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820 L. Di Sarno et al. / Journal of Constructional Steel Research 63 (2007) 819832
steel base plate connection. The latter was designed in
compliance with the rules utilized for the composite frame
tested at JRC Ispra laboratory [3,14]. Several tests under either
monotonic or cyclic lateral loads and different levels of axial
loads were performed. This work focuses on the response
of composite columns under monotonic regime, i.e. pushover
tests. The results of the experiments carried out on specimenswith welded base steel plate (traditional) and socket-type joints
are discussed. It is found that for the traditional connections,
concentrations of inelastic demand occur in the anchorage
bolts and relies chiefly on bond type mechanisms. This type
of structural response is assessed with a simplified fibre-
based numerical model. The latter allows the evaluation of the
different contributions of the lateral deformation at the top of
the column specimens. Such model points out the large inelastic
demand imposed on the anchorage bolts of the steel end
plate in traditional base column connections. Conversely, the
socket-type system, derived from precast reinforced concrete
structures, leads to large energy dissipation. Plastic hinges form
at column base and the strength capacity is not lowered even atlarge lateral drifts (greater than 5%6%). As a consequence,
socket-type foundations may be reliably utilized for steel
and composite steelconcrete framed structures, especially in
regions with moderate-to-high seismicity, to achieve adequate
seismic structural performance.
2. Experimental program and test set-up
Several research programmes were recently launched in
Europe, the US and Japan to investigate the inelastic response
of bare steel and composite steel and concrete composite
structures and/or their subassemblages, both for new andexisting buildings (e.g. [22,7], among others). In Italy, a number
of experimental programmes were funded through national
and European grants to provide technical support for the
implementation and improvement of seismic provisions and
guidelines (e.g. [4,20,11]). These test programmes included
the assessment of the seismic response of composite slabs,
beamcolumns and connections, either beam-to-column or
base columns. In particular, the authors of the present work
investigated the inelastic static and dynamic (seismic) response
of base column connections. In so doing, a number of partial
encased column specimens, with different base joints, were
tested in the laboratory of the Department of Structural
Analysis & Design (DAPS, University of Naples, Italy).The sample specimens included monotonic and cyclic tests;
different levels of axial loads were considered during the
tests to simulate the seismic load combinations implemented
in European standards [11] for composite building structures.
These axial loads were computed with regard to the exterior and
interior composite steelconcrete columns frame tested at JRC
Ispra laboratory [3,14], as shown in Fig. 1. In addition, pull-
out tests were carried out to define the forceslip relationships
of the hooked anchorage bolts; the results of the pull-out
tests were utilized to investigate the deformation capacity of
the base column traditional joint. In this work the results of
the monotonic (pushover) tests are discussed in details. The
experiments were carried out on two types of partially encased
composite columns: HEB260 and HEB280 (see Fig. 1). The
test specimens employed two layouts for the base column
joints as per Fig. 2: traditional (bolted steel base plate) and
innovative (socket-type) joints. The former consist of tapered
steel plates welded onto base plates and anchored to the
foundation block through steel bolted bars (see also [14]). Thelatter is an alternative and innovative socket type joint in which
the column is fixed to the foundation block utilizing a special
concrete filler; such joint was developed and designed to benefit
of composite action. Socket type connections are generally
utilized in precast reinforced concrete systems for residential,
office and industrial framed structures.
The first set of specimens (traditional base column
connections) correspond to the columns and base joints
designed in compliance with the guidelines of European
standards [911] and used for the full-scale composite frame
tested in ISPRA [3]; the layout of the latter is pictorially
displayed in Fig. 1. The socket-type foundation was designed
in compliance with Eurocode 2 [8] using strut-and-ties design
approach.
In the experimental tests, the lateral loads were applied
at two different locations along the height of the column,
namely at 1.6 m (traditional joint) and 1.7 m (socket type)
above the foundation block to account for the different location
of the restraint. Traditional base plates are generally placed
at floor level, conversely socket type solution enables to use
pavings that cover the foundation block. The horizontal load T,
simulating the earthquake loading, was applied by means of a
500 kN-hydraulic jack; the test was under displacement control.
As a consequence, the maximum flexural moments M, located
at the base connection, was increased until failure occurred. The
displacement controlled loading regime allowed the softening
branch of the response (capacity) curve to be investigated. The
connection of the jack to the column is ensured by two steel
plates 30 mm thick bolted on two opposite faces of the column.
The reaction wall for the horizontal load is a stiff tapered
cantilever bolted to a steel system; its layout is shown in Fig. 3
along with the test set-up. The cantilever system is connected to
the laboratory floor slab (strong floor) by means of large steel
rebars crossing the slab and the steel elements. These rebars are
loaded in tension to prestress the connection. The vertical load
N is applied by two hydraulic jacks connected with two bars
at the hinges placed at the foundation level. The axial load Nacts along the column centroid axis. The reaction system for
N consists of a steel plate located under the foundation block
and connected to the hinges. This layout ensures that the load
remains along the member axis during the column deformation.
The transversal beam, at the column top, is connected to the
jack by means of large stiffeners. An adequate lateral restraint
along four prestressing bars at the corners were used to prevent
slip and rocking of the foundation block and to guarantee the
transfer of the shear forces to the strong floor level.
The investigation of the interface behaviour between anchor-
age bolts and concrete block was carried out by means of pull-
out tests; the set-up employed to perform the tests is provided
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Fig. 1. Geometry of the full-scale composite frame tested at Ispra.
in Fig. 4. It consists of a rigid steel frame, designed to prevent
the onset of local buckling. The jack is bolted at the horizon-
tal beam of the frame and the anchoring device is gripped with
a tension bar. The anchoring devices were instrumented with a
transducer and strain gauges. It is instructive to note that the use
of strain gauges may, however, modify the mechanism associ-
ated to bond and influence significantly the results due to local
stresses acting on the device.
3. Test specimens
The experimental programme carried out at the laboratory
of DAPS, in Naples, focused on two sample partially encased
composite steel and concrete columns: HEB260 and HEB280
(see also Fig. 1). The sample specimens tested were cantilever
systems summarised as below (Table 1): partially encased
columns with a steel HEB 260 member and traditional
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Fig. 2. Layout of the sample base column connection: traditional (left) and socket-type (right).
Table 1
Sample column specimens
Specimen Axial load (kN) Loading type Connection
HEB260 330 Monotonic Traditional
HEB260 170 Monotonic TraditionalHEB260 330 Monotonic Socket
HEB280 520 Monotonic Socket
connection to foundation (stiffening plates and anchoring
devices) and innovative socket-type system; HEB280 with
socket-type foundation. The specimens were instrumented in
a refined fashion to characterize reliably the concentration of
inelastic demand at the base column. Fig. 5 provides a close-up
view of the displacement transducers (LVDTs) located at the
base of the partially encased columns with either traditional or
innovative joints.
The values of axial load N used in the tests were equal to170 kN, 330 kN and 520 kN. The latter values correspond to
the axial loads of the design load combinations of the full-
scale composite framed building tested in the ELSA laboratory
of JRC in Ispra [3]. The grade of the structural steel of the
specimens was S235; the reinforcing bars grade was B450-
C and the concrete was class C25/30. Actual values of the
mechanical properties of steel and concrete were estimated
from tensile (for steel) and compression (for concrete) tests.
The values computed for the column members are summarized
in Table 2. The width-to-thickness ratios of the sample columns
(b/tf = 20.8 for HEB260 and b/tf = 21.5 for HEB280)
fulfil the limit (b/tf = 44) given in European standards [10]
to prevent local buckling in partially-encased I-sections madeof steel grade S253. Further details on the results of such tests
can be found in [3]. The values of material overstrength ( fu/fy)
relative to the element flange are on average higher than those in
the webs. This result is in agreement with similar experimental
data for steel members produced in Europe (e.g. [2]).
The details of the foundation system are shown in Fig. 2. The
design of such foundation block was based on the ultimate limit
state corresponding to the stress values and distribution at the
column failure. The layout of the composite column specimen
employing HEB 260 steel section and traditional base joint is
provided in Fig. 6, while the specimen HEB260 with socket
type connection is provided in Fig. 7. The thicknesses of the
Table 2
Mechanical properties of the sample columns
Property HEB260 HEB280
Web Flange Web Flange
fy (MPa) 406 341 341 300fu (MPa) 480 449 450 430
fu/fy 1.18 1.32 1.32 1.43
u (%) 31.8 35.7 34.5 37.1
base of the socket is equal to 300 mm, while the walls of the
socket are 250 mm thick. The total height of the foundation
is 1050 mm. The values of the internal actions, i.e. axial load
NSd,j , flexural moment MSd,j and shear VSd,j used for the
design are NSd,j = 308 kN, MSd,j = 906 kN m and VSd,j =
444 kN. These values were derived from the ultimate flexural
capacity of the HEB 280; the evaluation of the resistance was
based on conservative assumptions. In fact, for the above cross-
section, the ultimate bending moment is MRd,col = 755 kN m;
note that the design value MSd,j accounts for the overstrength
( fu/fy = 1.20) at the base column connection.
For each foundation block, two anchoring devices were
embedded in addition to the ones used for the base column joint
details (Fig. 8). These embedded elements were instrumented
with transducers and were utilized for pull-out tests. Thus,
forceslip relationships were estimated for each anchorage
bolts; these relationships were then utilized to compute the
momentcurvature relationship of the base column section,
as discussed in Section 5. The results of the experimental
tests carried out on composite columns employing innovative
(socket-type) are discussed below.
4. Test results
The test results of the partially encased composite columns
were computed in terms of both local (momentcurvature
M and momentrotation M ) and global (lateral loadtop
displacement, F) response parameters. The capacity curves
of the specimens HEB260 with axial load N = 330 kN, both
traditional and innovative base connections, are provided in
Fig. 9. Such curves are expressed in terms of lateral force
(F)-drift (d/H), where H is the distance of the centreline
of the hydraulic jack from the foundation and d the total
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Fig. 3. Layout of the test set-up (top) and reaction wall (bottom).
horizontal displacement at the jack height. It is observed that
the traditional connection layout exhibits higher lateral strength
(600 kN m vs. 510 kN m) due to the steel stiffeners used
at the base of the column and to overstrength for seismic
design. Conversely, the ultimate deformation capacity of the
socket type connection is about 75% higher than the counterpart
traditional (about 0.05 rad vs. 0.09 rad); in both cases the
threshold value of 35 m rad given by Eurocode 8 [11] is
exceeded. The latter limit value is more stringent than the 3%
drift, which is assumed as the onset of the ultimate limit state in
steel and composite frames in the US practice [16]. Furthermore
the failure mode of the specimen with steel end plate is related
to anchorage bolt fracture (Fig. 10), while in the case of the
socket type a very ductile mechanism is observed (Fig. 11).
At serviceability, the stiffness of the traditional connection
is slightly higher than that of the socket connection. It can
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Fig. 4. Set-up of pull-out tests: layout (left) and actual system (right).
Fig. 5. Close-up view of the electrical displacement transducers (LVDTs): traditional (left) and innovative (right) joint.
thus be argued that the experimental tests carried out both
on traditional bolted steel end plate and innovative socket-
type connections demonstrate that the former experience brittle
failure modes. Rupture of anchorage bolts as per Fig. 10 was
observed for traditional connections. The composite partially
encased columns with traditional joint yield at about 310 kN,
which corresponds to a lateral drift of 26 mm (d/h 1.65%).
The maximum force is equal to 375 kN for HEB260 with axial
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Fig. 6. Traditional base column joint.
loads N = 330 kN and 340 kN for N = 170 kN. The lower
value found in the second specimen (340 kN) is related to
the premature rupture of the base joint, probably caused by
technological defects of the threaded bars. In both specimens,
i.e. with N = 170 kN and N = 330 kN, under monotonic
regime, the column strength and energy dissipation do not
exhibit significant loss for drift d/h 5%6%. The thick steel
plate and the stiffeners used at the column base ensure that
the end section of the column remains plane (rigid rotation).
Additional tests carried out by the authors under cyclic loads
have demonstrated that the crushed concrete and the inelastic
deformations in the steel components (anchorages), both at
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Fig. 7. Socket-type base column joint.
the column base, endanger the global lateral stiffness of
the composite column [15]. Bond-related phenomena give
rise to degrading effects, especially at large drifts, thus
reducing significantly the energy dissipation capacity of the
member. These findings point out that traditional connections
are not fully satisfactory, especially when the contribution
of reinforced concrete component increases, i.e. due to
cross section dimensions. Conversely, innovative socket-type
connections possess adequate ductility. Under monotonic load
conditions, the test results show strain hardening of the base
column equal to 1.32. This is due chiefly to the material
overstrength of structural steel (compare, for instance, values of
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Fig. 8. Steel reinforcement used for the traditional base column joint and layout of the additional anchorage bolts for pull-out tests.
Fig. 9. Capacity curves for the traditional and socket-type connections for
HEB260 (N= 330 kN).
fu/fy of the member flange in Table 2). The contribution of the
hardening of the longitudinal reinforcement bars is in fact very
small (1.13 vs. 1.32). The tests carried out on the specimens
employing the socket joint do not exhibit strength deterioration
even at large lateral drifts, e.g. d/h > 0.040.05 rad. The
formation of the plastic hinge occurs at the base column, as
observed during the tests. Fig. 11 shows the occurrence of
inelastic deformations at the base column during the test on the
HEB 260 with N= 330 kN; the spreading of such inelasticity is
also evident. At very large drifts, the flange plate of the column
tends to bend outwards and the bond between the inner concrete
and the exterior steel plate is broken as demonstrated by the
close-up view of the Fig. 11, without any relevant effect on
the global response. The tensile resistance of the concrete is
exceeded and inclined (flexural) cracks initiate and propagate
above the foundation block.
To shed light on the structural performance of the sample
columns with either traditional or innovative socket-type
Table 3
Material mechanical properties used to computed the interaction curves
Property Design Yield Collapse
fcd (MPa) 14.2 21.3 38.0
fsd (MPa) 391.3 450.0 450.0
fad (MPa) 213.6 235.0 450.0
c 1.50 1.00 1.00
s 1.15 1.00 1.00
a 1.10 1.00 1.00
Key: fcd = concrete compression strength at yield ( f c); fsd = compres-
sion/tensile strength of reinforcement steel (bars); fad = compression/tensile
strength of reinforcement steel (profiles); c = partial safety factor for con-
crete; s = partial safety factor for steel (bars); a = partial safety factor for
steel (profiles).
member-to-foundation joint, the bendingaxial load MN
interaction curves (Fig. 12) were computed for the specimens
HEB 260 and HEB 280. These MN curves were derived
in compliance with the simplified method of design for
doubly symmetric and uniform cross-section over the member
length with rolled steel sections implemented in the European
standards for composite steel and concrete structures [10]. Theresistance of the cross-section to combined compression and
bending is calculated assuming stress blocks (plastic stress
distribution); the tensile strength of the concrete is neglected
and the interaction curve is replaced by a polygonal diagram
(the dashed line in Fig. 12). The material mechanical properties
used to derive the simplified interaction curves are provided in
Table 3. The values of the simplified M N interaction curves
for the test with HEB260 (N= 170 kN and N= 330 kN) and
HEB280 (N = 330 kN and N = 520 kN) estimated at yield
and collapse are summarized in Tables 4 and 5, respectively.
The computed values of the MN curves demonstrate that
the failure mechanism is controlled in the case of traditional
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Fig. 10. Close-up view of the deformations of the specimen HEB260 with N= 330 kN (traditional connection).
Table 4
Values of interaction curves for the test with HEB260 (N= 170 kN and N= 330 kN) and traditional joint at different performance state
Yield Collapse Test
M (kN m) N (kN) M (kN m) N (kN) M (kN m) N (kN)
Section above stiffening plate 343 330 633 330 488 330
339 170 628 170 433 170
Column base connection 440 330 550 330 596 330
395 170 510 170 530 170
Table 5
Values of interaction curves for the test with HEB280 (N = 330 kN and
N = 520 kN) and socket-type joint at different performance state (at column
base connection)
Yield Collapse Test
M (kN m) N (kN) M (kN m) N (kN) M (kN m) N (kN)
390 330 740 330 608 330
420 520 750 520 762 520
joint by the base column component. By contrast, for socket-
type connections, the failure mode is due to the formation of
the plastic hinges in the columns (see also Fig. 11, where the
spreading of inelasticity due to flexure is evident).
The inelastic response of the critical region of the specimens
with socket-type joints was monitored carefully through six
electrical displacement transducers located at the column base
as displayed in Fig. 5. These LVDTs record the lateral
displacement of several points of the column flanges in
order to define reliably the various inelastic phenomena,e.g. yielding, local buckling, fracture initiation, occurring
at those locations during the tests. Fig. 13 provides force-
rotation curves computed at the base column of the socket-
type connection under monitonic loads. All monitored sections
shows significant nonlinear response as the deformations
increase; this behaviour characterizes also the cyclic tests [15].
Comparison between total cord rotation and the base column
one derived from LVDT #6 measures enable to recognise that
the higher is the drift, the higher is the contribution to the total
drift given by the deformation of the socket type connection.
Furthermore, due to the location of LVDT #1, 45.0 cm far
from the foundation, the socket type shows a yielding spreading
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Fig. 11. Close-up view of the deformations of the specimen HEB260 with N= 330 kN (socket-type connection).
Fig. 12. Simplified interaction curve for combined axial loaduniaxial bending
moment (NM).
that is nearly twice the width of the section (45.0 cm vs.
52.0 cm). As far as seismic design is concerned, the longer
the spreading of inelasticity the higher the energy dissipation.
By contrast, traditional connection systems, employing bolted
steel end-plate generate high concentrated inelastic demand on
the anchorage bolts, which fail prematurely in a brittle manner.
Fig. 13. Forcerotation of the base column of the socket-type connection:
LVDTs #1 to LVDT #6.
5. Numerical simulations
Numerical simulations of the pushover tests were carried out
through a fibre-based model, which may accommodate both
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Fig. 14. Evaluation of fixed end rotation: system response (top-left), cross-section response (top-right) and computational model (bottom).
Fig. 15. Momentcurvature of the columns and the base joints: HEB260 with N= 330 kN (left) and N= 170 kN (right).
mechanical (material nonlinearity and residual stresses) andgeometrical (P effects) nonlinearities. The effects of the
fixed end rotations are also accounted for; the computational
model is pictorially shown in Fig. 14. The constitutive
relationships employed in the numerical simulations were
based on experimental tests carried out on structural steel
and reinforcement bars (see Table 2); concrete stress strain
including size effect was used to simulate behaviour of
concrete. The collapse of the sample columns was caused
by base column joint failure in the case of the traditional
connection as shown in Fig. 10. The ultimate resistance of
the columns is thus controlled by this failure mechanism.
Consequently, the ultimate bending moment of the base
joint was computed for purpose of comparisons. Themomentcurvature relationship of the base column was derived
through the constitutive relationship of anchorage bolts and the
Manders model for unconfined concrete for the high strength
material which supports the rigid steel end-plate. The peak
of resistance for concrete ( fc = 70 MPa) is assumed at
c = 0.002, which corresponds to the upper bound of the
compressive strength estimated via experimental tests. The
actual yield and ultimate strengths of the bolts are equal to
400 MPa and 570 MPa, respectively; the strain hardening
starts at sh = 0.01. Fig. 15 provides the momentcurvature
curves of the columns and the base joints for the sample
specimens HEB260 with both N = 170 kN and N = 330 kN.
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Fig. 16. Pull-out tests for anchorage bolts used for HEB260 with N= 330 kN (left) and N= 170 kN (right).
Reference lines relative to the onset of the yield in the base
joint and the column are also plotted in Fig. 15. For both
specimens HEB 260, the former component yields at a value of
about 300 kN m, while the column elastic threshold is higher
(452 kN m and 475 kN m for HEB260 with N = 330 kNand N = 170 kN, respectively). The total deformation of the
columns were computed by integrating the momentcurvature
along the member. This relationship was derived by assuming
that member cross-sections remain plane and slip between
steel and concrete does not take place (ideal bond); the
computational model employed for the numerical simulation
is outlined in Fig. 14. The fixed end rotation was estimated
through the momentcurvature of the base joint ( position of
neutral axis) and a fitting curve of experimental bond-slip
constitutive relationships provided in Fig. 16 (slip effects).
Comparisons between numerical simulations and experi-
mental test are provided in Fig. 17 for the sample column HEB
260 with N = 330 kN; results for traditional and socket-type
joints are displayed. For the former, it may be observed that the
value of the yield of the base joint is close to the elastic thresh-
old measured during the experiment, thus demonstrating that
the failure of the base column controls the global response of
the composite steel and concrete column. This behaviour was
also experienced by the sample HEB 260 with N = 170 kN.
Forcedeformation curves of the tested columns are also shown
in the same figure. The total lateral deformations and the con-
tributions caused by the fixed-end rotation and the column flex-
ibility are included. The deformation of the column was esti-
mated with respect to the length of 1310 mm, i.e. the distance
between the column tip and the steel stiffening end plate atthe base. The comparison between experimental and numerical
simulation of the socket-type joint provided in Fig. 17 demon-
strates that also for this type of connection layout the fixed
end rotation can be significant. However, the contribution of
the end rotation to the overall lateral displacement of the spec-
imen with socket-type joint tends to be lower than in the case
of the traditional bolted base plate connection. The end rotation
in the case of socket joints is caused by concretesteel interface
mechanisms that occur within the socket, i.e. slip in the embed-
ded length. This slip is a function of the concrete strength, the
number and configuration of studs, if any, welded on the col-
umn web. Moreover, for socket-type connections the bending
Fig. 17. Comparisons between numerical simulations and experimental test for
HEB260 with N = 330 kN for traditional (H = 1310 mm) (top) and socket
type (H= 1700 mm) (bottom) joints.
moment computed through the numerical simulation of the can-tilever column with height of 1700 mm is smaller than the value
estimated experimentally; the difference between these bending
moments is about 25%. Similarly, Fig. 17 shows that the yieldof the sample column evaluated numerically is higher than thatmeasured experimentally. The latter may be caused by the inter-
action between the composite column and the surrounding con-
crete. It is observed that for the socket type connection the fixity
of the column is not located at the interface between the end-plate and the foundation block, but it penetrates in the socket,
as proved by the profile of the lateral displacement determined
by means of the LVDTs shown in Fig. 5.
6. Conclusions
Experimental tests carried out on composite steel and
concrete columns were presented in this paper. Two layouts
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for the base column connections were assessed: the traditional
system employing the bolted steel end plate and the innovative
socket type. The experimental results demonstrate that the
socket system is beneficial for the spreading of inelasticity
at the base of the composite columns. To assess the
inelastic structural performance, the composite specimens
were subjected to monotonic loads at increasing lateral drifts(pushover experimental tests). It was found that the maximum
drift of the socket-type connection is nearly 75% higher than the
traditional bolted steel end plate. Traditional base connections
fail in a less ductile fashion because of the fracture of the
anchorage bolts. Conversely, socket connections exhibit a
ductile response due the formation of the plastic hinge at the
base of the column, which extends over a length much higher
than the cross section depth. As a result, socket-type joints can
be reliably used for design of structures which may experience
significant inelastic excursions, such as those in earthquake
prone regions.
Acknowledgements
The present work was funded by the Italian Ministry
of Research, through the grant PRIN-COFIN04 (Composite
steel and concrete earthquake-resistant frames: advanced
dissipative joint systems, methods for damage assessment and
seismic design guidelines); the financial support is gratefully
acknowledged.
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