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Due to several uncertainties  which  affect structural responses of Reinforced concrete (RC) frames,  it is sensibly required  to  apply  a vulnerability analysis tool such as fragility curve. To  construct an analytical fragility curve, the incremental dynamic analysis (IDA) method has been extensively used as an applicable seismic analysis tool. To employ the IDA method for constructing fragility curves of RC frames,  it is important  to know  how many records will  be  adequate  to  assess  seismic  risk  analysis  properly?  Another  issue  is  to  know  how many IDA steps  are  required for developing an accurate fitted fragility function? For this purpose,  two 3D RC frames  called  3STRCF and 5STRCF have been nonlinearly modeled and 200 2-componets actual records have been considered for  the  IDA. The results  reveal that at least 15 IDA steps  are  required  to  reduce  fragility function error to less than 5% and 10  IDA  steps  are  required  to  yield  less  than  10%  error.  In  addition,  it  is  revealed  that  a selection of 100 records is completely adequate to be certain   to have  an accurate fragility curve. It is concluded that at least 25 records are required to decrease fragility curve error to less than 5% and 15 records to  have  less than 10%. The closeness of fragility curve error variation for two models and in all limit states show that these results can be generalized to other RC frames.

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In structural design, either the experience of designer is used or a uniform grouping is usually utilized to group the elements. This type of grouping affects the fundamental cost of the buildings, including the cost of concrete, steel and formwork, as well as secondary costs such as laboratory, checking, fabrication and etc. However, the secondary costs are not usually considered in the cost function. Strategies can also be used to automate the grouping of members in structural design. In this strategy beams and columns are automatically grouped into a limited number of groups to achieve the lowest cost. In this study, enhanced colliding bodies optimization algorithm is used to automatically group the beams and columns of the reinforced concrete structures and also to optimize their cost. The proposed procedure applied to three reinforced concrete frames with four, eight and twelve stories and the influence of automatic grouping of the members in optimal cost is investigated. Using this method, the beams and columns are automatically grouped and the results show that the optimal cost obtained from the automatic grouping is less than the manual grouping of the members.