International Journal of Civil Engineering and Technology (IJCIET), ISSN (Print), AND TECHNOLOGY (IJCIET)

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1 INTERNATIONAL ISSN (Online), Volume JOURNAL 5, Issue 6, June (2014), OF pp CIVIL IAEME ENGINEERING AND TECHNOLOGY (IJCIET) ISSN (Print) ISSN (Online) Volume 5, Issue 6, June (2014), pp IAEME: Journal Impact Factor (2014): (Calculated by GISI) IJCIET IAEME OUT-OF-PLANE BUCKLING OF HIGHLY REINFORCED SEISMIC WALLS: DISPLACEMENTS AND MODE OF FAILURE Theodoros A. Chrysanidis *1, Ioannis A. Tegos 2 *1 Aristotle University of Thessaloniki, School of Engineering, Department of Civil Engineering, Division of Structural Engineering, Thessaloniki, Greece. (T. A. Chrysanidis, Dr. Civil Engineer, MSc, MSc DIC). 2 Aristotle University of Thessaloniki, School of Engineering, Department of Civil Engineering, Division of Structural Engineering, Thessaloniki, Greece. (I. A. Tegos, Professor) ABSTRACT In the past few years, a concern is observed internationally regarding the seismic mechanical behavior of reinforced concrete walls, especially against their transverse instability under extreme seismic loads. The present work is experimental and examines the influence of tensile strain on outof-plane buckling of reinforced concrete seismic walls. The experimental work herein consists of 5 scaled test specimens simulating the end regions of structural walls. These specimens were reinforced with the same high longitudinal reinforcement ratio (3.68%). The degree of tensile strain applied was different for each specimen and it took values equal to 0, 10, 20, 30 and 50. The present work tries to investigate the influence of the degree of tensile strain to the displacements (horizontal and vertical) and the modes of failure of test specimens. Keywords: Buckling, Seismic Walls, Out-of-Plane, Tensile Strain. 1. INTRODUCTION A sufficient number of reinforced concrete walls is good to be utilized during the process of seismic design of reinforced concrete buildings. Buildings with a large number of structural walls, have demonstrated exceptional behaviour against seismic action, even if these walls had not been reinforced according to the modern perceptions [1]. Structural walls designed to be in a high ductility category according to modern international codes such as EC8: 2004 [2] and NZS 3101: 2006 [3] or designed with increased ductility requirements according to Ε.Κ.Ω.Σ (Greek Concrete Code 2000) [4], are expected to present extensive tensile deformations, especially in the plastic hinge 101

2 region of their base. According to Chai and Elayer [5], tensile deformations up to 30 are expected at the walls of the bottom storey height depending on their geometric characteristics and the level of ductility design of the walls. These tensile deformations, depending on their size, can cause out-ofplane buckling of walls. Prominent researchers, like Paulay [6], propose the use of flanges or enlarged boundary elements in the extreme regions of walls, which provide protection to the bending compression regions against transverse instability. Moreover, these elements are easier to be confined. New Zealand Concrete Code (NZS 3101: 2006) [3] and other modern international codes propose the construction of such elements. The phenomenon of lateral buckling of R/C walls depends basically on the size of tensile deformations which are imposed at the extreme regions of walls at the first semi-cycle of seismic loading and not so much on the size of flexural compression which is imposed at the reversal of seismic loading, according to Paulay and Priestley [7]. The authors of the present work have conducted an initial research work on the phenomenon of lateral buckling of reinforced concrete structural walls strained under seismic action [8][9]. The progress of research resulted to some promising results regarding the influence of the factor of elongation degree to the maximum failure load of seismic walls [10] and to the displacements of seismic walls [11]. The work presented herein focuses on the influence of the degree of tensile strain to the wall displacements. The present work on the phenomenon of out-of-plane buckling constitutes a small part of an extensive research program that took place at the Laboratory of Reinforced Concrete and Masonry Structures of the Department of Civil Engineering of Aristotle University of Thessaloniki. It has to be noted the fact that degree of tensile strain, degree of elongation and degree of tensile deformation refer to the same thing (meaning the size of the tensile deformation that longitudinal reinforcement is subjected to) at the present paper. 2. EXPERIMENTAL INVESTIGATION 2.1. Test specimen characteristics The test specimens were constructed using the scale 1:3 as a scale of construction. The dimensions of specimens are equal to 7.5x15x90 cm. The reinforcement of specimens consists of 4 bars of 10 mm diameter and 2 bars of 8 mm diameter. The total number of specimens is equal to 5. Each specimen was submitted first in tensile loading of uniaxial type up to a preselected degree of elongation and then was strained under concentric compression loading. The differentiation of specimens lies in varying degrees of elongation imposed on each one of them. Fig. 1 presents their front view both for tensile and compressive loading, while all specimen characteristics are brought together in Table 1. Table 1: Test specimens characteristics Name of specimens Υ-4Ø10+2Ø Υ-4Ø10+2Ø Υ-4Ø10+2Ø Υ-4Ø10+2Ø Υ-4Ø10+2Ø Dimensions (cm) Longitudinal reinforcement Transverse reinforcement Longitudinal reinforcement ratio (%) Concrete cube resistance at day of compression test (MPa) Degree of elongation ( ) 15x7.5x90 15x7.5x90 15x7.5x90 15x7.5x90 15x7.5x90 4Ø10+2Ø8 4Ø10+2Ø8 4Ø10+2Ø8 4Ø10+2Ø8 4Ø10+2Ø8 Ø4.2/3.3cm Ø4.2/3.3cm Ø4.2/3.3cm Ø4.2/3.3cm Ø4.2/3.3cm

3 Fig. 1: Sketch of front view of specimens having longitudinal reinforcement 4Ø10+2Ø8 for: (a) Tension test, (b) Compression test 2.2. Loading of specimens The experimental setups used in order to impose to the specimens in the first semi cycle of loading a uniaxial tensile load and in the second semi cycle of loading a concentric compressive load are shown in Fig. 2. Fig. 2: Test setup for application of: (a) Tensile loading, (b) Compressive loading 103

4 3. EXPERIMENTAL RESULTS Fig. 3 refers to the concentric compression test and shows the change of transverse displacement relative to the applied compressive load this time, while Fig. 4 depicts the residual transverse displacement in relation to the normalized specimen height. Finally, Fig. 5 shows the various failure modes of all specimens after the completion of the compressive loading. Transverse displacement δ m (mm) Υ-4Ø10+2Ø Υ-4Ø10+2Ø Υ-4Ø10+2Ø Υ-4Ø10+2Ø Υ-4Ø10+2Ø Load P (kn) Ratio P/(f c ' A g ) Normalized transverse displacement δ m /b Fig. 3: Diagram of compressive load [P(kN), P/(f c A g )] transverse displacement at the midheight of test specimens [δ m /b, δ m (mm)] Normalized residual transverse displacement δ/b Υ-4Ø10+2Ø Υ-4Ø10+2Ø Υ-4Ø10+2Ø Υ-4Ø10+2Ø Υ-4Ø10+2Ø Normalized specimen hegight y/h Normalized specimen height y/h Residual transverse displacement δ (mm) Fig. 4: Diagram of normalized specimen height [z/h] residual transverse displacement [δ(mm), δ/b] 104

5 Fig. 5: Modes of failure of specimens after the completion of the compression test: (a) Υ- 4Ø10+2Ø , (b) Υ-4Ø10+2Ø , (c) Υ-4Ø10+2Ø , (d) Υ- 4Ø10+2Ø , (e) Υ-4Ø10+2Ø ANALYSIS OF RESULTS The observations from the conduct of the experimental investigation are as follows: 1. It becomes readily apparent that there is substantial variation of the maximum failure load by varying the degree of elongation. It is generally observed that increasing the degree of elongation results to a decreased maximum failure load. This trend, however, is valid under certain conditions. 105

6 2. The evaluation of maximum residual transverse displacements indicates that there is not a clear tendency for these types of displacements to be increased or decreased by increasing the degree of elongation (Figs. 6, 7). Maximum residual transverse displacement δ max (mm) Elongation Δh ε (mm) Fig. 6: Diagram of maximum residual transverse displacement [δ max (mm), δ max /δ max,0 ] elongation [ h ε /h( ), h ε (mm)]. Ratio δ max /δ max,o % Nominal degree of elongation Δh ε /h ( ) 4Ø10+2Ø8 4Ø10+2Ø8 4Ø10+2Ø8 4Ø10+2Ø8 4Ø10+2Ø8 75% Longitudinal reinforcement Fig. 7: Column diagram of maximum residual transverse displacement [δ max /δ max,0, δ max (mm)] elongation and type of longitudinal reinforcement [ h ε /h( )]. 29% 350% 258% Nominal degree of elongation Δh ε /h ( ) Maximum residual transverse displacement δ max (mm) Ratio δ max /δ max,o 106

7 5. CONCLUSIONS The following conclusions have derived from the analysis and evaluation of experimental results: 1. Behaviour of test specimens during the second semi cycle of the compressive loading are drastically affected by the imposed degree of tensile strain during the first semi cycle of tensile loading when imposed tensile deformation exceeds a certain value (in our case this value is 30 ). 2. As far as transverse displacements (maximum residual transverse displacements) are concerned, it seems that there is not a clear relation between degree of elongation and transverse displacements. So, no clear conclusion has been derived on this matter. REFERENCES [1] J.W. Wallace, J.P. Moehle (1992), "Ductility and detailing requirements of bearing wall buildings", ASCE Journal of Structural Engineering, Vol. 116, No. 6, pp [2] European Committee for Standardization (2004), "EN :2004, Eurocode 8: Design of structures for earthquake resistance - Part 1: General rules, seismic actions and rules for buildings", Brussels, Belgium. [3] Standards New Zealand (2006), "NZS 3101:2006, Concrete structures standard: Part 1 The design of concrete structures", Wellington, New Zealand. [4] Ministry of Environment, Planning and Public Works (2000), "Greek Code for the Design and Construction of Concrete Works". Athens, Greece. (In Greek). [5] Y.H. Chai, D.T. Elayer (1999), "Lateral stability of reinforced concrete columns under axial reversed cyclic tension and compression", ACI Structural Journal, Vol. 96, No. 5, pp [6] T. Paulay (1986), "The design of ductile reinforced concrete structural walls for earthquake resistance", Earthquake Spectra, Vol. 2, No. 4, pp [7] T. Paulay, M.J.N. Priestley (1993), "Stability of ductile structural walls", ACI Structural Journal, Vol. 90, No. 4, pp [8] T. Chrysanidis, I. Tegos, G. Pallogos, S. Christodoulou (2008), "Lateral instability of alternating tensile and compressive flanges of RC shear walls due to intense seismic flexure", Proceedings of 3 rd Panhellenic Conference on Earthquake Engineering, Athens, Greece, ID (In Greek). [9] T. Chrysanidis, I. Tegos, V. Gkagkousis (2009), "The influence of longitudinal reinforcement ratio of boundary edges of R/C shear walls in their lateral stability", Proceedings of 16 th Greek Concrete Conference, Pafos, Cyprus, ID (In Greek). [10] T. Chrysanidis (2014), "Size of seismic tensile strain and its influence on the lateral buckling of highly reinforced concrete walls", IOSR Journal of Mechanical and Civil Engineering, Vol. 11, No. 1, pp [11] T. Chrysanidis (2014), "The influence of tensile strain of the high-reinforced end sections of seismic walls to their displacements and mode of failure", Proceedings of the International Conference on Recent Trends in Engineering and Technology 2014 (IJBRMM s ICRTET 2014), Cochin, Kerala, India, ID CC1266. [12] Prerna Nautiyal, Saurabh Singh and Geeta Batham (2013), A Comparative Study of the Effect of Infill Walls on Seismic Performance of Reinforced Concrete Buildings, International Journal of Civil Engineering & Technology (IJCIET), Volume 4, Issue 4, pp , ISSN Print: , ISSN Online: