Dynamic collapse test on eccentric reinforced concrete structures with and without seismic retrofit Yousok Kim a,⇑ , Toshimi Kabeyasawa a , Shunichi Igarashi b a Earthquake Research Institute, University of Tokyo, 1-1-1 Yayoi Bunkyo-Ku, Tokyo 113-0032, Japan b Structural Quality Assurance Inc., 2-7-10 Samikicho Chioda-Ku, Tokyo 101-0061, Japan article info Article history: Received 31 May 2010 Revised 5 September 2011 Accepted 6 September 2011 Available online 4 November 2011 Keywords: Reinforced concrete Torsional response Seismic retrofit Shear failure Axial collapse Shake table abstract Reconnaissance reports on structures damaged or collapsed by severe earthquakes have revealed several common characteristics in their structural members and systems, such as insufficient reinforcement details in beam-column joints and transverse confinements, low aspect ratios, soft and or weak stories, and eccentric plans. Dynamic tests were carried out to investigate the collapse process of reinforced con- crete structures that had seismically deficient reinforcement details (light transverse reinforcement) and seismic systems (soft/weak stories and eccentric plans). A comparison of collapse behaviors with and without seismic retrofits also verified the effectiveness of the SRF (super reinforced with flexibility) strengthening method, which was developed to prevent the loss of axial load carrying capacity even at excessive lateral deformation. The columns of one specimen were strengthened with polyester fiber belts and its shear walls with polyester fiber sheets, while the members of the other were not. Each specimen was designed following old (1970s) reinforcement detail practice in Japan, and is a one-third-scale rein- forced concrete structure with considerable stiffness and strength eccentricity in the first story. The spec- imens were composed of independent column frame and shear wall frame. Torsional response resulting from the eccentricity in the 1st story induced a displacement concentration on the weak frame, and even- tually the independent columns of the RC specimen failed in shear and lost their axial load carrying capacity. On the other hand, the SRF specimen survived not only an identical earthquake load to the one that caused the RC specimen to fail, but also three additional earthquake loads, although significant strength deterioration and considerable lateral and vertical deformation were generated at the end of the test. The following conclusions were drawn from the comparison of the two specimens’ responses: axial column collapse cannot be predicted from vertical responses since the vertical behavior of bare RC col- umns was not discernibly different from that of SRF columns until axial collapse was initiated, and the SRF strengthening method is effective in confining the column and preventing the cracking progress, thus modifying the failure mode of the RC columns from brittle shear failure after flexural yield to flexural dominant behavior. Ó 2011 Elsevier Ltd. All rights reserved. 1. Introduction Collapsing buildings have caused numerous casualties during recent major earthquakes, even in countries with advanced earthquake engineering technologies. Understanding the collapse mechanisms of reinforced concrete (RC) structures subjected to earthquake attack is a crucial aspect of one of the goals of earth- quake engineering, which is to protect people’s lives and safety, and has been a challenging task in many researches [1–3]. Many RC structures have suffered severe damage in devastating earth- quakes in the past, leading to unrecoverable damages and the collapse of structures. Reconnaissance reports describing these damaged or collapsed structures have revealed several common characteristics in their structural members such as insufficient reinforcement details in beam-column joints, transverse confine- ment designed following old seismic codes, and low aspect ratio (shear span to depth ratio) members that are susceptible to shear failure [4–7]. In addition to these seismic element deficiencies, the unbalanced layout of structural members in elevation or plan (i.e. buildings with soft/weak stories or eccentric plans) caused poor seismic performance leading buildings to collapse. Although research themes in earthquake engineering are turn- ing to innovative technologies for new structures, it is still impor- tant to continue developing economical retrofitting methods for existing buildings that have seismically deficient members and systems in order to reduce injuries and loss of life and property. To perform safely in an earthquake, vertical members (columns and walls) must maintain their gravity load carrying capacity even 0141-0296/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.engstruct.2011.09.017 ⇑ Corresponding author. Tel.: +81 3 5841 5783; fax: +81 3 5841 1765. E-mail address: [email protected](Y. Kim). Engineering Structures 34 (2012) 95–110 Contents lists available at SciVerse ScienceDirect Engineering Structures journal homepage: www.elsevier.com/locate/engstruct
Transcript
Engineering Structures 34 (2012) 95–110
Contents lists available at SciVerse ScienceDirect
Dynamic collapse test on eccentric reinforced concrete structureswith and without seismic retrofit
Yousok Kim a,⇑, Toshimi Kabeyasawa a, Shunichi Igarashi b
a Earthquake Research Institute, University of Tokyo, 1-1-1 Yayoi Bunkyo-Ku, Tokyo 113-0032, Japanb Structural Quality Assurance Inc., 2-7-10 Samikicho Chioda-Ku, Tokyo 101-0061, Japan
a r t i c l e i n f o a b s t r a c t
Article history:Received 31 May 2010Revised 5 September 2011Accepted 6 September 2011Available online 4 November 2011
Reconnaissance reports on structures damaged or collapsed by severe earthquakes have revealed severalcommon characteristics in their structural members and systems, such as insufficient reinforcementdetails in beam-column joints and transverse confinements, low aspect ratios, soft and or weak stories,and eccentric plans. Dynamic tests were carried out to investigate the collapse process of reinforced con-crete structures that had seismically deficient reinforcement details (light transverse reinforcement) andseismic systems (soft/weak stories and eccentric plans). A comparison of collapse behaviors with andwithout seismic retrofits also verified the effectiveness of the SRF (super reinforced with flexibility)strengthening method, which was developed to prevent the loss of axial load carrying capacity even atexcessive lateral deformation. The columns of one specimen were strengthened with polyester fiber beltsand its shear walls with polyester fiber sheets, while the members of the other were not. Each specimenwas designed following old (1970s) reinforcement detail practice in Japan, and is a one-third-scale rein-forced concrete structure with considerable stiffness and strength eccentricity in the first story. The spec-imens were composed of independent column frame and shear wall frame. Torsional response resultingfrom the eccentricity in the 1st story induced a displacement concentration on the weak frame, and even-tually the independent columns of the RC specimen failed in shear and lost their axial load carryingcapacity. On the other hand, the SRF specimen survived not only an identical earthquake load to theone that caused the RC specimen to fail, but also three additional earthquake loads, although significantstrength deterioration and considerable lateral and vertical deformation were generated at the end of thetest. The following conclusions were drawn from the comparison of the two specimens’ responses: axialcolumn collapse cannot be predicted from vertical responses since the vertical behavior of bare RC col-umns was not discernibly different from that of SRF columns until axial collapse was initiated, and theSRF strengthening method is effective in confining the column and preventing the cracking progress, thusmodifying the failure mode of the RC columns from brittle shear failure after flexural yield to flexuraldominant behavior.
� 2011 Elsevier Ltd. All rights reserved.
1. Introduction
Collapsing buildings have caused numerous casualties duringrecent major earthquakes, even in countries with advancedearthquake engineering technologies. Understanding the collapsemechanisms of reinforced concrete (RC) structures subjected toearthquake attack is a crucial aspect of one of the goals of earth-quake engineering, which is to protect people’s lives and safety,and has been a challenging task in many researches [1–3]. ManyRC structures have suffered severe damage in devastating earth-quakes in the past, leading to unrecoverable damages and thecollapse of structures. Reconnaissance reports describing these
ll rights reserved.
: +81 3 5841 1765.
damaged or collapsed structures have revealed several commoncharacteristics in their structural members such as insufficientreinforcement details in beam-column joints, transverse confine-ment designed following old seismic codes, and low aspect ratio(shear span to depth ratio) members that are susceptible to shearfailure [4–7]. In addition to these seismic element deficiencies,the unbalanced layout of structural members in elevation or plan(i.e. buildings with soft/weak stories or eccentric plans) causedpoor seismic performance leading buildings to collapse.
Although research themes in earthquake engineering are turn-ing to innovative technologies for new structures, it is still impor-tant to continue developing economical retrofitting methods forexisting buildings that have seismically deficient members andsystems in order to reduce injuries and loss of life and property.To perform safely in an earthquake, vertical members (columnsand walls) must maintain their gravity load carrying capacity even
96 Y. Kim et al. / Engineering Structures 34 (2012) 95–110
when earthquake intensity exceeds the members’ design limits. Insome old reinforced concrete buildings, the columns can lose theiraxial load carrying capacity due to inadequate transverse rein-forcement, then the buildings collapse at the weakest story orcompletely pancake. The development of a simple and economicalstrengthening method to prevent these brittle columns fromgravity load collapse during strong earthquakes would be verybeneficial.
A new strengthening method for RC columns called the superreinforced with flexibility method (SRF) focuses particularly onmaintaining the columns’ axial load carrying capacity even underexcessive lateral deformation. It was developed and has beendemonstrated using static tests on columns with a variety ofcross-sections under different loading patterns [8–11]. In the firstphase of this testing, eight specimens were tested, with 14 in a sec-ond phase. The specimens were constructed to simulate reinforcedconcrete columns in old buildings in Japan or Turkey with lighttransverse reinforcement and relatively low member aspect ratios.Test results showed that the columns strengthened by the SRFmethod could maintain relatively high gravity loads through driftratios of more than ten percent, while the bare specimens failedin shear at small drift ratios, simultaneously losing axial load car-rying capacity. Through such testing, the retrofit scheme has beenimproved to be effective in preventing loss of capacity against notonly axial loads but lateral load reversals as well. In the shake tabletest presented in this paper, we simultaneously tested two eccen-tric wall-frame specimens with identical section details and mate-rial properties. One specimen was strengthened using the SRFmethod while the other was not.
This study investigates several characteristics of structures thatsuffer from severe damage or collapse during earthquakes, includ-ing: (1) the lack of sufficient shear strength in RC columns designedfollowing 1970s Japanese reinforcement detail practice, whichleads to shear failure after flexural yielding and the loss of axialload carrying capacity; (2) irregular layout along the height (i.e.soft/weak first stories); and (3) asymmetric 1st story plans com-posed of independent column and wall frames that generate con-siderable stiffness and strength eccentricity. In addition, anotheraim of this experimental program is to confirm the effectivenessof SRF strengthening under dynamic loading conditions to supportstatic tests that have demonstrated its value.
The primary objectives of this experiment, therefore, are toinvestigate and understand the collapse process of reinforced con-crete structures with both seismically deficient sections (poortransverse reinforcement) and structural member layout (soft/
0 10 20 30 400
100
200
300
400
Strain (%)
Stre
ss (M
pa)
Sheet
Fig. 1. Tensile test resul
Table 1Material properties of SRF belt and sheet.
Thickness (mm) Width (mm) Tensile strength
Belt 3 50 358.1Sheet 0.9 169.7
weak stories and eccentric plans) and to confirm the effectivenessof the proposed SRF retrofit scheme by comparing the seismicbehaviors and collapse modes of two specimens that are identicalexcept for being strengthened or not. We also investigate the influ-ence of stiffness and strength eccentricity on elastic and inelasticearthquake responses under severe earthquake loads by observingthe seismic behaviors of the two specimens.
2. Shake table test description
2.1. SRF strengthening method
The polyester fiber reinforcing method used here was originallydeveloped to improve vertical members’ ability to sustain axialloads under large lateral deformation and to prevent the collapseof structures in severe earthquake loadings. The important charac-teristics of the SRF material are its toughness, durability, heat-resistance and flexibility.
The results of tensile tests on SRF sheets and belts are given inFig. 1 and the average material properties obtained from threetensile tests are summarized in Table 1. These results show an al-most linear relationship between stress and strain until the sheetand belt fail at very large strains of 16% and 35%, respectively.The tensile strength of belt is almost twice as high as that of sheet,while the Young’s moduli are almost same in both materials andlower than that of concrete. Much higher tensile force is expectedfrom belt (3 mm thick) than sheet (0.9 mm thick).
Externally bonded steel or fiber reinforced polymer (FRP) arealso very effective in strengthening the structural members ofexisting buildings which cannot satisfy the current designdemands [12,13]. FRP strengthening method, of which advantagesover steel plate bonding method are easy installation due to thelight weight, chemical resistance and lower labor cost, was pro-posed to overcome the shortcomings of steel plate bonding meth-od. In special, CFRP (carbon fiber reinforced polymer) retrofitmethod enhanced both stiffness/strength and ductility of struc-tural members. Improved structural behaviors are resulted fromthe high strength and stiffness of CFRP materials. Furthermore,adhesive which is essential in the external plating/wrapping meth-od is also stiffer and stronger than concrete. However, some exper-imental researches have also revealed that peeling off and shearcracks at the plate ends resulted in premature, brittle failure ofRC members externally bonded with FRP plates [14–16]. Therefore,it has been recommended that in strengthening applications, theexternal FRP should fail in tension after yielding the internal steel
0 10 20 30 400
100
200
300
400
Strain (%)
Stre
ss (M
pa)
Belt
ts of SRF materials.
(MPa) Strain at max. strength (%) Elasticity modulus (MPa)
34.42 0.85 � 103
16.17 0.76 � 103
Y. Kim et al. / Engineering Structures 34 (2012) 95–110 97
but before failure of the concrete in the compressive zone, sincethis would ensure a more ductile failure mode.
On the other hand, SRF strengthening method, which adopts al-most similar strengthening process to FRP method (i.e. plating orwrapping through adhesive), do not show the sudden failure re-sulted from premature failure or debonding from concrete. Themain difference between SRF and FRP is strengthening materialproperties. That is, polyester belt/sheet and urethane adhesivewhich are applied in SRF strengthening method are much moreflexible than concrete and steel. Therefore, its adherend such asconcrete surface is not damaged due to the fracture or peeling ofstrengthening materials, which can be observed from stiff andstrong strengthening materials used in FRP retrofit method.
The other different characteristic of SRF to FRP is that SRFmaterials (polyester belt/sheet) resist only in tension not incompression. In shear resistant behavior of RC members, two dif-ferent stresses are generated, which are tensile and compressivestresses across and along shear crack, respectively. In case of sim-ply supported beam subjected to flexural bending moment, twodifferent stress conditions which are tension in lower part andcompression in upper part of beam side are also generated in thesame surface. While the SRF materials resist in tensile directionin which it exerts very high and ductile resistance, very low com-pressive stress is generated in SRF materials.
However, in FRP materials which are very stiff and resist both incompression and tension, high compressive stress is concentratedand cause debonding or peeling of FRP from concrete surface. Itwas noted that peeling and shear cracks at the plate ends wereresponsible for causing premature, brittle failure of FRP materials.
RC members strengthened with SRF materials show very highperformance in terms of ductility, since the SRF materials includingadhesives can endure more than 10% tensile strain without anysudden failure. Furthermore, axial deformation can be expectedbeyond 2% of column height, which means that axial load carryingcapacity at extremely large deformation can be preserved in thevertical members strengthened by SRF materials and preventsstructures subjected to severe earthquake attack from collapse(e.g. pancake collapse).
From the comparisons of SRF with FRP strengthening methoddescribed above, it can be said that SRF retrofit scheme is moreeffective for improving post-peak behavior and axial load carryingcapacity of vertical members compared to FRP strengthening
(a)
West
744
80
0 5
00
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5344
Loading di
1st
600 1500 600
2700
600
15
00
600
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350
150
0 3
50
500 1500 500
2200
2500
2nd Floor
1st Floor
X1 X2
Y1
Y2
Y1
Y2
X
Y
Fig. 2. Specimen plan and eleva
method at large deformation, although considerable enhancementof initial stiffness and ultimate strength cannot be expected asthose of FRP strengthening method.
2.2. Specimens
Two specimens with identical properties and dimensions wereconstructed simultaneously. Fig. 2 shows plan and elevation ofthe specimens. They had wall and column frame construction inthe first story but had wall frames only in the second story. A largedamage concentration was expected in the 1st story independentcolumns due to the irregular layout of structural members in bothplan and elevation. Each specimen was constructed in two parts(connection level: 2044 mm, Fig. 2(b)) because of height limita-tions imposed on specimens constructed outside and moved intothe shake table testing facility.The specimens were built at 1/3scale based on a prototype structure and thus should meet simili-tude requirements. Adding steel plates on the top of the specimenpreserves the axial stresses and shear coefficients of the prototypesix-story building. These steel plates (WS, 148.3 kN) together withtwo concrete masses W1 and W2 (284.6 kN) produce an axial loadstress of 0.15 AgFc (Ag: gross area of the column section, Fc: concretestrength, 18 MPa) in the first story independent columns.
The total height of each specimen is 4544 mm, which is the sumof the base (500 mm), load cells (244 mm), the 1st story (800 mm),W1 (1100 mm), the 2nd story (800 mm) and W2 (1100 mm), asshown in Fig. 2. The top of concrete mass W1 does not correspondto the 2nd floor height of the prototype building, although it will bereferred to as the 2nd story throughout this paper. The 2nd storyresponse described here, therefore, just represents a responseabove the 1st story where the upper story has a symmetric planwith two walls.
An asymmetric plan comprised of columns and a wall frame inthe first story generates considerable stiffness eccentricity for seis-mic motion in the X direction. The stiffness eccentricity in the firststory is 0.71, as calculated from Eq. (1). The stiffness of the columnsand wall frame, 9.1 � 104 N/mm and 4.57 � 105 N/mm, respec-tively, were obtained from elastic analysis. The stiffness of the wallframe in the 1st story is almost five times greater than that of thecolumn frame.
tances between centers of stiffness and mass, lY : center of stiffness,gY : the center of mass, KX , KY : the frame stiffness in the X and Ydirection, respectively, Y ¼ Y � lY , X ¼ X � lX , X, Y: membercoordinate in the X- and Y-axis.
The independent columns in the first story were designedfollowing 1970s Japanese reinforcement detail practice (Fig. 3(a)).Their shear strength at flexural yield and shear strength calculatedfrom Arakawa’s [17] empirical equations are 62.8 and 60.7 kN,respectively. These calculated strengths show that the differencebetween their shear and flexural capacities is small. The shearstrength of the shear wall, however, calculated from Hirosawa’sempirical equation [17] is 430 kN and its shear strength at flexuralyield is 770 kN. As with the stiffness difference, the shear strengthof the wall frame is almost four times that of the independent col-umn frame. The cross-sectional details of the wall and columns areshown in Table 2, while the material properties of the concrete andsteel, obtained from cylinder and coupon tests, are summarized inTables 3 and 4, respectively.
One of the two specimens was strengthened with polyesterfiber sheets and belts, while the other was not. The strengthenedspecimen will hereafter be referred to as the ‘SRF specimen’ whoseindependent columns and shear wall with boundary columns inthe 1st story were strengthened. The latter will be called the ‘RCspecimen’ throughout this paper.
The reinforcing methods and material properties of indepen-dent columns and walls are somewhat different (Fig. 4). Indepen-dent columns in the Y1 frame were spirally wrapped with asingle layer of 3-mm-thick polyester belt material. Fig 5 showsthe SRF column retrofitting process, which is similar to that donein previous static tests. The shear wall, with its boundary columns,was wrapped with a double layer of polyester sheeting 0.9 mmthick and 0.8 m high that covered the entire clear height of thewall. Epoxy-urethane adhesive was used in both cases, appliedbetween the concrete surface of the members and the polyester
2 B (width),D (depth) 200, 200Longitudinal reinforcement 12-D10Transverse reinforcement 2-D6@50
1 B (width),D (depth) 200, 200Longitudinal reinforcement 12-D10Transverse reinforcement 2-D4@50
materials. The shear wall had two layers of sheeting, so adhesivewas also applied between the first and second layers. It shouldbe noted that in this experiment, it took only a half-day for fournon-technicians (students) to reinforce two independent columnsand one shear wall in the 1st story without using any specialdevices or machines.
2.3. Experimental setup and instrumentation
Fig. 6(a) shows the arrangement of the specimens on the shaketable, which is symmetric about the table’s centroid. The two spec-imens, therefore, were subjected to identical earthquake loadingsuntil the RC specimen collapsed.
As described previously, the upper and lower sections of thespecimens were constructed separately. Two sub-structures wereassembled on the shake table during setup as shown in Fig. 7.The lower part of each specimen was mounted on the shake tablefirst and then connected to the upper part by tensioning highstrength steel bars that penetrated through holes in W1. Steelweights (WS) were connected to top of W2 in the same manner.
(b) (c)
10 hoop: 2-D4@50 (1F) hoop: 2-D6@50 (2F)
strain gauge
No.1 No.2 No.3
No.4
No.5 No.6 No.7
n. (b) Attached column to shear wall. (c) Hoop.
Wall (unit: mm)
Thickness 50Vertical and horizontal reinforcement D6@100
Thickness 50Vertical and horizontal reinforcement D6@100
Fig. 6. Instrumentation on 1st and 2nd floor plans. (a) Specimen arrangement on shake table. (b) Specimen overview and instrumentation. (c) Instrumentation on 1st and 2ndfloor plans.
Y. Kim et al. / Engineering Structures 34 (2012) 95–110 99
The responses of the specimens (acceleration, displacement,the strain of the steel bars and shear and axial forces) when sub-jected to earthquake excitation were recorded with a samplingrate of 1000 Hz using accelerometers, displacement transducers,strain gauges and load cells. Instrument placement is shown in
Fig. 6(b and c). To record horizontal and vertical accelerationsof concrete masses W1 and W2, accelerometers were installedin boxes located inside of W1 and W2 as shown in Fig. 1. In addi-tion, input acceleration was recorded by accelerometers on theshake table.
Fig. 7. Specimen setup on shake table.
100 Y. Kim et al. / Engineering Structures 34 (2012) 95–110
Laser displacement sensors, which require no physical connec-tion between the target point and sensor, were used to measurehorizontal and vertical displacements of the 1st story where largedisplacements were expected, while strain-type displacementtransducers were installed in the 2nd story. Strain gauges wereattached to the longitudinal and transverse column reinforcementsin the 1st story (Fig. 2). Bi-directional load cells capable of measur-ing both axial and lateral forces in the X direction were installed atthe bottom of the independent columns of the first story. Instru-ments were located symmetrically about the geometric center ofthe specimen in order to compare torsional responses betweencolumns (Y1 frame) and wall frames (Y2 frame). Identicalinstrumentation was installed in both specimens.
In addition to installing data recording instrumentation, protec-tion must be provided to prevent the specimen from falling downonto the shake table after collapse, since this experiment wasdesigned to simulate the structure’s collapse. In this test, anH-shaped steel column (in Y1 frame) was built to contain the firstfloor where axial collapse of columns was expected to occur.H-shaped steel beams spanned the top of the steel frames, andsteel wires connected to the specimen hung from the beams.
2.4. Base motion input plan
Records of four different historical earthquakes were employedas seismic excitations; the Miyagi-ken Oki earthquake recorded atTohoku university in 1978 (TOH), the Imperial Valley earthquakerecorded at El Centro in 1940 (ELC), the Hyogo-Ken Nambu earth-quake recorded at the Japan Meteorological Agency in 1995 (JMA)and the Chile earthquake recorded at Viña del Mar in 1985 (CHI).North–south components of these earthquake records wereapplied unidirectionally to the specimens in the X direction.Table 5 shows the amplitudes of the input base motions, scaledon the basis of preliminary analysis results, under which the RCspecimen experienced gradual damage ranging from elastic behav-ior through final collapse. The duration of the base motion wascompressed by a factor of 1=
ffiffiffi3p
to satisfy the similitude law, keep-ing the acceleration scale factor at unity. Here, the number in eachloading name (e.g. 12.5 in TOH12.5 or 50 in JMA50, etc.) indicatesthe maximum velocity (kine, cm/s) corresponding prototype
Table 5Base motion input plan.
Earthquake data Scale factor Max. acceleration tospecimen (m/sec2)
records. Fig. 8(a and b) show the time history and accelerationresponse spectra of each input base motion.
Although the shapes of the acceleration response spectra of theactual input base motions recorded from the shake table areslightly different than those of the original data, the difference isacceptable. White noise excitations, whose maximum accelerationis about 0.3 m/sec2, were also performed before the earthquakeloadings to evaluate changes in the natural frequency of the dam-aged specimens.
3. Test results
3.1. Lateral displacement response
Fig. 9 shows the drift ratio of the columns (Y1 frame) and wallframe (Y2 frame) of the RC specimen subjected to loading ELC37.5.Displacement through the 1st story was recorded from laserdisplacement sensors set between the top of the specimen baseand the bottom of concrete mass W1. The 2nd story displacementwas recorded between the top of W1 and the bottom of W2. Asshown in Fig. 9(a), drift ratio of the Y1 frame in the 1st story isalmost 10 times larger than that of the Y2 frame because theconsiderable eccentricity in the 1st story induced large torsionalresponses. In the second story, which was symmetrical, the driftratio of the Y1 frame is slightly larger (about twice) than that ofthe wall frame, which might also have been affected by the tor-sional response generated in the 1st story. The other loading inputsproduced similar results as shown in Fig. 9 (i.e. deformation con-centration on the Y1 frame) in both the RC and SRF specimens.The maximum lateral drift ratio of the Y1 and Y2 frames are com-pared in Fig. 10. The difference of the two frames’ maximum driftratio is almost constant throughout all the input stages and in bothspecimens. Note that different scales on the vertical axis for the Y1and Y2 frames are used in Figs. 9(a) and 10.
A comparison of the drift ratio of the two specimens when sub-jected to the JMA50 loading is shown in Fig. 11. The drift ratio ofthe RC specimen is slightly larger than that of the SRF specimenfor both the Y1 and Y2 frames. This is consistent with the resultsobserved from all the other input stages, except for TOH12.5,which can be seen in Fig. 12 where ratios comparing the maximum
102 Y. Kim et al. / Engineering Structures 34 (2012) 95–110
drift ratio of the RC and SRF specimens are displayed for the vari-ous loadings. The difference between two specimens is most out-standing in loading CHI50-1 where the RC specimen collapsed.Since the SRF specimen exhibited smaller deformations than theRC specimen when subjected to identical earthquake loadings, thistest showed the SRF retrofitting method to be also effective underdynamic loading conditions.
(a) (b) θ Rotation angle
Fig. 13. Response center distance, r. (a) Definition of r. (b) Calculation of r.
3.2. Torsional response
As shown previously, the displacement of the specimens isdependent on the torsional response generated by eccentricity inthe 1st story. Although considerable torsional responses inducingdisplacement concentration on the weak 1st story Y1 frame wereobserved in both specimens for all inputs, these responses differedfor every loading input as well as for the strengthened or bare con-dition of the frame. Quantitative assessment of torsional responsein each input stage is needed to evaluate the characteristics of tor-sional behavior in both the elastic and inelastic ranges. The effec-tiveness of the SRF strengthening method can also be furtherverified by quantitatively comparing those two specimens’ tor-sional responses.
The extent of torsional response is evaluated by the index r,(Fig. 13(a)) representing the distance of a member’s response cen-ter from its center of gravity. This index can be calculated from Eq.(2), which is the relationship between lateral displacement androtation angle at the center of mass. Thus r can be thought of as
the slope of a line fitted from the relationship between angle ofrotation and translational displacement at the center of gravity(Fig. 13(b)).
r ¼ dh
ð2Þ
here, d is the displacement of the center of a specimen, obtained byaveraging the displacements of the Y1 and Y2 frames, and h iscalculated from displacement difference divided by the distancebetween the two frames.
For example, the value of r is zero in pure torsion and infinite forpure translation. Thus, the torsional response becomes dominantas the value of r decreases.
translation
torsion
small specimen damage large
TOH25 ELC37.5 JMA50 CHI50 TAK125 CHI63 CHI50 0.6
0.8
1
r (m
)
RC specimenSRF specimen
Fig. 14. Change of response center distance, r.
Y. Kim et al. / Engineering Structures 34 (2012) 95–110 103
Fig. 14 shows variations in r-values for both specimensproduced by the different input loadings. It can be observed thatr becomes smaller as load levels increase in both specimens, whichmeans that the torsional response becomes more dominant in theinelastic range rather than in the elastic range. This may be ex-plained by the fact that the strength eccentricity of the specimen,which governs the characteristics of torsional response in theinelastic range, is so high that the wall did not yield even thoughthe columns did. As a result, the increased strength eccentricityin the inelastic range might cause the specimen to exhibit moretorsional behavior, which was observed in both specimens. The rvalue of the SRF specimen is slightly larger than that of the RCspecimen because the SRF strengthening reduced damage and pre-vented the columns from losing their lateral load carrying capacityas compared to the RC specimen.
3.3. Shear force distribution
The base shear force was computed by summing the externalforces in the following manner: the masses of W1, W2 and WSwere multiplied by their accelerations, while the shear forces car-ried by the shear walls were obtained by subtracting the shearforce on the independent columns recorded by the load cells fromthe base shear force. Fig. 15(a and b) illustrate the shear forces car-ried by the columns and the wall in the RC and SRF specimens,respectively. The ratio of the column shear force to the base shearforce is shown in Fig. 15(c). Here, the shear forces are the values atthe moments when maximum base shear force was attained inboth directions (positive and negative).
From this figure, it can be seen that the shear force carried bythe columns is relatively smaller than that carried by the wall,and they degrade gradually as the load level increases. The col-umns of the SRF specimen carried a larger shear force than theRC columns, which attests to the efficacy of the SRF strengtheningmethod. It is also noteworthy that the maximum shear force car-ried by the shear wall is smaller than its calculated shear strength
TOH25 JMA50 -550
0
550ColumnsWall
(a) (b)
Fig. 15. Shear force carried by columns and wall. (a) RC specime
(430kN, about 30% shear strength), while shear force resisted bythe independent columns exceeded their capacity.
3.4. Hysteretic relations
The relationships between lateral displacement and shear forcein the Y1 frame, subjected to five consecutive earthquake loadings(TOH12.5 through CHI50-1) are shown in Fig. 16, where the scalesof all the plots are identical. The solid lines in the figure indicatethe calculated shear strength (121.5 kN) of the two independentcolumns, while dotted lines show their flexural strength(125.5 kN).
Fig. 17 shows changes in period experienced as the loadingstages progressed. As a system identification scheme for calcula-tion of period, the ARMAX method [18] was used. Accelerationsrecorded from the shake table and mass W2, subjected to whitenoise inputs were used as input and output values, respectively.As the load level increased, the stiffness degraded (or the periodwas lengthened) and lateral drift grew. This is more pronouncedin the RC specimen than in the SRF specimen. The specimens’ ini-tial period and stiffness were similar, although the SRF specimenwas slightly stiffer and had a shorter period than the RC specimen.As described previously, torsion-induced concentrated displace-ment in the Y1 frame and ultimately caused the collapse of theRC specimen’s independent columns. Shear forces shown in thisfigure were obtained from load cells installed at the bottom ofthe columns in the Y1 frame. During the TOH12.5 and TOH25 load-ings, the relationships between lateral drift ratio and shear forceare almost linearly elastic. In the RC specimen, initial crack was ob-served on the wall panel but not on the independent columns afterTOH25, which was unexpected and might be due to out-of-planedeformation resulting from the torsional response. SRF sheetsand belts covering the surfaces of the SRF specimen made it impos-sible to observe crack.
During the ELC37.5 loading input, a diagonal crack that haddeveloped in the wall panel during the previous loading step grew,
TAK125 CHI50-2
0
25
50
TOH12.5 ELC37.5 CHI50-1
0
25
50
SRFRC
(c)
n. (b) SRF specimen. (c) Ratio of column shear to base shear.
-0.01 0 0.01-150
-100
-50
0
50
100
150
Shea
r for
ce (K
N)
TOH12.5TOH25
-0.01 0 0.01
ELC37.5
-0.02 0 0.02Lateral drift ratio (%)
JMA50
-0.05 0 0.05
CHI50-1
dashed:flexural strengthsolid:shear strength
SRFRC
Fig. 16. Hysteretic relation between shear force and lateral displacement.
104 Y. Kim et al. / Engineering Structures 34 (2012) 95–110
but no cracks were observed on the independent columns of the RCspecimen. Yielding of the columns’ longitudinal reinforcement wasobserved in both specimen, but no transverse reinforcementyielded. The hysteretic behaviors of both independent columns inthe RC and SRF specimens are compared in Fig. 19(a), where thecircular and square markers indicate the instants when the longi-tudinal reinforcement first yielded in the RC and SRF columns,respectively. Varying axial load effects on the shear force carriedby the columns can be observed in both compressive and tensiledirections. The longitudinal reinforcements yielded in tension.
After the JMA50 loading, the first cracking was observed on theindependent columns, although this is not well explained, consid-ering the number of steel bars that had yielded as determined fromstrain gauge records. However, there is a possibility that crack wasconcentrated at the critical section nearest top of the load cell (Figs.1b and 18b) which is rough surface and therefore could not befound by visual observation after ELC37.5 loading. In addition,the yielding of reinforcement was also recognized from the straingauge attached at the bottom of longitudinal reinforcement (sameheight with critical section).
The crack patterns that developed on the columns and the wallduring the JMA50 loading is illustrated in Fig. 18. In bothspecimens, all the longitudinal reinforcements yielded and the
Fig. 18. Crack patterns. (a) Shear w
maximum shear force was recorded. The maximum shear forceof the SRF specimen is higher than that of the RC specimen. Therecorded maximum shear force in the RC specimen was almostthe same as its calculated strength, while the SRF specimen carriedhigher loads than its calculated strength (Fig. 16).
Yielding of transverse reinforcements was also observed in bothspecimens. Fig. 19(b) shows the relationship between shear forceand lateral drift ratio for each independent column, together withthe instants the transverse reinforcements yielded during theJMA50 loading. It can be seen from this figure that the differenceof the two specimens’ maximum column shear force is larger incompression than in tension, where the transverse reinforcementyielded. Furthermore, the maximum shear strength developed inan SRF column (east column) increased even after the transversereinforcement yielded, while in the RC column the maximum shearforce is almost same as the shear force recorded at the moment thetransverse reinforcement yielded, meaning that the shear strengthof the RC column did not increase after transverse reinforcementyielding. From these results, it might be concluded that SRFstrengthening is effective in confining the concrete, which is aprimary role of transverse reinforcement, and therefore SRF ismore effective in improving compressive response than tensileresponse.
During the CHI50-1 loading, the stiffness and strength of the RCspecimen significantly degraded under reversed cyclic loadings,resulted in collapse at an elapsed time of about 20 s. In the SRFspecimen, on the other hand, the hysteretic relations were stable,without strength deterioration.
All the transverse reinforcements of the RC columns yielded inthis loading, but only two transverse reinforcements yielded in SRFcolumns as recognized from the strain gauge records. The differ-ence in the number of yielded transverse reinforcements betweenthe two specimens also supports the effectiveness of the SRFretrofit method.
all. (b) Independent columns.
-0.005 0 0.005
-50
0
50
Shea
r for
ce (k
N)
Drif t ratio
West column
SRFRC
-0.005 0 0.005
-50
0
50
Drif t ratio
East column
SRFRC
(a)
-0.02 -0.01 0 0.01 0.02
-100
-50
0
50
100
Shea
r for
ce (k
N)
Drif t ratio
West column
SRFRC
-0.02 -0.01 0 0.01 0.02
-100
-50
0
50
100
Drif t ratio (%)
East column
SRFRC
(b)
Fig. 19. Shear force-drift hysteretic relation for each column. (a) ELC37.5 input. (b) JMA50 input.
Y. Kim et al. / Engineering Structures 34 (2012) 95–110 105
Fig. 20 shows the columns in both specimens after the CHI50-1loading input. The front and side views of the collapsed RC speci-men are shown in Fig. 21, where the W1 mass landed on the safetyframe (H-shaped steel frame) installed around the 1st floor andrested inclined toward the Y1 frame after the collapse of the inde-pendent columns. These figures indicate that the RC specimenwould have fallen onto the shake table if the safety frame (H-shapesteel frame) had not been installed.
Fig. 20. Column conditions after CHI50-1 loadin
Fig. 21. Collapsed RC specimen. (a) Fron
3.5. RC column collapse process
To investigate the collapse process of the east column in the RCspecimen during the CHI50-1 input, time histories and hystereticrelations of both horizontal and vertical responses for a period of10 s (from 12 to 22 s) are illustrated in Fig. 22. The responses ofthe SRF specimen’s east column are also plotted and comparedwith the RC specimen. Two reference times, 16.7 and 19.77 s,
g input. (a) RC specimen. (b) SRF specimen.
t view. (b) Side view (from West).
-5
0
5
(m/s
ec2 )
(a) Base motion
0
1000
2000
3000
( µ)
(b) Strain of hoop (No.4)SRFRC
-50
0
50
100
(kN
)
(c) Shear force
-40
-20
0
20
40
(mm
)
(d) Horizontal displacement
-400
-200
0
200
400
(kN
)
(e) Axial force
12 16.7 19.77 22-2
0
2
4
(mm
)
Time (sec.)
(f) Vertical displacement
-40 -20 0 20 40-50
0
50
100 12sec.-16.7sec.
Shea
r for
ce (K
N)
Horizontal displ. (mm)
(g)
-40 -20 0 20 40
16.7sec.-19.77sec.
Horizontal displ. (mm)-40 -20 0 20 40
19.77sec.-22sec.
Horizontal displ. (mm)
SRFRC
-40 -20 0 20 40-400
-200
0
200
400 12sec.-16.7sec.
Axia
l for
ce (k
N)
Horizontal displ. (mm)
(h)
-40 -20 0 20 40
16.7sec.-19.77sec.
Horizontal displ. (mm)-40 -20 0 20 40
19.77sec.-22sec.
Horizontal displ. (mm)
SRFRC
-40 -20 0 20 40
-6
-4
-2
0
2
4 12sec.-16.7sec.
Verti
cal d
ispl
. (m
m)
Horizontal displ. (mm)
(i)
-40 -20 0 20 40
16.7sec.-19.77sec.
Horizontal displ. (mm)-40 -20 0 20 40
19.77sec.-22sec.
Horizontal displ. (mm)
SRFRC
Fig. 22. Responses during CHI50-1 loading input. (a) Base motion time history. (b) Strain time history of hoop (No. 4 in Fig. 2). (c) Shear force time history. (d) Horizontaldisplacement time history. (e) Axial force time history. (f) Vertical displacement time history. (g) Horizontal displacement vs. shear force. (h) Horizontal displacement vs. axialforce. (i) Horizontal displacement vs. vertical displacement.
106 Y. Kim et al. / Engineering Structures 34 (2012) 95–110
shown by the black and white triangles in Fig. 22, were selected todivide the responses into three parts.
At 16.7 s, when the highest shear force experienced during theCHI50-1 input (i.e. 70% of the maximum shear strength developedduring the JMA50 loading) was recorded, considerable stiffness and
strength degradation were initiated and hoop strain at the mid-height of the column started to increase (Fig. 22b). The criticalshear cracking associated with the yielding of the hoop might havecaused the residual hoop strains and shear strength decay. Further-more, the loss of interface shear transfer along the widened shear
Y. Kim et al. / Engineering Structures 34 (2012) 95–110 107
cracks which was resulted from yield and fracture of hoop mighthave caused the fatal loss of axial load carrying capacity in column.From the above results, it should be noted that the inelastic hoopstrain, accumulated over cyclic load reversals, might be one ofthe main causes of shear and axial failure of RC columns. The hor-izontal hysteretic relationship of the RC column in Fig. 22g showsrelatively stable behavior until 16.7 s and a remarkable pinchingeffect between 16.7 and 19.77 s. In addition, the decay of stiffnessand strength in the RC column during this time zone resulted in theconsiderable increase in the lateral drift at 19.77 s compared tothat at 16.7 s. On the other hand, the lateral drift of SRF columnat 19.77 s is only slightly larger than its first peak drift at 16.7 s,since the decay of stiffness and strength were very small(Fig. 22b and g). At 19.77 s, when the mid-height hoop might haveruptured (Fig. 22b), almost all the lateral stiffness and strength ofthe RC column were lost. It can be seen from Fig. 22h that the max-imum axial force of RC column observed from 16.7 to 19.77 s ishigher than that of SRF column, which might be due to larger lat-eral drift of RC column than that of SRF specimen. However, after19.77 s, axial force in RC column could not exceed that of SRFone, although lateral drift of RC column was much larger than thatof SRF one. These results indicate that axial failure of RC columninitiated soon after shear failure around 19.77 s at which the lateraldrift ratio was 0.036. That is, the loss of axial load-carrying capac-ity, which was precipitated by shear failure, induced collapse of theRC specimen.
Fig. 24. Height-wise distribution of transverse ste
Fig. 23 shows the RC column at each reference time, showingthe collapse process, which was captured on video.
The SRF column was stable throughout the entire duration ofthe CHI50-1 input, although lateral strength and stiffness bothdeteriorated slightly compared to the previous input (JMA50). Bothspecimens showed that shear strength in the positive direction(compression) was higher than that in the negative direction (ten-sion), arising from the variable axial force. The lateral strength dif-ference between compression and tension is more pronounced inthe SRF column rather than in the RC column. This result, as men-tioned previously, indicates that the SRF retrofit method is moreeffective in compression than tension because of its ability to con-fine the concrete, which becomes more apparent with increasedvariable axial loads.
There was no distinguishable difference in the two specimens’vertical responses, both vertical displacement and axial force, untilthe 19.77 s reference time (Fig. 22e and f). Relatively large com-pressive deformation was generated just before 19.77 s and down-ward deformation drastically increased with the progress of axialfailure in the RC columns, which was followed by collapse of thewhole specimen. Comparing two specimens’ vertical responses inthis test, it could be concluded that axial collapse cannot be pre-dicted from the vertical responses since the RC column exhibitedno discernable difference in its vertical behaviors compared tothe SRF column until column collapse was initiated. Furthermore,it is also observed from the time history response (Fig. 22) that
(b) 19.77 s; (c) 20.03 s; (d) 20.3 s; (e) 20.9 s.
0 2000 4000
No.1
No.2
No.3
No.4
No.5
No.6
No.7
Max. strain (µ)0 20 40
Yield strain
Time (sec.)
Yield strain
Yield strain
Yield strain
Yield strain
Yield strain
Yield strain
(b)
el strain. (a) RC specimen. (b) SRF specimen.
108 Y. Kim et al. / Engineering Structures 34 (2012) 95–110
phase deviation between the two specimens until 19.77 s is signif-icantly smaller vertically than laterally. This might be due to thefact that there is no neighboring vertical member to sustain axialload redistributed from the collapsed column since there is justone span in each direction (only two columns per frame).
Fig. 24 illustrates the height-wise strain distribution recordedduring the CHI50-1 input. As described before, the transverse steelstrain at the mid-height of RC column (No. 4 in Fig. 2) started to in-crease at 16.7 s with rupture following at 19.77 s when the slidingof the shear failure plane might have occurred. However, trans-verse reinforcements at the bottom and top of the column (Nos.1, 2, 6, and 7 in Fig. 2) did not yield and the strains of the Nos. 3and 4 hoops slightly exceeded their yield strain. These strain re-cords indicate that shear damage to the RC columns was concen-trated at mid-height rather than at the plastic hinge regions atthe bottom and top of the columns. In the SRF column, however,which was strengthened by the polyester belting, shear damage,represented by transverse steel strain, was severe at the bottomand top but small at mid-height. The hoop at mid-height did notyield and its strain was about 7.76 � 10�4 m/m, far below the yieldstrain. This result also substantiates the effectiveness of the SRFretrofit method’s confining effect and prevention of crack progress.That is, using the SRF strengthening method changed the failuremode of RC columns with light transverse reinforcement fromshear failure after flexural yield to flexural dominant behavior.
3.6. Retrofit effectiveness
After removing the collapsed RC specimen from the shake table,the SRF specimen was subjected to three additional base motions:TAK125, CHI63 and CHI50-2. Fig. 25 shows the relationship be-tween lateral force and displacement of the Y1 frame in the SRFspecimen for those three loadings.
In the TAK125 input, the maximum strength had decreasesslightly from the previous input stage (CHI50-1), but the hysteretic
-0.1 -0.05 0 0.05 0.1-150
-100
-50
0
50
100
150
Shea
r for
ce (K
N)
TAK125
-0.1 -0.05Lateral d
Fig. 25. Hysteretic relations of SRF columns s
0 10 20 30 40 50
-200
-100
0
100
200
Time (sec.)
Axia
l for
ce (k
N)
(a)
Fig. 26. Axial force carried by east and west columns in
relationship showed relatively stable behavior. In the CHI63 input,strength deterioration and pinching effects can be observed fromthe hysteretic relationship. Furthermore, residual deformationwas generated in the positive direction, which might be due tothe characteristics of that particular earthquake load. The gravityload distribution between the two columns also changed due topermanent lateral deformation in the positive direction. The grav-ity load on the east column increased and that on the west columndecreased by about 50 kN as shown in Fig. 26.
In the final input, CHI50-2, maximum lateral strength degradedby almost one-third of the maximum strength recorded during theJMA50 loading cycle. Permanent deformation in the positive direc-tion could not be recovered during this input stage, and the maxi-mum lateral drift exceeded 10% of story height. Nevertheless, theSRF specimen showed stable behavior against axial collapsealthough its members lost almost all their lateral stiffness andstrength.
Fig. 27(a) shows the relationship between axial force and verti-cal displacement of the SRF columns. A remarkable compressivedeformation was observed from loading CHI63 through CHI50-2(Fig. 27(b)), and maximum axial deformation in the compressivedirection was 19.57 mm during CHI50-2 a 2.5% axial deformationratio. The columns in the Y1 frame of the RC specimen started tocollapse drastically at an axial deformation of 5 mm (axial defor-mation ratio of 0.063%), while the SRF specimen showed a gradualincrease in compressive deformation of about 4-times to that of RCcolumn and finally survived throughout all loading stages.
The state of the SRF columns after testing is shown in Fig. 28,from which it can be seen that severe damage was concentratedat the ends of the columns. At mid-height, however, the straintime-history for the SRF column shown in Fig. 29 indicated thatthe maximum strain only slightly exceeded the yield strain of itstransverse reinforcement. From these results, it is obvious thatthe midsections of the retrofitted columns were not damagedwhile both ends suffered severe damage.
0 0.05 0.1rift ratio (%)
CHI63
-0.1 -0.05 0 0.05 0.1
CHI50-2
dashed:flexural strengthsolid:shear strength
ubjected to additional earthquake loads.
0 10 20 30 40 50
-200
-100
0
100
200
Time (sec.) (b)
SRF specimen. (a) West column. (b) East column.
Fig. 28. SRF specimen after test.
0 50 100 150 200-500
0
500
1000
1500
stra
in (
µ)
Time (sec.)
CHI50-1 TAK125 CHI63 CHI50-2
0 50 100 150 200-500
0
500
1000
1500
Time (sec.)
CHI50-1 TAK125 CHI63 CHI50-2
(a) (b)
Fig. 29. Strain time histories at mid-height of SRF column. (a) West column. (b) East column.
-300 -200 -100 0 100 200 300-25
-20
-15
-10
-5
0
5
10
Ver
tical
dis
palc
emen
t(m
m)
Axial f orce (KN)
TAK125CHI63CHI50-2
-300 -200 -100 0 100 200 300Axial f orce (KN)
(a)
0 20 40 60 80 100 120 140-20
-10
0
10
Time (sec.)
Ver
tical
dis
pl.
(mm
) TAK125 CHI63 CHI50-2
(b)
Fig. 27. Vertical responses of SRF specimen. (a) Hysteretic relations between vertical displacement and axial force. (b) Vertical displacement of east column.
Y. Kim et al. / Engineering Structures 34 (2012) 95–110 109
4. Conclusions
Two identical specimens having several seismic deficienciesincluding low transverse reinforcement ratios, weak stories andeccentric plans were tested on a shake table. One specimen was
reinforced using the SRF strengthening method. The followingconclusions can be drawn from the results of the earthquakesimulations.
The first story plan of the specimens was eccentric, having oneside with independent columns only, while the other side included
110 Y. Kim et al. / Engineering Structures 34 (2012) 95–110
a shear wall. Throughout all the input stages, the lateral displace-ment of the side having only independent columns was muchlarger than that of the shear wall side in both specimens. Thecolumn-only side displacement was almost 10 times the shearwall-side displacement in the 1st story, about twice that of the2nd story, which had a symmetric plan. In addition, lateral dis-placement in the RC specimen is larger than that of the retrofittedspecimen, where shear strength and stiffness increased slightly byvirtue of the SRF retrofit, which may have decreased its deformation.
The response center distance (r) showed that torsional responseis more pronounced in the inelastic range than in the elastic range,which may be due to the increased strength eccentricity that re-sulted from damage concentration on the independent columnframe. The comparison of r between the two specimens indicatedthat the torsional response of the SRF specimen is smaller than thatof the RC specimen. This was also due to the SRF strengthening,which reduced the damage on the SRF columns.
The shear forces carried by independent columns are relativelysmaller than those carried by the wall and they decrease graduallywith increasing load level as column damage increases. The SRFcolumns carried larger shear forces than the RC columns. Further-more, the maximum shear strength of the SRF columns increasedeven after transverse reinforcement yield, while the maximumshear carried by the RC columns was very near that recorded atthe moment the transverse reinforcement yielded. These resultsindicate that the SRF strengthening method is effective in confiningthe concrete, a primary role of transverse reinforcement.
During the CHI50-1 input, the SRF columns showed stable hys-teretic relations without any considerable damage while the RCcolumns experienced severe shear strength deterioration and axialload failure along with inelastic load reversal and final collapse.Vertically, however, comparing the axial force and vertical dis-placement of the two specimens revealed that axial collapse couldnot be predicted from the vertical responses since bare RC col-umns, showing signs of drastic collapse, exhibited no discerniblydifferent vertical behaviors than the SRF columns until axial col-lapse was initiated.
The SRF specimen was subjected to three additional severeearthquake loads. It was capable of sustaining its gravity loadand survived all loading stages despite considerable lateralstrength deterioration (about one-third of maximum strength), ahigh lateral drift ratio (about 10%) and a large compressive axialdeformation ratio (about 2.5%) during the final loading.
The height-wise strain distribution was recorded from straingauges attached to transverse reinforcements in the RC columns.The strain was highest at mid-height and lowest at both ends. Thiscondition was reversed by strengthening columns with the SRFpolyester belting, so that the strain was highest at both ends andlowest at mid-height. This changed the failure mode of the columnfrom shear failure after flexural yield in the RC column to flexuralfailure showing minor damage at mid-height and concentrateddamage on both ends of the SRF column.
Acknowledgements
This study was carried out as part of the ‘‘Development of post-earthquake performance evaluation using practical accelerome-ters,’’ research project (Grant No. 12308018, PI: Toshimi Kabeyas-awa) under the support of the JSPS and Monbukagakusho. Thedynamic test was conducted at NIED, Tsukuba, with Dr. NobuyukiOgawa and Mr. Atsushi Kato of NIED. The support and cooperationwe received in conducting the tests are gratefully acknowledged.
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