13th World Conference on Earthquake Engineering
Vancouver, B.C., Canada
August 1-6, 2004
Paper No. 2398

AN ANALYSIS OF THE BEHAVIOR OF HYBRID STEEL BEAM – RC
COLUMN CONNECTION
Jose C. ADAJAR1, Toshiyuki KANAKUBO2, Masahiro NONOGAMI3, Nobuo KAYASHIMA3,Yasuhisa SONOBE4, Masami. FUJISAWA5SUMMARY
A hybrid connection has been proposed to connect reinforced concrete columns and steel beams.
Analytical and experimental studies have been conducted to investigate the performance of the proposedconnection. The connection is designed to consist of welded plates that form an encasement to confine theconcrete and the reinforcing bars. Results of analyses and experiments on loads, displacements, stresses
and strains are presented to describe the general structural performance of the connection. Comparisonsamong specimens have been conducted to study the effects of parameter changes. Results indicate that thehybrid connection provides adequate strength and ductility.
INTRODUCTION
A hybrid framing design scheme has been considered to build
a nine-story building in Japan. As illustrated in Fig. 1, theframe consists of steel beams that are connected to reinforced
concrete (RC) columns. The beam-column connection isherein called the “hybrid connection”. A particular design idea
for the hybrid connection has been proposed.
As shown in Fig. 2, the connection consists of a welded steel
plate encasement confining the concrete and the reinforcing
bars. The encasement has openings that allow reinforcing bars
to pass through the connection. A square tubing at the centerof the encasement serves as the inner core confinement and1Figure 1. Framing System

Structural Engineer / Owner, AJIISS Engineering, Dallas, Texas, USA. Email: jadajar@ajiiss.com
Professor, University of Tsukuba, Ibaraki, Japan. Email: kanakubo@kz.tsukuba.ac.jp
3Research Engineer, Daisue Construction Corporation, Osaka, Japan.
4
Professor, Ashikaga Institute of Technology, Tochigi, Japan
5
Professor, Tsukuba College of Technology, Ibaraki, Japan2

the outer plates act as an outer confining
reinforcement.
Objective
This research aims to study the behavior of theproposed hybrid connection through analytical andexperimental investigations.

Figure 2. Beam-column connections.
ANALYTICAL INVESTIGATION
Basic Structural Macro Models
Preliminary calculations have been conducted using macro models to determine the approximate
governing applied loads and material stresses. Considering the beam-column connection at the second
floor of a nine-story building and the testing laboratory capabilities, the calculations were based on twomodel types that were classified according to mode of failure. The first model type, termed herein as“beam-yielding model”, was for a failure mode governed by the steel beam. The second type, herein called“concrete-crushing model” was for a concrete compression failure mode. All model sizes areapproximately one third of the actual structures. (Show two models with properties indicated.)

Figure 3. Preliminary analytical models.
Sizes and Material Properties
The lengths of the models for beams and columns are shown in Fig. 3. The beam sizes have been
considered based on the commercially available steel sections. The column cross section is 320 mm x 320
mm. Beam size is 200 mm x 100 mm. The column/beam depth proportion Dc/Db is 1.6 and the width
proportion Bc/Bb is 3.2. The sizes and material properties of the structural elements are as follows:Beam-yielding model: A 320 mm x 320 mm concrete section was used for the column. The beam size was
H 200 (depth in mm) x 100 (height) x 5.5 (web thickness) x 8 (flange thickness). A compressive strengthof 400 kgf/cm2 (5600 PSI) was assumed for concrete. The yield strength of beam was assumed to be 3000
kgf/cm2 (42,700 PSI)
Concrete-crushing model: The size of the column was also 320 mm x 320 mm. Two sizes of beams,
namely: BH 200 x 100 x 6 x 25 and BH 200 x 100 x 12 x 25, were used. The design compressive strengthof concrete was assumed to be 200 kgf/cm2 (2800 PSI). The yield strength of the beam was assumed to be4000 kgf/cm2 (57,000 PSI)
Loads and assumed Boundary Conditions
The bottom of the column was restrained to move upward ordownward as shown in Fig. 3. Lateral restraints were placed at 200
mm from both ends of the column. An axial load was applied on
top of each column. The additional stresses (Load/concrete section)
on concrete due to axial loads were not presented in order tosimplify the approximation. Beams were subjected to antisymmetrical bending moments by applying loads at beam-ends.
Results of Preliminary Calculations
Figure 4. Boundary conditions.
Using the sizes and material properties mentioned above, the beamyielding model indicates that the steel beam yields when theapplied load reaches approximately 3.2 tonf as shown in Fig. 3. In the vicinity of the hybrid connection,
the maximum stress in the steel beam is 3,055 kgf/cm2. The indicated maximum stress in concrete near
the area of the connection is approximately 105 kgf/cm2.
On the other hand, each concrete-crushing model indicates that concrete governs when the applied load isapproximately 7.0 tonf. When the maximum stress in concrete is 231kgf/cm2, the indicated highest stresslevel in beam BH200x100x6x25 is 2,660 kgf/cm2. For beam BH200x100x12x25 with a thicker flange, thehighest stress level is 2498 kgf/cm2.
Steel Connector Micro Model Analyses
In order to study the flow of stresses from steel beams to steel connectors, micro models of designed
specimens were created. Considering the interaction of the concrete column, a set of boundary conditions
A, B, and C, as shown in Fig. 4, were assumed. Boundary conditions A and B indicate that the encasement
perimeter and the inner tube are restrained to move upward or downward. Nodes in line with the middleof the tube are assumed to be pin-connected. The governing loads obtained in the preliminary calculationsof the macro models were used to analyze the micro models. Using the properties of the designed
experimental specimens, nine micro models were made as shown in Table 1.
Results of Micro Model Analyses
JP1 and JP2 Comparison
Figure 5 shows the stress results for beam-yielding model JP1 as compared to JP2. The properties of JP1
are the same as those of JP2 except that JP1 is cross-shaped with both beam ends loaded while JP2 is halfcross with only one beam end loaded. When the applied load at each beam end is 3.2 tonf, flanges of JP1
indicate a maximum stress of 6,931 kgf/cm2 as compared to 7,077 kgf/cm2 in JP2. Flanges that are justoutside the connection encasement are within the assumed 3,000 kgf/cm2 yield stress. However, thestresses on flanges inside the encasement are more than twice the yield strength particularly at corners
where flanges change in shape.

The maximum stresses on JP1 and JP2 webs are 3,149 kgf/cm2 and 3,097 kgf/cm2 respectively.
Connection encasements indicate stress levels of up to 504 kgf/cm2 for JP1 and 470 kgf/cm2 for JP2. The
inner tube of JP1 shows a higher stress of 2,076 kgf/cm2 as compared to only 1,997 kgf/cm2 in JP2.

Figure 5. Stress results comparison for JP1 and JP2
JS1 and JS2 Comparison
The stress results of concrete-crushing model JS1 as compared to JS2 are shown in Fig. 6. JS1 and JS2 are
identical except the encasement thickness that is 2.3 mm for JS1 and 4.5 mm for JS2. When the applied
load at each beam end is 7.0 tonf, the maximum indicated stress on flanges of JS1 is 7,108 kgf/cm2 ascompared to 6,990 kgf/cm2 in JS2. Flanges that are just outside the connection encasement are below theassumed 3,000 kgf/cm2 yield stress. However, the stresses on flanges inside the encasement jump to morethan twice the yield strength particularly at corners where flanges change in shape.

The maximum stresses on the webs of JS1 and JS2 webs are 3,064 kgf/cm2 and 3,035 kgf/cm2respectively. Connection encasements indicate stress levels of up to 1180 kgf/cm2 for JS1 and 1129
kgf/cm2 for JS2. The inner tube of JS1 shows a higher stress of 2,768 kgf/cm2 as compared to only 2,719
kgf/cm2 in JS2.

Figure 6. Stress results comparison for JS1 and JS2
JS3, JS4 and JS5 Comparison
JS3, JS4 and JS5 are concrete-crushing models that have the same properties except for an encasement
thickness of 0.0 mm, 2.3 mm and 4.5 mm for JS3, JS4, and JS5, respectively. As shown in Fig. 7, when
the applied load at each beam end is 7.0 tonf, the maximum indicated stress on flanges of JS3 is 5,611
kgf/cm2 as compared to 5,404 kgf/cm2 in JS4 and 5,322 kgf/cm2 in JS5. As in other models that arementioned above, flanges that are just outside the connection encasement are below the assumed 3,000
kgf/cm2 yield stress. However, the stresses are more than double the yield strength on flanges inside theencasement where there are corners.

Figure 8 shows that the maximum stresses on the webs of JS3, JS4 and JS5 webs are 2,548 kgf/cm2, 2,410
kgf/cm2, and 2,481 kgf/cm2 respectively. Model JS3 without encasement is compared to JS4 and JS5 witha maximum stress of 1018 kgf/cm2 and 838 kgf/cm2, respectively. The inner tube of JS3, JS4 and JS5
indicates a peak stress of 2,364 kgf/cm2, 1,816 kgf/cm2 and 2,217 kgf/cm2, respectively.

Figure 7. Stress results comparison for JS3, JS4 and JS5 flanges.

Figure 8. Stress results comparison for JS3, JS4 and JS5 components.

JS6 and JS7 Comparison
Figure 9 shows the stress results for concrete-crushing model JS6 as compared to JS7. The difference
between the two models is the casing thickness. JS6 has 2.3 mm and JS7 has 4.5 mm. When the applied
load at each beam end is 7.0 tonf, flanges of JS6 indicate a maximum stress of 6,843 kgf/cm2 as comparedto 6,731 kgf/cm2 in JP2. Stresses on flanges that are just outside the connection encasement are below theassumed 3,000 kgf/cm2 yield strength. The stress levels are consistent with the results of the macro model
analysis. It is to be noted that the stress levels in some areas inside the encasement are more than twice theyield strength. Stress concentrations occur at plate corners where shapes change.

Figure 9. Stress results comparison for JS6 and JS7 components.
EXPERIMENTAL INVESTIGATION
Material Test Results
Table 2 shows the material test results. In JP1 and JP2, D13(SD345) (diameter=13mm; yieldstrength=345Mpa) reinforcing bars and high strength ties U5.1(SBPD1275/1420) were used for RCcolumns. The concrete strength was designed to be 360kgf/cm2. H200x100x5.5x8 (SS400) was selectedfor the beams. In specimens JS1-JS7, D16(SD390) steel bars and high strength stirrups

U4.6(SBPD1275/1420) were utilized for columns. The design strength was 210kgf/cm2 for concrete. Steelbeams were BH200x100x6x25 (SM490) and BH200x100x12x25 (SM490).

Table 1. Properties of specimens.

Table 2. Material test results.
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
D13D16U6.4

Steel Plates
ultimateyield
2kgf/cm
3610
2900
4030
3780
3500
3650
3640
4060
4170

22kgf/cm
5510
6880
14900

kgf/cm4440
3800
5720
5550
5400
5440
5320
5620
5650
Steel Bars
ultimateyield

kgf/cm
3680
4820
13900

2elongation
part%
34
outer plate
42
28
beam web28
48
flange31
diaphragm
28
steel tube
34
32

specimencomp.
strength

JP1
JP2
JS1
JS2
JS3
JS4
JS5
JS6
JS7

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

2ConcreteE
2kgf/cm
-5x10
2.49
2.50
2.14
2.35
2.23
2.47
2.12
2.37
2.27

tensilestrengthpart2kgf/cm

22.0
20.6
23.7
23.3
21.1
21.5
21.1

columnelongation
part%
26
main/core
19bars9
stirrupsLoading Method and Sequence
As illustrated in Fig. 3, an oil jack was used to apply the axial load on the column, and actuators at beamends provided the antisymmetrical bending moments. The loading cycle and sequence, as shown in Fig.

10, started from a drift angle R=±1/800 and ended at R=±1/25. The drift angle was doubled after every
two loading cycles. At R=±1/25, the cycle was done only once.
78 mm

+1/25

R (radian)
39 mm

Drift Angle, R (radian)+1/50

19.5 mm+1/100+1/200+1/400+1/800-1/800-1/400-1/2009.75 mm
2.44 mm

4.88 mmcycle-1/100

-1/50

-1/25

1X 1X

2X2X

2X

1X

134562

Figure 10. Loading cycle and sequence.

Displacement and Strain Gauges
Figure 11 shows the setup to measure relative displacements and curvatures in beams and column. Gauges
to record shear deformations within the beam-column connection are shown in Fig. 12. Strain gauges
placed at preferred locations to monitor strains on steel plates are shown in Fig. 13.

Figure 11. Setup of displacement gauges.

Figure 12. Shear deformation gauges.

GG’

EE’

12 4 5 6 7 8312 4 5 6 7 8
3G -G’

E - E’

Figure 13. Strain gauges on steel plates.

TEST RESULTS
Loads, Displacements, Cracking, and Yielding
Load-displacement diagrams are shown in Figure 14. In specimens JP1 (cross shape) and JP2 (half crossshape), the maximum loads for 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 minimal and 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 wasyielding. In specimen JS1 that has steel encasement around the beam-column connection, cracks due tobending occurred in the column during a drift R=1/100 cycle. During this cycle, the end of the steel beamand some parts inside the beam-column connection were yielding. Encasement plates yielded during thelast cycle when R=1/25. Shear cracks on the column was seen 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 in thecolumn occurred when R=1/200. During this cycle, shear and flexural cracks were noticed at the beamcolumn connection. Yielding of steel plates inside the beam-column connection and at beam-ends
occurred during R=1/100. A maximum load of 6.2 tonf was attained at R=1/25. The ultimate flexuralstrength of specimen JS3 was not reached. In specimen 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 reached when R=+1/25. This maximum load was almost
equal to the beam ultimate bending capacity.

Figure 14. Load – displacement diagrams.
Strain Distributions
The maximum indicated strain inthe square tubing is 900µ for crossshape specimen JP1 and 400µ for
half cross shape specimen JP2. The
outer plates of the beam-column
1000µ1000µJP1JP1connection indicated a maximum
Section G-G’
Section E-E’strain of 500µfor JP1 and 300µfor
JP2. Figure 15 shows the strain
values on the plates inside theFigure 15. Typical strain values in beam-yielding type specimens.
connection when R=1/25. Strain

readings for specimens JS1, JS3 and JS4 when R=1/25 are plotted in Fig. 16. The maximum load wasdesigned to be governed by the beam-column connection. This was true for specimens JS1 and JS3 where
the inner plates and square tubing indicated large strains. However, in JS4, the specimen that wasdesigned 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 occurredon the outer plates.

JS12000µ

2000µ
JS1

Section G-G’

Section E-E’

2000µ

2000µ

JS3JS4

Section G-G’

Section G-G’

Figure 16. Typical strain values in concrete-crushing type specimens.

Results of Hybrid Connection Analytical Investigation
A structural model was created using ADINA finite element analysis program to simulate the behavior ofa specimen JP1. The model indicating the load and boundary conditions is shown in Fig. 17. Full fixity
was assumed at the bottom end of the column. It was also assumed that there was no translation along yaxis at intermediate points near the top and bottom ends of the column where confining oil jacks werelocated. The model consisted of 2-node line elements for steel reinforcing bars, 4-node shell elements forsteel plates, and 8-node solid elements for concrete for a total of 1340 nodes. Bilinear model with Von
Mises yield condition was used for steel elements. Figure 18 shows that the experimental and theoreticalload-displacement relations for the beam of specimen JP1 are in good agreement. Both analytical andexperimental investigations indicated that when R=1/25, the deflection of the beam is about 95% of thetotal deformation of the specimen as can be observed in Fig. 19. During testing, cracks were observed on
the surface of the column in the area of the beam-column connection but analytical results indicated thatcracks formed beneath the surface as shown in Fig. 20. 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 may account for the difference between analytical andactual stress results in the inner part of the connection.

CONCLUSIONS
It can be concluded that the proposed design for the hybrid beam-column connection provides an adequate
strength and ductility when the encasement is present. The additional resistance due to confinement ofconcrete enhances the capacity of the connection. The hybrid connection method can be an alternative
design in building frame type structures. Design estimates can be made using finite element analyticalmodel of the steel beam and steel plates in the beam-column connection with the reinforced concrete
column action assumed as the boundary supports. The shear strength range of the connection can becalculated using Sakaguchi equation. Further investigation of the connection performance considering thebond between steel and concrete is recommended.