NONLINEAR RESPONSES OF LOW STRENGTH RC COLUMNS RETROFITTED WITH EXTERNAL HIGH-STRENGTH HOOPS

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1 SECED 215 Conference: Earthquake Risk and Engineering towards a Resilient World 9-1 July 215, Cambridge UK NONLINEAR RESPONSES OF LOW STRENGTH RC COLUMNS RETROFITTED WITH EXTERNAL HIGH-STRENGTH HOOPS Wencong LI 1 and Oh-Sung KWON 2 Abstract: In the Standard for Seismic Evaluation of Existing RC Building of Japan (21), the lower limit value of concrete compressive strength for the existing RC building is defined as 13.5 MPa, and the RC buildings are considered unsuitable for seismic retrofit if the compressive strength of concrete is lower than the limit. In this study, a novel seismic retrofit technique using external pre-tensioned high-strength hoops and steel plates is applied to the low strength RC column with smooth rebar and poor shear reinforcement under axial force ratio of.31 and.57. The experimental results show that both the ductility and the lateral capacity of the specimens can be improved significantly. Moreover, nonlinear analyses of the non-retrofitted and retrofitted specimens are carried out using VecTor2. The results from numerical models agree well with the experimental results. Introduction In the Standard for Seismic Evaluation of Existing RC Building of Japan (21), the lower limit value of concrete compressive strength for the existing RC building is defined as 13.5 MPa, and the RC buildings are fundamentally considered to be out of seismic retrofit if the compressive strength of concrete is lower than the limit. However, many existing RC building structures have compressive strengths lower than 13.5 MPa. It is too costly to rebuild the structures that are not qualified for retrofit. If an appropriate seismic retrofit technique can be developed, it is preferable to retrofit the structures than demolishing and rebuilding the structures. In this study, a novel seismic retrofit technique using external pre-tensioned highstrength hoops and steel plates is applied to the low strength RC column with smooth rebar and poor shear reinforcement under axial force ratio of.31 and.57. The following sections present the experimental program, numerical modelling strategies, and the significance of the research. Specimens Four low strength RC column specimens with smooth reinforcement bars and poor shear reinforcement were investigated in this study. The mechanical properties of the reinforcement used in the specimens are listed in Table 1. The dimensions and layout of the reinforcement bars are illustrated in Figure 1. The details of seismic retrofit method are shown in Figure 2. The experimental parameters of all the specimens are summarized in Table 2. Each specimen has sectional dimension of 2 mm x 2 mm and height of 8 mm. The shear span to depth ratio of every column is 2.. Table 1. Mechanical properties of material Reinforcement a(cm 2 ) f y(mpa) y (%) E s(gpa) Rebar Hoop High-strength steel bar Steel plate 2.3 mm (thickness) mm (thickness) Note: a = cross-sectional area, f y = yield strength of steel, y = yield strain of steel, E s = modulus of elasticity. 1 Assistant Professor, Fukuoka University, Fukuoka, liwencong@fukuoka-u.ac.jp 2 Assistant Professor, University of Toronto, Toronto, os.kwon@utoronto.ca

2 Gap (2mm) PL-2.3x18x PL-3.2x18x76 8 Gap (2mm) All dimensions in mm. Column section 4 -@155 (p w =.8%) (p g = 1.33%) Figure 1. Dimensions and reinforcement properties LC-PS LC-PSh 49 MPa Steel corner block 22 5 Steel corner block High-strength steel bar 2 All dimensions in mm. High-strength steel bar Figure 2. Details of retrofitted specimens LC-PS and LC-PSh Four 13 reinforcement bars are used as longitudinal reinforcements with longitudinal reinforcement ratio, p g, of 1.33%. Transverse reinforcement bars consist of 4 bars spaced at 155 mm interval (transverse reinforcement ratio, p w =.8%). The RC column test specimens introduced in this report is 1/3 scale model of the original column (6 mm x 6 mm) in the general mid-rise RC buildings of Japan. The axial force ratio of test specimens LC-P and LC-PS is.15, and the axial force ratio of specimens LC-Ph and LC-PSh is.3 in which the design compressive strength of concrete is assumed as 18 MPa. The actual compressive strength of specimens LC-P and LC-PS is turned out to be 8.7 MPa which resulted in the actual axial force ratio of.31. In addition, in 2

3 Steel plate High strength steel bar W. LI and O. KWON the specimens LC-Ph and LC-PSh whose actual concrete compressive strength B is 9.5 MPa, the actual loading axial force ratio becomes.57. In this retrofitted specimen, PS means the specimen retrofitted by high-strength hoops and steel plates, and h means the specimen under high axial force ratio. Table 2. Column specimens LC-P LC-PS LC-Ph LC-PSh Specimen B 8.7 MPa 9.5 MPa N/(bD B) Prestress 49 MPa 49 MPa Common details M/(QD) = 2., Hoop:4 -@155 (p w =.8%), Rebar:4-13 (p g = 1.33%) LC-P and LC-Ph are standard column specimens without seismic retrofit. LC-PS and LC- PSh are retrofitted with external pre-tensioned high-strength hoops and steel plates of 76 mm (height) x 18 mm (width). The thickness of steel plate of LC-PS and LC-PSh was 2.3 mm and 3.2 mm, respectively. In this retrofit technique, the steel plates were attached to the four column faces. Then, steel corner blocks were placed at the four corners of the column. The steel blocks have holes through which high-strength steel were penetrated. Finally the columns were confined by pre-tensioned high-strength steel bars. The steel bars work as external hoops as shown in Figure 2. The prestressing is introduced into the high-strength steel bars by fastening nuts with a torque wrench. This technique can be applied quickly and easily without utilizing heavy machinery on site. In this study, in order to prevent the compressive failure of the column, the confinement in the end of column was increased. The diameter of all high-strength steel bars used in this study had diameter of 5.4 mm. The prestressing strain of all steel bars was approximately 245 which is about 1/3 of yield strain. The prestressing force was about 11.3 kn per steel bar. Experimental results Quasi-static cyclic tests were carried out with constant axial force. The lateral deformation was controlled in a cyclic manner. The measured shear force (Q) and drift ratio (R) relationship are presented in Figure 3. The dotted lines drawn in these curves represent the calculated flexural strength by the simplified equation of Architectural Institute of Japan (199) including the P- effect and ignoring the active confinement effect of external highstrength hoops. Images of the failed column specimens after removing the steel plates after tests are shown in Figure 4. In every specimens, initial crack was flexural one occurred at the boundary region between the column and beam when drift ratio R was approximately.5%. In the non-retrofitted test specimen LC-P with axial force ratio of.31, the experimental shear force reached the maximum strength (36.4 kn) at the drift ratio of.5%. The maximum strength of 36.4 kn was smaller than the calculated flexural strength. After that, flexural compressive failure of cover concrete happened at the both ends of the column. As a result, lateral capacity declined gradually. In the test specimen LC-PS retrofitted with pre-tensioned external high-strength hoops and steel plates under the axial force ratio of.31, the experimental shear force did 3

4 not decrease with the increase of drift ratio, R, and the experimental lateral capacity (5.2 kn) reached the calculated flexural strength. Because hinges formed at the both ends of the column, the hysteretic curve showed large ductility and remained stable until the final drift ratio of 5%. The strain of rebar at top region finally reached the yield strain and the experimental lateral capacity of this specimen was about 1.4 times of that of LC-P. In addition, there was no crack occurred in the retrofitted region of the column as shown in Figure 4. It is considered that the improvements of the seismic performance are attributed to the active and passive lateral confinement effect of external high-strength hoops and lateral confining pressure of steel plates. LC-P 8 Flexural strength LC-PS LC-Ph LC-PSh Figure 3. Measured shear force Q versus drift ratio R relationships. -8 LC-P LC-PS LC-Ph LC-PSh Figure 4. Failure Mode of specimens In the non-retrofitted test specimen LC-Ph with axial force ratio of.57, the failure mode was similar to the failure mode of LC-P. However, because the axial force ratio was higher, 4

5 Concrete confined by external high-strength hoops W. LI and O. KWON the flexural compressive failure of cover concrete at the both ends of the column became more remarkable, the spalling of cover concrete was also more violent (see Figure 4), and the shear force decreased faster after maximum strength. The strain of rebar at top region of LC-Ph reached the compressive yield level. On the other hand, in the test specimen LC- PSh retrofitted with external high-strength hoops and steel plates under the axial force ratio of.57, the experimental shear force reached the experimental lateral capacity (65. kn) at the drift ratio of 1.%. Simultaneously, the strain of rebar at top region approached the yield strain. Thereafter, the rapid degradation of shear force that was shown in LC-Ph was not observed. The experimental lateral capacity of this specimen was about 1.5 times of the capacity of LC-Ph. It is conceivable that the flexural strength of column becomes larger due to the confinement effect of pre-tensioned external high-strength hoops and steel plates. Numerical modelling with VecTor2 This section describes a nonlinear numerical studies on the specimens based on VecTor2 LC-P & LC-Ph LC-PS LC-PSh N N N High strength concrete Longitudinal reinforcement (truss elements) Concrete confined by hoop Unconfined concrete Pin Figure 5. FormWorks models for column specimens 5

6 (Vecchio et al., 24). VecTor2 is a nonlinear finite element program for the analysis of twodimensional reinforced concrete structures and has been developed at the University of Toronto. The theoretical bases of VecTor2 are the Modified Compression Field Theory (MCFT) proposed by Vecchio FJ and Collins MP (1986) and the Disturbed Stress Field Model (DSFM) proposed by Vecchio FJ (2). VecTor2 permits accurate assessments of structural performance (strength, post-peak behaviour, failure mode, deflections and cracking) of RC elements regardless of failure modes. The VecTor2 bundle includes: FormWorks, a graphics-based preprocessor program that simplifies the development of a numerical model; Augustus, a complete VecTor2 post-processor that provides all the global and local results in useful numerical or graphic formats. FormWorks is also able to display the specimen crack pattern at each stage of imposed displacement which is very useful to detect the failure mode from a numerical model. In VecTor2, the automatic mesh generation facility with the hybrid discretization type is used to create the mesh of the specimens as shown in Figure 5. In these models, the specimens are represented with rectangular elements for the concrete and truss bar elements for the longitudinal reinforcing bars. Boundary nodes at all the bottom stubs are restrained in both X and Y directions. However, all other nodes are unconstrained. In order to constrain the rotation of upper stubs and prevent localized failure from stress concentration where the displacement load is applied, the stubs are modelled with high strength concrete. In this study, several reinforced concrete material types are used for the column regions. One material type represents the unconfined concrete for the cover concrete elements of the two non-retrofitted test specimens LC-P and LC-Ph. Another material type models the concrete confined by hoop only for the core concrete elements of LC-P and LC-Ph. And the others stand for the concrete confined by external pre-tensioned high-strength hoops and steel plates. For the core concrete elements of LC-P and LC-Ph, the confinement effects due to hoops are taken into account by means of the geometric percentage of in-plane and out-of-plane reinforcements according to the VecTor2 & Formworks User s Manual (Wong et al., 213). In this study, the column regions of test specimens LC-PS and LC-PSh were retrofitted by steel plates from four sides, and the prestressing was applied to every highstrength steel bar, the whole cross section of the retrofitted column would be confined by the steel plates. As a result, the effective area of confined concrete is the whole cross section of column. For the two retrofitted test specimens LC-PS and LC-PSh, the whole cross section of the column is considered as confined concrete, and confined concrete elements are divided into several parts based on the spacing of high-strength hoops. The external highstrength hoop is modelled with Prestressing Steel and the prestrain of 245 is applied to the material. The confinement effects due to external high-strength hoops are also taken into account by means of the geometric percentage of in-plane and out-of-plane reinforcements. However, the passive confinement effects of the internal hoops are neglected because of Table 3. Analysis parameters of VecTor2 Convergence Criteria Displacements - Weighted Concrete Bond Eligehausen Model Compression Base Curve Popovics (HSC) Concrete Creep / Relax Not Considered Compression Post-Peak Popovics / Mander Concrete Hysteresis Nonlinear w/ Offsets Compression Softening Vecchio 1992-A Steel Hysteresis Bauschinger (Seckin) Tension Stiffening Modified Bentz 23 Rebar Dowel Action Tassios (Crack Slip) Tension Softening Linear Rebar Buckling Refined Dhakal-Maekawa Concrete Dilatation Variable - Kupfer Slip Distortion Walraven (Monotonic) Cracking Criterion Mohr-Coulomb (Stress) Strain Rate Effects Not Considered Crack Stress Calculation Basic (DSFM/MCFT) Geometric Nonlinearity Considered Crack Width Check Crack Limit (Agg/2.5) Crack Allocation Uniform Spacing 6

7 poor internal hoop ratio. With regard to concrete models, Popovics (High Strength) model is selected for the pre-peak compression response, and Popovics / Mander model is selected for the post-peak compression response. For reinforcement model, ductile steel reinforcement is chosen for the reinforcement type, and Bauschinger Effect (Seckin) model proposed by Seckin (1981), is selected for the hysteretic response. Eligehausen Model proposed by Eligehausen et al. (1983), is assumed as bond stress-slip response between the bar and concrete element. Summation of the material properties for the concrete elements, reinforcement elements, bond elements, and the chosen analytical models for each material are shown in Table 3. Further details of the analytical parameters can be investigated in the VecTor2 & Formworks User s Manual. 8 8 LC-P Experimental result LC-PS Experimental result LC-Ph LC-PSh Figure 6. Comparisons of experimental results and numerical results of Vector2 The cyclic analyses are carried out by imposing lateral displacements to node 1 and applying axial force N to the top nodes of upper stub (see Figure 5) for these specimens. The comparisons of experimental results and numerical results are shown in Figure 6. The flexural stiffness of each specimen based on VecTor2 agrees well with the experimental one. This high level of accuracy in the numerical model is because VecTor2 considers the loaddeformation behaviour of cracked reinforced concrete subjected to shear. The hysteretic response of numerical models of non-retrofitted test specimens LC-P and LC-Ph match the experimental ones well. The numerical response of retrofitted test specimens LC-PS and LC-PSh shows good ductility and remains stable. However, for LC-PS and LC-PSh, the accumulated absorbed energy of numerical response is smaller than that of experimental one. This fact suggests that Eligehausen Model underestimates the actual bond behavior -8 7

8 between the mooth rebar and concrete due to active confinement effect of external highstrength hoops. Conclusions (1) When the four sides of the low strength concrete RC column with smooth rebar were prestressed with the steel plates and pre-tensioned high-strength hoops, the lateral capacity and ductility of column were increased remarkably. The retrofit technique introduced in this study is expected to be applied to existing RC columns with low concrete strength. (2) When the proposed retrofit technique is used, the expansion of concrete was controllable even though the column has low strength concrete, smooth rebar and high axial force ratio. (3) The numerical model predicted the response of the experimental specimen accurately. Thus, more extensive parameteric study would be possible through numerical simulation before applying the retrofit technique to real practice. Acknowledgments The authors would like to thank Professor F.J. Vecchio at the University of Toronto for providing VecTor2 software utilized for numerical study. Moreover, the first author is deeply indebted to Dr. Yamakawa who is an emeritus Professor of University of the Ryukyus, Doctors Esaki who is a former professor of Fukuoka University and Kimura who is a professor of Fukuoka University for their valuable help. Furthermore, supports on the loading test are provided by Mr. S. Honda who is an Assistant Professor of Fukuoka University, Mr. T. Tanaka who is Research Associate of Fukuoka University, H. Hirakuni who is a technical staff of Fukuoka University and former students of Fukuoka University. REFERENCES The Japan Building Disaster Prevention Association (21) Standard for Seismic Evaluation of Existing RC Building, The Japan Building Disaster Prevention Association, Tokyo, Japan (In Japanese) Architectural Institute of Japan (199) Ultimate Strength and Deformation Capacity Buildings in Seismic Design, Architectural Institute of Japan, Tokyo, Japan (In Japanese) Vecchio FJ, Bentz EC and Collins MP (24) Tools for forensic analysis of concrete structures, Computer and Concrete, 1(1): 1-14 Vecchio FJ and Collins MP (1986) The Modified Compression Field Theory for Reinforced Concrete Elements Subject to Shear, ACI Journal, 83(2): Vecchio FJ (2) Disturbed Stress Field Model for Reinforced Concrete: Formulation, ASCE Journal of Structural Engineering, 126(9): Wong PS, Vecchio FJ and Trommels H (213) VecTor2 & FormWorks User s Manual, 2 nd Ed, University of Toronto, Canada Seckin M (1981) Hysteretic Behaviour of Cast-in-Place Exterior Beam-Column-Slab Subassemblies, Ph.D. Thesis, Department of Civil Engineering, University of Toronto, Canada Eligehausen R, Popov E and Bertero V (1983) Local Bond Stress-Slip relationship of Deformed Bars under Generalized Excitations, Report No. UCB/EERC-83/23, Earthquake Engineering Center, University of California, Berkeley, U.S.A. 8