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Rivets, HS-bolts, and Bolting Specifications

CE671 – Lecture 4

Discussion

Rivets

A307 Bolts

A325 HS-Bolts

A490 HS-Bolts

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Rivets

One of the oldest forms of fastener in structural jointsCommon up until 1960’s– Used in buildings and bridges– Used in wrought iron and steel

Replaced by HS bolts and welding– Required 4-5 skilled laborers

FiremanToss manCatcherRiveter (2)

Many Types of Rivets

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Rivet Installation

Installed though multiple platesGenerally installed as “hot” rivets– About 1,800 F

Components drawn together with clamps or boltsRivet inserted into hole and the “driven”– Pressed or hammered

Head is formed by driving processAs the rivet cools, it shrinks thereby introducing precompression into the joint

Rivet Crew

HOT

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Rivet Installation

When rivet is driven it expands laterally into the hole

Expansion “fills” the hole and reduced likelihood of slip– May only occur on outer plies

When rivet cools, it will contract longitudinally and diametrically– Diametric contraction which can increase

clearance some small amount

– Longitudinal contraction compresses joint

Riveting

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Riveting – Close-up

3/4 7/8

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Rivet Performance

Precompression depends on:– Joint stiffness– Driving and finishing temperature– Driving pressure.

Hot-driven rivets can develop clamping forces that approach the yield load of a rivetConsiderable variation in clamping forces is generally observed – Can’t retightenAs the grip length is increased, the residual clamping force tends to increase

Effects of Driving

Driving generally increases the strength of rivets

For hot-driven rivets – machine driving increased the rivet tensile

strength by about 20%

– Pneumatic hammer increases rivet tensile strength about 10%

Hot working of the material

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Variability in Driving

Under driven rivets

‘Soldiers Cap’ head rivets

Offset rivets

Rivet Strengths / TypesThree structural rivet steels:– ASTM A502 grade 1, carbon rivet steel for general

purposes

– ASTM A502 grade 2, carbon-manganese rivet steelSuitable for use with high-strength carbon and high-strength low-alloy structural steels

– ASTM A502 grade 3, Similar to grade 2 but with enhanced corrosion resistance

Rivet steel strength is specified in terms of hardness requirements

There are no additional material requirements for strength or hardness in the driven condition

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Rivets Material Specifications

Strengths vary with type:– ASTM A502 Grade 1 about 60 ksi

– ASTM A502 Grade 2 & 3 about 80 ksi

Variations in driving temperature between 1,800 F and 2,300 F does not affect strength– May effect surface quality – pitting

– Too cold, very difficult to drive

A307 Bolts

A307 are low-carbon fasteners

Typically available ¼ to 4 inch diameter

Min. specified tensile strength of 60 ksi

May be galvanized without concern

Will develop some pretension force– No specified tightening procedures

– Relatively low strength

– Hence, can’t design as friction connection

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A325 HS-Bolts

A325 is a carbon steel bolt

Available in ½ to 1-½ inch diameters

Can be specified in two types– Used to be three

Head is heavy hex – Head must indicate bolt type

Minimum tensile strength– 120 ksi up to 1 inch

– 105 ksi > 1 inch

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)

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A325 HS-Bolts

A325 used to be available in three types– Type 1 – Medium carbon steel (high temp)– Type 2 – Low carbon martensitic steel– Type 3 – Corrosion resistant for use with

weathering steels

Type 2 was could only be used for “atmospheric” temperature conditions, but was discontinued since no need for itIf not specified, Type 1 or Type 3 can be provided by supplier– Usually Type 1 will be specified

A325 HS-Bolts

Can be reused as long as nut can be run up the threads by hand– Usually 1 or 2 times

– If galvanized, reuse is not permitted

Can be hot-dipped or mechanically galvanized– Galvanized bolts and nuts must be thought of as an

assembly

– Can’t mix types of galvanizing

Galvanizing will affect tightening procedures

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A490 HS-Bolts

A490 is a alloy steel bolt that is heat treated

Available in ½ to 1-½ inch diameters

Can be specified in two types– Used to be three

Head is heavy hex – Head must indicate bolt type

Minimum tensile strength– 150 min ksi all sizes

– 170 ksi max (No upper limit on A325)

A490 HS-Bolts

A490 used to be available in three types– Type 1 – Medium carbon steel (high temp)

– Type 2 – Low carbon martensitic steel

– Type 3 – Corrosion resistant for use with weathering steels

Type 2 was discontinued since no need for it

If not specified, Type 1 or Type 3 can be provided by supplier– Usually Type 1 will be specified

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A490 HS-BoltsGalvanizing is prohibited if metallizing or mechanical coating used– High strength steel is susceptible to hydrogen

embittermentUsually issue with steels with strengths

over 200 ksi

But considered close enough to 170 ksi– Within about 10% of minimum specified strength

A490 HS-Bolts CAN NOT BE REUSED

Approved Corrosion Protection for A490

Zn/Al Inorganic Coating, when applied per ASTM F1136 Grade 3 to ASTM studied– Does not cause delayed cracking by internal

hydrogen embrittlement

– Does not accelerate environmental hydrogen embrittlement by cathodic hydrogen absorption

Also approved for A325

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A Few Words about Galvanized HS Bolts

Galvanized and Zn/Al Inorganic coated bolts and nuts must be treated as a fastener assembly– Ensures overtapping of nut accounted for

– Nuts and bolts have to be tested together using same lubricant as in field

Nuts & bolts must be shipped together in the same shipping container

Nuts & bolts must be galvanized by same process, can’t mix

See RSCS Spec’s (August 1, 2014)

Why Can’t We Reuse Some HS Bolts?

Pretension involves deformation of the bolt, primarily in the threads

Black A-325 bolts can tolerate multiple cycles of elongation

However, A490 and galvanized A325 are not consistently capable of undergoing more than one pretensioning cycles

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Multiple Cycles on “Black” A325

Multiple Cycles on A490

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Connected Plies

Can only apply specifications if:– All plies are steel

– There are no compressible layersi.e., no gaskets

Must only have steel to ensure precompression can be achieved

Compressible layers will result in loss of pre-tension

Installation Techniques

Generally four accepted procedures:– Turn-of-the nut– Calibrated wrench– Twist-off-type bolt– Direct-tension-indicator (DTI)

All have been researched and are acceptable for most applicationsAll require a minimum preload of 70% of specified minimum tensile strength

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Minimum Bolt Preload

105 ksi

Turn-of-the Nut

After snugging the joint, the bolt shank and nut is marked and then a specific amount of rotation is induced between the nut and the boltGenerally results in more uniform bolt pretensions than torque-controlled methodsBased on strain control– Deformation of bolt related to angle of thread

Requires joint is “compacted” prior to tightening (All plies in contact)Current guidance on rotation requirements does not apply to bolts longer than 12db

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Turn-of-the Nut

Calibrated WrenchRepresentative sample of three bolts must be selected to calibrate a wrench– For each diameter, length, and grade

Tensioned in a Skidmore-Wilhelm on siteWrench is then set to cut out at that torque – The wrench becomes "calibrated"– Rotation must be limited to a specific value – Calibration is required every day

Actual results of tension in bolts produced by this method are acknowledged to be highly variable– Not allowed in Canada

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Skidmore-Wilhelm

Calibrated Wrench

Twist-off-type Bolt

Covered by ASTM F1852 (Equiv. A325) or F2280 (Equiv. A490)

Function by calibrating the torque needed to twist off a splined extension manufactured into the bolt shank

Made correctly, the "twist-off" will occur at a bolt tension above the minimum preload

Can be tightened from one side

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Twist-off-type Bolt

Require special equipmentMore expensive than regular bolts– But less labor

Hard to find in galvanized– Rumored

Direct-tension-indicator (DTI)

Covered by ASTM F959

Can be thought of a load cell

Independent of torque

Issues with oversized holes– Need washer “plate”

Issues with “dry” washers

Very consistent pre-tension

Allows for easy inspection

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Direct-tension-indicator (DTI)

Direct-tension-indicator (DTI)

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Use of Hydraulic Wrenchfor Large Fasteners

Rotational Capacity Test“Ro-cap”

Intended to evaluate:

1. the presence of a lubricant

2. the efficiency of the lubricant

3. the compatibility of fastener assemblies

In other words, verify the fastener assembly will function properly as a unit

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Rotational Capacity Test

Snug bolt to 10% of min. pretension

Match mark bolt

Tighten to min. pretension

Measure torque at this pretension

Calculate Maximum Permitted Torque

–T = 0.25PDT = Torque

P = Tension in pounds

D = Diameter of bolt in feet (inch/12)

Maximum Permitted Torque (ft-lb)

Rotational Capacity Test

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Rotate Nut from Match Mark

Rotational Capacity Test

Record Tension after Nut Rotation Final tension shall equal or exceed 1.15

times the minimum tension

Rotational Capacity Test

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Examine Bolt for the following: Thread Shear Failure

Thread Stripping

Torsion Failure

Rotational Capacity Test

Test Failure if: Exceed maximum allowable torque in

torque/tension comparison Cannot achieve required rotation Bolt breaks

Cannot achieve required tension at rotation

Thread failure Not being able to turn nut with fingers

Rotational Capacity Test

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Bolt Specifications

See www.boltcouncil.org– Research Council on Structural Connections

– Established January 1947

Provides general standards on bolting, bolts, installation, and inspection

Current specifications available for free on website (you should download it)

Also, the most important book on connections is available for free from the RCSC website

2014 Spec. Available