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CONTENTS
Volume 7, Number 5, November 2014
 

  • Part 1
  • Behavior of concrete and composite structures subjected to earthquake-simulated loading; Guest Editor: Thomas Kang
    Abstract; Full Text (92K) pages -.

Abstract


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Abstract


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Abstract
The capabilities of a high-resolution Digital Image Correlation (DIC) system are presented within the context of deformation measurements of full-scale concrete columns tested under reversed cyclic loading. The system was developed to have very high-resolution such that material strains on the order of the cracking stain of concrete could be measured on the surface of full-scale structural members. The high-resolution DIC system allows the measurement of a wide range of deformations and strains that could only be inferred or assumed previously. The DIC system is able to resolve the full profiles of member curvatures, rotations, plasticity spread, shear deformations, and bar-slip induced rotations. The system allows for automatic and objective measurement of crack widths and other damage indices that are indicative of cumulated damage and required repair time and cost. DIC damage measures contrast prevailing proxy damage indices based on member force-deformation data and subjective damage measures obtained using visual inspection. Data derived from high-resolution DIC systems is shown to be of great use in advancing the state of behavioral knowledge, calibrating behavioral and analytical models, and improving simulation accuracy.

Key Words
digital image correlation; deformations; strains, damage index; concrete

Address
Drit Sokoli and William Shekarchi: Department of Civil Architectural and Environmental Engineering, University of Texas at Austin 301 E. Dean Keeton St. STOP C1700, Austin, TX, USA

Eliud Buenrostro and Wassim M. Ghannoum: JQ Dallas, 2015 Commerce St., Dallas, TX, USA

Abstract
Using OpenSees as a framework, constitutive models of reinforced, prestressed and prestressed steel fiber concrete found by the panel tests have been implemented into a finite element program called Simulation of Concrete Structures (SCS) to predict the seismic behavior of shear-critical reinforced and prestressed concrete structures. The developed finite element program was validated by tests on prestressed steel fiber concrete beams under monotonic loading, post tensioned precast concrete column under reversed cyclic loading, framed shear walls under reversed cyclic loading or shaking table excitations, and a seven-story wall building under shake table excitations. The comparison of analytical results with test outcomes indicates good agreement.

Key Words
constitutive models; reinforced and prestressed concrete structures; finite element analysis;

Address
Arghadeep Laskar: Department of Civil Engineering, Indian Institute of Technology Bombay, Powai, Mumbai 200076, India


Liang Lu and Xilin Lu: Institute of Structural Engineering and Disaster Reduction, Tongji University, Shanghai, 200092, China

Feng Qin and Feng Fan: School of Civil Engineering, Harbin Institute of Technology, Harbin, 150090, China

Y.L. Mo and Thomas T.C. Hsu: Department of Civil and Environmental Engineering, University of Houston, Houston, 77204-4003, USA



Abstract
Squat reinforced concrete walls require enough shear strength in order to promote flexural yielding, which creates the need for designers of an accurate method for strength prediction. In many cases, especially for existing buildings, strength estimates might be insufficient when more accurate analyses are needed, such as pushover analysis. In this case, estimates of load versus displacement are required for building modeling. A model is developed that predicts the shear load versus shear deformation of squat reinforced concrete walls by means of a panel formulation. In order to provide a simple, design-oriented tool, the formulation considers the wall as a single element, which presents an average strain and stress field for the entire wall. Simple material constitutive laws for concrete and steel are used. The developed models can be divided into two categories: (i) rotating-angle and (ii) fixed-angle models. In the first case, the principal stress/strain direction rotates for each drift increment. This situation is addressed by prescribing the average normal strain of the panel. The formation of a crack, which can be interpreted as a fixed principal strain direction is imposed on the second formulation via calibration of the principal stress/strain direction obtained from the rotating-angle model at a cracking stage. Two alternatives are selected for the cracking point: fcr and 0.5fcr (post-peak). In terms of shear capacity, the model results are compared with an experimental database indicating that the fixed-angle models yield good results. The overall response (load-displacement) is also reasonable well predicted for specimens with diagonal compression failure.

Key Words
squat wall; panel model; strength; backbone; reinforced concrete; shear

Address
Leonardo M. Massone and Marco A. Ulloa: Department of Civil Engineering, University of Chile, Blanco Encalada 2002, Santiago, Chile

Abstract
Structural walls (also known as shear walls) are one of the common lateral load resisting elements in reinforced concrete (RC) buildings in seismic regions. The performance of RC structural walls in recent earthquakes has exposed some problems with the existing design of RC structural walls. The main issues lie around the buckling of bars, out-of plane deformation of the wall (especially the zone deteriorated in compression), reinforcement getting snapped beneath a solitary thin crack etc. This study compares performance of a typical wall designed by different standards. For this purpose, a case study RC shear wall is taken from the Hotel Grand Chancellor in Christchurch which was designed according to the 1982 version of the New Zealand concrete structures standard (NZS3101:1982). The wall is redesigned in this study to comply with the detailing requirements of three standards; ACI-318-11, NZS3101:2006 and Eurocode 8 in such a way that they provide the same flexural and shear capacity. Based on section analysis and pushover analysis, nonlinear responses of the walls are compared in terms of their lateral load capacity and curvature as well as displacement ductilities, and the effect of the code limitations on nonlinear responses of the different walls are evaluated. A parametric study is also carried out to further investigate the effect of confinement length and axial load ratio on the lateral response of shear walls.

Key Words
reinforced concrete; shear wall; design codes; comparative performance; cnfinement length; axial load ratio

Address
Farhad Dashti, Rajesh P Dhakal and Stefano Pampanin: Department of Civil and Natural Resources Engineering, University of Canterbury, Private Bag 4800, Christchurch 8140, New Zealand

Abstract
Reinforced concrete (RC) shear walls are commonly used for building structures to resist seismic loading. While the RC shear walls can have a high load-carrying capacity, they tend to fail in a brittle mode under shear, accompanied by forming large diagonal cracks and bond splitting between concrete and steel reinforcement. Improving seismic performance of shear walls has remained a challenge for researchers all over the world. Engineered Cementitious Composite (ECC), featuring incredible ductility under tension, can be a promising material to replace concrete in shear walls with improved performance. Currently, the application of ECC to large structures is limited due to the lack of the proper constitutive models especially under shear. In this paper, a new Cyclic Softening Membrane Model for reinforced ECC is proposed. The model was built upon the Cyclic Softening Membrane Model for reinforced concrete by (Hsu and Mo 2010). The model was then implemented in the OpenSees program to perform analysis on several cases of shear walls under seismic loading. The seismic response of reinforced ECC compared with RC shear walls under monotonic and cyclic loading, their difference in pinching effect and energy dissipation capacity were studied. The modeling results revealed that reinforced ECC shear walls can have superior seismic performance to traditional RC shear walls.

Key Words
constitutive model; engineered cementitious composite, shear walls, nonlinear finite element, pinching effect

Address
Mo Li, Hieu C. Luu, Chang Wu, Y.L.Mo and Thomas T.C. Hsu: Department of Civil & Environmental Engineering, University of Houston, Houston,Texas 77204-4003, United States

Abstract
This paper presents an analytical model for determining the transverse reinforcement required for reinforced concrete exterior beam-column joints subjected to reversed cyclic loading. Although the joint aspect ratio can affect joint shear strength, current design codes do not consider its effects in calculating joint shear strength and the necessary amount of transverse reinforcement. This study re-evaluated previous exterior beam-column joint tests collected from 11 references and showed that the joint shear strength decreases as the joint aspect ratio increases. An analytical model was developed, to quantify the transverse reinforcement required to secure safe load flows in exterior beam-column joints. Comparisons with a database of exterior beam-column joint tests from published literature validated the model. The required sectional ratios of horizontal transverse reinforcement calculated by the proposed model were compared with those specified in ACI 352R-02. More transverse reinforcement is required as the joint aspect ratio increases, or as the ratio of vertical reinforcement decreases; however, ACI 352R-02 specifies a constant transverse reinforcement, regardless of the joint aspect ratio. This reevaluation of test data and the results of the analytical model demonstrate a need for new criteria that take the effects of joint aspect ratio into account in exterior joint design.

Key Words
exterior beam-column joints; joint aspect ratio; joint shear strength; transverse reinforcement; strut-and-tie model

Address
Sung Chul Chun: Division of Architecture and Urban Design, Incheon National University, Incheon, Korea

Abstract
The equivalent frame method (EFM) is widely used for the design of two-way reinforced concrete slab structures, and current design codes of practice permit the application of the EFM in analyzing the flat plate slab structures under gravity and lateral loads. The EFM was, however, originally developed for the flat plate structures subjected to gravity load, which is not suitable for lateral loading case. Therefore, this study, the first part of series research paper, proposed the structural analysis method for the flat plate slab structures under the combined gravity and lateral loads, which is named as the unified equivalent frame method (UEFM). In the proposed method, some portion of rotation induced in the torsional member is distributed to the flexibility of the equivalent columns, and the remaining portion is contributed to that of the equivalent slabs. In the consecutive companion paper, the proposed UEFM is verified by comparing with test results of multi-span flat plate structures. Also, a simplified nonlinear push-over analysis method is proposed, and verified by comparing to test results.

Key Words
flat plate; slab; lateral load; gravity load; combined load; equivalent frame method; torsion

Address
KangSu Kim, Seung-Ho Choi, Hyunjin Ju and Deuck Hang Lee: Department of Architectural Engineering, University of Seoul, 163 Siripdaero, Dongdaemun-gu, Seoul,130-743, Korea

Jae-Yeon Lee: Division of Architecture, Mokwon University, 88 Doanbuk-ro, Seo-gu, Daejeon, 302-729, Korea

Myoungsu Shin: School of Urban and Environmental Engineering, UNIST, 50 UNIST-gil, Ulsan, 689-798, Korea

Abstract
In the previous paper, authors proposed the unified equivalent frame method (UEFM) for the lateral behavior analysis of the flat plate structure subjected to the combined gravity and lateral loads, in which the rotations of torsional members were distributed to the equivalent column and the equivalent slab according to the relative ratio of gravity and lateral loads. In this paper, the lateral behavior of the multi-span flat plate structures under various levels of combined gravity and lateral loads were analyzed by the proposed UEFM, which were compared with test results as well as those estimated by existing models. In addition, to consider the stiffness degradation of the flat plate system after cracking, the stiffness reduction factors for torsional members were derived from the test results of the interior and exterior slab-column connection specimens, based on which the simplified nonlinear push-over analysis method for flat plate structures was proposed. The simplified nonlinear analysis method provided good agreements with test results and is considered to be very useful for the practical design of the flat plate structures under the combined gravity and lateral loads.

Key Words
flat plate; lateral load; gravity load; equivalent frame method; torsion; stiffness degradation, push-over analysis

Address
Seung-Ho Choi, Deuck Hang Lee, Jae-Yuel Oh and Kang Su Kim: Department of Architectural Engineering, University of Seoul, 163 Siripdaero, Dongdaemun-gu, Seoul,
130-743, Korea

Jae-Yeon Lee: Division of Architecture, Mokwon University, 88 Doanbuk-ro, Seo-gu, Daejeon, 302-729, Korea

Myoungsu Shin: School of Urban and Environmental Engineering, UNIST, 50 UNIST-gil, Ulsan, 689-798, Korea

  • Part 2
  • Recent developments in seismic design of reinforced concrete structures toward urban resilience; Guest Editor: Myoungsu Shin
    Abstract; Full Text (92K) pages -.

Abstract


Key Words


Address


Abstract
This study investigates the behavior of precast concrete cantilever wall systems with new vertical connections under cyclic loading. C-type steel connections for PC wall systems are proposed for the transfer of bending moments between walls in the vertical direction, whereas a shear key in the center of the wall is prepared to transfer shear forces by bearing pressure. The proposed connections are assembled easily because the directions of the slots are different at the edges of the walls. Structural performance characteristics such as the strength, ductility, and failure modes of test specimens were investigated. The longitudinal reinforcing steel bars, which are connected to the C-type connections, yielded first. Ultimate deformation was terminated owing to premature failure of the connections. The strength and deformation obtained from the cross-sectional analysis were generally similar to experimental data.

Key Words
precast concrete wall; C-type connections; failure mode; strength; deformation

Address
Woo-Young Lim and Sung-Gul Hong: Department of Architecture and Architectural Engineering, Seoul National University,
1 Gwanak-ro, Gwanak-gu, Seoul 151-742, Republic of Korea

Abstract
This experimental investigation was conducted to examine the behavior and response of high-strength material (HSM) reinforced concrete (RC) columns under combined high-axial and cyclic-increasing lateral loads. All the columns use high-strength concrete (fc\' =100 MPa) and high-yield strength steel (fy =685 MPa and fy=785 MPa) for both longitudinal and transverse reinforcements. A total of four full-scale HSM columns with amount of transverse reinforcement equal to 100% more than that required by earthquake resistant design provisions of ACI-318 were tested. The key differences among those four columns are the spacing and configuration of transverse reinforcements. Two different constant axial loads, i.e. 60% and 30% of column axial load capacity, were combined with cyclically-increasing lateral loads to impose reversed curvatures in the columns. Test results show that columns under 30% of axial load capacity behaved much more ductile and had higher lateral deformational capacity compared to columns under the 60% of axial load capacity. The columns using closer transverse reinforcement spacing have slightly higher ductility than columns with larger spacing.

Key Words
high-strength concrete; high-yield strength steel; high-axial load, RC column, cyclic loads, ductility

Address
Muhammad Y. Bhayusukmaand and Keh-ChyuanTsai: Department of Civil Engineering, National Taiwan University,
No. 1, Sec. 4, Roosevelt Road, Taipei, Taiwan

Abstract
This research aimed to investigate retrofitting methods for damaged RC columns with SRF (Super Reinforced with Flexibility) and aramid composites and their impacts on the seismic responses. In the first stage, two original (undamaged) column specimens, designed to have a flexural- or shear-controlled failure mechanism, were tested under quasi-static lateral cyclic and constant axial loads to failure. Afterwards, the damaged column specimens were retrofitted, utilizing SRF composites and aramid rods for the flexural-controlled specimen and only SRF composites for the shear-controlled specimen. In the second stage, the retrofitted column specimens were tested again under the same conditions as the first stage. The hysteretic responses such as strength, ductility and energy dissipation were discussed and compared to clarify the specific effects of each retrofitting material on the seismic performances. Generally, SRF composites contributed greatly to the ductility of the specimens, especially for the shear-controlled specimen before retrofitting, in which twice the deformation capacity was obtained in the retrofitted specimen. The shear-controlled specimen also experienced a flexural failure mechanism after retrofitting. In addition, aramid rods moderately fortified the specimen in terms of the maximum shear strength. The maximum strength of the aramid-retrofitted specimen was 12% higher than the specimen without aramid rods. In addition, an analytical modeling of the undamaged specimens was conducted using Response-2000 and Zeus Nonlinear in order to further validate the experimental results.

Key Words
SRF composites; aramid rods; hysteretic behavior; displacement ductility; seismic retrofit

Address
Hoang V. Dan: Department of Architectural Engineering, Sejong University, 98 Gunja-dong, Gwangjin-gu, Seoul 143-747, Republic of Korea

Myoungsu Shin and Kihak Lee: Department of Architectural Engineering, Hanyang University, Wangsipli-ro 222, Seongdong-gu, Seoul 133-791, Republic of Korea

Sang Whan Han: Department of Architectural Engineering, Hanyang University, Wangsipli-ro 222, Seongdong-gu,
Seoul 133-791, Republic of Korea



Abstract
ASCE/SEI 41-13 provides modeling parameters and numerical acceptance criteria for various types of members that are useful for evaluating the seismic performance of reinforced concrete (RC) building structures. To accurately evaluate the global performance of a coupled wall system, it is crucial to first properly define the component behaviors (i.e., force-displacement relationships of shear walls and coupling beams). However, only a few studies have investigated on the modeling of RC coupling beams subjected to earthquake loading to date. The main objective of this study is to assess the reliability of ASCE 41-13 modeling parameters specified for RC coupling beams with various design details, based on a database compiling almost all coupling beam tests available worldwide. Several recently developed coupling beam models are also reviewed. Finally, a rational method is proposed for determining the chord yield rotation of RC coupling beams.

Key Words
ASCE/SEI 41-13; modeling parameters; coupling beam; chord yield rotation

Address
Seongwoo Gwon and Myoungsu Shin: School of Urban and Environmental Engineering, Ulsan National Institute of Science and Technology (UNIST), Ulsan, Korea

Benjamin Pimentel: Rosenwasser Grossman Consulting Engineers, 485 Seventh Avenue, New York, NY, USA

Deokjung Lee: Interdisciplinary School of Green Energy, Ulsan National Institute of Science and Technology (UNIST),
Ulsan, Korea

Abstract
In the present study, the seismic performance of structural walls with boundary elements confined by conventional tie hoops and steel fiber concrete (SFC) was investigated. Cyclic lateral loading tests on four wall specimens under constant axial load were performed. The primary test parameters considered were the spacing of boundary element transverse reinforcement and the use of steel fiber concrete. Test results showed that the wall specimen with boundary elements complying with ACI 318-11 21.9.6 failed at a high drift ratio of 4.5% due to concrete crushing and re-bar buckling. For the specimens where SFC was selectively used in the plastic hinge region, the spalling and crushing of concrete were substantially alleviated. However, sliding shear failure occurred at the interface of SFC and plain concrete at a moderate drift ratio of 3.0% as tensile plastic strains of longitudinal bars were accumulated during cyclic loading. The behaviors of wall specimens were examined through nonlinear section analysis adopting the stress-strain relationships of confined concrete and SFC.

Key Words
structural wall; boundary element; transverse reinforcement; steel fiber concrete; seismic performance

Address
Taesung Eom: Department of Architectural Engineering, Dankook University,152 Jukjeon-ro, Suji-gu, Yongin-si, Gyeonggi-do, 448-701, Republic of Korea

Sumin Kang and Okkyue Kim: Department of Architectural Engineering, Chungbuk National University,
410 Seongbong-ro, Heungdeok-gu, Cheongju Chungbuk 361-763, Republic of Kiorea

Abstract
This paper studies the response of seismic behavior of reinforced concrete exterior beam-column joints under reversal loading with different anchorages and joint core details. The joint core was detailed without much confinement (group-I) and/or with proposed X-cross bars in the core (group-II). The beam longitudinal reinforcement\'s anchorages were designed as per ACI 352 (headed bars), ACI 318 (conventional 90o bent hooks) and IS 456 (90o bent hooks with extended tails). The nonlinear finite element analysis response of the beam-column joints was studied, along with initial and progressive cracks up to failure. The experimental and

Key Words
reinforced concrete; exterior beam-column joint; headed bars; hooked bars; nonlinear finite element analysis; crack

Address
S. Rajagopal and S. Prabavathy: Department of Civil Engineering, Mepco Schlenk Engineering College, Sivakasi, Tamil Nabu 626-605, India

Abstract
The purpose of this study is to present adequate modeling solutions for squat and slender RC walls. ASCE41-13 (American Society of Civil Engineers) specifies that the aspect ratios of height to width for the RC walls affect the hysteresis response. Thus, this study performed non-linear analysis subjected to cyclic loading using two different macroscopic models: one of macroscopic models represents flexural failure of RC walls (Shear Wall Element model) and the other (General Wall Element model) reflects diagonal shear failure occurring in the web of RC walls. These analytical results were compared to previous experimental studies for a slender wall (> aspect ratio of 3.0) and a squat wall (= aspect ratio of 1.0). For the slender wall, the difference between the two macroscopic models was negligible, but the squat wall was significantly affected by parameters for shear behavior in the modeling method. For accurate performance evaluation of RC buildings with squat walls, it would be reasonable to use macroscopic models that give consideration to diagonal shear.

Key Words
macroscopic model; nonlinear analysis; reinforced concrete (RC) wall; aspect ratio

Address
Jiuk Shin: School of Civil and Environmental Engineering, Georgia Institute of Technology,
790 Atlantic Drive, Atlanta, GA 30332-0355, USA

JunHee Kim: Department of Architectural Engineering, Yonsei University, 50 Yonseiro, Seadaemun-gu, Seoul 120-749, Republic of Korea

Abstract
The purpose of this study is to propose a modification factor to reflect the lateral stiffness modification when a step is located in flat plates. Reinforced concrete slabs with steps have different structural characteristics that are demonstrated by a series of structural experiment and nonlinear analyses. The corner at the step is weak and flexible, and the associated rotational stiffness degradation at the corner of the step is identified through analyses of 6 types of models using a nonlinear finite element program. Then a systematic analysis of stiffness changes is performed using a linear finite element procedure along with rotational springs. The lateral stiffness of reinforced concrete flat plates with steps is mainly affected by the step length, location, thickness and height. Therefore, a single modification factor for each of these variables is obtained, while other variables are constrained. When multiple variables are considered, each single modification factor is multiplied by the other. Such a method is verified by a comparative analysis. Finally, a complex modification factor can be applied to the existing effective slab width.

Key Words
lateral stiffness; slab with step; flat plate, modification factor; effective slab width method

Address
Sanghee Kim, Thomas H.-K. Kang and Hong-Gun Park: Department of Architecture and Architectural Engineering, Seoul National University, Seoul 151-744, Korea

Jae-Yo Kim: Department of Architectural Engineering, KwangWoon University, Seoul 139-701, Korea


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