A structure is made of various members that if it exist appropriate joints between them, we can hope an action of structural uniform system under any loads. One of the important members of resistance system is floor's story and roof. The important action of floor's story and roof is bearing of gravity loads and transfer of these loads to structural vertical members such as columns and walls. In addition, floor systems make important role in summation of lateral loads in indicated levels and their suitable distribution between resistance system members again lateral forces. Usually, at the time of analyze and design of a structure members, it's floor diaphragm is modeled rigid. This type of modification decrease equations and analyzing time. In some of conditions, results error can take non-safe or non-economic design. Nowadays, the concrete as a primary material with appropriate caracteristiques have fond specific situation in building industry. At last decades, with progressing of reinforced concrete structures in the world and need to building construction at short time, use of prefabricated concrete segments has been very important. Hollow core floors are usually used in these buildings, due to their bay length, light weight and so one. The task of floor diaphragm is bearing of vertical loads and also summation and transfer of seismic loads to resistance vertical members against to earthquake forces such as shear walls, bearings or moment frames. So, design of floor diaphragm is important. At precast concrete buildings, this work is made by hollow core floors. Then, considering of behavior of these floors, in attention of their connections type has specific importance. In this research, we study the rigidity of precast concrete structures floors that are made by two methods, a) modeling of diaphragm by using of finite elements method, b) modeling of diaphragm by using of substance usual methods of finite elements. That is inclusive of force s in main structural members like walls, columns and also calculation of displacements. Various structures have been studied including three sets of plan shape; rectangular, U shape and L shape, and three sets of story numbers of 4, 8, 12 stories. Also two sets of lateral resistance systems; moment frame (without shear wall) and frame with simple beam to column connection (with shear wall) and two sets of floor diaphragm system; rigid and flexible. In building with shear wall, for studying rigidity of diaphragm of precast floors, 4 thicknesses of 15, 20 25 and 30 centimeters for shear walls have been studied.
