1
Suitable Stiffening System
s
for LiteSteel Beam
s
with Web Opening
s
Subjected to Shear
Poologanathan Keerthan and Mahen Mahendran
Science and Engineering
Faculty
Queensland University of Technology, Brisbane, QLD 4000, Australia
Abstract:
LiteSteel beam
(LSB) is a new cold

formed steel hollow flange channel section
produced using
a
simultaneous cold

forming and dual electric resistance welding
process
.
It is
commonly used as floor joists and bearers with web openings in residential, industrial and
commerc
ial buildings.
Their shear strengths are considerably reduced when web openings are
included for the purpose of locating building services.
A cost effective method of eliminating
the detrimental effects of a large web opening is to attach
suitable
stiffeners around the web
opening
s of LSBs
.
E
xperimental and numerical studies were undertaken to investigate the
shear beha
viour and strength of LSBs with
circular
web openings
reinforced using
plate,
stud
,
transverse and sleeve
stiffeners with varying s
izes and thicknesses
. Both welding and
varying
screw

fastening arrangements
were used to attach these stiffeners to the web
of LSBs
.
F
inite
element models of LSBs with
stiffened
web openings in shear were developed to simulate
the
ir
shear behaviour and str
ength of LSBs. They were t
hen validated by comparing the
results with experimental test results and used in a detailed parametric study.
These studies
have
shown
that
plate stiffeners were the most suitable, however, their
use
based on
the
current American
standards
was found to be
in
adequate.
Suitable s
crew

fastened plate
stiffener arrangements
with optimum thicknesses
have been proposed for LSB
s
with web
opening
s
to
restore the
ir
original
shear capacity.
This paper presents the details of the
numerical study and the results.
Keywords:
LiteSteel beam, Web openings,
Finite element analysis
, Shear strength,
Plate
stiffener
,
LSB stud stiffener, Sleeve stiffener, Transverse stiffener,
Hollow flanges
,
Cold

formed steel structures.
Corresponding author’s email address:
m.mahendran@qut.edu.au
2
1. Introduction
The use of cold

formed steel members in
low rise
building construction has increased
significantly in recent times. There are many
significant
benefits associated with the use of
lightweight cold

formed steel sections in residential, industrial and commercial buildings.
Thinner cold

formed steel sections
with varying geometry are contin
uously developed to suit
various
requir
ements including higher moment
capacities.
The
LiteSteel Beam (LSB)
shown n
Figure 1(a)
is a new cold

formed steel
hollow flange channel
beam produced by OneSteel
Australian Tube Mills
[1]
.
It is manufactured from a single strip of high strength steel
using
a
combined cold

forming and dual electric resistance welding process. The effective
distribution of steel in LSBs with two rectangular hollow flanges results in a thin and
lightw
eigh
t section with good moment capacity
. The LSB has many applications
and
has
become a very popular choice in the flooring systems as shown in Figure 1(b)
[1]
.
Table 1
shows
the
details of
LS
B sections including their dimensions
.
Current practice in flooring
systems is to include openings in the web of floor joists or
bearers so that building services can be located within them. Without web openings, services
have to be located under the joists leading to increased floor height
s
.
Pokharel
and
Mahendran
[2]
re
commended the use of circular web openings in LSBs based on an
investigation using finite element analyses.
Three standard opening sizes of 60, 102 and 127
mm are used with
the
currently available LSBs
[3]
. The use of web openings in a beam
section signifi
cantly reduces its shear capacity due to the reduced web area.
Since about
88%
of the shear force is supported by the main web element of LSB
[4],
th
e use of web openings
can
lead to significantly reduced shear c
apacities
of
LSBs.
Keerthan and Mahendran
[5,
6]
investigated the shear behavior
and strength
of LSB
s
with
circular
web openings using
experimental and numerical studies.
They developed suitable
design equations for the shear
capacity of LSBs with web openings
by including both the enhanced bucklin
g coefficient and
the
post

buckling strength in shear
.
Since the loss of shear capacity of LSBs was found to be as high as 60
% [5]
when the
standard 127 mm web openings were used in 200x45x1.6 LSBs, the
LSB manufacturers and
researchers realized the need
to improve the shear capacity of LSB with web openings.
There
are several methods used to improve the shear capacity of beams with web openings. The
most practical method is to increase the web thickness. However, this may not be possible
3
with cold

formed
steel sections as the thickness is governed by the manufacturing process.
A
cost effective way to improve the detrimental effects of a large web opening is to attach
appropriate stif
feners around the web openings
.
Currently available cold

formed stee
l desi
gn
standard
s
[7,8]
and
steel framing standard
s
[9]
do not provide adequate guidelines to facilitate
the design and construction of stiffeners for
LSBs
with large web openings. Hence
experimental
and numerical
studies were conducted to develop the
most
effective and
economical stiffener arrangement for LSB
s
with circular web openings subjected to shear.
Details of the
experimental study and the results are presented in
[10].
In the numerical study,
suitable finite element models of LSBs with stiffened we
b openings were developed to
simulate their shear behaviour and capacity, and were validated by comparing thei
r results
with experimental
results reported in
[10]
.
A detailed parametric study was then undertaken
using the validated finite element model
to
develop the optimum stiffening
system for the
shear capacity of LSBs with web openings.
This paper presents the details of the
development of finite element models of LSBs with stiffened
circular
web openings
subject
to shear,
and the results. It includes
a compar
ison of finite element analysis
and experim
ental
results
as well as the d
etails of
the
new optimum
plate
stiffener
arrangement
for LSBs.
2. Experimental Study
of LSBs with
Stiffened
Web Openings
This section presents the important details of
17 shear tests
of
simply supported
back to back
200x45x1.6
LSBs
under a three

point loading arrangement
as
shown in Figure 2.
The main
focus
was
on
the use of plate stiffeners with varying fastening arrangements
while
two tests
included the use of LSB stud
stiffeners.
Table 2 shows the details of test specimens while
Figure
3 shows
the
m
with various stiffener arrangements.
The first four s
pecimens
were not stiffened
as seen in Table 2
.
In Specimen 5, the web
openings were stiffened with plate stiffeners based on
AISI’s [9] minimum stiffening
requirements
.
The
plate
stiffener
thickness
was equal to th
at
of 200x45x
1.6 LSB while the
plate stiffener
extended 25 mm beyond
the
web opening
edge
s. The plate stiffener was
fastened to the
LSB
web with No.12 T
ek screw
s at 25 mm spacing
with
an
edge distance of
12.5
mm
as shown in
Figure
3
(a)
.
This stiffener arrangement was defined as “Arrangement
1” (20
screws).
Test Specimen 6 was assembled simil
ar to Test Specimen 5
, but with a total
of 8
screws at 63.5 mm
spacing
along the plate stiffener
edges
with a
n edge distance of 12.5
mm (
Figure
3
(b)). This s
crew
fastening
arrangement was defined as “Arrangement 2”. Since
4
the plate stiffeners with a thick
ness equal to the
LSB web thickness (1.6 mm) did not restore
th
e original shear capacity
, two and three
1.6 mm plate stiffeners
were used in
Specimen
s
7
and 8, respectively. The plate stiffeners’ heights were also increased to match the clear LSB
web heigh
t of 168 mm, which led to plate stiffener sizes of 152x168x3.2 mm and
152x168x4.8
mm. These two specimens we
re fastened using
Arrangement
2
with screws
locate
d
in the middle as implied by AISI [9] recommendations. Hence
the edge distance
along the horizont
al edges was 16.5 mm instead of 12.5 mm while its spacing along the
vertical edges was 67.5 mm instead of 63.5 mm due to the increased height of plate stiffeners.
In Specimen
9
,
200x45x1.6 LSB stud stiffeners
were
used
with 102 mm web openings
while
200x45x1.6 LSB stud stiffener and 177x168x1.6 mm plate stiffener were
used in
Specimen
10
with 1
27
mm web openings.
In these tests, the stiffener heights were again increased to
that of clear web. Arrangement 2 of eight screws wa
s used in
Specimen 9
, but the edge
distances and screw spacings w
ere 16.5 mm and 67.5 mm
.
Improved Arrangement 3 with
four additional screws in the diagonal direction (12 screws in total) was used in
Specimen 10
.
The additional
screws
in
the diagonal direction were located at
10 mm from the
web opening
edge
. T
o increase the shear capacity further, 3 mm thick and
202 mm wide plate stiffeners
were use
d for the full web height of
Specimens 11 and 12 (Figures 3
(c) and (d)). As in
Specimen 10, four
additional screws were
used to
a
ttach
these 202x168x3.0 mm plate
stiffeners
along the diagonal direction.
The
screws were
located in the middle on each side of
the plate stiffener, which led to the
edge distance
s
of 25 mm
and 16.5 mm and spacings
of
67.5 mm and 76 mm in Test 11
(
Figure
3
(c
))
. However, in
Specimen 12, the
edg
e distances
were 12.5 mm
and 16.5 mm
(
Figure
3
(d
)).
This stiffener arrangement of using 12 screws
with a reduced edge distance of 12.5 mm was defined as “Arrangement 4”.
In Specimen 13,
202x168x3.0
mm
plate stiffeners were welded to LSB
s to determine whether welding instead
of screw

fastening would produce higher shear capacities
. Specimen 14 was
used to
investigate the use of thicker (5 mm) and wider (227 mm) plate stiffeners
for
larger 127 mm
web ope
nings. Two 2.5 mm plates of 227x168 mm dimensions were screw fastened using 12
screws in Arrangement 3 as in
Specimen 11. Specimen
15 wa
s similar to
Specimen 14, but
the plate stiffeners were attached using
screws
located on a
circular format as shown in
F
igure
3
(
e
)
(
Arrangement 5
)
.
In
Specimen 16 the plate stiffener
width
was reduced to 177
mm based on AISC’s [9] recommendations while three 1.6 mm plate stiffeners were used
.
Specimen 17
with
the smallest web opening of 60 mm
was stiffened
with only one
1
.6 mm
plate stiffener
.
5
3
. Finite Element Analyses of LSBs with
Stiffened
Web Openings
3
.1. Description
of
Finite Element Model
This section describes the development of
suitable
finite element models to investig
ate the
ultimate shear
behaviour
and strength
of LSBs with
stiffened
web openings. For this purpose,
a general purpose finite element
program, ABAQUS Version 6.7
[11]
, which has the
capability of undertaking geometric and material non

linear analyses of three dimensional
structures,
was u
sed. Finite element models were developed first with the objective of
accurately simulating the actual test members’ physical geometry,
loads, constraints and
mechanical properties
reported in
the experimental study
[10]
.
However, they
were
also
developed
for LSBs with
other
stiffener
type
s such as transverse and sleeve stiffeners.
S
hear
test results of
back to back
LSBs were similar to those obtained from
single LSBs with a shear
centre loading
[4]
.
Hence in this study, finite element models of single LSBs
with
a
shear
centre loading and simply supported boundary conditions were used to simulate the shear tests
of
back to back
LSB with stiffened web openings
.
The cross

section geometry of the finite element model was base
d on the measured
dimensions,
thicknesses
and yield stresses
of
17
tested LSBs reported in
[10].
Table 2 gives
the measured dimensions
of the
test beams made of
200x45x1.6
LSB
s
, where
t
w
and d
1
are
the base metal thickness
and
the clear web height.
The measured yield stresses of web,
and
inside and outside flange elements were 452.1, 491.3 and 536.9 MP
a
, respectively.
For LSBs
d
1
is defined as the clear height of web instead of the depth of the flat portion of web
measured along the plane of the web as defined in AS/NZS 4600
[7]
for co
ld

formed channel
sections. The reasons for this are given in
[4]
.
Table 2 also provides the diameters of web
openings (d
wh
) used in the models. Since the effect of including the rounded corners in LSBs
on the shear buckling behaviour and capacity was foun
d to be negligible
[12]
, right angle
corners were used in the finite element models used in this study.
ABAQUS has several element types to simulate the shear behaviour of beams with
stiffened
web openings.
The shell element in ABAQUS called S4R5
was sele
cted as it has the
capability to simulate the shear behaviour of thin steel beams such as LSBs. This element is
6
thin, shear flexible, isometric quadrilateral shell with four nodes and five degrees of freedom
per node, utilizing reduced integration and
bili
near interpolation scheme.
R3D4 rigid body elements were used to simulate the restraints and loading in the finite
element models
of LSB
s
with stiffened web openings
. The R3D4 element is a rigid
quadrilateral with four nodes and three translational
degrees of freedom per node. Finite
element modelling was
undertaken
using MD PATRAN R2.1 pre

processing facilities using
which the model was created and then submitted to ABAQUS for the analysis. The results
were also viewed using MD PATRAN R2.1 post

proc
essing facilities.
In order to fully und
erstand the shear behaviour of L
SB
s with stiffened web openings
,
several
important issues were considered when deciding the
se parameters such as the ratio
of the
depth of web openings to clear height of web (d
wh
/d
1
)
,
type
s and thicknesses
of stiffeners
,
number of self

drilling Tek screws
used to fasten the stiffeners to the web
and their spacing
.
3
.2. Finite Element Mesh
In
finite element analyses (
FEA
)
, selection of mesh size and layout is critical. It is
desirable to
use as many ele
ments as possible in the analysi
s. However, such an analysis will requi
re
excessive computing time and resources
. In this
research
, adequate numbers of elements were
chosen for both flanges and web based o
n detailed convergence studies
to obtain sufficient
accuracy of results without excessive use of computing time. Convergence studies
showed
that an elemen
t size of 5mm x 5
mm provided an accurate representation of shear buckling and
yielding deformations. A
n element length of 5 mm in the longitudinal direction was found to
provide suitable accuracy for all the
LSB
sections. In order to get accurate results, Paver Mesh
was applied around the
LSB
web
and stiffener
openings. The geometry and finite element
mesh
of a typical LSB with
stiffened
web
openings is shown in Figure
4
.
Plate and LSB
stud
stiffeners were connected to the web elements of LSB using screw
fasteners as shown in Figure 5 (c). These stiffeners were modelled using 5mm x 5mm S4R5
elements and we
re connected to the web of LSB using Tie MPC. Tie MPC makes the global
displacements and rotations as well as all other active degrees of freedom equal at the two
connected nodes. If there are different degrees of freedom active at the
se
nodes, only those
in
common will be constrained. This Tie MPC was used to simulate
all the
screw connections
used in installing the stiffeners
.
7
3
.3. Material Model and Properties
of LSB
The ABAQUS classical metal plasticity model was used in all the analyses. This model
implements the von Mises yield surface to define isotropic yielding, associated plastic flow
theory, and either perfect plasticity or isotropic hardening behaviour.
When t
he measured
strain hardening in the web element was used in FEA, the shear capacity improvement was
less than 1%
[4]
. Hence it was not considered in our analyses.
Tensile coupon tests were
conducted for the batch of LSB
s
from which the test beam specimens
were taken. Tensile
coupons taken from the web and inside and outside flanges of
200x45x1.6
LSB sections were
tested to determine the averag
e
yield stresses reported earlier
. The
se
measured yield stresses
were used in
FEA
.
Since
the
plate stiffeners were t
aken from the LSB web, the measured web
yield stresses were used in the modeling of plate stiffeners. The LSB stud stiffeners used in
the tests were taken from the same batch of LSBs. Hence the measured
yield stresses of
web,
inside and outside flange
elem
ents
of LSBs were used in the modeling of LSB stud stiffeners.
The elastic modulus and Poisson’s ratio
were taken as 200
,000 MPa and 0.3, respectively.
3
.4. Loads and Boundary Conditions
Simply supported boundary conditions were implemented in the finite
element models of
LSBs with
stiffened
web
openings
. They were used at the supports to provide the following
requirements:
Simply supported in

plane

Both ends fixed against in

plane vertical deflection but
unrestrained against in

plane rotation, and one
end fixed against longitudinal
horizontal displacement.
Simply supported out

of

plane

Both ends fixed against out

of

plane horizontal
deflection, and twist rotation, but unrestrained against minor axis rotation.
In order to provide simply supported conditions for the shear panel,
the following boundary
conditions were employed.
Left and right supports:
u
x
= 0
x
= 1 Mid

span loading point:
u
x
= 1
x
= 1
u
y
= 1
y
= 0
u
y
= 0
y
= 0
u
z
= 1
z
= 0
u
z
= 1
z
= 0
8
Note:
u
x
,
u
y
and
u
z
are translations and θ
x
, θ
y
and θ
z
are rotations in the x, y and z directions,
respectively. 0 deno
tes free and 1 denotes
restrained
.
The vertical translation was not restrained at the loading point. Figure
5
shows the applied
loads and boundary conditions of the model. Single point constraints and concentrated nodal
forces were used in the finite element models to simulate
the experimental boundary
conditions and applied loads as closely
as possible
. In order to prevent twisting, the applied
point load and simply supported boundary conditions were applied at the shear centre using
rigid body reference node. Shear test speci
m
ens included a
10 mm thick and
75 mm wide
plate at each support to prevent lateral movement and twisting of the section. These stiffening
plates were modelled as rigid bodies using R3D4 elements. In ABAQUS
[11]
a rigid body is a
collection of nodes and ele
ments whose motion is governed by the motion of a single node,
known as the
rigid body reference node
. The motion of the rigid body can be prescribed by
applying boundary conditions at the rigid body reference node.
Hence
simply supported
boundary conditio
ns were applied to the node at the shear centre in order to provide an ideal
pinned support.
3
.5
.
Fastener Modelling
Fasteners play an important role in
the shear behaviour of LSBs with stiffened web openings
.
T
his study assumed that screw
fastener
failure is unlikely to occur as confirmed by o
ur
experimental study [10].
Considering this
observation, the screw fasteners connecting the
stiffeners to the LSB were not explicitly modelled. Instead they were simulated using Tie
MPCs, which make all activ
e degrees of freedom equal on both sides of the connection.
The web side plates at the supports were connected using h
igh strength steel bolts (M16
8.8/S) to avoid bolt failure
s
during testing. Our
shear
tests
[10]
confirmed
that there were no
bolt
or p
late
failures. Therefore these web side plates were modelled as rigid bodies using
R3D4 elements.
3
.6
. Initial Geometric Imperfections
T
he local plate imperfections
in LSBs were found to be
less than the currently accepted
fabrication tolerance of d
1
/150
[4]
. However, the fabrication tolerance limit of d
1
/150 was
9
used in the numerical modelling of LSBs as the preliminary analyses showed that the effect of
local plate imperfection (from d
1
/300 to d
1
/150) on the shear capacity of LSB
with stiffened
web open
ings
was small. The critical imperfection shape was introduced by ABAQUS
*IMPERFECTION option with the shear buckling eigenvector obtained from an elastic
buckling analysis.
3
.7
. Residual Stresses
The residual stresses in the LSB sections produced using
the dual electric resistance welding
and cold

forming processes have unique characteristics. The residual stress models of
conventional steel sections are therefore not suitable for LSB sections. Details of the residual
stress tests and an idealized residu
al stress model developed for computer a
nal
yses are
presented in
[13]
.
Preliminary
FEA
showed that the effect of residual stresses on the shear
capacity of LSBs without openings is less than 1%
[14]
. Therefore the effect of residual
stresses on the shear c
apacity of LSBs with
stiffened
web openings is
also
lik
ely to be very
small. I
t was
thus
decided to neglect the residual s
tresses in the FEA
.
3
.8
. Analysis Methods
Both
elastic buckling and nonlinear static
analyses were used
. Elastic buckling analyses
were
used to obtain the eigenvectors for the inclusion of
initial
geometric imperfe
ctions. Nonlinear
static analyse
s, including the effects of large deform
ation and material yielding, were
used
to
investigate the shear
behaviour of LSB
s
until
failure. The
RIKS method in ABAQUS was
also
i
ncluded in the nonlinear analyse
s.
It is generally used to predict geometrically unstable
nonlinear collapse of structures. In using the RIKS method in this study, the solution of
nonlinear equations was achieved by the Newt
on

Raphson method, in conjunction with a
variable arc

length constraint to trace the instability problems associated with nonlinear
buckling of beams.
Following parameters were used in t
he non

linear analyses
of
LSB with
stiffened web openings
:
Maximum num
ber of load increments = 100,
Initial increment size =
0.01,
Minimum increment size = 0.000001,
Automatic increment reduction enabled, and large
displacements enabled.
10
4
.
Validation of Finite Element Models
of LSB with Stiffened Web Openings
T
he
accuracy of the
developed finite element model
s
of LSBs with stiffened web openings
was investigated by
comparing the non

linear a
nalysis results with those
obtained from
the
shear tests of LSBs with
stiffened web openings
[10].
Seventeen finite element
mo
dels were
constructed using the material and geometric properties from experimental testing (
Table 2)
and the
results were compared
with test results,
with pa
rticular attention given to the ultimate
load,
load

deflection curves
and
f
ailure m
odes.
These
com
parison
s were
intended to establish
the validity of the shell element model in the modelling of initial geometric imperfections and
shear deformations, and
associated material yielding.
The accuracy of local plate imperfection
magnitude and finite element
mesh density was also established.
Table 2
presents the ultimate shear capacity results
from FEA
and a comparison of these
results with the
corresponding experimental
results. The mean and COV of the ratio of test to
FEA ultimate shear capacities
are 0.97
and
0.021
. This indicates that the finite element model
developed in this study is able to predict the ultimate shear capacity of LSBs with
stiffened
web openings with very good accuracy.
In the experimental study, five screw fastening arrangements were considered as shown in
Figure 3 [10]. Figure 6 shows the FEA results in the form of load versus deflection for
200x45x1.6 LSB with 127 mm stiffened web openings (Test Specimen 14) while Figu
res 7
(a) and (b) show them for 200x45x1.6 LSB with 102 mm stiffened web openings (Test
Specimens 11 and 12) and compare them with corresponding experimental results. Figures 8
and 9 show the shear failure modes of Test Specimens 11 and 12, respectively, w
hile Figures
10 (a) to (c) show the failure modes of Test Specimens 13, 15 and 17, respectively.
The shear
capacity of
Test specimen 13 was
considerably
increased due to the additional welded plate
stiffener
. Hence
it
failed due to combined shear and bendi
ng as shown in Figure 10 (a). These
figures demonstrate a good agreement between the results from FEA and experiments and
confirm the adequacy of the developed finite element model in predicting the ultimate load,
deflections and failure modes of LSBs with
stiffened web openings.
5
.
Finite Ele
ment Analyse
s of LSBs with
Different
Type
s
of
Stiffeners
The overall objective of this research is to develop the
most
e
ffective and economical
stiffening
arrangement
s
for LSB
s
with web openings subjected to shear.
In this
section
,
the
11
use of
different
types of stiffeners, namely,
plate stiffener
s
, LSB stud stiffener
s
, sleeve
stiffener
s
and transverse stiffener
s
,
was
investigated
by using
finite element analyses
.
5
.1
.
Transverse Stiffener
s
Transverse stiffeners a
re
generally used
in hot

rolled steel sections and are
welded
to
the web.
Welding in
cold

formed steel
sections is difficult and hence this stiffener is not a practical
option
. However, it
was
investigated due to its popularity in traditional steel design.
Transverse
stiffeners improved
the shear strength by reducing the aspect ratio of the web
panel.
The geometry and finite element mesh of a typical LSB with
transverse
stiffener
s
is
shown in
F
igure
4
(b)
,
which
shows
that
two
steel
plates are attached to the web on either side
of the open
ing
at
20
mm
from the edge of the
web
opening.
T
ransverse stiffeners can be welded to either the web only or both web and flange elements
.
Finite element analyses conducted
to determine the
effect of this variation showed that
additional restraint provided by welding the
transverse
stiffener
s
to the flange
s
gave only a
4% increase in
the shear capacity.
Hence
welding the
transverse
stiffener
s
to the web
of LSBs
is
considered
sufficient.
In order to investigate the effect
of
the
thickness of transverse stiffeners
on
the
shear
capacity
of LSB
with web opening
s
,
FEA
of 200x45x1.6 LSBs with 60 and 127 mm web openings
were undertaken
with varying
transverse
stiffener thicknesses
.
Figure 11
shows the failure
mode
s
of LSB
s
with
3 mm
transverse stiffeners while
Figure 12
shows the
FEA
results in the
form of shear capacity
of LSBs
versus thickness of transverse stiffeners
.
Here the transv
erse
stiffeners
were only welded
to the web
element of LSB
.
Table 3
shows the shear capacities
of LSB
s
with
5 mm
transverse stiffeners. In this table, the shear capacities of LSBs without
and with web openings and stiffened web openings are referred to as
V
v
,
V
nl
and
V
nls
,
respectively.
These
results show that
the thickness of the
transverse stiffeners
does
not play a
significant role on the shear
capacity
of LSB with web opening
s
(see Figure 12
)
. This is due
to the fact that the
transverse stiffeners
mainly
reduces the aspect ratio and does not
contribute to
increasing the
shear area
of LSB with web openings
.
Both
Figure 12
and Table
3
show that transverse stiffeners are
not adequate to restore the shear strengths of LSB
s
with
large
r
web openings
(127
mm)
.
Hence
the use of
transverse stiffeners
is not recommended
for
LSB
s
with large web openings.
12
5
.2
.
Sleeve Stiffener
s
The sleeve stiffener
was proposed
based on
its ability to
restrain the free edge of the
web
opening.
This
proposal
was
adopted from conventional cold

formed steel designs where
such
stiffeners are used to reduce the local buckling effects of free edge
s
. This stiffener is
likely to
have a greater effect on
the buckling strength of LSBs than the ultimate shear strength.
How
ever, increasing the
shear
buckling capacity is
also
important.
The geometry and finite
element mesh of a typical LSB with
sleeve
stiffener
s
is shown in Figure 4
(c)
.
In comparison
with
other
stiffeners, the sleeve stiffener is less practical as it require
s the sleeve to be formed
during manuf
acturing or welded to the web. However, w
elding
thin
cold

formed steel
sections
is
to be
avoided as it
can be detrimental in terms of heat effects and r
esidual stresses.
Hence
the
use of
sleeve stiffener
s
may
complicate
the fabrication phase of LSB joists and bearers.
Th
e th
ickness and length of sleeve stiffener
that affects its
rigidity
were considered as
possible variables in FEA. T
he
sleeve
stiffener thickness
was considered
to be the
same as
the
LSB web
th
ickness
.
However, its
length
was varied (10, 20 and 25 mm) to determine its
effect
on
the
shear capacity of
200x45x1.6
LSB
s with
60 mm
web openings.
Table
4
shows
the
shear capacity results, which
show that
the
sleeve stiffener
length
(10 to 25 mm)
did not
play a signif
icant role on the shear capacity
of LSB with web opening
s
.
Table
5
show
s
the
shear capacities
of LSB
s with varying web opening sizes and 20 mm
sleeve stiffener
s
,
which
indicate
that
sleeve
stiffeners are
not adequate to restore the shear strengths of LSB with
large
r
web openings
(102 and 127 mm)
.
Hence
the use of
sleeve
stiffeners
is not
recommended
for LSB
s
with large web openings.
Figure 13
shows the failure mode of
200x45x1.6 LSB with
60 mm web opening
s
and
20 mm sleeve stiffener
s
.
5
.3
. LSB Stud Stiffener
s
The
LSB stud stiffeners are LSBs
with web openings that are attached to the web
.
The
LSB
stud
stiffener is attached to the web around the opening by fastening
with No.12 T
ek screws
.
It
is capable of both reducing the aspect ratio and increasing the shear area
of LSB with web
openings
.
The geometry and finite element mesh of a typical LSB with
LSB stud
stiffener is
shown in Figure
4
(d).
13
E
ffect of LSB stud stiffeners on
the
shear cap
aci
ty of LSBs with web openings was
investigated using
FEA
and
Tests 9 and 10
, and the results are shown
in Table 2
.
As shown
by the experimental study, finite element analyses also showed that
LSB stud stiffeners
were
able to obtain
about
80%
of
the
shear
capacity of LSB without web openings
(52 kN) in the
case of 102 mm web openings
.
Since the thickness of LSB stud stiffener is equal to the
web
thickness,
LSB stud stiffeners are
not adequate to restore the shear strengths of LSB with
large
web openings.
He
nce
LSB stud stiffeners
are not
recommended
for LSB
s
with large
web openings.
5
.4
.
Plate Stiffener
s
Plate stiffeners are plates
with web opening
s
that are attached to the web
(see Figure
s
4
(
a
)
and 5
).
The
y are
attached to the web around the opening
s
by
fas
tening
with
No
.
12 Tek
screws
. In c
ontrast to the other
types of
stiffeners,
the
plate
stiffener is capable of both
reducing the aspect ratio and increasing the shear area
of LSB with web openings
.
Our
experimental studies
[10] showed
that plate stiffene
r
s were the best
stiffener for LSB
s
with
web openings.
However,
the number of shear tests was
limited
.
Hence in order to determine
the optimum
plate stiffener size
s
,
finite element models of LSBs
with web openings stiffened
with
plate stiffener
s
in shear were developed to simulate th
eir shear behaviour and strength
.
They were then validated by comparing their results with available test results
(Test
Specimens 5 to 17) and used in a parametric study (see Section 5.5)
.
In
Test Specimen 5,
plate s
tiffener dimensions and screw fastening arrangement were
adopted based on AISI
[9] (Arrangement 1)
. However, FEA and experimental results show
ed
that it
only reached
about
65% of t
he shear capacity of LSB
without web openings (
3
4.8 and
3
3.6
kN
vs 52 kN). H
ence FEA and test
results
showed that
the plate sti
ffeners
based on
AISI
[9]
recommendations
are not adequate to restore the shear st
rengths of LSBs
.
Test
Specimens
5 and 6
with 152x152x1.6
mm
plate stiffeners
were considered to investigate
the effect of
different
screw
spacing
s (Arrangements 1 and 2)
.
Both FEA and tests showed
that using
more screws (20 versus 8 screws) increased the
shear capacit
y
of LSBs
by
only
5%
as shown in Table 2
.
Test
Specimen
s 7 and 8
were
conducted
to investigate the effect of
using thicker (3.2 and 4.8 mm) plate stiffeners of full web height (152x168 mm).
In this case,
both
FEA and test results
showed that the shear cap
acities increased considerably (Table 2).
14
Tests 11 and 12 of LSBs
with 102 mm web openings were
considered
with 3 mm plate
stiffeners that were 202 mm wide (50 mm on either side of the edge of web opening) and 168
mm height (full web height).
Both FEA and Test results showed that thicker and wider
stiffeners of full web
height as used in these tests were able to fully restore the shear capacity
of LSBs
(
56.0
and
55.0
versus 54
kN
in FEA and 54.5 and 52.5 kN vs 52 kN in tests
).
T
est
and FEA
results
also showed that Arrangement 4 with reduced edge distance
s
of 12.5 mm led
to a small reduction in the shear capacity. Hence screw fastening Arrangement 3
used in Test
11
is recommended.
Test
Specimen
13
with
welded pl
ate stiffeners
failed at
a higher load d
ue to combined shear
and bending as
shown in
Figure 10
.
Both
FEA and
test results
(66.5 and 67 kN)
showed that
the shear capacity of LSB with web openings can be improved to levels beyond the shear
capacity of a solid LSB section (
54 and
52 kN) by welding suitable plate stiffeners.
Generally
horizontal, vertical and incline
d
stiffeners are
welded to the web around the openings
in hot

rolled sections
.
Our
FEA and
test
results show that plate stiffeners can also be welded to
improve the shear capacity of cold

formed sections with large web openings. However,
welding is not rec
ommended to avoid excessive heat an
d residual stress effects on
thin

walled cold

formed steel sections.
Test
Specimen
s 14 a
nd 15 with
larger 127 mm web openings
were investigated to determine
the required stiffener thickness
.
Five
mm plate stiffeners that
were 227 mm wide (50 mm on
either side of the edge of web opening) and 168 mm height (full web height)
were used
.
Both
FEA and test
results
showed that these plate stiffeners fastened using Arrangements 3 and 5
were almost able to restore the full shear c
apacity (
93
% of the shear capacity of LSB without
web openings
). In this case, the depth of web opening to the clear height of web ratio (d
wh
/d
1
)
is 0.75, which is more
than
the limiting value of 0.7 given in AS/NZS 4600 [7]. Hence it is
unlikely that such
large openings will be used in practice.
Arrangement 5
(Figure 3(e))
is
architecturally appealing, h
owever, it is not recommended
due
to
additional installation costs.
Test
Specimen
16
included
177x168x4.8 mm plate stiffeners, and the shear capacity
from
FEA
was only
36.8
kN due to the use of plate stiffeners with reduced width (177 mm versus
227 mm). However, for Test Specimen 17 with 60 mm web openings, 1.6 mm thick plate
stiffeners of 160x168 mm were able to restore
the original shear capacity (52
.5
an
d 50.5
vs
15
52 kN).
Both
FEA and test results show
that AISI’s [9] recommendation for the minimum
width of plate stiffeners to be based on 25 mm on each side of web openings is not adequate.
This research has shown that the plate stiffeners should extend 50
mm beyond all the edges of
web openings.
In summary,
FEA and
test results
show that plate stiffeners with dimensions equal to web
opening width and depth plus 100 mm, screw fastened using Arrangement 3, are needed to
restore the original shear strength o
f 200x45x1.6 LSBs. Their thicknesses have to
be a
minimum of 1.6 mm and
3.0 mm
for these LSBs with 60 mm and
102 mm web openings,
respectively.
However, detailed parametric studies
are needed
to
determine these parameters
for
other
LSB sections. Details of
these
parametric studi
es are given next
.
5
.
5
.
Parametric Study of LSB
s
with Stiffened Web Openings
(Plate Stiffener
s
)
In this
parametric
study
based on validated finite element models
,
five LSB sections,
150x45x1.6 LSB,
150x45x2.0 LSB, 200x45x1.6 LSB, 300x75x2.5 and 300x75x2.0 LSB, with
four web opening sizes (60, 102, 119 and 127 mm)
,
were
selected
with an aim to determine
the optimum plate
stiffener thickness that increases the shear capacity to that of LSB without
web
openings in each case
.
The plate stiffener thickness was varied from 1.6 to 8 mm in the
models with an
aspect ratio of 1.5.
T
he ultimate shear capacities
obtained
for varying ratios of
d
wh
/d
1
are given in Tables 6 (a) to (
d
)
.
Figure 14 shows the FEA resu
lts in the form of shear capacity of LSB with 102 mm stiffened
web openings versus number of screws while Figure 15 shows the FEA results in the form of
shear capacity of LSB with stiffened web openings versus stiffener thickness for 200x45x1.6
LSB. Figure
14 indicates that plate stiffeners with 12 screws (Arrangement 3) provide the
optimum screw fastening arrangement. Figure 15 (b) shows that the optimum thickness of
plate stiffener is 3.0 mm for 200x45x1.6 LSB with 102 mm web openings.
Similarly, Figures
15 (a) and (c)
show that 1.6 mm and 4 mm are the optimum plate stiffener thicknesses for
200x45x1.6 LSB with 60 mm and 119 mm web openings, respectively. Experimental results
also confirmed that plate stiffeners with Arrangement 3 (12 screws) provided
the
optimum
arrangement
and 1.6 mm and 3.0 mm were the optimum stiffener thicknesses
for 60 mm and
102 mm
web openings
, respectively. Figures 16 (a) to (c) show the FEA results in the form of
shear capacity of LSB versus stiffener thickness for 300x75x2.5 LSB
and 300x75x2.0 LSB
16
while Figure 17 shows the
se
results
fo
r 150x45x1.6 LSB and 150x45x2.0 LSB.
The optimum
plate stiffener thickness in each case can be obtained from these figures. Table 7 shows the
optimum plate stiffener thickness
es
for the different LSB
sections and web opening sizes.
Table 7 and Figure 18 show
that
5 mm
plate stiffeners fastened using Arrangements 3 were
almost able to restore the full shear capacity
(93%) of 200x45x1.6 LSB with 127 mm web
openings.
In this case, the depth of web o
pening to the clear height of web ratio (d
wh
/d
1
) is
0.75, which
exceeded
the limiting value of 0.7 in AS/NZS 4600 [7].
In order to obtain the full
shear capacity, the
depth of web opening to the clear height of web ratio (d
wh
/d
1
)
was limited
to 0.7 based o
n FEA results.
6
. Optimum Stiffener System
s
for LSB with Web Openings
Subject
ed
to Shear
In this section the optimum plate stiffener system was proposed based on the numerical and
experimental results reported in the previous sections. It is proposed
that t
he width of the
optimum plate stiffener is d
wh
+100 mm and
its
height is
lesser
of clear web height (d
1
) and
d
wh
+100 mm. These dimensions have been chosen for practicality and allow a distan
ce of 50
mm between the
free edge
s of the web opening
and
the
plate
stiffener. This optimum stiffener
arrangement
is
an improvement of
the recommendations of AISI
[9]
and
Sivakumaran
[15].
Table 8 shows the shear capacities
of LSB
s using
optimum plate stiffener arrangement
(Arrangement 3 with No.12 screws)
. It shows
that
LSBs were able to restore the original
s
hear s
trength when
the
optimum stiffener arrangements were used around the web openings.
Keerthan and Mahendran [14] proposed suitable predictive equations for the shear capacity of
LSB without web opening (V
v
)
. These equations can be used for LSBs with stiffened web
openings when the optimum stiffening system proposed here is used
around the web
openings.
Keerthan and Mahendran’s [14] design equations
are also given in Appendix A of
this paper for the sake of c
ompleteness.
Figure 19 shows the plot of optimum plate stiffener t
hickness to
web thickness ratio (t
Siff
/t
w
)
versus depth of web opening to clear height of w
eb
(d
wh
/d
1
).
Figure 20
shows the
schematic
diagram of
optimum plate stiffener arrangement for
LSBs with web openings.
Suitable
equations are also proposed to predict the sizes of optimum plate stiffeners (Equations 1 to 3).
Equations 1 to 3 were developed so that they predict the required
plate stiffener thickness
(t
Stiff
)
conservatively
and thus
ensure a safe design of LSBs with stiffened web openings.
17
w
wh
Stiff
t
d
d
t
035
.
0
52
.
3
1
70
.
0
24
.
0
1
d
d
wh
(1)
(2
)
Stiff
h
Lesser of
1
d
and
100
wh
d
(3)
w
her
e
w
stiff
x h
stiff
= Width x Height of plate stiffener
7
. Conclusions
This paper has presented a detailed investigation into the shear behaviour of LSBs with
stiffened
web openings using finite element analyses. Suitable finite element models were
developed and validated by comparing their results with experimental test results. The
developed nonlinear finite element model was
able
to predict the shear capacities of LSBs
with
stiffened
web openings and associated deformations and failure modes with very good
accuracy.
Both finite element analysis
and experimental
results
show
that
the plate stiffeners
based on the recommendations of
AISI
[9]
are not adequate to restore th
e shear strengths of
LSB
s
with web openings.
New
plate
stiffener
systems
with optimum sizes and screw

fastening arrangements have been proposed
to restore the shear capacity of LSB
s
with web
openings
based on both experimental and
numerical
parametric study results.
It was found
that
t
he width of the optimum plate stiffener is d
wh
+100 mm and
its
height is
lesser
of
the
clear web height (d
1
) and d
wh
+100 mm
while the associated screw fastening is to be based on
Arrangement 3 with
12 screws.
S
uitable predictive equations have been proposed for LSB
designers to determine the optimum plate stiffener thickness
as a function of d
wh
/d
1
.
Acknowledgements
The authors would like to thank Australian Research Council and OneSteel Australian Tube
Mills
for their financial support, and the Queensland University of Technology for providing
the necessary facilities and support to conduct this research project.
They
would also like to
thank
Mr Christopher Robb and
Mr Jamie Sc
ott

Toms
for their
valuable
assistance in
performing the
shear tests of stiffened LSBs
.
The
authors
would also like to thank Mr Ross
100
wh
Stiff
d
w
18
Dempsey, Manager

Research and Testing, OneSteel Australian Tube Mills
,
for his technical
advice and
support to this research project.
References
[
1]
LiteSteel Technologies (LST), OneSteel Australian Tube Mills, Brisbane, Queensland,
viewed 10 March, 2009, <
www.litesteelbeam.com.au
>.
[2]
Pokharel, N. and Mahendran, M. (2006), Preliminary Investigation
into the Structural
Behaviour of LSB Floor Joists Containing Web Openings, Research Report, Queensland
University of Technology, Brisbane, Australia.
[3]
OneSteel Australian Tube Mills, (OATM) (2008), Design of LiteSteel Beams, Brisbane,
Australia.
[4]
K
eert
han, P. and Mahendran, M. (2010
), Experimental Studies on the Shear Behaviour
and Stre
ngth of LiteSteel Beams, Engineering Structures, Vol.
32, pp. 3235

3247.
[5] Keerthan, P. and Mahendran, M. (2012), Shear Behaviour and Strength of LiteSteel
Beams w
ith Web Openings, Journal of Advances in Structural Engineering, Vol.
15, pp.197
–
210.
[6]
Keerthan, P. and Mahendran, M. (2012), New Design Rules for the Shear Strength of
LiteSteel Beams with Web Openings, Journal of Structural E
ngineering, ASCE, In Pres
s.
[7]
Standards Australia/Standards New Zealand (SA) (2005), Australia/New Zealand
Standard
AS/NZS4600 Cold

Formed Steel Structures, Sydney, Australia.
[8]
American Iron and Steel Institute (AISI) (2007), North American Specification for the
Design of
Cold

formed Steel Structural Members, AISI, Washington, DC, USA.
[9]
American Iron and Steel Institute (AISI) (2004), Supplement to the Standard for Cold

Formed Steel Framing
–
Prescriptive Method for One and Two Family Dwellings, 2001
Edition, American I
ron and Steel Institute, Washington, DC
, USA
.
19
[10] Mahendran, M. and Keerthan, P. (2012), Experimental Studies of the Shear Behavior
and Strength of LiteSteel Beams with Stiffened Web Openings,
Research Report, Queensland
University of Technology, Brisban
e, Australia.
[11]
Hibbitt, Karlsson and Sorensen, Inc. (HKS)
(2007)
, ABAQUS User’
s Manual
, New
York, U
SA
.
[12]
Keerthan, P. and Mahendran, M. (2010), Elastic Sh
ear Buckling Characteristics of
LiteSteel Beams, Journal of Constructional Steel Research, Vo
l
.
66, pp. 1309

1319.
[13]
Seo, J.K, Anapayan, T and Mahendran, M.
(2008)
, Imperfection Characteristic of Mono

Symmetric LiteSteel Beams for Numerical Studies, Proc. of the 5
th
I
nternational
Conference
on Thin

Walled Struc
tures, Brisbane, Australia
,
pp. 451

460.
[14]
Keerthan, P. and Mahendran, M.
(2011)
, New
Design Rules for the S
he
ar Strength of
LiteSteel Beams,
Journal of Con
structional Steel Research
,
Vol. 67
, pp.
1050
–
1063
.
[15]
Sivakumaran, K.S., (2008), Reinforcement Schemes for CFS Joists
Having Web
Openings, Research Report, Department of Civil Engineering, McMaster University, Ontario,
Canada.
20
Appendix A:
Proposed Design Equations for the Shear Strength of LSBs
[14]
yw
v
for
yw
LSB
w
f
Ek
t
d
1
(Shear yielding)
(A

1
)
i
yw
i
v
25
.
0
for
yw
LSB
w
yw
LSB
f
Ek
t
d
f
Ek
508
.
1
1
(Inelastic shear buckling)
(A

2
)
for
yw
LSB
w
f
Ek
t
d
508
.
1
1
(Elastic shear buckling)
(A

3
)
where
where
yw
yw
f
6
.
0
For LSBs
)
(
87
.
0
ss
sf
ss
LSB
k
k
k
k
(A

4)
2
1
34
.
5
4
d
a
k
ss
for
1
1
d
a
(A

5)
2
1
4
34
.
5
d
a
k
ss
for
1
1
d
a
(A

6)
1
1
2
1
39
.
8
44
.
3
31
.
2
34
.
5
d
a
d
a
d
a
k
sf
for
1
1
d
a
(A

7)
3
1
2
1
99
.
1
61
.
5
98
.
8
d
a
d
a
k
sf
for
1
1
d
a
(A

8)
where k
ss
, k
sf
= shear buckling coefficients of plates with simple

simple and simple

fixed
boundary conditions.
2
1
905
.
0
w
LSB
e
t
d
Ek
e
yw
e
v
25
.
0
w
yw
LSB
i
t
d
f
Ek
1
6
.
0
21
Direct Strength Method (DSM)
1
yw
v
≤ 0.815
(A

9
)
815
.
0
1
25
.
0
815
.
0
yw
v
0.815<
≤ 1.23
(A

10
)
2
2
1
1
25
.
0
1
yw
v
> 1
.23
(A

11
)
where
yw
yw
f
6
.
0
(A

12
)
2
1
2
2
1
12
d
t
E
k
w
LSB
cr
(A

13
)
LSB
yw
w
cr
yw
Ek
f
t
d
1
815
.
0
(A

14
)
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