Advances in Steel Structures - part 32 ppt

290
T.F. Nip and J.O. Surtees
Moment rotation curves for whole-connections FB2 and FB4 are presented in Figure 6. These display
a rigid initial characteristic but are sufficiently ductile to develop 30 x 10 -3 rad rotation before failure.
A summary of cruciform connection test results is shown in Table 3. A maximum applied moment of
1.2 Mp was recorded for both tests. The compressibility of the connections at compression flange level
correlated well with corresponding values obtained from test CB5. Although FB4 is not fully
equivalent to tests CB 1,CB2 or CB3, it was clear in the case of whole connections that sidesway
buckling of the compression zone is totally inhibited by several factors. The carrying capacities
obtained in the isolated compression tests understated the true capacities of whole connections
TABLE 3
SUMMARY OF WHOLE-CONNECTION TEST RESULTS
Test l[ Stiffening method
FB2 8 (4 off-line) ~24HTS bars
FB4 4 ~24 HTS bars
Failure moment + Mp Failure mode
1.229 Beam flange and web local buckling
1.2715 Beam flange and web local buckling
CONCLUSION
Threaded bar compression stiffening has been shown to be an effective and viable alternative to
traditional welded plate stiffening. Tests have confirmed that bearing strengths much in excess of
those required for current typical end plate connections are possible. Use of threaded bar as an
effective form of tension stiffening has been considered incidentally in this paper because of its
application to the whole-connection tests. The case for tension stiffening is strong and its eventual
practical acceptance will undoubtedly increase the appeal of threaded bar compression stiffening.
ACKNOWLEDGEMENTS
The research described herein was funded by the Engineering and Physical Sciences Research Council
and British Steel. Further support and advice was provided by the Steel Construction Institute and
British Constructional Steelwork Association Ltd.
REFERENCES
Grogan W. and Surtees J. O. (1995) Column flange reinforcement in end plate connections using

bolted backing angles.
Nordic Steel Construction Conference,
Malm6, Sweden, pp 87-94.
Grogan W. and Surtees J. O. (1999) Experimental behaviour of end plate connections reinforced with
bolted backing angles. J.
construct. Steel Research,
vol. 50, pp71-96.
Grundy P., Thomas I. R. and Bennetts I.D. (1980) Beam-to-column moment connections. J.
Struct.
Div., Am Soc. Civ. Engnrs,
pp313-330.
Murray T. M. and Kukreti A. R. (1988) Design of 8-bolt stiffened moment end plate.
Engineering
Journal, AISC,
Vol.25, Pt. 2, pp45-53.
Murray T. M. (1988) Recent developments for the design of moment end-plate connections.
J. Construct. Steel Research,
Vol. 10, pp 133-162.
Surtees J.O. and Yeung K.W. (1996) A new form of high moment beam-to-column connection.
EPSRC Final Report, Grant reference GR/J70758,
University of Leeds.
Experimental Study of Steel I-Beam to CFT Column Connections
S.P. Chiew & C.W. Dai
School of Civil and Structural Engineering, Nanyang Technological University,
50 Nanyang Avenue, Singapore 639798.
ABSTRACT
This paper focused on the experimental study of the composite behavior of steel
universal beam to concrete-filled tube (CFT) column connections. Eight specimens were
designed and tested to failure, of which four specimens are simple beam-column
connections and the rest are rigid connections with different type of stiffening details. For

simple connections, the parameters investigated are the thickness and diameter of the
steel tube and the beam size. For the rigid connections, the stiffening details investigated
include cover plate, shear plate, extemal ring and re-bar respectively. Experimental
results showed that the simple connections have weaker ultimate strength, ductility and
stiffness, and their behaviors are influenced by the parameters investigated. All stiffening
details improved the composite behavior, but to different extent. The specimen with the
re-bar detail that is easy to fabricate and costs effective exhibited excellent behavior in
terms of ultimate strength, stiffness and ductility.
KEYWORDS:
composite behavior, CFT column, re-bar stiffening detail.
1. INTRODUCTION
In the building construction industry, composite construction is gaining
widespread popularity in recent years. Its better structural performance and relatively
lower costs compared to conventional reinforced concrete construction makes it
especially attractive for high-rise building projects. In this connection, structural
291
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S.P. Chiew and C.W. Dai
engineers have been experimenting with different types of composite columns in a hope
to produce the most aesthetically impressive and futuristic buildings. Undoubtedly, new
breakthroughs with this form of construction will usher a new and exciting era into the
building construction industry.
Basically, there are two types of composite columns: concrete-encased structural
steel section and concrete-filled tube (CFT) columns. CFT column has many advantages
over other types of column. Architecturally, CFT columns have many attractive features;
for example, the concrete filling has no visual effect on their external appearance. The
advantages from a structural point of view are, firstly, the triaxial confinement of the
concrete within the section, and secondly, the fire-resistance of the column which largely
depends on the residual capacity of the concrete core. During construction, the steel tube
will dispense with the need for formwork and prevents spilling of the concrete. Although

the CFT column is an economical form of composite construction, their uses to date have
been limited due to the lack of design information on the beam-to-column connections
and to the limited construction experience. While extensive data is available on CFT
column behavior under different loading conditions, relatively less work has been done
on the connections to these columns.
Experimental results on CFT column connections can vary significantly
depending on the tube shape and other connection requirements. Broadly speaking,
details can be generalized into two categories, i.e. connections with the beams attached to
the face of the steel tube only and connections that use elements embedded into or passed
through the concrete core. Connections to the face of the steel tube include welding the
beam directly to the tube surface, using fin-plate
[ 1 ]
or cover plate to connect the beam to
the tube and providing diaphragms or external tings [2,3] to stiffen the connections.
Connections with embedded or passed elements include through bolting beam end plates
and continuing structural steel shapes into and through the column [4,5].
This paper summarized an experimental investigation to study the connection
details to circular CFT columns. The objectives are: a) to investigate the effect of
different parameters on the composite behavior of the steel I-beam to CFT column
~J
connections, and hence, an strength and stiffness prediction for this type of connection
can be given; b) to compare the effect of different stiffening details on the composite
Experimental Study of Steel I-Beam to CFT Column Connections
293
behavior, so that a relatively good stiffening detail can be recommended and c) to provide
test results to verify the finite element model built for this kind of composite connection.
Experimental results showed that the simple connections have weaker ultimate strength,
ductility and stiffness, and their behaviors are influenced by the parameters investigated.
All stiffening details improved the composite behavior, but to different extent. The
specimen with the re-bar detail exhibited excellent behavior in terms of ultimate strength,

stiffness and ductility. This detail which is easy to fabricate and cost effective proved to
be the most promising of all.
2. EXPERMENTAL PROGRAM
2.1 Specimen Details
A total of eight specimens were tested in this study. All specimens are modeled to
89 scale of the real size. Fig. 1 and Tables 1-2 show dimensions and details of the test
specimens. The materials used in the specimens are equivalent to BS4360 grade 43A
steel. Specimens consist of simple and rigid composite connections. For simple
connections, the parameters investigated are tube thickness (6.3mm and 8.0mm), tube
extemal diameter (219.1mm and 273mm) and beam size (203mm x 133mm x 31.3kg/m
and 254mm x 146mm x 31.25kg/m). For the rigid connections, the stiffening details
investigated include cover plate, shear plate, external ring and re-bar respectively.
Table 1. Details of Test Specimens
Specimen
UCN'I
Beam size
(mm x mm x kg/m) .
203 x 133 x 31.3
Column size
(mm x mm )
219.1 x 6.3
Stiffener type
No stiffener
Simple ucN-2 203 x 133 x 31.3 d~ 219.1 x 8.0 No stiffener
Connection UCN-3 203 x 133 x 31.3 d~ 273 x 6.3 No stiffener
UCN-4 254 x 146 x 31.25 qb 219.1 x 6.3 No stiffener
, , ,
UCN-5' "203 x 133 x 31.3 d~ 219.1 x 6.3 Cover plate
Rigid UCN-6 203 x 133 x31.3 ~ 219.1 x 6.3 Shear plate
Connection UCN-7 203 x 133 x 31.3 (~ 219.1 x 6.3 External ring I

UCN-8 ' 203 x 133'x'31.3 d~ 219.1 x 613 Re-bar
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S.P. Chiew and C. I41. Dai
Fig. 1 Overall Dimensions of Test Specimens
2.2 Load Application and Instrumentation
Monotonic static loads were applied as shown in Fig.2. Bonded strain gauges
were installed to observe the stress distribution in the flange, web and stiffeners (if
available). Also, 14 linear variable displacement transducers (LVDT) were used to
measure the vertical displacement, lateral displacement and the rotation of the 1-beam as
shown in Fig.3. In addition, two inclinometers were installed on the upper flange of the
steel 1-beam (near column) to measure the rotation of the steel I-beam. Loading was
terminated when the deformation was already excessive or when the composite
connection lost its ultimate capacity altogether.
Fig.2 Test Set-up Fig.3 Beam Rotation Measurement
Experimental Study of Steel I-Beam to CFT Column Connections
Table 2. Details of Rigid Connection
295
3. TEST RESULTS AND DISCUSIONS
3.1 Material Properties
Material tests were performed to determine the mechanic properties of steel and
concrete used in this experiment program. For concrete material, the 28 days cube
296
S.P. Chiew and C.W. Dai
strength is 39.88 N/mm 2 and the cylinder strength is 34.4
N/mm 2.
The properties of the
structural steel are summarized in table 3.
Coupon Thickness
(mm)
BF(203x133) 9.86

BW(203x133) 6.34
BF(254x146) 8.76
BW(254x146) 5.98
CL(219. lx6.3) 6.19
CL(219. lx8.0) 7.98
CL(273x6.3) 6.21
steel plate 10.47
Re-bar ~30
BF:
beam flange
Table 3 Steel Mechanical Properties
Gy
(N/mm 2)
355.9
385.0
351.2
407.3
357.4
,,
409.5
347.7
283.0
479.4
BW: beam web
6y E o. o]o. Ob EIo.
(xl0 "6) (N/mm 2) (N/mm 2) (%) (N/mm 2) (%)
1725 206357 498.1 71.5 371.0 25.9
1867 206201 507.2 75.9 401.2 26.4
1709 205560 483.8 72.6 362.2 27.8
2030 200620 503.5 80.9 404.6 21.6

1716 208290 443.8 80.5 321.07 26.4
1962 208764 499.3 82.0 377.0 24.4
1665 208850 455.87 76.3 357.6 24.9
1363 207566 429.9 65.8 336.13 37.0
2601 184338 596.8 80.3 430.6 25.1
CL: column Cy: yield stress
Ou: ultimate stress Elo.: elongation
3.2 Load carrying capacity
The moment-rotation relationships of all specimens are shown in Fig.4 and Fig.5.
Table 4 shows the experimental and numerical analysis results. The yield load in table 4
was determined as the value at an intersection point between an initial tangential line
from the origin point and a tangential line with a 1/3 slope of the initial tangential line [6]
as shown in Fig.5. The yield loads of all specimens are compared with the numerical
analysis results. In order to evaluate the load carrying capacity of the connection, the
yield moment of the test result is also compared with the plastic moment capacity of the
steel I-beam.
For simple connections, except specimen UCN-2, all other specimens had a weak
load carrying capacity. This was reflected on the coefficient a- the value of ot is just
between 0.40-0.48. This means these connections can not even achieve half the beam' s
capacity. The specimen UCN-2 had a higher load carrying capacity and this illustrated
that the thickness of the steel tube is an important parameter on the load carrying
capacity. It is also found that the load carry capacity will decrease when the outside
Experimental Study of Steel I-Beam to CFT Column Connections
E 297
Fig.4 Moment-Rotation relationship of Simple Connections
Fig.5 Moment-Rotation relationship of Rigid Connections
diameter of the steel tube increased by comparing specimens UCN-1 and UCN-3. Under
the same cross-section area, the selection of the higher and wider, but thinner steel I-
beam can improve the yield load of the connection about 42%, however, the value of the
coefficient c~ is almost the same (0.45 and 0.48 respectively). This means that the load

carrying capacity of the connection depends on the properties of the composite column,
the steel I-beam has lesser effect on it.
For rigid connections, all specimens have a higher load carrying capacity when
they are compared with the standard one (specimen UCN-1). This means the different
298
S.P. Chiew and C.W. Dai
Table 4 Comparison of Numerical and Experimental Results
Specimen
Test
result
M~ (kN.m)
UCN-1 95.2
UCN-2 128.2
My, (~.m)
FEA result
M. (kN.m)
My1/My2
1%
62.3 64.4 0.967 i37.95
87.8 0.968 137.95 90.7
55.3
UCN-3 96.1 54.8 0.991 137.95
UCN-4 140.4 88.5 91.7 ' 184.4
UCN-5 123.8

UCN-6 118.8
UCN-7 138.8
UCN-8 229.4
81.9
88.8

78.8
0.965
0.962
0.887 78.8
90.8
137.95
137.95
137.95
o~ = M,,,/~,
0.45
0.64
0.40
0.48
0.57
0.57
86.1 1.054 0.66
175.5 189.9 0.924 137.95 1.27
,.
Mu: ultimate moment
Myl, My2:
yield moment
Mp: plastic moment capacity of steel I-beam
Load
113
initial stiffness
Initial sty"
Yield
Load
Deflection
Fig. 5 Determination of yield load

stiffeners are useful on improving the connection' s load carrying capacity, but their
effects are different. The yield load of specimen UCN-5 and UCN-6 are 1.26 times of that
of the specimen UCN-1. The external ring stiffener can enhance 46% of the yield load
and the degree can be higher if the external ring had a higher strength (the yield strength
is only 283N/mm 2 in this experimental project). The re-bar stiffener is the most effective
one in improving the load carrying capacity the yield load is 2.82 times of the specimen
UCN-1.
The numerical analysis was carried out with MARC version K7.0 software
Experimental Study of Steel I-Beam to CFT Column Connections
299
package a general purpose finite element analysis program for nonlinear or linear stress
analysis in the static and dynamic regimes [7]. Three kinds of element are used in the
analysis: the 8-node doubly curved thick shell element is used to model the steel tube and
beam, the 8-node isoparametric three dimensional elements is used for the concrete and
the friction and gap link element is used to model the interaction between the steel tube
and the in-filled concrete. The Von Mises yield criterion and Buyukozturk yield criterion
are adopted for steel and concrete material respectively. The Newton-Raphson iterative
procedure is used in the analysis. The yield load was obtained by inputting actual material
properties into the program.
From table 4, it can be found the numerical results agree well with the test results
for all simple connections. For rigid connections, the prediction by numerical analysis is
acceptable, except specimen UCN-7, where the prediction is not so good 11% lower
than the test result. The good agreement between the numerical and test results shows that
it is feasible to use finite element analysis to predict the load carrying capacity of this
kind of composite connection.
3.3 Initial Stiffness and Ductility
Initial stiffness and ductility of each specimen is represented in table 5. They are
all compared with specimen UCN-1. For simple connections, from the table, it can be
found that the investigated parameters affect the initial stiffness and ductility in different
ways. When the thickness of the steel tube wall was increased, both the initial stiffness

and ductility of the connection were improved. The increase in the diameter of the steel
tube caused the increase of ductility, but it decreased the initial stiffness. The selection of
higher and wider but thinner beam (having the same cross-section area with UCN-1)
improved the initial stiffness of the connection, but it almost had no effect on ductility.
For rigid connections, the use of cover plate stiffener (UCN-5) improved the initial
stiffness of the connection, but it had no effect on ductility. Shear plate stiffener (UCN-6)
increased the ductility greatly, also had some useful on improving the initial stiffness.
The use of external ring stiffener (UCN-7) improved the ductility of the connection
obviously, but the initial stiffness decreased. The reason caused the reduction in initial
stiffness may be the use of lower strength steel plate (refer to table 3) in fabricating the