connections in steel structures.pdf

8/14/2019 connections in steel structures.pdf

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How critical a connection in steel structures safety, cost, and performance

Connections | Introduction

The structural designer leads the connectiondesign, but should work with steel fabricators tooptimize the total cost of the project

a) Block Shear Rupture

b) Bolt Bearing

c) Bolt Shear

Connections | General Limit states

d) Bolt Tension Fracture

e) Concentrated Forces

f) Flexural Yielding

g) Prying Action

h) Shear Yielding and Shear Rupture

i) Tension Rupture

j) Whitmore Section Yielding / Buckling


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Slide: 3

Limit states | (a) Block Shear Rupture

(Photo by J.A. Swanson and R. Leon, courtesy of

Georgia Institute of Technology)

Limit states | (b) Bolt Bearing (againstthe bolt hole edge)




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Limit states | (c) Bolt Shear

(Photo by P.S. Green)

Limit states | (d) Bolt Tension Fracture




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Limit states | (e) Under Concentrated

Forces

When forces are transferred from one

member to another, some form of

localized deformation (due to yielding or

buckling) occurs, depending on types of

connections, as illustrated in the following

slides .

Limit states | Under ConcentratedForces (compression due to bending)

Flange Local Bending Limit State

(Beedle, L.S., Christopher, R., 1964)


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Limit states | Under ConcentratedForces (Shear force)

Web Crippling Limit State

(Photo by T. Murray, Virginia Tech)

Limit states | UnderConcentrated Forces

(Compression)

Web Local Buckling Limit

State

(SAC Project)


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Module IContents:

OverviewLimit states | (j) Whitmore Section Yielding /

Buckling in Gusset Plate

Gusset

plate

P

(Beedle, L.S. and Christopher, R., 1964)

a) Commonly used bolt

b) Bolt types

c) Bolt shear strength (LRFD/ASD)

Module IContents:OverviewConnections | Bolt related limit states and

detailing

o o e an a ure mo es

e) Bolt minimum spacing and edge distance


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Bolt | Commonly used bolts

Bolt| Commonly used bolts

A307 machine bolts (unfinished bolts or common bolts:

bolt)

Fnt = 310 MPa (45 ksi)

A325 high strength bolts (can be pretensioned)

F nt = 620 MPa (90 ksi)

F nt = 780 MPa (113 ksi)

F nt : nominal tension strength (can be pretensioned)


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Bolt| Commonly used bolts (AISC 360-05)

Bolt| Bolt types: N, X, and SC

T es of Connections:

(a) Bearing Type (A307, A325, A490)

N - threads iNcluded in shear planeX - threads eXcluded from shear plane

SC - slip critical

Ex: 19 mm ( in.) A325 - N


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Bolt | nominal shear strength in bearing-type (N and X)

Nominal Shear Strength per bolt per

shear plane:

rn = Fnv x Ab , MPa, (Ab is nominal bolt area = db 2/4)

Nominal Shear Strength of the connection:

R = r x Number of Bolts x Number of Shear

Planes (either 1.0 or 2.0)

Bolt | nominal shear strength in bearing-type (N and X)

Design Shear Strength of the Connection:

LRFD : Rn = 0.75 Rn

ASD : Rn = Rn/ 2.00(AISC 360-05, J3.1)


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Module IContents:

OverviewBolt | Bolt hole and failure modes

For all hole related limit states except tear out,

the effective hole diameter used in

calculations is

dh = dh + 2mm (AISC360-05, Table J3.3)The additional 2mm accounts for damage.

, .

For bearing, the bolt diameter is used.

Module IContents:OverviewBolt | Bolt hole and failure modes

TuBearing

Tear OutTu

Le Lc


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Module IContents:

OverviewBolt | Bolt hole and failure modes

Bearing

Tear Out

Le Lc

Module IContents:OverviewBolt | Bolt hole and failure modes

Section J3.10 Bearing Strength at Bolt

Holes

For standard, oversized, and short-slotted

holes

Rn = 1.2 L ct Fu < 2.4 db t Fu

1.2 L ct Fu is the tear out strength

2.4 db t Fu is the bearing strength

Lc = clear distance


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Module IContents:

OverviewBolt | Minimum Spacing and Edge Distance

e s

s

e

Section J3.3 Minimum Spacing:

Preferred: S = 3d; and e = S/2

d = the nominal diameter of the bolt.(commonly S = 75mm and e =38mm)

Module IContents:OverviewBolt | Minimum Spacing and Edge Distance

e s

s

e

Section J3.5 Maximum Spacing and Edge Distance:

S


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Module IContents:

Overview

Bolt | Minimum Spacing and Edge Distance (TS648)

a) Fillet weld strength

b) Effective width in Fillet weld

Module IContents:OverviewConnections | Weld related limit states

and detailing

c) Minimum size, t, of fillet welds

d) Base metal rupture strengthe) Example: Determine design strength Td for Welds


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Module IContents:

OverviewWeld | Fillet weld strength

Nominal Strength Rn = Fw Aw (AISC360-05, Eq.J2-4)

Fw = 0.60 FEXX (1.0 + 0.50 sin1.5) (AISC360-05, Eq.J2-4)

Fw = nominal strength of the weld metal per unit area, MPa

FEXX = electrode strength, MPa

= angle of loading measured from the weld longitudinal axis, degrees2

w

T

Weld

Weld RuptureWeld | Fillet weld strength

T T

= 0 =90

= 00 Fw = 0.60 FEXX = 900 Fw = 0.60 (1.5 FEXX)


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Weld | Effective width in Fillet weld

t

tt

eff .(ASIC360-05, J2a)

t : leg dimension

teff: effective throat of a fillet weld

Weld | Minimum size, t, of fillet welds


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Weld | Minimum size, t, of fillet welds

Maximum Fillet Weld Size (AISC360-05, J2.b):

tp < 6mm tw = tp

tp > 6mm tw = tp 2

mm

1/16

tp :thickness of the plate

tw :weld size

Weld | Minimum length of fillet welds

Minimum length of fillet welds(AISC360-05, J2.b):

4tw =< 6mm

Minimum length of fillet welds(AISC360-05, J2.b):

=1.2-0.002(L/tw) 1.0

If the length, L, exceeds 100 times the weld size,

the actual length is reduced to an effective length by

multiplying the actual length by. When the length of the weld exceeds

300 times the leg size, =0.60.

tw :weld size

L


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Weld | End Returns of Fillet Welds

The reason for end returns is to ensure

that the weld size is maintained over

. -

does not require end returns.

Weld | Base Metal Rupture Strength at weld

AISC 360-05 Section J4.2 Shear Rupture

Strength

The desi n shear ru ture stren th for the

limit state of rupture along a shear failure

path in the affected and connecting:

Rn = (0.6 Fu Anv)

Anv : welded area subjected to shear (the same for base metal rupture and

weld rupture)


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Base metal force is carried by one fillet weld. The

largest effective fillet weld size will be the size

where the weld strength equals the base metal

strength:

(0.6FEXX) teffL(0.6Fu) tBML

teff=0.707t (Fu / FEXX) tBM

Any weld size larger than the above

value doesnot contribute to the strength

.


Base metal force is shared between two equal

size fillet welds (one weld on each side of the base

metal). The largest effective fillet weld size will be

the size where the weld strength equals the base

metal stren th:

2(0.6FEXX) teffL(0.6Fu) tBML

teff=0.707t 0.5(Fu / FEXX) tBM

Any weld size larger than the above

value doesnot contribute to the strength

.


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Weld |Example: Determine design

strength Td for Welds

E70XX

PL 3/8" x 8"E70XX Electrod, Fexx = 485 MPa

STI (A36) steel Fu = 400 MPa

PL 10mm x 200mm

6mm

Td

PL 5/16" x 5"5"

Weld Rupture:

Tn=(0.6x485MPa)(0.707x6mm) )(125mmx2)

= 308.6 kN

Base Metal:

Tn= (0.6 Fu Anw)

PL 8mm x 125mm125mm

= (0.6x400MPa)(8mm)(125mmx2) = 480 kN Tn = 308.6 kN (weld rupture governs)

Td = (0.75)(308.6kN)=231.5 kN (LRFD)

Td = (0.50)(308.6kN)= 154.3 kN (ASD)

a) Groove weld strength

b) Effective area in Groove weld

Module IContents:OverviewConnections | Weld related limit states

and detailing


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Weld | Groove weld strength

Base Metal Nominal Strength

=

Weld | Groove weld strength

n - , . -

FBM

= nominal strength of the base metal per unit area, MPa

ABM=croos-sectional area of the base metal (mm2)

e om na reng

Rn = FwAw (AISC360-05, Eq.J2-3)

Fw = nominal strength of the weld metal per unit area, MPa

Aw=effective area of the groove weld (mm2)


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Weld | Effective Area of Groove Weld

Aw=teLte

Weld | Effective Area of Groove Weld

For CJP welds, the limit state of weld metal strength will never control since both the

weld and the base metal have the same effective area and the filler metal is

constrained to be stronger than the base metal. As a result, only the capacity of the

.

For PJP welds, the effective areas for the weld and base metals differ, with the weld

effective area being less than the base metal. If the welds effective throat is small

enough, then the weld strength will control over the base metal strength.


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Weld | Effective Area of Plug and Slot

Welds

The effective shearing area of plug and slot welds is

determined as the nominal cross-sectional area of

e o e or s o n e p ane o e ay ng sur ace.

Weld | Effective Area of Plug and Slot

Welds

(AISC 360-05, J4-2)


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Weld | Summary

Weld | Summary


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Weld | Summary

Weld | Summary


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Shear Connections | Double-Angle Connection

Module IContents:Overview


All Bolted Double-Angle Connection

Girder B1

Beam B1B

Beam B1BGirder B1

Girder B1


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Module IContents:

Overview

Shear Connections | Double-Angle ConnectionAll Bolted Double-Angle Connection

(continued from the previous slide)

Girder B1 supports Beam B1B by an all-bolted, double-angle

connection.

These double-angles are field bolted to the supporting girder and

shop bolted to the supported beam. This eliminates "knifed"

erection. (Lowering the supported beam web into place between the

angles).

The offset bolt rows between the in-plane and outstanding angle

legs provide better entering and tightening clearances.

Since both of the members are the same depth, the beam is

double coped to accommodate the flanges of the girder.



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Shear Connections | Single Plate (Shear Tab) Connection

becoming more popular

Shear Connections | Single Plate (Shear Tab) Connection


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Shear Connections | Unstiffened Seated Connection

Shear Connections | Stiffened Seated Connection


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Shear Connections | Stiffened Seated Connection

Shear Connections | Single Angle


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Shear Connections | Single Angle

Shear Connections | Tee Connection


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Shear Connections | Tee Connection


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Limit States associated with All Bolted Double-Angle Connection

Nominal Strength of the connection, Rn in kN, from

each of the following limit states:

a) Block Shear Rupture

b) Bolt Bearing

c) Bolt Shear

d) Shear Yielding

e) Shear Rupture

f) Flexural strength

The governing nominal strength of the

connection, Rn is the smallest among

all.


Shear Connections | Double-Angle ConnectionPossible limit States in a typical beam-to-

girder connection

1 25

2L

a) Block Shear Rupture of the beam web (1-1) or the angles (2-2)

1 2

3, 45

A A

o ear ng o e eam we or ang es

c) Bolt Shear (4)

d) Flexural Yielding of the coped web

e) Shear Yielding of the gross area of angles along 5-5

f) Shear Rupture of the net area of angles along 5-5


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Module IContents:

Overview

Shear Connections | Double-Angle ConnectionLimit State | Block-shear rupture


Shear Connections | Double-Angle ConnectionLimit State | Bolt Bearing

Rn = 1.2 L ct Fu < 2.4 db t Fu , kN

1.2 L ct Fu : tear out strength

2.4 db t Fu : bearing strength

Lc : clear distance

Rn Rn

Rn/2

Rn/2= 0.6 Fu L ct

Lc Lc

nn

Rn/2


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Module IContents:

Overview

Shear Connections | Double-Angle ConnectionLimit State | Bolt Shear

Rn = Fnv x Ab x Number of Bolts in the web x

um er o ear anes = or ou e

angle connections), kN

Ab = db2/ 4,

db : bolt diameter.



Shear Yielding: Rn = (0.6 Fy)Ag

=

Limit State | Shear yielding and rupture

n . u n

Fy = yield stress;

Fu = tensile strength

Ag = gross area in shear; and

An = net area of the angles


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Module IContents:

Overview

Shear Connections | Double-Angle ConnectionLimit State | Flexural Strength with Coped

Beams ( Rupture and local buckling)

Required flexural strength:

u u a a , ee ons ruc on anue , on

Ru or Ra = beam end reaction force, kN

Available Strength based on Flexural Rupture:

Mn = Fu Snet (for single or double coped beam cases)b = 0.75, b= 2.00 Available Strength based on Flexural Local Web Buckling:

Mn = FcrSnetb = 0.90, b= 1.67


Shear Connections | Double-Angle ConnectionLimit State | Flexural Strength with Coped

Beams ( Rupture and local buckling) (contd)

Snet = net section modulus, mm3 , tabulated in Table 9-2

in AISC 13th Edition Manual

Fcr= available local web buckling stress as given in thefollowing slides


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Limit State | Flexural Strength local web buckling

stress Fcr with only top flange coped

Limitations: c < 2 d Ru or Ra

dc < d / 2

22

2

012(1- )

wcr y

tEF fk F

v h

=

1.65

E = 29,000 ksi, 0.3=E=200,000 MPa, v=0.3

2 when 1.0

1 when >1.0

c cd d

fc c

d d

=

+

0

0

0

0

2.2 when 1.0

2.2when >1.0

cc h

kh c

c h

=

f= plate buckling model adjustment factor

k=platebucklingcoefficient



Limit State | Flexural Strength local web buckling

stress Fcr with both top and bottom flanges coped

Limitations: c < 2 d

2

0.62 wcr d

tF E f=

dct < 0.2 d

dcb

< 0.2 d

0

fd = 3.5 7.5 (dc / d) (adjustment factor)

dc = the larger of (dct , dcb)


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Module IContents:

Overview


Limit State | Flexural Strength Example: Determine if adequate

mm200mm

12mm

R u =180 kNW14x30 STII (A992 Steel)




W14x30 STII (A992) Fy = 345 MPa Fu = 450 MPa

e = 212 mm

c = 200 mm

d = 351 mm

tw = 6.86 mm

dc = 75 mm

= =o .

Snet = 137,160 mm3 from Table 9-2 AISC 13 Ed.

Manual


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Module IContents:

Overview



22

2

012(1- )

wcr y

tEF fk F

v h

=

= = F 50 ksi, so F 50 ksi

=

= = MP345Fso,MPa345MPa12.476 cr=>=

)74.3)(14.1(276

86.6762,180

2

=

. .

So, f = 2 (c / d) = 2 x 0.57 = 1.14

c / ho = 200 / 276 = 0.72 < 1.0

So, k = 2.2 (ho / c)1.65 = 2.2 (276 / 200)1.65 = 3.74


Shear Connections | Double-Angle ConnectionLimit State | Flexural Strength Example: Determine if adequate

Use LRFD since Ru is given:

Required strength: Mu = Ru e = (180kN)(0.212m) = 38.16 kNm

Available Strength based on Flexural Rupture:

Mn

= Fu

Snet

= (450)(137,160) = 61.72 kNm

b Mn = (0.75)(61.72) = 46.29 kNm > Mu = 38.16 kNm Available Strength based on Flexural Local Web Buckling:

n = cr net = , = . mbMn = FcrSnet = (0.9) (47.32) = 42.59 kNm > Mu = 38.16 kNm


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References:

Shen, J., Advanced Steel Structures, Class Notes,

Fall 2009.

American Institute of Steel Construction (AISC)Specification: AISC 360-05 Chapter J (includedin the AISC Manual Part 16).

Design of Connections (Parts 9 through 13) of the

anua

AISC Documents on Teaching Steel Connections

Quimby, T.B., Steel Class Notes, 2008