Bolts and Bolted Connections - avant-garde.engineering · Bearing Type Connections!A Bolted...
Transcript of Bolts and Bolted Connections - avant-garde.engineering · Bearing Type Connections!A Bolted...
MORGAN STATE UNIVERSITY
SCHOOL OF ARCHITECTURE AND PLANNING
LECTURE X
Dr. Jason E. Charalambides
Bolts and Bolted Connections
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Topics
! Types and characteristics of structural bolts! Strength of bolts in tension, shear, and combined shear +
tension! Load transfer mechanisms in bolted connections under
shear loading:" Friction" bearing and shear
! Design of simple bolted connections under shear loading:" bearing type connections" slip critical connections
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Reading
! AISC Specification and Commentary: Section J3! (pp 16.1-118 to 16.1-128 and 16.1-400 to 16.1-411)! Specification for Structural Joints Using High Strength Bolts,
published by the Research Council on Structural Connections" (www.boltcouncil.org)." Most recent edition: December 31, 2009" Copy provided in AISC Manual - Section 16.2
! Design Aids: AISC Manual - Part 7
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Experimental Behavior of a Bolted Tension Splice
Reference: Bendigo, R., Hansen, R., and Rumpf, J., “Long Bolted Joints”, Journal of the Structural Division, ASCE, Vol. 89, No. ST6, December 1963.
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TYPES OF STRUCTURAL BOLTS
Bolts are identified by ASTM designation
Low Strength Bolts
A307 Fu = 60 ksi
High Strength Bolts
A325 Fu = 120 ksi for d 1-inch Fu = 105 ksi for d > 1-inch F1852 “Twist-off” A325 bolt
A490 Fu = 150 ksi F2280 “Twist-off” A490 bolt
Other Bolts and Threaded Fasteners
A449: Same material specification as A325, but available in larger diameters and available unheaded
A354 Gr BD: Same material specification as A490, but available in larger diameters and available unheaed
A36 and A572 Gr 50 are available as round bars and can be threaded (for anchor bolts, etc).
Identified as Group A bolts in AISC Manual
Identified as Group B bolts in AISC Manual
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General Characteristics of A325 and A490 Bolts
Types of A325 and A490 Bolts:
Type 1 -- available plain or galvanized (only plain for A490) Type 3 -- weathering steel
Bolt Diameter: 1/2" to 1-1/2" in increments of 1/8"(3/4" 7/8" 1" most common)
Bol
t Len
gth
(Ava
ilabl
e ap
prox
. 1"
to 8
" )
Thread Length:Standard for eachbolt diameter
Threads: dimensions per ANSI B1.1Thread Series UNC( Unified Stadard Series - Coarse)
Hex Nut: ASTM A563 -- standard dimensions -- standard markings
Hardened Washer: ASTM F436
Hex Head -- standard dimensions
Standard Head Markings: -- Grade ( A325 or A490 ) -- Manufacturer
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Load Transfer Mechanisms
! Load can be transferred through the bolted connection via two possible mechanisms
" Friction:" Bearing and Shear:
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Load Transfer Mechanisms
! Review of Concept of Friction:" Consider this cubical body subjected to gravitational load and to a
lateral load. " Naturally the gravitational load would not move it as it rests on the
ground. " What could stop this body from accelerating and continuing to
accelerate in a motion parallel to the horizontal?
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Load Transfer Mechanisms
! Review of Concept of Friction:" The body will slip at the point where the lateral load P will
surpass the resistance caused by the friction of the surface." Consider the the pressure distribution at the base of the block
as having a resultant N
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Load Transfer Mechanisms
! Review of Concept of Friction:" Increase load P until block slips" Coefficient of friction “µ” determinant factor for slip action:
µ=PslipN
ΟRP slip=µ×N
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Load Transfer By Friction
! Consider a simple bolted connection made of two plates and a bolt." Prior to applying external load P, bolt is tightened." Initial tension in bolt = T
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Load Transfer By Friction
! Envision a diagram of stresses of the bottom half of this arrangement." The applied tension multiplied by the slip factor “µ” will equal
the frictional shear stress P
" Note: On this diagram the shearing of the bolt was not addressed
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Load Transfer By Friction
! Prior to slip, P is transferred by frictional shear stresses at the contact surfaces of the plates. The bolt experiences no shear stresses. Only tension is what takes place.
! The load at which the connection slips is: P slip=µ×N
" For Steel plates, typical values of µ (also the symbol “ks” used) vary between 0.3 and 0.5.
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Load Transfer By Friction
! If the connection is constant, using more than one bolt:" P_slip" Where N_b is the number of bolts, T is the tension per bolt, and µ is
the slip coefficient
! If more than two plates are used:" 2 slip planes – each resisting P/2
! In general: " Where Ns is the number of slip planes
Pslip=µ×Nb×T
Pslip=2µ×N b×T
Pslip=N s× µ×Nb×T
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Load Transfer By Bearing and Shear
! Consider a simple bolted connection made of two plates and one bolt" Assume no initial tension on the bolt
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Load Transfer By Bearing and Shear
! Consider a simple bolted connection made of two plates and one bolt" Consider the Free Body Diagram of the plates
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Load Transfer By Bearing and Shear
! Consider a simple bolted connection made of two plates and one bolt" Consider the Free Body Diagram of the bolt
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Load Transfer By Bearing and Shear
! Consider a simple bolted connection made of two plates and one bolt" Consider the Free Body Diagram of the bolt
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Load – Deformation Behavior of Bolted Connection
! Consider a simple bolted connection made of two plates and a bolt" Assume that plates are strong enough to withstand the possibilities of
tension member failure (yield, fracture, block shear) prior to fracture of bolted connection:
Pslip= µ×Nb×T
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Load – Deformation Behavior of Bolted Connection
! Prior to slip (P<P_slip), load transfer is by friction only.! After slip (P>P_slip), load transfer is by combination of friction and
shear + bearing! As P approaches Pult, load transfer relies mainly on bearing and
shear, rendering the contribution of friction minimal to insignificant.
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Load – Deformation Behavior of Bolted Connection
! Pult is controlled either by bolt shear failure or by plate bearing failure (hole tear out or hole excessive elongation)
! What is “failure”?" Depending on the application it can be either “P_ult” or “P_slip”
Pslip= µ×Nb×T
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Design Criteria
! Bolted connections are classified as either of the two following types:" Bearing type Connections" Slip critical Connections
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Bearing Type Connections
! A Bolted connection is classified as “bearing type” if the occurrence of slip is not considered to be a limit state.– Applications" Connections in which the deformations due to slip are not
detrimental to the strength or serviceability of the structure" Most connections in building frames subject to static loads
and using standard size holes can be designed as bearing type, with a maximum slip of 1/16”
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Bearing Type Connections cont
– Structural Performance Criteria" Prevent bolt shear at factored loads" Prevent Plate bearing failure at factored loads– Installation Requirements" No special surface preparation" Bolts need not be fully tightened (snug tight is adequate)" Minimal inspection required
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Slip Critical Connections
! The connection is classified as “slip critical” if the occurrence of slip shall be detrimental to strength or serviceability of the structure – Applications" Oversize slotted holes (with load in the direction of the slot
! Slip will result in potentially large deformations that may result in structural failure or serviceability problems
" Cyclic loading! Slip may result in fatigue
" Bolts and welds are used together in the same connection! Use of slip critical design for the bolts leads to better
sharing of load with welds
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Slip Critical Connections
! Structural performance criteria " Prevent slip at factored loads" Prevent bolt shear failure at factored loads" Prevent plate bearing failure at factored loads
Usually do not control
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Slip Critical Connections
! Installation Requirements" Special preparation of steel is required (to attain the assumed µ in the
design)" Bolts must be fully tightened (to achieve T assumed in design)" Close inspection is required on surface conditions and bolt installation
! Slip critical connections are in generally considerably more expensive than bearing type due to: " Larger number of bolts; larger connection" Higher labor costs associated with surface preparation and bolt
installation" Higher inspection costs
The Dom-ino frame design by Le Corbusier during the early 20th century became a paradigm ,a datum in the history of architecture and a reference to the modernist movement.