ce 671 high strength bolts
DESCRIPTION
Civil Engineering, Structures BoltsTRANSCRIPT
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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