Hybrid structural steel beam – reinforced concrete column connection
J.C. Adajar

AJIISS, Dallas, Texas, USA

T. Kanakubo

University of Tsukuba, Ibaraki, Japan

M. Nonogami, N. Kayashima,

Daisue Construction Corp., Osaka, Japan

Y. Sonobe

Ashikaga Institute of Technology, Japan

M. Fujisawa

Tsukuba College of Technology, Ibaraki, Japan
ABSTRACT: Nine specimens were tested to evaluate the behavior of a hybrid steel beam – reinforced concrete column connection under axial and antisymmetrical cyclic loads. The connection is made of weldedplates forming an encasement that confines twelve reinforcing bars at the corners and four bars at the centercore. Experimental and theoretical results on loads, displacements, stresses and strains are presented to describe the general structural performance of the connection.
1 INTRODUCTION

2 EXPERIMENTAL INVESTIGATION

Newly developed hybrid frames made of reinforced
concrete (RC) columns connected to steel beams
have been used in Japan. These frames can provide
practical and economical merits by combining
longer steel beams with high compression resistant
RC columns. To further develop such hybrid structures, it is necessary to determine the strength andductility of the connection. This study focuses on
beam-column connections, as shown in Figure 1,2.1 Specimens

JP1,2; JS1,2,5,6,7

JS3

JS4

Figure 1. Beam-column connections.

that consist of welded steel plates forming an encasement confining the concrete and the reinforcing
bars. The connections are designed to effectively resist varying column axial loads especially in tension.
Openings in the encasement are made to provide thenecessary arrangements for reinforcing bars. Asquare tube at the center of the encasement serves asthe inner core confinement and the outer plates act
as outer confining reinforcements. The specimens
are modeled after some beam-column connections ofa nine-story building that can be subjected to antisymmetrical bending moments and varying axialloads.
1.1 Objective
The main objective of this research is to investigate
the general structural performance of the hybrid
connection and its design method.

The specimens are about 1/3 the actual size of thedesigned structural members at the second floor of anine-story building. Table 1 shows the properties ofthe specimens. The column cross section is 320 mm
Table 1. Properties of specimens.
Specimen
BxD(mm)main bars
core bars
pg(%)
stirrups
pw(%)
DxBxt1xt2
inner tube
outer plates
Specimen
BxD(mm)main bars
core bars
pg(%)
stirrups
pw(%)
DxBxt1xt2
inner tube
outer plates

JP1

JP2
JS1
JS2
320x320
12-D16
12-D13
4-D16
4-D13
column1.98
3.11
4-U5.1@50
4-U6.4@50
0.50
0.75
H-200x100x5.5x8 BH-200x100x6x25 beam
120x120x6
125x125x6
connx
2.3
4.5
3.2
JS30.0JS4JS5JS6JS7
320x320
12-D16
4-D163.114-U6.4@500.75
BH-200x100x12x25125x125x9125x125x4.5
2.3
4.5
2.3
4.5x 320 mm. Beam size is 200 mm x 100 mm. The
column/beam depth proportion Dc/Db is 1.6 and thewidth proportion Bc/Bb is 3.2.

2.2 Material Properties
Table 2 shows the material test results. In flexural
type specimens JP1 and JP2, D13(SD345) (diameTable 2. Material properties.
plate properties
SS400 t=2.3 mm
SS400 t=4.5 mm
SM490 t=6 mm
SM490 t=12 mm
SM490 t=25 mm
SM490 t=28 mm
SM490 t=9 mm
SN490 t=4.5 mm
SN490 t=6 mm
designation
D13
D16
U6.4

specimen

Steel Plates
ultimateyield
kgf/cm
3610
2900
4030
3780
3500
3650
3640
4060
4170

2

22

2

elongation
part%main/core
26
bars
19
9
stirrups

kgf/cm
5510
6880
14900
ConcreteE
comp.
2strength
kgf/cmkgf/cm
3680
4820
13900

2

kgf/cm
433
456
217
234
238
237
190
201
211

JP1
JP2
JS1
JS2
JS3
JS4
JS5
JS6
JS7

elongation
part%
34
outer plate
42
28
beam web28
48
flange31
diaphragm
28
steel tube
34
32

kgf/cm
4440
3800
5720
5550
5400
5440
5320
5620
5650
Steel Bars
ultimateyieldtensile
strength

part

2-5

kgf/cm

x10
2.49
2.50
2.14
2.35
2.23
2.47
2.12
2.37
2.27

22.0
20.6
23.7
23.3
21.1
21.5
21.1

moments. The loading cycle and sequence, as shownin Figure 3, started from a drift angle R=±1/800 andended at R=±1/25. The drift angle was doubled after
every two loading cycles. At R=±1/25, the cycle wasdone only once.
78 mm

+1/25

R (radian)
39 mm

+1/50
19.5 mm+1/1009.75 mm+1/200+1/400+1/800-1/800-1/400-1/2004.88 mm
2.44 mmcycle-1/100

-1/50

-1/25

Figure 2. Loading cycle and sequence.

2.3 Displacement and strain gauges
Figure 4 shows the setup to measure relative dis-column

ter=13mm; yield strength=345Mpa) reinforcing bars
and high strength ties U5.1(SBPD1275/1420) wereused for RC columns. The concrete strength was designed to be 360kgf/cm2. H200x100x5.5x8 (SS400)
was selected for the beams, In shear type specimens
JS1-JS7, D16(SD390) steel bars and high strength
stirrups U4.6(SBPD1275/1420) were utilized forcolumns. The design strength was 210kgf/cm2 forconcrete. Steel beams were BH200x100x6x25(SM490) and BH200x100x12x25 (SM490).

Figure 4. Setup of displacement gauges.

placements and curvatures in beams and column.
Gauges to record shear deformations within thebeam-column connection are shown in Figure 5.

2.2 Loading Method and Sequence
As illustrated in Figure 2, an oil jack was used toapply the axial load to the column, and actuators at
beam-ends provided the antisymmetrical bending

Figure 5. Shear deformation gauges.

200650Strain gauges placed at preferred locations to monitor strains on steel plates are shown in Figure 6.

1650

1Unit: mm245631790

Figure 3. Loading method.

320

1790

7824567

3Section E-E’

Section G-G’Figure 6. Strain gauges on steel plates.

8GG’

EE’

3 TEST RESULTS

3.2 Strain distributions

3.1 Loads, displacements, cracking, and yielding

In flexural type specimens, the maximum strain in
the square tubing was 900µfor cross shape specimen
JP1 and 400µfor half cross shape specimen JP2. Theouter plates of the beam-column connection indicated a maximum strain of 500µfor JP1 and 300µfor
JP2. Figure 8 shows the strain values on the plates

Load-displacement diagrams are shown in Figure 7.
In bending failure type specimens JP1 (cross shape)
and JP2 (half cross shape), the maximum loads for
10Q (tonf)
Q (tonf) 4

52-40

4020
R (x10-3 rad.)-20-2-4-40 -30 -20 -1010203040

R (x10-3 rad.)-5JP1-10JS1Q (tonf) 10

Q (tonf) 10

5-40 -30 -20 -10-5-1020 30 40
R (x10-3 rad.)-40 -30 -20 -10

JS3JP1Section G-G’

Section E-E’

5101000µ1000µ

JP1Figure 8. Typical strain indication in flexural type specimens.
10-520 30 40
R (x10-3 rad.)-10 JS4

inside the connection when R=1/25. Strain readings
for shear type specimens JS1, JS3 and JS4 when
R=1/25 are plotted in Figure 9. The maximum load

Figure 7. Load – displacement diagrams.

both specimens are 4.2 tonf during the positive cycle
and 4.4 tonf during the negative cycle. More visible
cracks occurred in the column of half-cross specimen JP2 than in JP1. Cracks in JP1 are very minimaland occurred only near the beam-column connection. There was no indication of yielding in the steel
reinforcements of the column. However, the web of
the beam was yielding. In a shear failure typespecimen JS1 that has steel encasement around thebeam-column connection, cracks due to bending occurred in the column during a drift R=1/100 cycle.
During this cycle, the end of the steel beam andsome parts inside the beam-column connection wereyielding. Encasement plates yielded during the lastcycle when R=1/25. Shear cracks on the column wasseen during R=1/25 cycle. A maximum load of 8.1
tonf was reached during this cycle. The maximum
load was still less than the ultimate flexural strengthof the column. In specimen JS3, where outer plate
encasement was not present, the flexural cracks inthe column occurred when R=1/200. During this cycle, shear and flexural cracks were noticed at thebeam-column connection. Yielding of steel plates
inside the beam-column connection and at beamends occurred during R=1/100. A maximum load of
6.2 tonf was attained at R=1/25. The ultimate flexural strength of specimen JS3 was not reached. Inspecimen JS4, flexural cracks started to occur when
R=1/200. The beam started to yield upon reaching
R=1/100. Plates inside the beam-column connection
began yielding during R=1/50. In this cycle, shear
cracks appeared in the area of beam-column connection. A maximum load of 11.1 tonf was reachedwhen R=+1/25. This maximum load was almost
equal to the beam ultimate bending capacity.

JS12000µ

2000µ

JS1

Section G-G’

Section E-E’

2000µ
JS3

Section G-G’

2000µ
JS4Section G-G’

Figure 9. Typical strain indications in shear type specimens.

was thought to be governed by the beam-columnconnection. This was true for specimens JS1 and JS3
where the inner plates and square tubing indicatedlarge strains. However, in JS4, the specimen thatwas thought to fail in bending at beam-ends, relatively small strains on the square tube were observed. Although strains on the outer plates of JS1
were smaller than those on the square tubing, yielding occurred on the outer plates.

Figure 10. FEM Model with load and boundary conditions.

4 ANALYSIS
4.1 Results of first analytical investigation

Q (tonf)

A structural model was created using ADINA finite
element analysis program to simulate the behavior ofa flexural type specimen JP1. The model indicating
the load and boundary conditions is shown in Fig.
10. Full fixity was assumed at the bottom end of thecolumn. It was also assumed that there was notranslation along y-axis at intermediate points near
the top and bottom ends of the column where confining oil jacks were located. The model consistedof 2-node line elements for steel reinforcing bars, 4node shell elements for steel plates, and 8-node solid
elements for concrete for a total of 1340 nodes. Bilinear model with Von Mises yield condition was

ing testing, cracks were observed on the surface ofthe column in the area of the beam-column connection but analytical results indicated that cracks

Figure 13. Theoretical crack formations in JP1.
42-50 -40 -30 -20 -10 0

10 20 30 40 50-3
R (x10 rad)-2-4FEA
Test

Figure 11. Analytical and experimental load-displacement
relations for JP1.

used for steel elements. Figure 11 shows that the experimental and theoretical load-displacement relations for the beam of specimen JP1 are in goodagreement. Both analytical and experimental investigations indicated that when R=1/25, the deflectionof the beam is about 95% of the total deformation of
the specimen as can be observed in Figure 12. Dur-formed beneath the surface as shown in Figure 13.
Theoretical results for stresses on plates inside theconnection were notably different from the experimental results. A response model for bond between
concrete and steel plates was not provided. This mayaccount for the difference between analytical andactual stress results in the inner part of the connection. Because of this, a second analytical investigation was done to reexamine the stress distributions
on steel plates.
4.2 Results of second analytical investigation
Finite element models for beams and beam-column
connections without concrete and reinforcing bars
were created as shown in Fig. 14. Since it is the reinforced concrete column that supports and restrains
the beam, the resistance of column was modeled asboundary pin supports along the outer and innerJP1

Figure 12. Analytical exaggerated deflection of JP1.

JS3

JS1

JS4

Figure 14. Typical models for beam-column connections.

square perimeters at the top and bottom of the connection. The main objective was to investigate thestress proportion and flow from the beam to theplates in the beam-column connection. Models thatwere identical in shapes and sizes to actual specimens were created using 4-node shell elements. Figure 15 and 16 show stress contours at maximum
loads obtained from one-load-step calculations. Itwas observed that in all specimens, the stresses indi-cated by the strain gauges were closely approximated by analytical results. Slight differences in thestress results were found in the square tube and in
the inner webs. The assumption for concrete as resisting boundary supports may account for the differences. However, with a good correlation in stressresults, finite element models formed using exact
sizes and shapes can be suggested to be the bases ofactual structural design of this hybrid connection.

JP1

JS2

JS3

JP2

JS1

Figure 15. Stress distributions in the connections of specimens JP1, JP2, JS1, JS2, JS3, and JS4.

JS4

JS5capacity was calculated using Urbon equation. The
ultimate flexural strength of the beam and the shear
strength were estimated using the equations provided by the AIJ. The shear strength of the beamcolumn connection was approximated using Sakaguchi equation. The actual shear strengths for specimens with outer confining plates were 1.04 to 1.31
times the calculated strengths using Sakaguchi
equation for an average factor of about 1.16. For
specimen JS3 without outer confining plates, thecalculated strength was closer to the actual at a ratioof about 0.96. It is therefore possible to use Sakaguchi equation to determine the range of the shear
strength of the hybrid connection.
Table 3. Experimental and analytical strengths.
SpecimenJ P1
a xia l(t on f)
flexu r a l(t on f) 29.5
sh ea r (t on f) 22.2
a pplied(t on f) 11.6
r a t io
1.91
b flexu r a l(t on f) 3.5
esh ea r (t on f) 30.6
a m a x. loa d(t on f) 4.2
mr a t io
1.2
csh ea r (t on f)
668oa pplied(t on f) 65.0
nxr a t io
10.3
column

JS6

J P2
34.4
23.0
5.8
3.97
3.5
30.6
4.2
1.2
690
32.1
21.5

Specimen
J S3 J S467.0
a xia l(t on fflexu r a l(t on f) 29.1 28.9
sh ea r (t on f) 30.2 30.1
a pplied(t on f) 17.1 15.3
r a t io0.59 0.53
b flexu r a l(t on f)esh ea r (t on f)a m a x. loa d(t on f) 6.2 11.1
mr a t io
0.54 0.97
csh ea r (t on f) 99.2 105
oa pplied(t on f) 94.9 84.9nx
r a t io0.96 0.72
columnJ S1 J S2
67.0
28.3 28.9
29.3 30.1
22.3 23.7
0.79 0.82
10.1
20.9
8.1
8.6
0.8 0.85
94.4 113.0
124 132
1.31 1.16
J S5
26.6
29.1
26.4
0.99
11.4
39.3
9.6
0.84
142
147
1.04

J S6
57.0
27.7
29.8
23.1
0.83

J S7
28
28.9
24.8
0.89

8.4
0.74
109
129
1.18

9.0
0.79
126
138
1.09

5 CONCLUSIONS
JS7
Figure 16. Stress distributions in the connections of specimens
JS5, JS6, and JS7.

4.3 Strength
Empirical and experimental strengths are presented
in Table 3. Existing equations were used to calculate
the bending and shear strengths of beams and columns. The e function method of the Architectural
Institute of Japan (AIJ) was used to calculate thebending moment capacity of the column. Its shear

It can be concluded from the observed general performance that the presented hybrid beam-column
connection provides an adequate strength and ductility. The hybrid connection method can be an alternative design in building frame type structures.
Design can be made using finite element analyticalmodel of the steel beam and steel plates in the beamcolumn connection with the reinforced concrete column action assumed as the boundary supports. The
shear strength range of the connection can be calculated using Sakaguchi equation. Further investigation of the connection performance considering thebond between steel and concrete is recommended.