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CONTENTS
Volume 14, Number 3, March 2018
 

Abstract
The paper describes a modified cyclic bar model including bond-slip phenomena between steel reinforcing bars and surrounding concrete. The model is focused on plain bar and is useful, for its simplicity, for the seismic analyses of RC structures with plain bars and insufficient constructive details, such as in the case of \'60s-\'70s Mediterranean buildings. The model is based on an imposed exponential displacements field along the bar including both steel deformation and slip; through the adoption of equilibrium and compatibility equations a stress-slip law can be deducted and simply applied, with opportune operations, to RC numerical models. This study aims to update and complete the original monotonic model published by the authors, solving some numerical inconsistencies and, mostly, introducing the cyclic formulation. The first aim is achieved replacing the imposed linear displacement field along the bar with an exponential too, while the cyclic behaviour is described through a formulation based on the results of parametric analyses concerning a large range of steel and concrete properties and geometric configurations. Validations of the proposed model with experimental results available in the current literature confirm its accuracy and the reduced computational burden, highlighting its suitability in performing nonlinear analyses of RC structures.

Key Words
bond slip; plain bars; existing structures; fiber elements; cyclic/seismic behavior

Address
Silvia Caprili: Department of Civil and Industrial Engineering, University of Pisa, 1 Largo L. Lazzarino, 56126, Pisa, Italy
Francesca Mattei: Department of Structural and Geotechnical Engineering, University of Rome La Sapienza, 18 Eudossiana St., 00184, Rome, Italy
Rosario Gigliotti: Department of Structural and Geotechnical Engineering, University of Rome La Sapienza, 18 Eudossiana St., 00184, Rome, Italy
Walter Salvatore: Department of Civil and Industrial Engineering, University of Pisa, 1 Largo L. Lazzarino, 56126, Pisa, Italy

Abstract
Predictive demand and collapse fragility functions are two essential components of the probabilistic seismic demand analysis that are commonly developed based on statistics with enormous, costly and time consuming data gathering. Although this approach might be justified for research purposes, it is not appealing for practical applications because of its computational cost. Thus, in this paper, Bayesian regression-based demand and collapse models are proposed to eliminate the need of timeconsuming analyses. The demand model developed in the form of linear equation predicts overall maximum inter-story drift of the lowto mid-rise regular steel moment resisting frames (SMRFs), while the collapse model mathematically expressed by lognormal cumulative distribution function provides collapse occurrence probability for a given spectral acceleration at the fundamental period of the structure. Next, as an application, the proposed demand and collapse functions are implemented in a seismic fragility analysis to develop fragility and consequently seismic demand curves of three example buildings. The accuracy provided by utilization of the proposed models, with considering computation reduction, are compared with those directly obtained from Incremental Dynamic analysis, which is a computer-intensive procedure.

Key Words
probabilistic seismic demand analysis; demand model; collapse function; Bayesian regression; incremental dynamic analysis

Address
M. Kia: Department of Civil Engineering, University of Science and Technology of Mazandaran, Behshahr, Iran; Department of Civil Engineering, Amirkabir University of Technology, Tehran, P.O. Box 15875-4413, Iran
M. Banazadeh: Department of Civil Engineering, Amirkabir University of Technology, Tehran, P.O. Box 15875-4413, Iran
M. Bayat: Young Researchers and Elite Club, Roudehen Branch, Islamic Azad University, Roudehen, Iran

Abstract
Supplemental passive control devices are widely considered as an important tool to mitigate the dynamic response of a building under seismic excitation. Nevertheless, a systematic method for strategically placing dampers in the buildings is not prescribed in building codes and guidelines. Many deterministic and stochastic methods have been proposed by previous researchers to investigate the optimum distribution of the viscous dampers in the steel frames. However, the seismic performances of the retrofitted buildings that are under large earthquake intensity levels or near collapse state have not been evaluated by any seismic research. Recent years, an increasing number of studies utilize genetic algorithms (GA) to explore the complex engineering optimization problems. GA interfaced with nonlinear response history (NRH) analysis is considered as one of the most powerful and popular stochastic methods to deal with the nonlinear optimization problem of damper distribution. In this paper, the effectiveness and the efficiency of GA on optimizing damper distribution are first evaluated by strong ground motions associated with the collapse failure. A practical optimization framework using GA and NRH analysis is proposed for optimizing the distribution of the fluid viscous dampers within the moment resisting frames (MRF) regarding the improvements of large drifts under intensive seismic context. Both a 10-storey and a 20-storey building are involved to explore higher mode effect. A far-fault and a near-fault earthquake environment are also considered for the frames under different seismic intensity levels. To evaluate the improvements obtained from the GA optimization regarding the collapse performance of the buildings, Incremental Dynamic Analysis (IDA) is conducted and comparisons are made between the GA damper distribution and stiffness proportional damping distribution on the collapse probability of the retrofitted frames.

Key Words
probability of collapse; genetic algorithms; steel MRFs; viscous dampers; interstorey drift

Address
Xiameng Huang: School of Engineering, University of Warwick, Coventry CV4 7AL, U.K.

Abstract
Under strong ground motion, pounding can be caused because of the different dynamic properties between two adjacent buildings. Using different structural systems in two adjacent structures makes a difference in the lateral stiffness and thus changes the pounding force between them. In this paper, the effect of the structural system of adjacent buildings on the amount of force applied by pounding effects has been investigated. Moment resisting frame systems (MRFs), lateral X-bracing system (LBS), shear wall system (SWS) and dual system (DS) have been investigated. Four different cases has been modelled using finite element (FE) method. The number of stories of the two adjacent buildings is different in each case: case 1 with 6 and 4 stories, case 2 with 9 and 6 stories, case 3 with 15 and 6 stories and case 4 with 10 and 10 stories. The structures have been modelled three-dimensionally. Non-linear time history analysis has been done on the structures using the finite element software SAP2000. In order to model pounding effects, the non-linear gap elements have been used.

Key Words
pounding effects; structural systems; time history analysis

Address
Ali Kheyroddin: Department of Structural Engineering, Semnan University, Semnan, Iran
Mahdi Kioumarsi: Department of Civil Engineering and Energy Technology, OsloMet – Oslo Metropolitan University,
PI 830, Pilestredet 35, Oslo, Norway
Benyamin Kioumarsi: Department of Structural Engineering, Semnan University, Semnan, Iran
Aria Faraei: Department of Structural Engineering, Semnan University, Semnan, Iran

Abstract
To study the mechanical properties of joints in ancient timber buildings in depth, the force mechanism of the through-tenon joints was analyzed, also the theoretical formulas of the moment-rotation angles of the joints with different loosening degrees were deduced. To validate the rationality of the theoretical calculation formulas, six joint models with 1/3.2 scale ratio, including one intact joint and five loosening joints, were fabricated and tested under cyclic loading. The specimens underwent the elastic stage, the plastic stage and the destructive stage, respectively. At the same time, the moment-rotation backbone curves of the tenon joints with different looseness were obtained, and the theoretical calculation results were validated when compared with the experimental results. The results show that the rotational moment and the initial rotational stiffness of the tenon joints increase gradually with the increase of the friction coefficient. The increase of the tenon section height can effectively improve the bearing capacity of the through-tenon joints. As the friction coefficient of the wood and the insertion length of the tension increase, the embedment length goes up, whereas it decreases with the increase of section height. With the increase of the looseness, the bearing capacity of the joint is reduced gradually.

Key Words
ancient timber buildings; through-tenon joints; looseness; seismic performance; low cyclic reversed loading tests; moment-rotation relationship

Address
Jianyang Xue: Department of Civil Engineering, Xi\'an University of Architecture and Technology, Xi\'an 710055, China
Liangjie Qi: Department of Civil Engineering, Xi\'an University of Architecture and Technology, Xi\'an 710055, China; Department of Civil and Environmental Engineering, Virginia Tech, Blacksburg, 24060, USA
Jinshuang Dong: Department of Civil Engineering, Xi\'an University of Architecture and Technology, Xi\'an 710055, China
Dan Xu: Department of Civil Engineering, Xi\'an University of Architecture and Technology, Xi\'an 710055, China

Abstract
This paper aims to assess the seismic risk of a plane moment-resisting frames (MRFs) consisting of concrete-filled double skin steel tube (CFDST) columns and I-section steel beams. Firstly, three typical limit performance levels of CFDST structures are determined in accordance with the cyclic tests of seven CFDST joint specimens with 1/2-scaled and the limits stipulated in FEMA 356. Then, finite element (FE) models of the test specimens are built by considering with material degradation, nonlinear behavior of beam-column connections and panel zones. The mechanical behavior of the concrete material are modeled in compression stressed condition in trip-direction based on unified strength theory, and such numerical model were verified by tests. Besides, numerical models on 3, 6 and 9-story CFDST frames are established. Furthermore, the seismic responses of these models to earthquake excitations are investigated using nonlinear time-history analyses (NTHA), and the limits capacities are determined from incremental dynamic analyses (IDA). In addition, fragility curves are developed for these models associated with 10%/50yr and 2%/50yr events as defined in SAC project for the region on Los Angeles in the Unite State. Lastly, the annual probabilities of each limits and the collapse probabilities in 50 years for these models are calculated and compared. Such results provide risk information for the CFDST-MRFs based on the probabilistic risk assessment method.

Key Words
concrete-filled double-skin steel tube; beam-column joint; unified strength theory; seismic fragility; risk assessment

Address
Yi Hu, Junhai Zhao and Dongfang Zhang: School of Civil Engineering, Chang\'an University, Xi\'an 710061, China
Yufen Zhang: School of Civil Engineering, North China University of Technology, Beijing 100041, China

Abstract
Fiber reinforced cementitious composites (FRCC) materials that exhibit strain-hardening and multiple cracking properties under tension were recently developed as innovative building materials for construction. This study aims at exploring the use of FRCC on the seismic performance of coupling beams with conventional reinforcement. Experimental tests were conducted on seven FRCC precast coupling beams with small span-to-depth ratios and one ordinary concrete coupling beam for comparison. The crack and failure modes of the specimens under the low cycle reversed loading were observed, and the hysteretic characteristics, deformation capacity, energy dissipation capacity and stiffness degradation were also investigated. The results show that the FRCC coupling beams have good ductility and energy dissipation capacities compared with the ordinary concrete coupling beam. As the confinement stirrups and span-to-depth ratio increase, the deformation capacity and energy dissipation capacity of coupling beams can be improved significantly. Finally, based on the experimental analysis and shear mechanism, a formula for the shear capacity of the coupling beams with small span-to-depth ratios was also presented, and the calculated results agreed well with the experimental results.

Key Words
fiber reinforced cementitious composites; coupling beam; span-to-depth ratio; quasi-static test; conventional reinforcement; shear capacity

Address
Xingwen Liang and Pengtao Xing: School of Civil Engineering, Xi\'an University of Architecture and Technology, No. 13 Yanta Road, Xi\'an 710055, P.R. China

Abstract
Previous experimental researches indicate that reinforced concrete beam-column joints play an important role in the mechanical properties of moment resisting frame structures, so as to require proper design. In order to get better understanding of the beam-column joint performance, a rational model needs to be developed. Based on the former considerations, two typical models for calculating the shear carrying capacity of the beam-column joint including the inelastic reinforced concrete joint model and the softened strut-and-tie model are selected to be introduced and analyzed. After examining the applicability of two typical models mentioned earlier to interior beam-column joints, several adjustments are made to get better predicting of the test results. For the softened strut-and-tie model, four adjustments including modifications of the depth of the diagonal strut, the inclination angle of diagonal compression strut, the smeared stress of mild steel bars embedded in concrete, as well as the softening coefficient are made. While two adjustments for the inelastic reinforced concrete joint model including modifications of the confinement effect due to the column axial load and the correction coefficient for high concrete are made. It has been proved by test data that predicted results by the improved softened strut-and-tie model or the modified inelastic reinforced concrete joint model are consistent with the test data and conservative. Based on the test results, it is also not difficult to find that the improved beam-column joint model can be used to predict the joint carrying capacity and cracks development with sufficient accuracy.

Key Words
beam-column joint; carrying capacity; cracks development; axial compression ratio; softened strut-and-tie model; inelastic joint model

Address
Guoxi Fan: School of Engineering, Ocean University of China, No.238 Songling Road, Laoshan District, Qingdao City, China
Debin Wang: School of Civil and Safety Engineering, Dalian Jiaotong University, No.794 the Yellow River Road, Shahekou District, Dalian City, China
Jing Jia: School of Engineering, Ocean University of China, No.238 Songling Road, Laoshan District, Qingdao City, China


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