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MORGAN STATE UNIVERSITY SCHOOL OF ARCHITECTURE AND PLANNING LECTURE X Dr. Jason E. Charalambides Bolts and Bolted Connections 2 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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Page 1: Bolts and Bolted Connections - avant-garde.engineering · Bearing Type Connections!A Bolted connection is classified as “bearing type” if the occurrence of slip is not considered

MORGAN STATE UNIVERSITY

SCHOOL OF ARCHITECTURE AND PLANNING

LECTURE X

Dr. Jason E. Charalambides

Bolts and Bolted Connections

2

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

Page 2: Bolts and Bolted Connections - avant-garde.engineering · Bearing Type Connections!A Bolted connection is classified as “bearing type” if the occurrence of slip is not considered

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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

4

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.

Page 3: Bolts and Bolted Connections - avant-garde.engineering · Bearing Type Connections!A Bolted connection is classified as “bearing type” if the occurrence of slip is not considered

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Page 4: Bolts and Bolted Connections - avant-garde.engineering · Bearing Type Connections!A Bolted connection is classified as “bearing type” if the occurrence of slip is not considered

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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

Page 5: Bolts and Bolted Connections - avant-garde.engineering · Bearing Type Connections!A Bolted connection is classified as “bearing type” if the occurrence of slip is not considered

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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

Page 6: Bolts and Bolted Connections - avant-garde.engineering · Bearing Type Connections!A Bolted connection is classified as “bearing type” if the occurrence of slip is not considered

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Page 7: Bolts and Bolted Connections - avant-garde.engineering · Bearing Type Connections!A Bolted connection is classified as “bearing type” if the occurrence of slip is not considered

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Page 8: Bolts and Bolted Connections - avant-garde.engineering · Bearing Type Connections!A Bolted connection is classified as “bearing type” if the occurrence of slip is not considered

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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?

Page 9: Bolts and Bolted Connections - avant-garde.engineering · Bearing Type Connections!A Bolted connection is classified as “bearing type” if the occurrence of slip is not considered

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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

Page 10: Bolts and Bolted Connections - avant-garde.engineering · Bearing Type Connections!A Bolted connection is classified as “bearing type” if the occurrence of slip is not considered

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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

Page 11: Bolts and Bolted Connections - avant-garde.engineering · Bearing Type Connections!A Bolted connection is classified as “bearing type” if the occurrence of slip is not considered

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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

Page 12: Bolts and Bolted Connections - avant-garde.engineering · Bearing Type Connections!A Bolted connection is classified as “bearing type” if the occurrence of slip is not considered

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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

Page 13: Bolts and Bolted Connections - avant-garde.engineering · Bearing Type Connections!A Bolted connection is classified as “bearing type” if the occurrence of slip is not considered

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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

Page 14: Bolts and Bolted Connections - avant-garde.engineering · Bearing Type Connections!A Bolted connection is classified as “bearing type” if the occurrence of slip is not considered

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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.

Page 15: Bolts and Bolted Connections - avant-garde.engineering · Bearing Type Connections!A Bolted connection is classified as “bearing type” if the occurrence of slip is not considered

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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

Page 16: Bolts and Bolted Connections - avant-garde.engineering · Bearing Type Connections!A Bolted connection is classified as “bearing type” if the occurrence of slip is not considered

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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

Page 17: Bolts and Bolted Connections - avant-garde.engineering · Bearing Type Connections!A Bolted connection is classified as “bearing type” if the occurrence of slip is not considered

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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

Page 18: Bolts and Bolted Connections - avant-garde.engineering · Bearing Type Connections!A Bolted connection is classified as “bearing type” if the occurrence of slip is not considered

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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.