High Performance Fiber Reinforced Cement Composites (HPFRCC7), Stuttgart, Germany – June 1-3, 2015 

409

SEISMIC PERFORMANCE OF FULL-SCALE HIGH-PERFORMANCE FIBER-REINFORCED (HPFRC) SPECIAL MOMENT FRAME SLAB-BEAM-COLUMN SUBASSEMBLAGE USING JOINTS AS THE MAJOR ENERGY DISSIPATION SOURCE

Y.-J. Choi and S.-H. Chao

University of Texas at Arlington, USA

Abstract This paper presents a study on the seismic performance of reinforced concrete (RC)

perimeter special moment frames (SMFs) which have high-performance fiber-reinforced concrete (HPFRC) in beam plastic hinge and joint regions. This research mainly focused on evaluating the feasibility of utilizing the HPFRC joints as major sources of energy dissipation. A full-scale HPFRC slab-beam-column (SBC) subassemblage with a weak joint was tested under large displacement reversals. This specimen eliminated the majority of steel hoops and ties in the beam plastic hinge and joint regions. A counterpart specimen made of conventional concrete was designed based on ACI 318-11 and tested under the same loading protocol. Both specimens showed stable hysteretic responses up to 3.5% column drift ratio without significant strength degradation, which meets collapse prevention structural performance according to ACI committee 374 (ACI 374 Committee, 2013). The response of the HPFRC specimen was controlled by joint shear mechanism, whereas the conventional RC specimen responded in a flexure mode forming plastic hinges at the end of the beams. This paper also compares the seismic performance of the HPFRC specimen with that of the conventional RC specimen by delineating the lateral force versus column drift ratio, damage state, joint distortion, and dissipated energy.

1. INTRODUCTION

Modern reinforced concrete (RC) buildings often use perimeter special moment frames (SMFs) to resist seismic force together with an interior post-tensioned slab-column system to carry the gravity loads. Since only a few moment frames are used to resist the lateral forces and provide lateral stiffness, the spandrel beams and columns in the perimeter frames are very large. In addition, for high-rise buildings, closely spaced columns (6 m or less apart) are used to maintain high lateral stiffness. However, due to the large bending moment of the beams developed over the relatively short span, large shear force can be generated, which may lead to early degradation in energy dissipation in the plastic hinge regions of the beams.

High Performance Fiber Reinforced Cement Composites (HPFRCC7), Stuttgart, Germany – June 1-3, 2015 

 

410

Recent research in steel SMFs aims at utilizing beam-column joints (panel zone) as major energy-dissipating components together with the beams to prevent concentrated damage in beams. This is achieved because the shear response of the steel panel zone shows highly ductile behavior [1]. This “balanced” design concept has been advocated for steel SMFs, so that high-level ductility can be obtained [2]. Unlike the steel SMFs, however, beam-column joints of RC moment frames are conventionally designed not to be the major energy-dissipating components so as to remain elastic during moderate earthquake events. This is because the response of beam-column joints is controlled by shear and bond mechanisms, both of which exhibit poor hysteretic properties when conventional reinforced concrete material is used [3]. On the other hand, high-performance fiber-reinforced concrete (HPFRC) materials provide very high shear and bond strength as well as tensile ductility when compared to conventional concrete; hence, HPFRC beam-column joints can be potentially used as major sources of energy dissipation.

Another advantage of using the HPFRC beam-column joints is to remove undue construction difficulties associated with conventional steel confining reinforcements. Previous research has demonstrated that fiber-reinforced concrete (FRC) can be an alternative to conventional steel confining reinforcements [4, 5].

This paper evaluates the feasibility of utilizing the HPFRC joints as major sources of energy- dissipating components in modern high-rise perimeter SMFs by comparing the seismic performance of the HPFRC specimen with that of the conventional RC specimen.

2. EXPERIMENTAL PROGRAM

2.1 Materials Details of HPFRC composition and fiber properties are given in Tables 1 and 2,

respectively. A hybrid fiber concrete mixture with long and short steel twisted fibers was used, and the volume fractions of the fibers were 1.2% and 0.5%, respectively. Figure 1 shows the tensile responses from three dog-bone-shaped tensile specimens prepared together with the HPFRC slab-beam-column (SBC) specimen. The cross section of the specimens was 102 mm (4 in.) by 102 mm (4 in.). Although the strain at peak strength varied from 0.26%~0.57%, the tendency of strength degradation was similar for all specimens. Average compressive strength of the concrete used in the plastic hinge and joint regions of the HPFRC and conventional RC specimen was 46 MPa and 50 MPa, respectively.

Table 1: Relative composition of concrete mixture by weight and compressive strength

Cement (Type ш)

Fly Ash*

Sand** Coarse

Aggregate † Super-

plasticizer Water Steel Fiber 

(MPa)

1 0.5 2.3 1 0.00077 0.55 0.316 46 *Class C; **Fine aggregate; † Maximum size of 9.5 mm

Table 2: Properties of steel fibers

Fiber Type Equivalent Diameter (mm), Length (mm), Tensile Strength

(MPa), Long Twisted 0.5 50 2450 Short Twisted 0.5 14 2450

High Performance Fiber Reinforced Cement Composites (HPFRCC7), Stuttgart, Germany – June 1-3, 2015 

411

Con

cret

eT

ensi

leS

tres

s(M

Pa)

102

mm

 

Figure 1: Tensile tests on HPFRC dog-bone-shaped specimens

2.2 Design of specimens The test subassemblages, designated as RC-SP1 and HPFRC-SP2, consisted of a column

(depth: 1067 mm × width: 813 mm), two beams (1067 mm × 813 mm) which framed into the column, and a post-tensioned slab (203 mm × 1321 mm) on only one side. Reinforcement details of the test specimens are shown in Figure 2.

Specimen RC-SP1 was designed in accordance with ACI 318-11 [6] and ACI 352-02 recommendation [7]. Longitudinal reinforcement ratio for the beams was 1.24%. Since the width of the beams and column was the same, the beam longitudinal bars, which were supposed to be located at each corner of beam transverse reinforcements (confining reinforcement), were moved about 70 mm toward the center of the beam section so as to avoid conflict with column longitudinal bars in the joint. No. 5 bars (construction bars) were then placed at the corners to support the beam transverse reinforcements, and additional six No. 5 bars (skin reinforcement) were evenly distributed along the beam depth to control crack width. These No. 5 bars were terminated immediately before the joint in order not to increase joint shear demand. The above arrangement is a common practice in the U.S. The sum of nominal flexure moment strength of the column was 2.17 times that of the beam, and the strength of the joint was greater than the maximum ultimate demand from the adjacent beams. The column depth was 27 times the diameter of the beam longitudinal bar. Post-tensioning force of 160 kN (36 kips) per rod was applied to the slab in direction parallel to the frame and 133 kN (30 kips) perpendicular to the frame, so that prestress in the slab was 0.86 MPa (125 psi), which is the minimum average prestress specified by ACI 318 for two-way slabs [6].

Specimen HPFRC-SP2 was designed in the same manner as the RC-SP1 except: 1) HPFRC was used in the beam plastic hinge and joint regions (Figure 2); 2) To utilize the inelastic shear deformation of the joint, the design was done such that the shear demand in the joint of the HPFRC-SP2 was the same as that of the design shear strength. This was done by extending the construction and skin reinforcements through the joint, thereby increasing joint shear demand; 3) The majority of the steel hoops and ties in the beam plastic hinge and joint regions were eliminated. The amount of the conventional confining steel reinforcement for the beam plastic hinge and joint regions in the HPFRC-SP2 was 29% and 15% of that used in the RC-SP1, respectively.

High Performance Fiber Reinforced Cement Composites (HPFRCC7), Stuttgart, Germany – June 1-3, 2015 

 

412

1622

584

1829

# 5

@ 1

14#

5 @

114 A

A

1067

# 5

@ 1

52#

5 @

152

1067

BB

AA

Figure 2: Reinforcement details of test specimens

2.3 Construction and test setup The upper column was spliced by mechanical splices at 559 mm (22 in.) above the top of

the beams, which allowed constructing the test specimens at two different places. The lower column, slab, and beams were constructed at the civil engineering lab of the University of Texas at Arlington, and the upper column and top block were built at the MAST lab of the University of Minnesota (Figure 3). As shown in Figure 3(b) and (c), four ancillary actuators were attached to the specimen by load transfer beams. The actuators allowed the beams to move horizontally and to rotate about z axis (direction perpendicular to the specimen), while vertical movements were restrained. The lower end of the column was attached to a clevis, which simulates an inflection point of the column. Lateral support was provided at the bottom of the column. The top concrete block was connected to the six-degree of freedom crosshead, which imposed lateral and axial loads on the specimens. The lateral load was applied based on the loading protocol shown in Figure 3(d), while the axial load was held vertically and constant throughout the testing. This load represented 10 percent of the product of the gross column area and the nominal concrete comprehensive strength.

High Performance Fiber Reinforced Cement Composites (HPFRCC7), Stuttgart, Germany – June 1-3, 2015 

413

3. EXPERIMENTAL RESULT

3.1 Overall response The story shear-column drift ratio response of the test specimens is shown in Figure 4(a).

Both specimens showed stable hysteretic responses up to 3.5% column drift ratio without significant strength degradation, which meets the collapse prevention structural performance according to ACI Committee 374 criterion [8].

          (a) Construction at civil eng. lab of UT-Arlington (b) Test setup at MAST lab of UM-twin cities

(c)  Test setup (d) Loading protocol

Figure 3: Construction, test setup, and loading protocol

In RC-SP1, the damage concentrated in the plastic hinges of the beams, whereas the response of the HPFRC-SP2 was controlled by joint shear mechanism (Figures 5 and 6). All longitudinal bars in the beams of the RC-SP1 yielded at 0.75% column drift ratio, and the yielding of the bars spread from the beam-column interface into the beam approximately one effective beam depth. On the other hand, yielding of beam longitudinal bars in HPFRC-SP2 commenced at 1.75% column drift ratio, spreading only half of effective beam depth from the interface. For both specimens strength degradation began after 2.75% column drift. In RC-SP1 it was due to the concrete spalling and significant buckling of beam longitudinal bars, while wide opening of the diagonal cracks in the joint led to the degradation in HPFRC-SP2. The strength at the first cycle of 3.5% column drift ratio was 95% and 87% of the peak strengths for the RC-SP1 and HPFRC-SP2, respectively.

High Performance Fiber Reinforced Cement Composites (HPFRCC7), Stuttgart, Germany – June 1-3, 2015 

 

414

Figure 4(b) shows the joint shear-distortion responses. The joint shear was calculated by dividing beam end moments by a moment arm equal to 7/8 of beam’s effective depth. The measured joint strength of the RC-SP1 was 95% of the nominal joint strength [6]. Through the RC-SP1 test, many diagonal cracks and some concrete spalling (see Figure 6a) were observed in the joint. However, the joint response was satisfactory. The measured joint strength of the HPFRC-SP2 was similar to the nominal joint strength [6].

  (a) Story shear vs. column drift ratio (b) Joint shear vs. joint shear distortion

Figure 4: Seismic responses of test specimens

              (a) RC-SP1 (cracks at beam-column interface) (b) HPFRC-SP2 (no cracks at beam-column interface)

Figure 5: Specimen damage at 1.75% column drift ratio

               (a) RC-SP1 (b) HPFRC-SP2

Figure 6: Specimen damage at 3.5% column drift ratio

A few visible diagonal cracks and lots of fine cracks around the diagonal crack were observed in the joint at 1.75% column drift, near where all the steel confining reinforcements

15

High Performance Fiber Reinforced Cement Composites (HPFRCC7), Stuttgart, Germany – June 1-3, 2015 

415

in the joint yielded. There was no noticeable damage in the joint at this drift level.It should be noted that although the joint distortion of HPFRC-SP2 at 1.75% column drift ratio was greater than that of the RC-SP1 at 3.5% column drift ratio, the joint of the RC-SP1 had more severe damage.

The width of the diagonal cracks in the joint of HPFRC-SP2 widened rapidly after 1.75% column drift ratio, and the joint eventually yielded during the first positive cycle at 2.75% column drift ratio. During this drift level, the response of the specimen was entirely controlled by joint shear mechanism. All confining reinforcement in the joint yielded, and the joint showed ductile behavior without significant strength degradation up to 3.5% column drift ratio.

In RC-SP1, concrete spalling and the buckling of beam longitudinal bars were observed at the bottom of the east beam during positive loading at 1.75% column drift ratio. The first vertical flexural crack of the RC-SP1 was observed at the beam-column interface during positive loading at 0.2% column drift, and this crack joined with another similar vertical crack that occurred during negative loading at 0.35% column drift. The crack became wide enough at 1.75% column drift ratio (Figure 5) to eventually separate the beams from the joint at 2.75% column drift ratio. In contrast, up to 1.75% column drift ratio the HPFRC- SP2 did not show any sign of the beam longitudinal bars buckling nor was there noticeable damage in the beams and joint.

3.2 Dissipated energy Figure 7 shows all the components contributing to the accumulated dissipated energy at

each drift level. The total dissipated energy in the RC-SP1 at the completion of the first cycle at 3.5% column drift ratio was 1.36 times that in the HPFRC-SP2. Once the column drift ratio reached 1.75%, both specimens dissipated a similar amount of energy. At this column drift level, RC-SP1 was more damaged than the HPRC-SP2 due to the buckling of the beams’ longitudinal bars and large vertical cracks at the beam-column interface. This is because the RC-SP1 dissipated most of energy in the beams, while the beams and joint of the HPFRC-SP2 nearly equally dissipated the energy, balancing damage between the beams and the joint. After 1.75% column drift ratio, as the joint distortion in the HPFR-SP2 increased, the dissipated energy in the joint surpassed that in the beams. Since the column was designed to behave in elastic range, the dissipated energy in the column was much smaller than that in the other components.

       (a) RC-SP1 (b) HPFRC-SP2

Figure 7: Accumulative dissipated energy of tests specimens

High Performance Fiber Reinforced Cement Composites (HPFRCC7), Stuttgart, Germany – June 1-3, 2015 

 

416

4. CONCLUSIONS

1. The HPFRC-SP2 eliminated undue construction difficulties associated with the conventional steel confining reinforcements in the beam plastic hinge and joint regions.

2. Compared to RC-SP1, there was no noticeable damage in HPFRC-SP2 up to 1.75% column drift ratio. This can eliminate the need for repair after a moderate earthquake event.

3. Although the response of the HPFRC-SP2 was controlled by joint shear mechanism, the HPFRC specimen showed ample ductile behavior up to 3.5% column drift ratio, which meets the collapse prevention structural performance requirement according to ACI Committee 374 [8]. The joint shear mechanism delays the yielding commencement of the beams (starting approximately from 1.75% column drift ratio). This research proves the feasibility of utilizing ductile HPFRC joint to dissipate seismic energy, thus balancing the damage between the joint and beams.

ACKNOWLEDGEMENTS

This research was sponsored by the U.S. National Science Foundation under award No. CMMI-1041633. Steel fibers were donated by TaiGu-NewFibers Technology in China and the couplers and threaded bars were donated from ERICO and CMC. Their support is gratefully acknowledged. The authors would also like to acknowledge the assistance of the staff at MAST Laboratory.

REFERENCES

[1] Krawinkler, H., 'Shear in beam-column joints in seismic design of steel frames', Engineering Journal. AISC. 15(3) (1978) 82-91.

[2] Shin, S. and Engelhardt, M. D., 'Cyclic Performance of Deep Column Moment Frames with Weak Panel Zones', Proceedings of a World Congress, Jeju, Korea, September, 2013 (Advanced in Structural Engineering and Mechanics, 2013)

[3] Paulay, T. and Priestley, M. J. N., 'Seismic Design of Reinforced Concrete and Masonry Buildings', 1nd Edn (John Wiley & Sons, Inc,. New York, 1992).

[4] Filiatrault, A., Pineau, S. and Houde, J., 'Seismic Behavior of Steel-Fiber Reinforced Concrete Interior Beam-Column Joints', Structural Journal. ACI. 92(5) (1995) 543-552.

[5] Parra-Montesinos, G.J., Peterfreund, S. W., and Chao, S.-H., 'Highly Damage Tolerant Beam-Column Joints Through the Use of High-Performance Fiber Reinforced Cement Composites', Structural Journal. ACI. 102(3) (2005) 487-495.

[6] ACI Committee 318, 'Building Code Requirements for Structural Concrete (ACI 318-11) and Commentary (318R-11)', American Concrete Institute (Farmington Hills, Mich., 2011).

[7] Joint ACI-ASCE Committee 352, 'Recommendations for Design of Beam-Column Connections in Monolithic Reinforced Concrete Structures (ACI 352R-02)', American Concrete Institute (Farmington Hills, Mich., 2002).

[8] ACI Committee 374, 'Guide for Testing Reinforced Concrete Structural Elements under Slowly Applied Simulated Seismic Loads (ACI 374.2R-13)', American Concrete Institute (Farmington Hills, Mich., 2013).