Joints in Steel Structures based on Eurocode 3

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Nov 15, 2013 (3 years and 1 month ago)

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Joints in Steel Structures based on Eurocode 3


Frans Bijlaard


Delft University of Technology, Faculty of Civil Engineering and Geosciences

PO Box 5048, 2600 GA Delft, The Netherlands

F.S.K.Bijlaard@citg.tudelft.nl








ABSTRACT

This paper deals
about the design aspects of joints based on Eurocode 3 and in
particular on Part 1
-
8 ”Design of Joints". An overview is given of design philosophies
named as "traditional" and "modern" design and the various design rules by which the
structural behaviour i
n terms of stiffness, strength and deformation capacity can be
predicted, with the aim of reducing the integral costs of steel structures.

Emphasis is given on the need of reliable software tools to make the use of the
Eurocode information easier for the d
esigner.



INTRODUCTION


Cost optimisation is one of the most important items in steel construction in order to
be competitive in the market of buildings. The joints determine almost 50% of the
total costs of a steel structure. The cost of joints can decr
ease substantially when
stiffeners between flanges can be avoided.


Consequences of detailing of joints for distribution of forces and moments in
joints and requirements with respect to stiffness, strength and rotation capacity
for joints

The distribut
ion of forces and moments in the structure due to the loading is a result
of the strength and stiffness distribution in the structure. So the structural properties of
the joints such as stiffness, strength and rotation capacity, together with those of the
structural components like beams and columns, produce these forces in the joints.
This means that the choices made by the designer in designing the joints including the
connecting parts are of direct influence on the level of forces and moments in these
jo
ints. In fact, construction is joining components such as columns and beams
together while designing is making choices for components taking the structural
properties such as strength and stiffness into account.


Traditional design

In traditional desig
n it is assumed that the joints are stiff and strong and that the forces
and moments in the structure are determined using the linear
-
elastic theory. Because it
was assumed that the joints were stiff, it needs to be checked weather the joints are
really st
iff. In many cases in practice this is neglected. The strength of the joints is
adjusted to the level needed. As a result most joints have low deformation or rotation
capacity. Last but not least, the fabrication costs are very high because of the
necessit
y of applying stiffeners between the flanges.


Modern design

In modern design the joints are considered as structural components such as columns
and beams with properties as stiffness, strength and deformation capacity. These
structural properties of the

joints are incorporated into the design on the same level as
those of columns and beams. The joint layout should only be influenced by
fabrication considerations and considerations for easy and safe construction on
-
site.
The structural safety verification

of all components including that of the joints is
dependent on the design method used to determine the distribution of forces and
moment in the structure.

a.

In case that the elastic theory is used, the beams need to be checked for strength
and for lateral t
orsional buckling, the columns need to be checked for strength and
for beam
-
column stability (incl. lateral torsional buckling) and the connecting
parts of the joints need to be checked to have sufficient strength to transfer
bending moments, shear forces
and tensile forces.

b.

In case that the plastic theory is used, the beams need to be checked for lateral
torsional buckling only, the columns need to be checked for beam
-
column
stability (incl. lateral torsional buckling) only and the joints need to be checke
d to
have sufficient deformation (in fact rotation) capacity.

c.

In case that the elastic
-
plastic
-
non linear theory is used, the beams and columns
need to be checked for lateral torsional buckling only and the joints need to be
checked to have sufficient defo
rmation (in fact rotation) capacity.


The Eurocode 3 "Common unified rules for steel structures" contains performance
-
based requirements to carry out these checks. In fact the Eurocode 3 Part 1
-
8 "Joints"
(see [1]) provides rules to determine the structur
al behaviour of joints in terms of
Strength (moment capacity), Stiffness (rotational stiffness) and Deformation capacity
(rotation capacity). The extend to which all types of joints can be checked using
Eurocode 3 depends on the creativity of the designer
to recognise components in the
connecting parts of these joints that are similar to the components given in the
chapters for joints in Eurocode 3 Part 1
-
8 "Joints". If necessary, because the designed
joints are out of the scope of Eurocode 3, experiments o
n these types of joints have to
be carried out and the results have to be evaluated statistically, in order to obtain
reliable design values for the stiffness, strength and rotation capacity of these joints,
to be equivalent to the safety level based on th
e Eurocode 3.


In next chapters a few important aspects about joints out of Eurocode 3 Part 1
-
8
"Joints" are mentioned and discussed.



SCOPE, TERMS AND DEFINITIONS AND DESIGN ASSUMPTIONS



Part 1
-
8 of EN 1993 gives design methods for the design of joints
subject to
predominantly static loading using steel grades S235, S275, S355 and S460. Part 1
-
12
provides additional information to what extend the rules of Part 1
-
8 is applicable for
steel grades up to S700. It appears that, due to lack of sufficient resea
rch results, in
this case only elastic design is allowed. In this context it is worthwhile mentioning
that in particular young bright researchers are looking into the subject of deformation
and rotation capacity of joints with end
-
plate connections in stee
l grade S690. As an
example see (
Girão Coelho
, 2004).


With respect to the wording "joint" and "connection", to avoid misunderstanding,
Eurocode 3 Part 1
-
8 gives the following definitions:




basic component
(of a joint): Part of a joint that makes a contr
ibution to one or
more of its structural properties.



connection
: Location at which two or more elements meet. For design purposes it
is the assembly of the basic components required to represent the behaviour
during the transfer of the relevant internal
forces and moments at the connection.



connected member
: Any member that is joined to a supporting member or
element.



joint
: Zone where two or more members are interconnected. For design purposes
it is the assembly of all the basic components required to
represent the behaviour
during the transfer of the relevant internal forces and moments between the
connected members. A beam
-
to
-
column joint consists of a web panel and either
one connection (single sided joint configuration) or two connections (double si
ded
joint configuration), see Figure 1.



joint configuration
: Type or layout of the joint or joints in a zone within which
the axes of two or more inter
-
connected members intersect, see Figure 2.




2
1
3



2
3
1
2


Joint

=

web panel in shear + connection

Left joint =

web panel in shear + left connection

Right joint=

web panel in shear + right connection


a) Single
-
sided joint configuration


b) Double
-
sided joint configuration


1 web panel in shear

2 co
nnection

3 components (e.g. bolts, endplate)


Figure
1
: Parts of a beam
-
to
-
column joint configuration



1
1
2
5
4
5
2
3
3


1

Single
-
sided beam
-
to
-
column
joint configuration;


2

Double
-
sided beam
-
to
-
column
joint config
uration;


3

Beam splice;


4

Column splice;


5

Column base.



a) Major
-
axis joint configurations









Double
-
sided beam
-
to
-
column



joint configuration




Double
-
sided beam
-
to
-
beam




joint configuration


b) Minor
-
axis joint configurations (t
o be used only for balanced moments
M
b1,Ed

=
M
b2,Ed

)


Figure
2
: Joint configurations


About resistance of joints it is stated:


(1)

The resistance of a joint should be determined on the basis of the resistances of
its basic

components.

(2)

Linear
-
elastic or elastic
-
plastic analysis may be used in the design of joints.

(3)

Where fasteners with different stiffness are used to carry a shear load the
fasteners with the highest stiffness should be designed to carry the design loa
d.
An exception to this rule is allowed only in case that preloaded class 8.8 and
10.9 bolts in connections, designed as slip
-
resistant at the ultimate limit state,
may be assumed to share the load with welds, provided that the final tightening
of the bolt
s is carried out after the welding is complete.


Design assumptions for joints are:

Joints should be designed on the basis of a realistic assumption of the distribution of
internal forces and moments. The following assumptions should be used to determine
t
he distribution of forces:

(a)

the internal forces and moments assumed in the analysis are in
equilibrium with the forces and moments applied to the joints,

(b)

each element in the joint is capable of resisting the internal forces and
moments,

(c)

the defo
rmations implied by this distribution do not exceed the
deformation capacity of the fasteners or welds and the connected parts,

(d)

the assumed distribution of internal forces should be realistic with regard
to relative stiffness within the joint,

(e)

the
deformations assumed in any design model based on elastic
-
plastic
analysis are based on rigid body rotations and/or in
-
plane deformations
which are physically possible, and

(f)

any model used is in compliance with the evaluation of test results (see
EN 199
0).


The application rules given in Eurocode 3 Part 1
-
8 "Joints" satisfy this design

assumptions



CONNECTIONS MADE WITH BOLTS



As a help to the designer Eurocode 3
-
1
-
8 provides a classification of connections in
categories, such that if the designer choo
ses a certain connection, he is confronted
with the consequences of his choice.


Bolted connections loaded in shear should be designed as one of the following:


a)
Category A: Bearing type

In this category bolts from class 4.6 up to and including class 1
0.9 should be
used. No preloading and special provisions for contact surfaces are required.
The design ultimate shear load should not exceed the design shear resistance,
nor the design bearing resistance.


b)
Category B: Slip
-
resistant at serviceability l
imit state


Slip should not occur at the serviceability limit state. The design serviceability
shear load should not exceed the design slip resistance. The design ultimate
shear load should not exceed the design shear resistance, nor the design bearing
re
sistance.


c)
Category C: Slip
-
resistant at ultimate limit state

In this category preloaded bolts should be used. Slip should not occur at the
ultimate limit state. The design ultimate shear load should not exceed the design
slip resistance, nor the desi
gn bearing resistance. In addition for a connection in
tension, the design plastic resistance of the net cross
-
section at bolt holes
N
net,Rd

should be checked at the ultimate limit state.

For Category B and C only bolt assemblies of classes 8.8 and 10.9 wi
th controlled
tightening may be used as preloaded bolts.



Bolted connection loaded in tension should be designed as one of the following:


a)
Category D: non
-
preloaded

In this category bolts from class 4.6 up to and including class 10.9 should be
used.

No preloading is required. This category should not be used where the
connections are frequently subjected to variations of tensile loading. However,
they may be used in connections designed to resist normal wind loads.


b)
Category E: preloaded

In t
his category preloaded 8.8 and 10.9 bolts with controlled tightening should
be used.


Eurocode 3 part 1
-
8 provides requirements for the positioning of holes for bolts like
spacing between bolt holes as well as for end and edge distances dependent on the
qu
estion whether the steel structure is under weather or corrosive conditions or not.
The code provides formulae for determining the design resistances of individual bolts
with respect to tension, shear and bearing. The situation of combined shear and
tensio
n is treated too.


For groups of bolts the following is stated.

The design resistance of a group of fasteners may be taken as the sum of the design
bearing resistances
F
b,Rd

of the individual fasteners provided that the design shear
resistance
F
v,Rd

of eac
h individual fastener is greater than or equal to the design
bearing resistance
F
b,Rd

. Otherwise the design resistance of a group of fasteners
should be taken as the number of fasteners multiplied by the smallest design
resistance of any of the individual

fasteners.

This statement is meant to persuade the designer to choose a balanced bolt pattern and
to avoid having a relative small end distance in combination with a relative large
pitch. A wrong design may lead to premature failure of the end bolts befor
e the inner
bolts reached their capacities. The capacity of the group of bolts will be over
estimated in such cases.


Without going into detail, the code pays attention to subjects as long joints, deduction
for fastener holes where aspects are treated like

block tearing, angles connected by
one leg. Attention is given to the presence of prying forces.


About the distribution of forces between fasteners at the ultimate limit state (ULS) the
following is said:


(1)

When a moment is applied to a joint, the di
stribution of internal forces may be
either linear (i.e. proportional to the distance from the centre of rotation) or plastic,
(i.e. any distribution that is in equilibrium is acceptable provided that the resistances
of the components are not exceeded and
the ductility of the components is sufficient).

(2)

The elastic linear distribution of internal forces should be used for the
following:



when bolts are used creating a category C slip
-
resistant connection,



in shear connections where the design shear resist
ance
F
v,Rd

of a fastener is less
than the design bearing resistance
F
b,Rd
,



where connections are subjected to impact, vibration or load reversal (except wind
loads).

(3)

When a joint is loaded by a concentric shear only, the load may be assumed to
be unifo
rmly distributed amongst the fasteners, provided that the size and the
class of fasteners is the same.



WELDED CONNECTIONS



Out of all information on welded connections in this context attention will be
focussed on the design resistance of fillet welds.

The design resistance of a fillet weld should be determined using either the
Directional method or the Simplified method.


Directional method

(1)

In this method, the forces transmitted by a unit length of weld are resolved into
components parallel and tra
nsverse to the longitudinal axis of the weld and
normal and transverse to the plane of its throat.

(2)

The design throat area
A
w

should be taken as
A
w

= ∑
a


eff

where ℓ
eff

is the
effective length of the weld.

(3)

The location of the design throat area sh
ould be assumed to be concentrated in
the root.

(4)

A uniform distribution of stress is assumed on the throat section of the weld,
leading to the normal stresses and shear stresses shown in Figure 3, as follows:



σ


is

the normal stress perpendicular to the

throat



σ


is

the normal stress parallel to the axis of the weld



τ


is

the shear stress (in the plane of the throat) perpendicular to the axis of
the weld



τ


is

the shear stress (in the plane of the throat) parallel to the axis of the
weld.



Figure
3
: Stresses on the throat section of a fillet weld

(5)

The normal stress
σ


parallel to the axis is not considered when verifying the
design resistance of the weld.


(6)

The design resistance of the fillet weld will be sufficient
if the following are
both satisfied:



[
σ

2

+ 3 (
τ

2

+
τ

2
)]
0,5

f
u

/ (
β
w

γ
M2

) and
σ


f
u

/
γ
M2


where:



f
u


is

the nominal ultimate tensile strength of the weaker part joined;


β
w

is

the appropriate correlation factor taken from Table 1.


(7
)

Welds between parts with different material strength grades should be designed
using the properties of the material with the lower strength grade.


Table
1
: Correlation factor
β
w

for fillet welds

Standard and steel grade

Co
rrelation factor
β
w

EN 10025

EN 10210

EN 10219

S 235


S 235 W

S 235 H

S 235 H

0,8

S 275

S 275 N/NL


S 275 M/ML

S 275 H


S 275 NH/NLH

S 275 H


S 275 NH/NLH


S 275 MH/MLH

0,85

S 355

S 355 N/NL


S 355 M/ML

S 355 W

S 355 H


S 355 NH/NLH

S 355 H


S 355 NH/
NLH

S 355 MH/MLH

0,9

S 420 N/NL

S 420 M/ML


S 420 MH/MLH

1,0

S 460 N/NL

S 460 M/ML


S 460 Q/QL/QL1

S 460 NH/NLH

S 460 NH/NLH

S 460 MH/MLH

1,0


Simplified method for design resistance of fillet weld

(1)

Alternatively to the directional method, the design

resistance of a fillet weld
may be assumed to be adequate if, at every point along its length, the resultant
of all the forces per unit length transmitted by the weld satisfy the following
criterion:



F
w,Ed


F
w,Rd



where:



F
w,Ed

is

the design value of the weld force per unit length;


F
w,
R
d

is

the design weld resistance per unit length.


(2)

Independent of the orientation of the weld throat plane to the applied force, the
design resistance per unit lengt
h F
w,Rd

should be determined from:



F
w,Rd

=
f
vw.d

a




where:


f
vw.d

is

the design shear strength of the weld.


(3)

The design shear strength
f
vw.d

of the weld should be determined from:



f
vw.d

=
2
3
/
M
w
u
f






where:


f
u


is

the nominal ultim
ate tensile strength of the weaker part joined;


β
w

is

the appropriate correlation factor taken from Table 1.




ANALYSIS, CLASSIFICATION AND MODELLING



Global analysis


The effects of the behaviour of the joints on the distribution of internal forces and
moments within a structure and on the overall de
formations of the structure, should
generally be taken into account, but where these effects are sufficiently small they
may be neglected.

To identify whether the effects of joint behaviour on the analysis need be taken into
account, a distinction may be m
ade between three simplified joint models as follows:



simple, in which the joint may be assumed not to transmit bending moments;



continuous, in which the behaviour of the joint may be assumed to have no effect
on the analysis;



semi
-
continuous, in which th
e behaviour of the joint needs to be taken into
account in the analysis.

The appropriate type of joint model should be determined from Table 2, depending on
the classification of the joint and on the chosen method of analysis.

The design moment
-
rotation ch
aracteristic of a joint used in the analysis may be
simplified by adopting any appropriate curve, including a linearised approximation
(e.g. bi
-
linear or tri
-
linear), provided that the approximate curve lies wholly below the
design moment
-
rotation characte
ristic.


Table
2
: Type of joint model

Method of
global analysis

Classification of joint

Elastic

Nominally pinned

Rigid

Semi
-
rigid

Rigid
-
Plastic

Nominally pinned

Full
-
strength

Partial
-
strength

Elastic
-
Plastic

Nominally pinn
ed

Rigid and full
-
strength

Semi
-
rigid and partial
-
strength

Semi
-
rigid and full
-
strength

Rigid and partial
-
strength

Type of

joint model

Simple

Continuous

Semi
-
continuous



Classification of joints

Classification of joints is a help to the designer, not an

obligation. The designer makes
choices about the type and geometrical layout of joint he wants to make. The
behaviour of that joint in terms of moment
-
capacity, rotational stiffness and rotation
capacity follows from the design and can be determined using

the rules of this
Eurocode part. These structural properties of all the joints, together with the structural
properties of the beams and columns form the basis for the calculation of the
structural response of the structure to the loading working on it. T
he joints than
always need to be modelled as a set of (rotational) non
-
linear springs. The question
whether or not the joints have influence on the response of the structure to loading
working on it, just follow from the calculation. If a joint is rigid an
d strong its
behaviour needs not to modelled other than by assuming a rigid and strong attachment
of the jointed members to each other. If a joint behaves like a hinge, this behaviour
needs not be modelled other than by assuming a hinged attachment between

the
jointed members.


It can be of help to the designer to make use of a classification system in determining
the behaviour (structural properties such as rotational stiffness, moment capacity and
rotation capacity) of the joints just by knowing the type
and layout of the joint alone.
The classification as given in Eurocede 3 Part 1
-
8 is based on the following criteria:

a)

A joint is classified as rotationally stiff if the Euler buckling load of the
structure is not less that 95% of the Euler buckling loa
d of that structure using full
rigid attachment of the members jointed together. In this analysis it is assumed that
the span
-
height
-
ratio of the beams is about 20. In terms of ultimate capacity of the
frame, this will lead to a reduction of not more than
2% because most steel framed
structures are in the slenderness range where the influence of instability and plasticity
is of almost equal importance.

b)

A joint is classified as full strength if the moment capacity is not less than the
moment capacity of t
he cross section of the attached beam.


Joints not fulfilling these criteria are called semi
-
rigid and partial strength and their
structural behaviour need to be taken into account in the calculation of the response of
the structure. Any other system of cl
assification can be used as long as the designer
takes the consequences into account in his design.


The starting points of the classification of joints in Eurocode 3 Part 1
-
8 are:

(1)

The details of all joints should fulfil the assumptions made in th
e relevant
design method, without adversely affecting any other part of the structure.

(2)

Joints may be classified by their stiffness and by their strength and lead to the
following criteria:


C
LASSIFICATION BY STI
FFNESS

(1)

A joint may be classified as
rigid, nominally pinned or semi
-
rigid according to
its rotational stiffness, by comparing its initial rotational stiffness
S
j,ini

with the
classification boundaries.

(2)

A joint may be classified on the basis of experimental evidence, experience of
p
revious satisfactory performance in similar cases or by calculations based on
test evidence.

Nominally pinned joints

(1)

A nominally pinned joint should be capable of transmitting the internal forces,
without developing significant moments which might adve
rsely affect the
members or the structure as a whole.

(2)

A nominally pinned joint should be capable of accepting the resulting rotations
under the design loads.

Rigid joints

(1)

Joints classified as rigid may be assumed to have sufficient rotational stiff
ness
to justify analysis based on full continuity.

Semi
-
rigid joints

(1)

A joint which does not meet the criteria for a rigid joint or a nominally pinned
joint should be classified as a semi
-
rigid joint.

NO
T
E:

Semi
-
rigid joints provide a predictable degr
ee of interaction between
members, based on the design moment
-
rotation characteristics of the joints.

(2)

Semi
-
rigid joints should be capable of transmitting the internal forces and
moments.


Classification boundaries for joints other than column bases are

given in Figure 4.






Zone 1:

rigid, if
S
j,ini


k
b
EI
b

/
L
b


where


k
b

= 8

for frames where the bracing system
reduces the horizontal displacement
by at least 80 %


k
b

= 25

for other frames, provided that in
every storey
K
b
/
K
c

≥ 0,1
*)

Zone 2:

semi
-
rigid





All joints in zone 2 should be classified as
semi
-
rigid. Joints in zones 1 or 3 may
optionally also be treated as semi
-
rigid.


Zone 3:

nominally pinned, if
S
j,ini

≤ 0,5

EI
b

/
L
b



*)

For frames where
K
b
/
K
c

< 0,1

the joints
should be classified as semi
-
rigid.

Key:

K
b

is

the mean value of
I
b
/
L
b

for all the beams at the top of that storey;

K
c

is

the mean value of
I
c
/
L
c

for all the columns in that storey;

I
b

is

the second moment of area of a beam;

I
c

is

the
second moment of area of a column;

L
b

is

the span of a beam (centre
-
to
-
centre of columns);

L
c

is

the storey height of a column.


Figure
4
: Classification of joints by stiffness

C
LASSIFICATION BY STR
ENGTH

(1)

A joint may be
classified as full
-
strength, nominally pinned or partial strength
by comparing its design moment resistance
M
j,Rd

with the design moment
resistances of the members that it connects. When classifying joints, the design
resistance of a member should be taken

as that member adjacent to the joint.


Nominally pinned joints

(1)

A nominally pinned joint should be capable of transmitting the internal forces,
without developing significant moments, which might adversely affect the
members or the structure as a whole
.

(2)

A nominally pinned joint should be capable of accepting the resulting rotations
under the design loads.

(3)

A joint may be classified as nominally pinned if its design moment resistance
M
j,Rd

is not greater than 0,25 times the design moment resista
nce required for a
full
-
strength joint, provided that it also has sufficient rotation capacity.

Full
-
strength joints

(1)

The design resistance of a full strength joint should be not less than that of the
connected members.

(2)

A joint may be classified as
full
-
strength if it meets the criteria given in Figure
5.

Partial
-
strength joints

(1)

A joint, which does not meet the criteria for a full
-
strength joint or a nominally
pinned joint, should be classified as a partial
-
strength joint.


a) Top of column




M
j,Sd

Either


M
j,Rd


M
b,pℓ,Rd


or


M
j,Rd


M
c,pℓ,Rd

b) Within column height



M
j,Sd

Either


M
j,Rd


M
b,pℓ,Rd


or


M
j,Rd

≥ 2

M
c,pℓ,Rd

Key:

M
b,pℓ,Rd

is

the design plastic moment resistance of a beam;

M
c,pℓ,Rd

is

the design plastic moment resistance of a column.


Figur
e
5
: Full
-
strength joints

Modelling of beam
-
to
-
column joints

For the modelling of beam
-
to
-
column joints many detailed rules are given in
Eurocode 3 part 1
-
8 to represent the actual behaviour. Figure 6, 7 and 8 gives an
impress
ion of the complexity.


Detailed rules for determining the structural properties

Eurocode 3 Part 1
-
8 provides a complete set of detailed rules to determine the
structural properties of beam
-
to
-
column joints and base
-
plate joints for I and H
sections. Thes
e rules are based on the so
-
called component method, where the
structural behaviour of the joint is composed out of the structural behaviour of all
relevant components out of which the joint is composed. One of the main components
is the equivalent T
-
stub,

see Figure 9. In Figure 10 and 11 it is shown how the
equivalent T
-
stub is positioned in column
-
side and in the beam
-
side of an end
-
plate
joint.









a) Values at periphery of web panel

b) Values at intersection of member centrelines

Direction o
f forces and moments are considered as positive in relation to equations (5.3) and (5.4)


Figure
6
: Forces and moments acting on the joint



M
b2,Ed
N
b2,Ed
V
b2,Ed
V
b1,Ed
M
b1,Ed
N
b1,Ed


a) Shear forces in web panel

b) Connections, with forces and moments in beams


Figure
7
: Forces and moments acting on the web panel at the connections


x
x
x
3
2
1


Single
-
sided joint configuration

Double
-
sided joint configuration


1 Joint

2 Joint 2: left side 3 Joint 1: right side


Figure
8
: Modelling the joint




eff




Figure
9
: Dimensions of an equivalent T
-
stub flange





1 End bolt row adjacent to a stiffe
ner

2 End bolt row

3 Inner bolt row

4 Bolt row adjacent to a stiffener


Figure 10: Modelling a stiffened column flange as separate T
-
stubs









b
p
w
e
x
m
x

eff

eff

eff



p
e
e

The extension of the end
-
plate and the portion
between the beam flanges are modelled as two
sep
arate equivalent T
-
stub flanges.


For the end
-
plate extension, use
e
x

and
m
x

in
place of
e

and
m

when determining the design
resistance of the equivalent T
-
stub flange.


Figure 11: Modelling an extended end
-
plate as separate T
-
stubs


In

the rules

for the determination

of the strength of an equivalent T
-
stub the effects of
prying forces are directly taken into account.


Eurocode 3 part 1
-
8 "Joints" also provides much information to determine the
Strength of Hollow Section Joints as given in Figure
12.


For the designer the advantage of Eurocode 3 is that this code provides extensive
information about how to calculate the structural behaviour of components like
columns, beams and joints. However, many times it is said that the Eurocode is too
compl
ex for use in day
-
to
-
day practice. In the opinion of the author this is not the case
but it is admitted that working with the Eurocode is a lot of work. And it is true that
the designer has not much time to do his job in a commercial and competitive
surrou
nding. Therefore it is necessary that user
-
friendly software is available to the
designer to take the time consuming rules of the code to determine the joint behaviour
out of his hands. In that situation the designer can spend his time to his profession
be
ing a designer looking for alternative structural solutions to reach a final design that
reaches minimum integral costs (design + material + fabrication + erection + end
-
of
-
life + re
-
use) and leave the number crunching to the computer using adequate
softwa
re. In that respect a warning should be made in using so
-
called expert
-
systems
from the market. The designer should be very alert on the correctness of the software
itself and on the correct use of that software. The term "expert
-
system" only means
that th
is software should be handled by experts only. In that case we can stop saying
"Simple rules sell steel" and replace that by saying "
Simple TOOLS sell Steel
".







K

joint

KT joint

N joint




T joint

X joint

Y joint



DK joint

KK joint



X joint

TT joint



DY joint

XX joint


Figure 12: Types of joints in hollow section lattice girders






CONCLUSIONS


In order to keep a competitive position in the market, the costs of steel structures, in
particular steel frames, need to be reduced

as much as possible. As the costs of steel
frame structures are determined for about 50% by its joints, the need to design
modern joints, preferably without stiffeners, is of increasing economic importance. In
this way the costs of steel structures can be

reduced significantly. Although design
codes like Eurocode 3 "Common unified rules for steel structures" are still based on
more traditional joints with bolts and welds, in many cases the design rules can also
be used for the design and verification of so
-
called plug and play joints, see
Brekelmans and Bijlaard (2000), in which traditional components can be recognised.
This is because these design rules for joints are related to the components in which
almost all joints can be sub
-
divided and because the r
equirements for stiffness,
strength and rotation capacity of joints are given in so
-
called performance based
requirements and are irrespective of the type of the joint. However, where non
-
traditional components like clamps and hooks are used, experiments h
ave to be
carried out and the results have to be evaluated statistically, in order to obtain reliable
values for the stiffness, strength and rotation capacity of these plug and play joints.



REFERENCES


EN 1993
-
1
-
8 : 2004 "Eurocode 3 : Design of Steel Str
uctures, Part 1
-
8 : Design of
Joints”, CEN Central Secretariat, Rue de Stassart 36, B
-
1050 Brussels, BELGIUM

Girão Coelho, A.M., Simões da Silva, L. and Bijlaard, F.S.K. (2004), "
Ductility
analysis of bolted extended end plate beam
-
to
-
column connections"
,

The Second
International Conference on Steel and Composite Structures, (ICSCS’04)


2
-
4 September 2004, Seoul, KOREA

Brekelmans, J.W.P.M. and Bijlaard, F.S.K. (2000), "Design requirements for plug and
play type joints in mixed and steel
-
concrete composite c
onstruction",

Connection
-
Workshop ECCS/AISC, 23
-
25 October 2000, Roanoke, Virginia, USA