calibration
-convert tool output to bolttension
-Torque-Control Wrenches
-Conventional Impact Wrenches
-Turn-of-Nut Method
Skidmore-Wilhelm Calibrator
Slide courtesy of David Ruby, Ruby & Associates
Bolt Installation
Alternative-design bolts



“Twist-off” or Tension-control bolts
Special wrench required
Spline designed to twist off at required
level of torque / tension
Spline
ANIMATION  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
Direct Tension Indicator Bolts
Slide courtesy of David Ruby, Ruby & Associates
Bolt Installation
Direct Tension Indicators (DTIs)
Direct Tension Indicator Washers
Slide courtesy of David Ruby, Ruby & Associates
Direct Tension Indicator Washers
Slide courtesy of David Ruby, Ruby & Associates
TENSION FAILURE
SHEAR FAILURE
Deformation /
elongation of bolt
hole
Shear rupture /
splitting of plate
BEARING FAILURE
Bolted Joint Failure Modes
Bearing
Yield
Bearing
Fracture
Bearing
Fracture
Bearing
Yield
• 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)
Slide courtesy of David Ruby, Ruby & Associates
Resistance Factor
Rn  Pu
  0.75
Use this for :
-- tension capacity
-- shear capacity
-- bearing resistance
AISC J3.6 & Table J3.2
Tensile Strength
Rn  FnAb
Nominal,
unthreaded cross
section (in2)
Fn  Ft  0.75F
b
u
Tensile stress
capacity
AISC J3.6 & Table J3.2
Shear Strength
Rn  FvAb
Rn  muAb  m(0.563F ) Ab
b
u
Number of shear
planes
P
P
P
m=1
P/2 P/2
Shear Strength
P/2
P
P/2
P/4 P/4
m=2
P
P/4 P/4
Shear Strength
Rn  muAb  m(0.563F ) Ab
b
u
Connection length effect = 0.9
shear factor (from tests) = 0.625
0.9 x 0.625 = 0.563
Shear Strength (threads included)
A325X
(threads excluded
from shear plane)
A325N
(threads included
in shear plane)
Rn  muAb  m(0.450F ) Ab
b
u
0.563 x 0.8 = 0.45
Threads in the Shear Plane
•
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 Included In The Shear Plane
Threads Excluded From The Shear Plane
Slide courtesy of David Ruby, Ruby & Associates
Bearing Limit State
t
d
Le
Rn = 2 t [Le- d/2] p
if Le = 2-2/3 d
Rn = 3.0Fud t
Can use similar derivation for Rn = 1.2 Lc t Fu on next slide
AISC J3.10
Design Bearing Resistance
Deformation IS a design consideration
(do not want hole elongation > ¼ inch)
Lc Lc
Rn  1.2LctFu  2.4dtFu
Clear distance (in)
AISC J3.10
Design Bearing Resistance
Plate / angle tensile
stress (ksi)
Plate / angle thickness (in)
Bolt diameter (in)
Rn  1.2LctFu  2.4dtFu
Design Bearing Resistance, cont’d
Deformation is NOT a design consideration
(can tolerate hole elongation > ¼ inch)
Rn  1.5LctFu  3.0dtFu
Design Resistance
Rn ( boltgroup)   Rn (individual)
Rn (individual)  min Rn ( shear), Rn ( bearing) 
See User Note, AISC J3.10 [16.1-128]
AISC J3.3
Minimum Spacing
2
s  2 dbolt
3
3dbolt preferred
s
AISC Table J3.4
Minimum Edge Distances
Bolt
Diameter
3/4”
Min. Edge
Distance
1”
7/8”
1-1/8”
1”
1-1/4”
Le
1.5dbolt
preferred
AISC J3.5
Maximum Edge Distances
Le  12t
Le  6"