ce470 lecture 10 bolts - purdue engineering · pdf filece470 lecture 10 bolts types of...
TRANSCRIPT
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
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
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
• 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
Head Shank
Washer
Nut Washer
Face
Grip
Thread
Length
Slide courtesy of David Ruby, Ruby & Associates
WASHER
goes under part you’re
using to tighten bolt
(head or nut)
A325
High-Strength Bolts
AISC Table 7-14
Standard dimensions
(F, H, W, thread length)
F H H W
Thread length
ASTM Material Fub
A325
(Group A)
Medium carbon steel
105 - 120 ksi
A490
(Group B)
Heat-treated alloy steel
150 ksi
Common Sizes
Buildings 3/4” and 7/8”
Bridges 7/8” and 1”
AISC Table 2-6
for 0.5” to
1” diameter
Markings
COR
A325
Material Specification
Manufacturer
(initials or abbreviation;
here“Cordova Bolt”)
Underline if Type 3 bolt
(weathering steel)
Otherwise, Type 1 – standard
(Type 2 discontinued)
SLIP-CRITICAL
“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)
Bolts tightened to
specified tensile
stress
• 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
Contact or
bearing on
plate
BEARING TYPE
Permitted to be “snug-tight” – all plies in a joint are in firm contact
May be PRE-TENSIONED [AISC J1.10]
• 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
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 3
Rotate nut specified
“Turn-of-Nut” amount
Step 2
“Match-Mark” bolt tip,
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
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
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
Bolt T
ensi
on
Turns from “Snug”
“Snug”
Proof Load = yield stress x tensile stress area
= approx. 70 – 80% of tensile capacity
A325
7/8” diameter
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
Calibrated Wrench Method
Slide courtesy of David Ruby, Ruby & Associates
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
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
ANIMATION http://www.tcbolts.co.uk/2_installation.html
Spline
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
BEARING FAILURE
Deformation /
elongation of bolt
hole
Shear rupture /
splitting of plate
• 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
Resistance Factor
un PR
75.0 Use this for :
-- tension capacity
-- shear capacity
-- bearing resistance
bnn AFR
Tensile Strength
butn FFF 75.0
AISC J3.6 & Table J3.2
Nominal,
unthreaded cross
section (in2)
Tensile stress
capacity
bbubun AFmAmR )563.0(
Shear Strength bvn AFR AISC J3.6 & Table J3.2
Number of shear
planes
P P
P
P/2 P/2
m = 1
Shear Strength
P/2
P/2 P
m = 2
P/4 P/4
P
P/4 P/4
Shear Strength
bbubun AFmAmR )563.0(
Connection length effect = 0.9
shear factor (from tests) = 0.625
0.9 x 0.625 = 0.563
bbubun AFmAmR )450.0(
Shear Strength (threads included)
A325X (threads excluded
from shear plane)
A325N (threads included
in shear plane)
0.563 x 0.8 = 0.45
• 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
Bearing Limit State
Le
t
d
Rn = 2 t [Le- d/2] p
Rn = 3.0Fud t if Le = 2-2/3 d
Can use similar derivation for Rn = 1.2 Lc t Fu on next slide
Design Bearing Resistance
uucn dtFtFLR 4.22.1
AISC J3.10
Deformation IS a design consideration
(do not want hole elongation > ¼ inch)
Lc Lc
Clear distance (in)
Design Bearing Resistance
uucn dtFtFLR 4.22.1
Bolt diameter (in)
Plate / angle thickness (in)
Plate / angle tensile
stress (ksi)
AISC J3.10
uucn dtFtFLR 0.35.1
Design Bearing Resistance, cont’d Deformation is NOT a design consideration
(can tolerate hole elongation > ¼ inch)
Design Resistance
)()( individualnboltgroupn RR
)(),(min)( bearingnshearnindividualn RRR
See User Note, AISC J3.10 [16.1-128]
Minimum Spacing
s
bolt
bolt
d
ds
3
3
22
preferred
AISC J3.3
Minimum Edge Distances
AISC Table J3.4
Le
Bolt Diameter
Min. Edge Distance
3/4” 1”
7/8” 1-1/8”
1” 1-1/4”
boltd5.1preferred
Maximum Edge Distances
"6
12
e
e
L
tL
AISC J3.5