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It is thus a basic fact of life that the structural resistance and structural loading can only be assessed with a degree of uncertainty. But, classical mathematics is not equipped to deal with uncertain or imprecise descriptions (Mistakidis and Georgiou, 2003). Various techniques are used to assess building vulnerability assessment. The techniques can be grouped into: empirical/ statistical method; heuristic method; and analytical/ mechanistic/ theoretical/ engineering methods. The techniques are grouped from simple scoring to more complex nonlinear structural analysis (Tesfamaraim et. al., 2008; Tesfamaraim and Saatcioglu, 2008). Recently, performance is the most popular concept about structural behaviors under earthquake effects. There are many researches, journals, codes, etc. about this subject. The concept performance is comprehensively expressed in "Prestandard and commentary for the seismic rehabilitation of buildings" called report 356 of Federal Emergency Management Agency (FEMA). Nonlinear static seismic analysis procedures are improved in FEMA440 (2004). For the performance evaluation of the buildings, the recommendations and related restrictions are given in different seismic codes and documents having very strict boundaries. When the approximations made in structural system modeling and in numerical analysis, generally the logical consequences of those restrictions are questionable. Using strict criteria at the end of the analysis which analysis contains probability and relativity is a deficiency. Referenced doctoral study has been done to eliminate this deficiency. A new mathematical and logical flexibility method is been described in the thesis named 'Flexible Performance'. In this study, a fuzzy logic based system is used to determine more realistic performance level of reinforced concrete buildings under the earthquake effects by considering the sections having plastic deformations and the level of deformation on the structural elements by using the displacement based design techniques and nonlinear structural analysis. Also, only flexible performance evaluation of performance criteria for primary beams controlled by flexure is given in this paper. This study will be a first step for seconder elements and shear controlled behaviors. It was shown that the performance levels for beams can define to be flexible by using certain weighted values depending on the number and the deformation levels of beams as used in the fuzzy set theory. In the numerical treatment of the presented method, the damages and the corresponding performance levels given in FEMA 356 (2000) are considered and MATLAB Fuzzy Toolbox and Simulink options are used. The method is tested over 400 data, the performance levels are determined and the representative results are given in tables are discussed, comparatively. Paper has six sections and some sub sections. The sections are introduction, performance based design, fuzzy logic and basic concepts, flexible performance for beams, system verification and results. In introduction, some general information about logic, fuzzy logic; vulnerability, performance based design and FEMA356 (2000), FEMA440 (2004), TDY2007 (2007), ATC40 (1996) was given. In performance based design section, seismic performance objectives, seismic performance description process and acceptance criteria, contradiction of building performance and component performance were explained briefly. Fuzzification with sets, fuzzy rule based systems and defuzzification were given in fuzzy logic section briefly. The most important part of the paper is flexible performance of beams. In this part, combination of the acceptance criteria and fuzzy logic was explained. Flexibilisation of performance criteria means fuzzification, fuzzy rules of model, defuzzification methods in model and processes in MATLAB were sub titles. Then, model verification was given in another section. After the section, assessments were given under the results caption. All of the engineers usually use their experiences and judgements when they are analyzing a project (Hong et. al., 2003). With "flexible performance model" analysis engineers will spend less effort and time to make decision when analyzing the projects. In other words, the flexible performance model is a kind of "computer based decision making system" to give limited responsibility to computers for decision making instead of human. Spending less time of educated employee means economy (profit) in business language. [ABSTRACT FROM AUTHOR]