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 – 28 – 38 38 ksi Clamping force varied Bad rivet? Difficult & expensive to remove Required crew of 4 skilled workers
Types of Fasteners Rivets
Mild carbon steel, Fy = 28 – 28 – 38 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, F u = 60 ksi “Common” bolts “Machine”, “Common” 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
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 (Group A) A490 (Group B)
Medium carbon steel
105 - 120 ksi
Heat-treated alloy steel
150 ksi
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
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
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
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
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
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
Turn-of-Nut Method
The turn-of-nut method of installation is reliable and produces bolt pretensions that are consistently above the prescribed values.
Proof Load = yield stress x tensile stress area = approx. 70 – 80% of tensile capacity Pretension = 70% of tensile capacity
55K
40K
n o i s n e T t l o B
10K “Snug”
Pretension 39K = Proof Load for A325
1/3 to 1/2
A325 7/8” diameter
3/4 to 1 ~1-3/4 Turns from “Snug”
Calibrated Wrench Method
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
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
Direct Tension Indicator Bolts
Bolt Installation Direct Tension Indicators (DTIs)
Direct Tension Indicator Washers
Direct Tension Indicator Washers
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)
Resistance Factor
Rn P u 0.75
Use this for : -- tension capacity -- shear capacity -- bearing resistance
AISC J3.6 & Table J3.2
Tensile Strength
Rn F nAb
Nominal, unthreaded cross section (in2)
b u
F n F t 0.75F Tensile stress capacity
AISC J3.6 & Table J3.2
Shear Strength Rn F vAb b u
Rn m u Ab m(0.563 F ) Ab Number of shear planes
P
P
P
m =
P/2
P/2
1
Shear Strength P/2
P
P/2 P/4
P/4 m =
P P/4
P/4
2
Shear Strength b u
Rn m u Ab m(0.563 F ) Ab 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)
b u
Rn m u Ab m(0.450 F ) Ab 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
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 R n = 1.2 Lc t F u on next slide
AISC J3.10
Design Bearing Resistance Deformation IS a design consideration (do not want hole elongation > ¼ inch)
Lc
Lc
Rn 1.2 LctF u 2.4dtF u Clear distance (in)
AISC J3.10
Design Bearing Resistance
Plate / angle tensile stress (ksi) Plate / angle thickness (in) Bolt diameter (in)
Rn 1.2 LctF u 2.4dtF u
Design Bearing Resistance, cont’d Deformation is NOT a design consideration (can tolerate hole elongation > ¼ inch)
Rn 1.5 LctF u 3.0dtF u
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
s 2
2 3
s
d bolt
3d bolt preferred
AISC Table J3.4
Minimum Edge Distances Bolt Diameter
Min. Edge Distance
3/4”
1”
7/8”
1-1/8”
1”
1-1/4”
Le
1.5d bolt preferred