simple guide to high strength bolting – background, design approaches
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Simple Guide to High Strength Bolting – Background, Design Approaches. Presented by: Hossein Bajehkian Payman Hosseini. Outline. Rivets Bolts Different methods of Pre- tenstioning of High-strength Bolts Tension Behavior of Bolts and Prying Action Shear Behavior of Bolts - PowerPoint PPT PresentationTRANSCRIPT
Simple Guide to High Strength Bolting – Background, Design Approaches
Presented by:Hossein BajehkianPayman Hosseini
2023-04-22 1 of 20High Strength Bolting by H. Bajehkian and P. Hosseini
Outline Rivets Bolts Different methods of Pre-tenstioning
of High-strength Bolts Tension Behavior of Bolts and Prying Action Shear Behavior of Bolts Typical Shear Connections
2023-04-22High Strength Bolting by H. Bajehkian and P.
Hosseini 2 of 20
Rivets
ASTM A502More expensive to install than bolts
More man power More equipment
Problematic when slip critical conditions are required as the amount of pretension depend on
Compactness of griped material Initial temperature
Pretension is limited to ultimate strength which is much lower than bolts
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Hosseini
BoltsTwo Types of Bolts
Common High-strength
Common Bolts ASTM A307 Min tensile strength 415 MPa Used for general applications Good for static loads Cheap
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High-Strength Bolts ASTM A325
Min Tensile Strength 830 Mpa for dia, up to 1 inch 725 Mpa for larger dia
ASTM A490 Min Tensile Strength 1030 Mpa Max Tensile Strength 1168 Mpa
Main advantage is the ability to have large magnitudes of pretension
.
High Strength Bolting by H. Bajehkian and P. Hosseini
Pretension of High-Strength BoltsPretension is required
Cyclic Loading Oversized or slotted holes Shear connections under seismic
loadFailure to achieve pretension can lead to unwanted displacement or fatigue-type failureIn seismic applications effectiveness of energy dissipation mechanism depends on preloading of the jointVibration loosening and loss of nut is prevented by adequate pretensioning
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Desired amount of pretension is 70% of bolt’s min specified tensile strength (2004 CSA Std.)Snug-tight condition is adequate for many applicationsFour methods of pretentioning
Turn-of-nut method Calibrated wrench Use of Tension-Control bolts Use of direct tension indicators
Turn-of-nut Method
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Pretension is achieved by turning the nut past the snug-tight conditionStrain controlled methodAmount of pretension depends on the friction between the bolt threads and nutAmount of pretension is about 35% higher than minimum specified
High Strength Bolting by H. Bajehkian and P. Hosseini
Calibrated Wrench Method
Tension controlled methodThere is relationship between the applied torque and magnitude of pretensionComplex relationship depends on
Thread pitch Thread angle Friction coefficient between the
nut and boltRCSC Specifications does not recognize the available formulas and tables to relate the torque to tensionWrenches are calibrated on site using a representative sample of bolts that are being used
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Hosseini
Tension-control BoltsAlso known as Twist-off BoltsASTM F1852They meet the requirements of A325 but have special featuresPretension depends on
Bolt material strength Diameter of annular groove Thread conditions Surface conditions at nut-washer
interfaceMin pretension achieved is likely to exceed the specified pretension by 13%.Three advantages
Single person can perform the installation Installation is quicker Inspection is simple as the spline end of
the bolt is twisted off
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Hosseini
Use of Direct Tension Indicators (DTIs)
ASTM F959Washer-type elements that have arch-shaped protrusionProtrusions compress in response to tension force in the boltTension force is determined by measuring the size of the gapDeformation controlled method and friction related variables do not affect the pretensionMin pretension achieved is likely to exceed the specified pretension by 12%.
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Hosseini
Tension Behavior of Single Bolt
Tensile Strength of a single bolt depends on:
Ultimate Tensile Strength Effective Area
Represented in S16-09 as:
Where: Ab is the nominal crosssectional
area of the bolt and 0.75 factor accounts for the reduction in the threaded section
Fu is the ultimate tensile strength of the bolt
φb is the resistance factor for the bolts which is equal to 0.80
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ubbn FAR 75.0
High Strength Bolting by H. Bajehkian and P. Hosseini
Tension Behavior of Group of Bolts
Bolts are normally pre-tensioned to their connected elements to provide slip resistanceThe force in a pre-tensioned bolt can be calculated assuming equal deformation of bolt and plate until the separation of connected elementsThe most important issue is prying action:
Occurs when the bolt force is amplified due to the deformation of connected elements
Depends on bolt deformation capacity. flange stiffness and location of bolts in the flange
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Shear Behavior of Single Bolts
Ultimate Shear Strength of bolts are closely related to:
Tensile Strength Type of Test Axial Load on the Fasteners
(Tension or Compression)Factors Affecting Shear Resistance of a Bolted Connection
Ultimate Shear Strength ~ 60% of Tensile Strength
Number of Shear planes and bolts
Shear AreaS16-01 Shear Resistance
Equation:ubbr FnmAV 60.0
High Strength Bolting by H. Bajehkian and P. Hosseini
Slip Condition in Bolts Loaded in Shear c1 is a coefficient that relates the specific initial tension and mean slip to a 5% probability of slip for bolts installed by the turn-of-the-nut method and depends on the type of bolt0.53nAbFu denotes the clamping force of the bolt in the S16-09 equation where:
0.53 comprises of 0.70 factor which reflects that the amount of pretension need to be 70% of the ultimate tensile strength and 0.75 factor accounts for the reduction in area
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To ensure low probability of slip during the life of structureJoints shall not slip due to serviceability loadUsed in connections subjected to stress reversals and fluctuationsSlip Load depends on:
ks = slip coefficient m = number of slip planes ΣT = Sum of bolt tensions
S16-09 Slip Resistance Equation:
is TmkP
ubss FmnAkcV 153.0
High Strength Bolting by H. Bajehkian and P. Hosseini
Shear SplicesPlates or parts of steel section connected to each other by series or rows of boltsVariations include: lap joints, shingle joints, gusset platesThe shear plane is parallel to the longitudinal axis of the memberReduction in ultimate capacity of the bolt due to uneven distribution of stresses of the boltsHigher stresses at the ends of the plates and lower stresses in the middle
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Hosseini
Gusset PlateUsed to transfer the load from one member to the other when the longitudinal axis of two or more member are inclined with respect to each otherInsignificant out of plane bendingClassical Analysis Procedure:
Taking sections parallel and perpendicular to the chord of the truss
identifying the bolt forces that had been delivered to the gusset plate, and utilize these forces to calculate the shear, normal force, and moment at the cut section using beam theory2023-04-22
High Strength Bolting by H. Bajehkian and P. Hosseini 15 of 20
Gusset PlateFinite Element Method Analysis
Several models have been developed and analyzed the gusset plate in the elastic range and also predicted its behavior up to its ultimate strength
Whitmore’s conclusions were confirmed by the results of the analysis
Inaccuracy of the beam theory employed in the classical approach was also confirmed however it appeared to be more conservative.
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Whitmore’s Analysis Approach. maximum normal stress can be
estimated by assuming that the member force was distributed uniformly over an effective area of plate material
This area was obtained by multiplying the thickness of the plate by an effective length which is obtained by constructing 30° lines from the outer fasteners to the bottom fasteners as shown.
High Strength Bolting by H. Bajehkian and P. Hosseini
Eccentrically Loaded JointsWhen the applied load results in a line of action passing outside the center of rotation of the fastener group.Common examples: bracket-type connections, web splices in beams and girders, and standard beam connections
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Hosseini
Eccentrically Loaded Joints
Design Assumptions: the connection rotates about an
instantaneous center of rotation.
Maximum slip resistance of all fasteners is reached at slip load.
The slip resistance of each fastener can be represented by a force at the center of the bolt acting perpendicularly to the radius of rotation
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Design Procedure:• Three equations of equilibrium
must be employed to determine the coordinates of the instantaneous center of rotation and the maximum value of the load that results in slip of the connection.
A trial centre of rotation is selected and iterated until the three above equations of equilibrium are satisfied.
0sin1
n
iisR
0cos1
PRn
iis
0)(1
0
n
isiRrreP
High Strength Bolting by H. Bajehkian and P. Hosseini
Questions
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Hosseini