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