Abstract—Flat double-layer grid is from category of space structures that are formed from two flat layers connected together with diagonal members. Increased stiffness and better seismic resistance in relation to other space structures are advantages of flat double layer space structures. The objective of this study is assessment and calculation of Behavior factor of flat double layer space structures. With regarding that these structures are used widely but Behavior factor used to design these structures against seismic force is not determined and exact, the necessity of study is obvious. This study is theoretical. In this study we used structures with span length of 16m and 20 m. All connections are pivotal. ANSYS software is used to non-linear analysis of structures. Keywords—Behavior factor, Double-layer, Intensified resistance, Non-linear analysis I. INTRODUCTION HERE is high risk of seismic occurrence in most of world’s countries and strong earthquakes cause human and financial losses in some countries yearly, so it is necessary to prepare codes to design earthquake resistant structures with enough accuracy and safety factor. Discussing about various methods of structural analysis and selecting various factors of design in these codes is inevitable.Only a nonlinear dynamic analysis is representative of exact and real behavior of structures during earthquake events but this type of analysis is very time-consuming and expensive, so using of this method to analyze and design of structural frames is not practical. Besides analyze and design of structure only based on elastic behavior without considering plastic behavior and energy absorbing or repelling capacity of structures during large earthquakes that are non-permanent and hazardous forces result in non-economical design with very heavy members. In order to considering positive effects of plastic behavior of structures to carry lateral forces almost all of authorized world’s codes use special factor called behavior factor of structure or reflection corrective factor to decrease calculated seismic forces and allows designers to perform elastic analysis of structure under reduced forces and design based on that results. This factor is function of various factors for example ductility of structure, material property, damping characteristics, non-structural member participation, structural degree of indeterminacy, members extra resistance, members over design. Behnam Shirkhanghah is Ph.D. student of Mohaghegh Ardabili university in structural engineering, (e-mail: behnam.shir60@gmail.com) Vahid Shahbaznejhad-Fard is M.Sc. student of mohaghegh ardabili university in structural engineering, (e-mail: v.sh.fard@gmail.com) Houshyar Eimani-Kalesar is professor of the technology and engineering department of the mohaghegh ardabili university ,Ardabil, Iran, (e-mail: HEK@uma.ac.ir) Babak pahlevan is professor of Meshkin Shar Islamic Azad university, Ardabil, Iran, (e-mail: ba_pahlevan@yahoo.com) Over few last decades method of calculating this factor for different structural systems was not specified and its value was specified based on engineering realization and guess of committee members of code preparation from behavior of structural systems. But over last few years some researchers proposed reasonable and reliable methods to calculate behavior factor. Basis of all methods of behavior factor calculation is force or energy level that a frame absorbs from first plastic hinge formation to reaching fully collapsible mechanism [1]. II. FINIT ELEMENT MODELING A. Determining required elements There are different elements with different specifications to perform various analyses. In this study we used Beam189 that is beam element based on Timoshenko beam theory. This element is suitable for non-linear problems. By specifying any cross- section geometric properties is calculated automatically. B. Material property It is customary to model steel behavior as stress-strain curve approximated by two straight lines. First line from origin with slope of E connects to point of yielding. To consider strain hardening behavior of steel second line is approximated by line with slope 5 % of initial stiffness. Modulus of elasticity and Poisson ratio of steel is 2.1E6, 0.3 respectively. C. Modeling Geometry of model by using FORMAIN software is created then is imported to ANSYS. After assigning elements to geometry, model is ready to analysis, but first structure is analyzed and designed [2]. D. Loading Now we specify supported nodes then constraint all translational degrees of freedom. Loads are imposed on top layer nodes. According to analysis type, it is necessary that value of loads is greater than critical load of structure, so during analysis load will be imposed increasingly to reach to critical load that solution will be divergent. E. Solution control Nonlinear static analysis is selected. Large displacement and arc-length method with convergence control based on limit point is used. To obtain behavior factor we need solution up to limit point. III. MODEL SELECTION In this study behavior factor is calculated for three models A, B, and C with plan view shown in Fig. 1. Each model has span of 16m and 20m and height of 2m, 2.5m, and 3m. Totally 18 different models is analyzed. Properties of models are shown in table I. Behavior Factor of Flat double-layer Space Structures T Behnam Shirkhanghah, Vahid Shahbaznejhad-Fard, Houshyar Eimani-Kalesar, Babak Pahlevan World Academy of Science, Engineering and Technology International Journal of Civil and Environmental Engineering Vol:6, No:9, 2012 740 International Scholarly and Scientific Research & Innovation 6(9) 2012 scholar.waset.org/1307-6892/9387 International Science Index, Civil and Environmental Engineering Vol:6, No:9, 2012 waset.org/Publication/9387