Steel Connections Part 1_Part1.pdf - Staff.zu.edu.eg

staff.zu.edu.eg

Steel Connections Part 1_Part1.pdf - Staff.zu.edu.eg

Steel Connections

Non‐pre‐tensioned Steel

Connections

A lecture prepared

p

By

Assis. Prof. Dr Ehab Boghdadi Matar


Acknowledgement

• I acknowledge photos found din this lecture to the

scientific teaching aids found in different sources,

especially ill

• AISC digital library

• ESDEP lecture notes

• AISC‐ connection teaching toolkit

• Personal photos taken in Germany, Holland,

Austria and Egypt.

Ehab matar

Assis. Prof. of steel structural


Objectives

Through the following 3 lectures, we shall study

together the steel connections. Our main

objectives will be:

1. Identify the different types of steel connections

2. Understanding the force transfer through steel

connections

3. Practicing the design of bolted and welded

connections through neat self explained

calculations and full dwgs details.


Definitions

• Every structure is an assemblage of individual

parts or members that must be fastened together

usually at the member ends.

Connections are required where the various

member ends must be attached to other

members sufficiently to allow the load to

continue in an orderly flow to the foundation.

• The connection design involves producing a joint

that is safe, economic and capable of being built.


Desseldorf airport Germany


Aachen banhof Gemany


Kolen banhof Germany


Connections classifications

Connections can be

classified into several

categories depending

on:‐

1. Type of connectors i.e.

rivets, bolts or welding


2‐ According to

type of

transferred

forces i.e.

concentric

simple shear

connections,

eccentric shear

connections,

moment

connections,

moments and

shear

connections,….

etc.


• 3‐ connection rigidity

i.e. rigid connection

(that transfer 100% of

moments at

connection) and semi

rigid connection (that

transfer a pre‐estimated

amount of moments

requested by designer)


4‐ According to

whether the

connections are

executed on

shop or in site

(shop splices are

usually welded

connections

while field

splices are

usually bolted

connections)


Economy in steel connections design

(ESDEP)

• The costs for a steel

Division iii Item

% of overall

structure can be divided

cost

into costs for material and Material Material 20‐40%

costs for lb labour as shown in

Calculation

table.

Drawings

• From this division of costs it

can be concluded that a Labour Fabrication 60‐80%

saving of labour costs has

Protection

potentially more influence

Erection

on the overall costs of steel

structures than saving on

material.

• An influencing factor is the

relation between cost per

kg steel and cost per man

hour.


• A major part of labour costs has a

direct relation to the design and

fabrication of connections. It is often

better in design to save labour at the

expense of material. This fact can be

illustrated with some simple

examples. To estimate the costs, the

following assumptions are made:

• the costs for 1cm 3 of weld is

equivalent to 0,7 kg of steel.

• the costs for fabrication of stiffening

plates are equal to the welding costs.

• the costs per hole are equivalent to 2

kg of steel.

• The shown example illustrates that

simple connection resulted in larger

beam size, cheaper cost, and

continuous connection resulted in

lower beam size and extra cost for

connections. Alternative A is the

cheapest, while Alternative C may be

cheaper for spans > 10m

Example


Connecting Structural Steel (AISC)

• A fabrication shop will have a desired fastening method suited to its equipment and

fabrication methods

• Field connections are typically bolted

• Welding may be used for field connections where bolting is either impractical or

undesirable

• Welding is better suited to the controlled environment of a fabrication shop

15


Our Course shall handles:‐

• Bolted connections (bearing type connections

and friction type connections)

• Welded steel connections

The study of these connections will be for

1. Simple shear connections

2. Eccentric shear connections

3. Moment connections


Bolted steel connections

A‐ Non pre‐tensioned dbolted connections (bearing type

connections)

• Bearing type connections are those connections

that use either ordinary bolts or high strength

bolts and transfer the forces in the connections

depending on bolts and connected plates

capacity.

• Usually most of the connections are bearing type

except those subjected to dynamic or repeated

fluctuated stresses and in this case friction type

connections that t uses only high h strength thbolts are

only allowed. This connections transfer the forces

through friction between connected plates.


Bearing and friction type bolted

connections


Bolted Joint Types (AISC)

• There two basic bolted joint types:

• Bearing

19

o

• Slip‐critical

o

The load is transferred between members by bearing on the bolts

The load is transferred between members by friction in the joint


Bolts Grades According to ECP

Bolt Grade

4.6 4.8 5.6 5.8 6.8 8.8 10.9

F yb (t/cm 2 ) 2.4 3.2 3.0 4.0 4.8 6.4 9

F 2 ub (t/cm )

4.0

4.0

5.0

5.0

6.0

8.0

10.00

The bolts grade is distinguished by two characters

the first identify the ultimate strength of the bolts

and the second identifies the percentage of yield

stress relative to the ultimate and thus bolts grade

5.6 identifies a bolt with ultimate strength of

5t/cm 2 and yield stress of 5x0.6=3t/cm 2


ASTM Bolt Types (AISC)

• A307 – Low carbon steel

• Not commonly used

• Only used for secondary members

• A325 – High‐strength medium carbon steel (above left)

• Most common bolts used in building construction

• A490 – High‐strength heat treated steel (above right)

21

(AISC & NISD 2000)

• Cost more than A325’s, but are stronger so fewer bolts may be necessary

• Note that the ASTM designation is indicated on the head of the bolts above


Common Bolt Sizes (AISC)

• A325 and A490 bolts are available in diameters ranging from 1/2” to 1‐1/2”

• The most common sizes are 3/4”, 7/8”, and 1”

• High‐strength bolts are commonly available in incremental lengths up to 8”

22

(AISC)


Washers (AISC)

• Hardened steel washers are used in many structural connections to spread pressure

from the bolt tightening process over a larger area

• Washers may also be used to cover an oversized or slotted hole (RCSC 2000)

• Flat washers are most commonly used

• Tapered washers (above left) are used when the surface being bolted has a sloped

surface, such as the flange of a channel or an S shape

• A325 bolts require a washer under the element (head or nut) being turned to tighten

the bolt (shown under the nut, above right)

23

• A490 bolts require a washer under both the head and nut (AISC & NISD 2000)


Parts of the Bolt Assembly (AISC)

Grip

Washer

Face

Washer

Nut

Head

Shank

Length

Thread

• Grip is the distance from behind the bolt head to the back of the nut or washer

• It is the sum of the thicknesses of all the parts being joined exclusive of washers

• Thread length is the threaded portion of the bolt

• Bolt length is the distance from behind the bolt head to the end of the bolt

24

(AISC & NISD 2000)


Samples of bolted connections


Gallery of column and beam splices


Modes of failure of bearing type

• Failure of bolted

connections may

happen either for the

connected plates or for

the bolts.

• Failure of the plates

may be due to

shearing, tearing or

bursting.

• Fil Failure of the bolts may

be due to shear,

bending or bearing.

• These modes of

failures may be avoided

through proper

arrangement of bolts

connections


Bolted Joint Failure Modes (AISC)

Bearing

Bearing

Bearing

Yield Fracture

Fracture

Bearing

Yield

• Bolts in bearing joints are designed to meet two limit states:

1. Yielding, which is an inelastic deformation (above left)

2. Fracture, which is a failure of the joint (above left)

• The material ilthe bolt bears against is also subject to yielding ildi or fracture if it is

undersized for the load (above right)

• Tension connections act similarly to bearing connections

29

• Many times, connections in direct tension are reconfigured so that the bolts act

in shear

(AISC)


Bearing Joints (AISC)

• In a bearing joint the connected elements are assumed to slip into bearing against the

body of the ebolt

• If the joint is designed as a bearing joint the load is transferred through bearing

33

whether the bolt is installed snug‐tight or pretensioned

(AISC)


• Bolts in a

connection is

to be arranged

in such a way

to avoid either

of plate

failure, or uneven

distribution of

forces in the

bolts that

leads to bolts

failure

Bolts arrangement


• If the joint is too

long, it is evident

that the first bolt

will carry more

than the medium

bolt. It is preferred

that no more than

6 bolts should be

arranged in a single

gauge otherwise a

reduction in the

resistance of the

bolts is expected as

the load

distribution on

each bolt will be

non‐uniform

Limiting max. no. of bolts in a

connection


Allowable stresses for Bearing type

connections

1. Simple shear

Rleast

min.

R

sh,

Rb


connections:‐ Rsh

m.

Ab

. qb

m.


Rb


.

tmin.

fb

n= F d /R least

where

Where

R

sh


qb


n= no. of bolts ≥2

and equal

F d= design force

Ab

bolt

R least = least

m no. of

resistance of t

min


fb


the bolted

strength of

connection in

shear and

bearing

shear resistance

20%.F

2


. q

4

b

of bolts (tons)

allowableshear stress for bolts (t/cm

ub

2

) 25%.F

for bolts grades 4.8, 5.8,

ub

6.8 and10.9

2

2

bolt cross sectional

area cm



4

shear plans 1for single shear, 2 for double shears

min. thickness of

connected parts

allowable bearing strength t/cm

connected plates

2

0.6.F

u

where F

for bolts grades 4.6,5.6,8.8

u

ultimate tensile


2‐ for bolts subjected to

tension

n= F d /R t

Where

n= no. of bolts

F d = design tensile force (t)

R t = A b x0.33xF ub (t)

A b = bolt cross sectional area

(cm 2 )

F ub = ultimate strength of

bolts material (t/cm 2 )


• 3‐ Bolts subjected to

tension and shear

The no. of bolts are

assumed and then the

shear force in each bolt

2

2

Q

/ b

Text

/ b

and the tensile force in



1.

0


R sh


Rt


each bolt is calculated

where

and checked against the

shown interaction

equation:

Q

T

R

R

shear force per one bolt (t)

tensile force per one bl bolt () (t)

shear resistance of one bolt (t)

tensile resistance of one bolt (t)

/b


ext/b

sh

t


Threads in the Shear Plane (AISC)

• The shear plane is the plane

between two or more pieces

under load where the pieces

tend to move parallel from

each other, but in opposite

directions

• The threads of a bolt may

either be included in the shear

plane or excluded from the

shear plane

• The capacity of a bolt is greater

with the threads excluded

from the shear plane

• The most commonly used bolt

is an ASTM A325 3/4” bolt with

the threads included in the

shear plane

39

(AISC & NISD 2000)

Threads Included In The Shear Plane

Threads Excluded From The Shear Plane


General Requirements Regarding Bolted

Connections in Trusses

1. Min. no. of bolts ≥22

2. Max. no. of bolts /

single gauge line ≤6 6

otherwise, for long

joint a reduction factor

Lf is multiplied by the

bolt capacity R least

where

B Lf =1‐(L i ‐15)/200

[ 0.75 B L 1.0]


Continue general requirements

• The minimum thickness of gusset

plate is 8mm.

• The minimum angle of inclination

between the member and the

gusset plate is 20 o .

• In the case of large number of

bolts that exceed the maximum

permissible number of bolts, then,

four choices may be adopted:

• increasing the bolt diameter

increasing the bolt material

strength increasing the member

size to adopt double gauge lines

and finally using a splices for

chord members and lug angles for

web members as shown in Fig.

(5.4)┘└.


• Gusset plates

should be

checked in a

critical sections

as shown to

avoid the

rupture of the

gusset plate

itself.


Design Procedure of truss bolted

connections

1. Define the design force.

2. Choose the gusset plate thickness.

3. Calculate the least resistance of the bolt R least

4. Calculate the required number of bolts n =

Force / R least .


Examples

Example 5.1

• Given: The shown

steel truss

connection in a Fig.

(5.5)

• Required: Design

this connection

using M16 bolts of

Grade 4.6 using a

gusset plate of

10mm thickness of

St. 37


• 1‐ Least resistance of a bolt R least :

• For grade 4.6, q bolt = 0.25F ub .

• All members are 2Ls b.t.b, i.e. bolts

subjected to double shear, therefore

R

R

R

2 2

sh

2x

xFsh

2x

x1.6

x0.25x4

4

4


t xxF


1.0

x

1.6

x

0.6

x

3.6


3.456

t

d.

b

least

min

3.456t

b

4.02t

• For member 1:

• F1 = +18.937t, 2 ┘└ 75x50x7, then n1

= 18.937 / 3.456 = 5.48 6, arranged

in a single gauge as a‐t/ < 4.5.

• For member 2:

F2 12 748t 2┘└ 90 90 9 th 2

• F2 = ‐12.748t, 2┘└ 90x90x9, then n2 =

12.748 / 3.456 = 3.69 4, arranged

in a staggered pattern as a‐t/ > 4.5

& < 6.


• For member 3:

• F3 = +23.371t,

2┘└11011010, then n3 =

23.371 / 3.456 = 6.76 8,

arranged in a double gauges

as a‐t/ > 6.

• 5‐ For member 4:

• F4 = +41.569t,

2┘└13013012, then n4

=

41.569 / 3.456 = 12.1 14,

arranged in a double gauges

as a‐t/ > 6.

• As the number of bolts in

each gauge line exceeds 6

bolts then, there is a need

to reduce the capacity of

the bolt by a reduction

factor , therefore R L =

0.9853.456 = 3.4t , and

the new number of bolts is

n = 41.569/3.4 = 12.21

14 and there is no further

reduction.


Analysis of eccentric shear

connections


Example for an eccentric connection

N

Mx

My


Another example for eccentric shear

connections:‐ Beam splices

T

T=Mf/yct

M

Q

Mf

Mf

M

Mw

Mw

Q

yct

C

C=Mf/yct

M=Mf+Mw

Mw=M*Iw/I, Mf=M*If/I


Example


Analysis of sections subjected to

Bending moments


Example 5.4

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