Seismic Behavior of Hybrid Concrete Beam-Column Connections with Composite Beams and Cast-in-Place Columns

617ACI Structural Journal/May-June 2014

ACI STRUCTURAL JOURNAL TECHNICAL PAPER

The objective of this study is to develop a new type of moment- resisting hybrid beam-column connections that are econom-ical and could be easily constructed. The new type of moment- resisting hybrid concrete beam-column connections proposed for the seismic region consisted of cast-in-place (CIP) concrete columns and composite concrete beams with precast U-shaped beams. An experimental study of two full-scale hybrid concrete beam-column connections with composite beams and CIP columns, including an exterior and interior connection and two control CIP connections under reversed cyclic loading, is presented. Results revealed that two hybrid concrete beam-column connections, as expected, exhibited a strong-column/weak-beam failure mecha-nism, and were capable of matching the performance of monolithic connections. Test specimens behaved in a ductile manner, and exhibited stable hysteresis loops. The difference in load-carrying capacity between the hybrid concrete connections and their control specimens was less than 10%. A restoring force model was devel-oped based on the test results.

Keywords: concrete composite beams with precast U-shaped beams; cyclic loading; energy dissipation; hybrid concrete beam-column connection; restoring force model.

INTRODUCTIONThe potential benefits of precast concrete in terms of

high quality control, construction efficiency, and time and cost savings are well recognized, and precast concrete frame structures are widely used in the United States, New Zealand, and Japan.1 As one of the most appropriate struc-tures for residential buildings, particularly low-cost housing, precast concrete frame structures have been booming in China since 2009.

Although precast concrete construction provides high-quality structural members, the performance of precast concrete frame structures is mostly governed by seismic behavior of the beam-column connections. Beam-column connection design is one of the most important consider-ations for successful construction of precast concrete frame structures. The detailing and structural behavior of the beam-column connections affects the strength, deformation, ductility, and constructability. It was therefore necessary to evaluate seismic behavior of precast concrete beam-column connections used in seismic zones. Up until now, many experimental and analytical studies have been conducted on seismic performance of reinforced monolithic concrete beam-column connections subjected to reversed cyclic loading. There have been only a limited number of studies, however, on seismic performance of precast concrete beam-column connections.

Park and Bull2 tested three full-scale exterior precast concrete beam-column connections consisting of precast beams and columns. The test specimens detailed for seismic loads performed satisfactorily in terms of strength, ductility, and energy dissipation, and could be used in ductile moment-resisting frames. A test program conducted by Khaloo et al.3 studied the characteristics of a simple moment-resisting precast connection with precast beams and columns. The connections transferred bending moment by a combination of lap-splicing and end anchorage of bars. The main conclusions drawn from the study were that the simple precast connection matched the performance of monolithic connection. Priestley et al.4 presented a paper about a test of two ungrouted, post-tensioned, precast concrete beam-column connections with precast beams and columns under cyclic loading. It was reported that satisfactory seismic performance could be expected from the specimens. Studies on the seismic behavior of precast concrete beam-column connections with precast beams and columns and CIP high strength concrete joint or with bolted joint were conducted respectively by Zhao et al.5 at Tongji University, China. Results from the tests indicated that the connection with cast-in-place (CIP) high-strength concrete joint performed seismically as well as the monolithic control specimen, and the seismic behavior of the connection with bolted joint was quite different from the monolithic control specimen, partic-ularly in energy dissipation. Lin et al.6 conducted a test of four ductile precast concrete connections with precast beams and columns and concluded that the detailing of the connec-tions had a major influence on the seismic performance of the connections.

Generally, existing studies were mainly focused on the seismic behavior of precast concrete beam-column connec-tions with precast beams and columns. The experimental investigations on hybrid concrete connections composed of composite concrete beams and CIP concrete columns, however, were very scarce. In addition, there is an absence of prescriptive seismic code provisions for this kind of precast concrete structures in ACI 318,7 EC 8,8 NZS 310,9 and the Chinese GB 50011.10

Since 2005, a test of precast concrete connections and frames composed of composite concrete beams and CIP

Title No. 111-S51

Seismic Behavior of Hybrid Concrete Beam-Column Connections with Composite Beams and Cast-in-Place Columnsby Weichen Xue and Bin Zhang

ACI Structural Journal, Vol. 111, No. 3, May-June 2014.MS No. S-2012-282.R1, doi: 10.14359/51686577, was received September 7,

2012, and reviewed under Institute publication policies. Copyright © 2014, American Concrete Institute. All rights reserved, including the making of copies unless permission is obtained from the copyright proprietors. Pertinent discussion including author’s closure, if any, will be published ten months from this journal’s date if the discussion is received within four months of the paper’s print publication.

618 ACI Structural Journal/May-June 2014

concrete columns under monotonic and cyclic loading was carried out by Xue et al.11,12 A type of composite concrete beam, which had precast inverted T-beams and a section of CIP concrete at the beam end, was adopted in the test connections and frames. The test results indicated that this type of precast concrete connections and frames showed the good integrity. Nevertheless, the shortcoming is that more formwork and shoring are needed in the construction process. In this paper, a new type of connections composed of concrete composite beams with precast U-shaped beams and CIP concrete columns was proposed, in which the precast U-shaped beam was directly supported on the column instead of a section of CIP concrete at beam end (refer to References 11 and 12). The improved type of connection offers advantages over the previous one: better construction efficiency due to the less formwork and shoring and smaller deadweight of precast beams because of the U-shaped section adopted in the composite beam.

So far, the new type of connections with composite concrete beams and CIP concrete columns proposed in the paper has been implemented in some pilot projects in China. An experimental program was carried out to study seismic behavior of this type of hybrid beam-column connections subjected to reversed cyclic loading in this paper.

RESEARCH SIGNIFICANCEThe new type of hybrid concrete beam-column connections

proposed in the paper consisted of CIP concrete columns and composite concrete beams with precast U-shaped beams. This study presented experimental data that showed the seismic performance of the proposed beam-column connec-tion was as good as the conventional CIP connection. The results presented in the paper will contribute to the much-needed database of knowledge on the expected performance of precast concrete structures subjected to cyclic loading as well as the instructions for applications of the precast concrete structures.

EXPERIMENTAL PROGRAM

Design basisCompared with the hybrid concrete beam-column connec-

tions11,12 investigated previously, the new type of connec-tion proposed in the paper was composed of CIP concrete columns and composite beams with precast U-shaped beams, in which the precast U-shaped beam was directly supported on the column instead of a section of CIP concrete at the beam end. The hybrid concrete specimens tested were full-scale concrete connections from the bottom story of a proto-type building structure in China, which has a large beam cross section of 13.78 x 25.59 in. (350 x 650 mm), and the column dimensions were 31.50 x 31.50 in. (800 x 800 mm). The prototype was designed according to a strong column-weak beam seismic design philosophy. The joint region of each connection had sufficient strength to prevent the joint shear failure before beam and column yielding. In addition, the specimens were detailed according to the requirements for special moment frames in ACI 318.7

Specimen detailsOf the four specimens, two were hybrid concrete beam-

column connections made with composite concrete beams and CIP concrete columns, and the others were the CIP control specimens. Specimens HIC and HEC represented a hybrid concrete interior connection and a hybrid concrete exterior connection, while Specimens MIC and MEC were the CIP control specimens of HIC and HEC, respectively. The dimensions and reinforcement details of test specimen are given in Fig. 1. The specimens were designed assuming that the slab within an effective width equal to one-third of the span length would be participating in the moment strength. This effective width was defined in the Chinese Code10 as the calculated width of the slab for T-beam flange, which was 33.3% larger than the effective slab width suggested by ACI 318.7

Specimens HIC and HECSpecimens HIC and HEC consisted of composite concrete

beams and CIP columns, and the composite beams were composed of precast U-shaped beams, precast slabs, and CIP slabs. For the interior connection Specimen HIC, butt-welding was used to connect the longitudinal reinforcements of the beams in the connection region, while for the exterior connection Specimen HEC, anchoring bolts were used in the end of the longitudinal reinforcements in the joint region instead of the conventional 90-degree hooked anchorage.

To improve the integrity of hybrid beam-column concrete connections, the following measures were taken: a) a new type of hybrid beam, as shown in Fig. 1(e), was adopted, which was made up of a precast concrete U-shaped beam, precast concrete slabs, and CIP concrete; b) the precast slab was directly supported on the precast concrete U-shaped beam; c) the average roughness amplitude of the precast components was 0.20 in. (5 mm); d) the truss steel rein-forcements, as shown in Fig. 1(e), were placed between the precast concrete slab and CIP concrete; e) the reinforcing bars were used between the precast slab and CIP concrete in beam; and f) for Specimen HIC, the tie bars were placed along the beam above the joint of the precast concrete slabs, as shown in Fig. 2.

Specimens MIC and MECSpecimens MIC and MEC had the same dimensions as

Specimens HIC and HEC and were cast monolithically.

Construction processThe construction sequence of the new type of hybrid

connections was as follows. The precast slabs and U-shaped beams first were prefabricated in the factory. The longitu-dinal bars and transverse hoops of column were tied in the construction site. The precast components prefabricated in the factory were then transported to the construction site, and the precast U-shaped beams were set in position with a temporary brace. The precast slabs were placed on the either side of the beams. After this, the longitudinal reinforcing bars in the CIP topping and stirrups in the joint region were placed on the top of the beam and slab and in the joint core area, respectively. In the end, the concrete was placed in


the column and on the top of the precast slabs. It should be noted that, compared with the hybrid beam-column connec-tions11,12 investigated previously, less shoring and formwork were needed in the construction process of this new type of hybrid connections.

MaterialsSamples representing all sizes of reinforcing bars were

tested in tension to failure. Table 1 summarizes the properties

of the reinforcing bars. The concrete mixture was designed for a cubic compressive strength of 7252 psi (50 MPa) for all specimens. Details of the ingredients in concrete are as follows: cement = 22.16 lb/ft3 (355 kg/m3); sand = 39.33 lb/ft3 (630 kg/m3); aggregate = 72.10 lb/ft3 (1155 kg/m3); water = 9.99 lb/ft3 (160 kg/m3); high-range water-reducing admix-ture = 0.22 lb/ft3 (3.54 kg/m3); and fly ash = 5.49 lb/ft3 (88 kg/m3).

Fig. 1—Specimen geometry and steel details. (Note: Dimensions in mm; 1 mm = 0.0394 in.)


The nominal maximum size of the coarse aggregate was 0.98 in. (25 mm). The mixture had a slump of 1.57 in. (40 mm). The concrete was vibrated when placed to ensure good workability in the mixture. Table 2 lists the material properties of concrete at the time of testing.

Test setup and loading procedureThe adopted geometry of specimens was determined by

the span of beam and column in the prototype structure as well as the loading space and ground anchors in the labora-tory. The boundary conditions are presented in Fig. 3. The column was supported by a pinned connection at its base, and the top of the column was free to move. The beam

end was designed as a roller support. A constant axial load was applied to the column by using a vertical 2248 kip (10,000 kN) hydraulic actuator, which could automatically trace the column top when loading. The axial compressive ratio n was 0.5, representing the vertical load experienced in the prototype building. Herein, the axial compression ratio n, is defined as n = N/(fc · A), where N is the axial load, fc is the axial compressive strength of concrete, and A is the area of column cross section. According to the Chinese Code GB 50011-2010, the relationships between standard cube compressive strength and the axial compressive strength are as follows10: fcu = 0.88αc1αc2 fcu, where fcu is the cubic compressive strength of concrete, αc1 is a conversion factor between axial compressive strength and cube compres-sive strength (αc1 = 0.76 and 0.82, for fcu ≤ 50 MPa and fcu ≥ 80 MPa, when 50 MPa < fcu < 80 MPa, (αc1 = 0.76 + 0.06 × [fcu – 50]/30), αc2 is a reduction factor considering the disadvantage of brittleness of high strength concrete (αc2 = 1.0 and 0.87, for fcu ≤ 40 MPa and fcu ≥ 80 MPa, when 40 MPa < fcu < 80 MPa, αc2 = 0.87 + 0.13 × [80 – fcu]/40). After the application of the column axial load, the lateral load was applied to the column top. The loading history was divided into two phases. The first phase was a load-controlled phase. The second phase was a displacement-controlled phase consisting of cycles of increasing magnitude 0.5% story drift, namely 0.59 in. (15 mm), with three cycles applied at each drift level. The loading history is shown in Fig. 4. The constant axial load was first applied to the top of the column through a vertical 2248 kip (10,000 kN) hydraulic actuator, which could auto-matically trace the column top when loading.

Applied loads and lateral displacements were monitored through load cells and linear variable differential transducers (LVDTs), respectively. LVDTs were mounted on the spec-imens to measure the story drift, joint rotation, beam curva-ture, and slip between precast beam and precast slab and between precast slab and CIP slab. Electrical resistive strain gauges were mounted at critical locations, including the cross sections in the beams and column with larger bending moment. The net displacement at the column top was calcu-lated by subtracting the column base lateral displacement. Top displacement of the column was measured using LVDTs mounted at the level of the hydraulic actuator. Column base displacement was measured at the pinned support.

EXPERIMENTAL RESULTS AND DISCUSSIONAfter the design and construction of the four beam-

column connection specimens, they were subjected to the basic loading history described previously. During the test, the progress of the cracking was recorded, and pictures were taken at each level of story drift. The behavior of the speci-mens was evaluated on the basis of failure pattern, displace-ment ductility, restoring force model, stiffness degradation, energy dissipation, and slip.

Behavior of specimensInterior connections: Specimens HIC and MIC—When

the specimens were tested under the lateral load after the application of the column axial load at the top of the column,

Fig. 2—Schematic diagram of tie bar location in Specimen HIC. (Note: Dimensions in mm; 1 mm = 0.0394 in.)

Table 1—Properties of reinforcing bars

Bar type

Yield strength fy, ksi (MPa)

Ultimate strength fu, ksi (MPa)

Young’s modulus,Es, ksi (× 105 MPa)

Elongation at fracture, %

φ10 52.3 (361) 72.8 (502) 26,100 (1.80) 27.1

φ12 48.4 (334) 76.3 (526) 25,665 (1.77) 28.3

φ14 55.5 (383) 80.9 (558) 27,695 (1.91) 28.6

φ18 54.4 (375) 82.7 (570) 28,855 (1.99) 28.9

φ25 64.4 (444) 92.7 (639) 29,290 (2.02) 24.8

φ32 61.5 (424) 85.7 (591) 29,000 (2.00) 27.5

Notes: Nominal diameter of bars: φ10 = 0.39 in. (10 mm); φ12 = 0.47 in. (12 mm); φ14 = 0.55 in. (14 mm); φ18 = 0.71 in. (18 mm); φ25 = 0.98 in. (25 mm); φ32 = 1.26 in. (32 mm).

Table 2—Material properties of concrete

Specimens

MIC MEC HIC HEC

CIP CIP CIP Pre CIP Pre

Cube strength fcu, ksi (MPa)

9.0 (62.1)

8.7 (60.1)

9.2 (63.6)

8.6 (59.6)

8.8 (60.7)

8.5 (58.3)

Split strength ft, ksi (MPa)

0.64 (4.4)

0.52 (3.6)

0.67 (4.6)

0.55 (3.8)

0.54 (3.7)

0.58 (4.0)

Elastic modulus Ec,ksi (× 104

MPa)

5220(3.6)

5365(3.7)

4930(3.4)

4495(3.1)

5655(3.9)

4930(3.4)

Notes: CIP is cast-in-place concrete; Pre is precast concrete.


for Specimen HIC, a flexural crack appeared first on the tension side of the beam at a distance of 6.3 in. (160 mm) from the column face at the 0.06% story drift level when the applied lateral load was 22.48 kip (100 kN). For Spec-imen MIC, the first hairline flexural crack occurred on the tension side of beam at a distance of 7.9 in. (200 mm) from the column face at the 0.11% story drift level when the applied lateral load reached 26.98 kip (120 kN), which indicated that Specimen MIC possessed the higher cracking resistance than Specimen HIC. The cracks on the beam opened wider as the bending moment level was increased. In the later stage of test, for both Specimens HIC and MIC, as expected, the diagonal and flexural cracks were observed on the beam in the vicinity of the joint, while no visible cracks were observed in the column and the joint region under the reversed cyclic loading. Specimens HIC and MIC reached their yield state defined according to the criteria for equiva-

lent elasto-plastic energy absorption used by Park13 at a drift of 1.07 and of 1.29%, respectively.

At the yield stage, the average strain of the steel bars in the bottom of the beam was 2150 με for Specimen HIC, while it was 2459 με for Specimen MIC. It was indicated that the deformation of Specimen HIC was close to that of the control specimen. Specimens HIC and MIC failed at a story drift of 2.64 and 3.12%, respectively, as the lateral load of both specimens reached the 85% of their lateral peak load. During the test, of the two specimens the maximum strains of the steel bar in the columns and stirrups in the core regions were below the yield strains measured, which indi-cated that the steel bars in the columns and the stirrups in the core regions remained elastic.

Exterior connections: Specimens HEC and MEC—For Specimen HEC, the first hairline flexural crack was observed on the tension side of the beam at a distance 13.0 in. (330 mm) from the column face at the 0.32% story drift level when the applied lateral load reached 35.52 kip (158 kN), while for Specimen MEC, the first hairline flexural crack occurred on the tension side of the beam at a distance of 6.3 in. (160 mm) from the column face at the 0.2% story drift level when the applied lateral load reached 29.67 kip (132 kN). In the later stage of the test, for each specimen, as expected, the diagonal and flexural cracks were observed at the beam-column interface, while no visible cracks were observed in the column and the joint region under the reversed cyclic loading. This was similar to that of Spec-imen HIC and MIC. Specimen HEC reached its yield state at a drift of 0.95%; Specimen MEC reached its yield state

Fig. 3—Boundary conditions. (Note: 1 mm = 0.0394 in.)

Fig. 4—Loading history.


at a drift of 1.13%. The Specimens HEC and MEC failed at a story drift of 2.04 and 2.21%, respectively, as the lateral load of both specimens reached the 85% of their lateral peak load. At the yield phase, the average strain of the steel bars in the bottom of the beam was 2058 με for Specimen HEC, while it was 2380 με for Specimen MEC. It was indicated that the deformation of Specimen HEC was close to that of the control specimen. In the whole loading process, the steel bars in the columns and the stirrups in the core regions remained elastic.

Failure patternThe failure pattern of the four specimens, as expected,

involved the formation of a plastic hinge in the beam at the column face. The formation of plastic hinges caused severe cracking of the concrete near the beam end of each spec-imen. No cracks, however, were observed on the column and the joint region. In other words, the precast beam-column connections exhibited a strong column-weak beam failure mechanism. There were also some prominent shear cracks in the hinge region, mainly due to the large depth of the beam. At the end of the test, concrete damage was visible in the beam near the face of column, particularly in the beam plastic hinge zone, and the reinforcements fractured at the bottom of the beam end near the column face. This could be due to the fact that the neutral axis of beams moved upward owing to the presence of the concrete slab, which caused the large strain demand for bottom longitudinal bars.

It should be noted that there were no horizontal cracks observed between CIP concrete and precast concrete in the failure cross section in hybrid concrete connections until the lateral load reached the peak load, and in the later stage of the test, the horizontal cracks occurred in the interface

between the CIP concrete and precast concrete in the slabs. It could be seen that the measures, that is, roughing the contact surface and having truss steel reinforcements in precast concrete slabs, were effective for the integrity of the hybrid beam. Figure 5 shows the final failure patterns observed at the end of testing.

Hysteresis characteristicsThe lateral load-versus-drift hysteresis curves of all spec-

imens are given in Fig. 6. The envelope curves are plotted in Fig. 7. The hysteresis loops of Specimens HIC and HEC were quite similar to those of their control specimens. The reason lies in the few differences with seismic performance between the precast concrete specimens and the monolithic specimens. This behavior is considered to be an indication of satisfactory performance. It can be seen that the areas of the hysteresis loops became larger with increasing drift. The curves exhibited slight pinching, as well as some stiffness and strength degradation, during same-drift repeat cycles, which is mainly attributed to bond degradation between concrete and reinforcement, concrete cracking, reinforce-ment yielding, or a combination of these. The results showed that the yield load, peak load, and corresponding drift of Specimens HIC and HEC were very close to those of its control specimen. For the exterior connections, the hyster-esis curves were not symmetrical due to the presence of the concrete slab.

Displacement ductilityThe lateral load versus displacement envelope curve

was used to define the yield and ultimate displacements according to the criteria for equivalent elasto-plastic energy absorption used by Park.13 As shown in Fig. 8, when area

Fig. 5—Crack patterns and failure modes: (a) Specimen MIC; (b) Specimen HIC; (c) Specimen MEC; and (d) Specimen HEC.


S1 is equal to area S2, the position of Point C is determined. The line CG, perpendicular to the transverse axis, intersects the P-Δ curve at Point E. The displacement corresponding to Point E is used as Δy, the yield displacement. The ultimate displacement Δu was determined as corresponding to a 15% drop of the lateral peak load. The displacement ductility for all the specimens is listed in Table 3. In this table, Δcr was the displacement corresponding to the lateral cracking load, and Δmax was the displacement corresponding to the lateral peak load. The monolithic Specimen MIC exhib-ited excellent ductile manner. Take the positive direction, for example. The measured yield displacement was 1.52 in. (38.7 mm) at a lateral load of 190.90 kip (849.2 kN). The ultimate displacement was recorded at 3.68 in. (93.5 mm), for a ductility factor of 2.42. The load attained at this ductility level was 189.30 kip (842.1 kN), or 0.99 times the measured yield load. According to the results in Table 3, the following conclusions could be drawn. The displacement ductility factor of Specimen MIC was 2.42 to 2.55, of Spec-

imen HIC was 2.47 to 2.57, of Specimen MEC was 1.95 to 2.16, and of Specimen HEC was 2.05 to 2.14. These results indicated that both the hybrid concrete connections behaved comparably under the cyclic loading. It should be noted that the displacement ductility of the specimens was deter-mined by the deformation of the beam end at the column face due to no damage in the columns and the joint regions, and the beam end had shear deformation as well as flexure deformation, which lessened the displacement ductility of the specimens.

Stiffness degradationThe secant stiffness is used to evaluate stiffness degrada-

tion at each drift level. The secant stiffness is calculated on the last cycle for each successive story drift level according to the method used by Soubra.14 The secant stiffness was defined as the slope of the straight line connecting the maximum drift levels of that specific load cycle. The secant stiffness is plotted against drift in Fig. 9. The stiffness of each

Fig. 6—Hysteresis curves.

Fig. 7—Envelopes of hysteresis loops.


specimen continuously decreased as story drift increased and was close to zero at the end of test, as shown in Fig. 9. The stiffness degradation trend of the hybrid concrete connec-tions was very similar to that of their control specimens. The stiffness of each of the hybrid concrete connections was almost equal to that of its corresponding control spec-imen. Stiffness degraded rapidly before the drift of 1% for all specimens, which was probably because most concrete cracking and reinforcement yielding occurred in this stage. For hybrid connections, the connection details had a small effect on its initial stiffness, however, there was no effect on its latter stiffness.

Restoring force modelBy analysis of P-Δ hysteresis curves, skeleton curves and

characteristic loads of the four connections, a four-linear restoring force model for test specimens was proposed, as shown in Fig. 10. The normalized characteristic parameters for the P-Δ hysteretic model are listed in Table 4, and in this table, Pcr, Py, Pmax, and Pu represented cracking load, yielding load, peak load, and failure load, respectively; Δcr, Δy, Δmax, and Δu were column-tip lateral displacement corre-sponding to these loads, respectively. In Fig. 10, “+” and “–” at the top right corner of letters denoted the characteristic values in positive and negative directions, respectively.

Main hysteretic rules were expressed as follows: a) the envelopes of the test specimens were simplified into a four-fold line in the positive direction and negative direction, and the descending branch was taken into account. The char-acteristic points were cracking point, yielding point, peak point, and failure point; b) before beam end cracking, the initial stiffness K1 (K1 = Pcr/Δcr), was taken as the loading stiffness, and stiffness degradation and residual deformation were not taken into account during unloading, and reloading rules in the negative direction were that the curves directly pointed to cracking point in negative direction; c) during the stage between the cracking point and yield point, K2 (K2 = (Py – Pcr)/(Δy – Δcr)), was taken as loading stiffness, and stiffness degradation and residual deformation were considered; d) post-yielding stiffness K3 (K3 = (Pmax – Py)/(Δmax – Δy)), was defined as loading stiffness after yield point, and loading stiffness K4 (K4=(Pu – Pmax)/(Δu – Δmax)) became negative stiffness after ultimate load point; e) the initial stiff-ness, K1, was taken as unloading stiffness by reduction factor β. Herein, β = (Δy/Δm)ν, where Δy was yield displacement, and Δm was the maximum displacement experienced, and ν was regressed from the test results; and f) reloading rules in negative (positive) direction after post-cracking unloading were that the curves directly pointed to pivot pinching point, J (K), the ordinates of which were listed in Table 4, in positive (negative) direction then pointed to the maximum previous displacement point, and then took K2, K3, or K4 as the loading stiffness pointing to the skeleton curve.

Energy dissipation capacityThe energy dissipation capacity of beam-column connec-

tions is a function of the area under the load-displacement

Table 3—Displacement ductility values of all specimens

SpecimenΔcr, in. (mm)

Δy, in. (mm)

Δmax, in. (mm)

Δu, in.(mm) Δu/Δy

MICPOS 0.13 (3.37) 1.51 (38.74) 2.35 (60.32) 3.65 (93.53) 2.41

NEG — 1.43 (36.60) 2.36 (60.39) 3.64 (93.37) 2.55

HICPOS 0.07 (1.85) 1.25 (32.04) 1.76 (45.16) 3.09 (79.16) 2.47

NEG — 1.21 (31.00) 1.76 (45.01) 3.11 (79.77) 2.57

MECPOS — 1.32 (33.84) 1.76 (45.01) 2.58 (66.15) 1.95

NEG 0.03 (0.65) 1.22 (31.41) 2.40 (61.61) 2.65 (67.98) 2.16

HECPOS 0.37 (9.46) 1.11 (28.40) 1.83 (46.82) 2.39 (61.23) 2.16

NEG 0.02 (0.63) 0.82 (21.00) 2.38 (60.99) 2.73 (70.01) 3.33

Notes: POS is positive direction; NEG is negative direction.

Fig. 9—Stiffness degradation.

Fig. 8—Method used to define yield and ultimate displacements.


curve, and indicates the degree of effectiveness of the connection to withstand earthquake loading. The cumulative energy dissipation is the commonly used method to calcu-late the energy dissipated during the loading.15 The energy dissipated per cycle is defined as the area enclosed by the load-displacement curve. The cumulative energy dissipated at each of the drift levels in the test is given in Fig. 11. Generally, both the hybrid concrete connections and their control specimens demonstrated an almost similar pattern of energy dissipation, namely, energy dissipation increased with the increasing story drift. During the first loading cycle, the dissipated energy was very small, showing that the test specimen exhibited elastic behavior. At a drift of 3%, the cumulative energy dissipated by Specimen MIC was 14.3% lower than that of Specimen HIC. This was also the drift at which Specimen MIC failed. The precast Specimen HIC did not fail at this drift, and underwent additional cycles until failure occurred. The cumulative energy dissipated by Spec-imen MEC was only 2.1% lower than that of Specimen HEC at a drift of 2%, at which the Specimen MEC failed. The energy dissipation capacity of the interior connection was obviously higher than that of the exterior connection. This

might be because there was only one beam in the exterior connection and two in the interior connection. The energy dissipation of the test specimens was mainly in the beam end hinging.

SlipThe slip between precast beam and precast slab and the

slip between precast slab and CIP slab were measured during the test of both the hybrid concrete connections. The slip at a particular drift level is presented in Table 5. It was shown that both the slip between precast beam and precast slab and the slip between precast slab and CIP slab had an increasing trend with drift level increasing. At yield state, the slip between precast slab and CIP slab in posi-tive and negative directions was 0.00254 and 0.00924 in. (0.065 and 0.237 mm), respectively, in Specimen HIC; and was 0.0094 and 0.00956 in. (0.241 and 0.245 mm), respec-tively, in Specimen HEC. This showed that the slip between precast slab and CIP slab was small, and those in the exterior connection were larger than those in the interior connection. The slip between precast beam and precast slab in posi-tive and negative directions was 0.00511 and 0.01026 in. (0.131 and 0.263 mm), respectively, in Specimen HIC; and was 0.00971 and 0.00402 in. (0.249 and 0.103 mm), respectively, in Specimen HEC, when the yield state was reached. These slips, in general, are small, indicating that the measures to improve the integrity of cross section and prevent the shear failure between precast slab and CIP slab were very effective.

CONCLUSIONSBased on the cyclic loading tests of hybrid concrete beam-

column connections and their CIP control specimens, the following conclusions can be drawn from the study.

1. Each specimen, as expected, developed plastic hinge in the vicinity of the beam-column interface without damage in the column and joint region, and exhibited a strong

Fig. 10—Restoring force model.

Table 4—Normalized characteristic parameters for hysteretic model

Specimen

MIC MEC HIC HEC

POS NEG POS NEG POS NEG POS NEG

Pmax 1.00 1.00 1.00 1.00 1.00 1.00 1.00 1.00

Py 0.92 0.83 0.85 0.86 0.78 0.71 0.87 0.83

Pcr 0.13 — — 0.18 0.12 — 0.45 —

Pu 0.91 0.72 0.85 0.85 0.82 0.79 0.85 0.85

Δmax 1.00 1.00 1.00 1.00 1.00 1.00 1.00 1.00

Δy 0.64 0.61 0.75 0.51 0.71 0.69 0.61 0.34

Δcr 0.06 — — 0.01 0.04 — 0.20 —

Δu 1.55 1.55 1.47 1.10 1.75 1.76 1.31 1.15

γ

2Δ –0.58 –0.22 –0.05 0.00 –0.08 –0.06 0.07 1.19

3Δ 0.25 0.23 0.11 0.14 0.31 0.34 0.62 1.14

4Δ 0.53 0.44 0.20 0.24 0.45 0.48 0.63 1.12

5Δ 0.61 0.53 0.22 0.25 0.51 0.53 0.43 0.98

6Δ 0.63 0.55 — — 0.53 0.55 — —


column-weak beam failure mechanism. The design intention was achieved;

2. Both Specimens HIC and HEC with composite beams and CIP columns exhibited a stable lateral load versus drift hysteretic response. The hysteresis curve of each of the hybrid concrete connections was quite similar to that of its corresponding control specimen;

3. The displacement ductility of Specimens HIC and HEC was more than 2, and very similar to that of the corresponding control Specimens MIC and MEC, indicating that both hybrid concrete connections behaved in a ductile manner;

4. For both hybrid concrete connections and their control specimens, the trend of stiffness degradation is similar. Although there are obvious differences in details between hybrid concrete connections and their control specimens, all specimens exhibited a similar energy dissipation capacity. The energy dissipating capacity of each hybrid concrete connection was almost identical to its control specimen; and

5. Both the slips in the composite beams were very little, showing that the measures to improve the integrity of cross section and prevent the shear failure between precast slab and CIP slab were very effective.

By comparison of load-carrying capacity, displacement ductility, and energy dissipation between hybrid concrete connections with composite beams and CIP columns and their control specimens, it could be concluded that the seismic behavior of hybrid concrete connections with composite beams and CIP columns were similar to those of the respective control specimens. The results of this inves-

tigation could enrich the data available that document the behavior of hybrid concrete moment-resisting frames, and contributed to enlarge the application of the hybrid concrete structures in seismic zones. The connection details have been adopted in Chinese code.

To fully understand the seismic behavior of the buildings with the connection details proposed herein, it is necessary to perform a large-scale, pseudo-dynamic test and propose design guidelines in the future.

ACKNOWLEDGMENTSThe authors gratefully acknowledge the financial support provided by

the Key Project of Scientific Supporting Plan in the Chinese “Twelfth Five-Year” Period (No. 2010BAK69B28), the program for New Century Excel-lent Talents in University (No. NCET-10-0636), the Fundamental Research Funds for the Central Universities, and the project of Shanghai Science and Technology Commission (No. 10dz0583700).

AUTHOR BIOSWeichen Xue is a Professor of structural engineering at Tongji Univer-sity, Shanghai, China. His research interests include precast, prestressed concrete structures; composite structures; and fiber-reinforced polymer used in concrete structures.

Bin Zhang is a PhD Candidate in the Department of Building Engi-neering at Tongji University. His research interests include precast concrete structures.

NOTATIONPcr = cracking loadPmax = peak loadPu = 0.85Pmax

Py = yield loadΔc = displacement corresponding to Py

Δcr = displacement corresponding to Pcr

Table 5—Slip in hybrid concrete connections

Drift, % 0.5 1.0 1.5 2.0 2.5 3.0 3.5 Yielding

Slip 1× 10–3 in.(mm)

HICPOS 0.16 (0.004) 1.99 (0.051) 3.00 (0.077) 5.27 (0.135) 4.21 (0.108) 2.85 (0.073) 14.86 (0.381) 2.54 (0.065)

NEG 2.30 (0.059) 6.40 (0.164) 9.83 (0.252) 19.89 (0.510) 20.94 (0.537) 34.20 (0.877) 56.43 (1.447) 9.24 (0.237)

HECPOS 2.46 (0.063) 11.39 (0.292) 29.60 (0.759) 29.41 (0.754) 41.93 (1.075) — — 9.40 (0.241)

NEG 2.61 (0.067) 7.88 (0.202) 19.58 (0.502) 37.32 (0.957) 26.83 (0.688) — — 9.56 (0.245)

Slip 2× 10–3 in.(mm)

HICPOS 1.37 (0.035) 4.45 (0.114) 5.89 (0.151) 8.15 (0.209) 45.44 (1.165) 33.38 (0.856) 30.19 (0.774) 5.11 (0.131)

NEG 1.21 (0.031) 8.54 (0.219) 27.11 (0.695) 90.95 (2.332) 157.56 (4.040) 208.73 (5.352) 92.04 (2.360) 10.26 (0.263)

HECPOS 0.16 (0.004) 10.80 (0.277) 20.36 (0.522) 23.13 (0.593) 41.34 (1.060) — — 9.71 (0.249)

NEG 4.49 (0.115) 4.95 (0.127) 10.96 (0.281) 18.68 (0.479) 34.24 (0.878) — — 4.02 (0.103)

Notes: Slip 1 is slip between precast slab and CIP slab; Slip 2 is slip between precast slab and precast beam; yielding is test specimen has reached its yield state; POS is positive direction; NEG is negative direction.

Fig. 11—Cumulative energy dissipation.


Δmax = displacement corresponding to Pmax

Δu = displacement corresponding to 0.85Pmax

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NOTES:

Documents

Seismic Behavior of Hybrid Concrete Beam-Column Connections with Composite Beams and Cast-in-Place Columns