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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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L. Di Sarno et al. / Journal of Constructional Steel Research 63 (2007) 819832 821

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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