# This irregularity is created by factors like various usages of floors in the structure

### This irregularity is created by factors like various usages of floors in the structure

This irregularity is created by factors like various usages of floors in the structure, and additional massive mass in some floors or mass change because of dimension change in plan in different floors. According to fourth edition standard 2800 11 in some cases that mass of each floor have difference with adjacent floor mass (up and down) more than 50%, is a mass irregular structure. In this case, standard 2800 states that the purpose of mass is the same as effective mass seismic, and in this control, roof and arc are the exceptions.
2.4 Plastic hinges
When talking about nonlinear structures behavior, we mean two features entitled geometry nonlinear and Materials nonlinear. In most structures only P-? can be critical and considered by todays software of doing linear analysis. According to this thing, most of the time the meaning of nonlinear analysis refers to the analysis that attention is reflected to the nonlinear effect of materials. Being nonlinear material of a structure causes some restrictions in structure members named pasty joints.
In steel members, plastic hinges include a section with special attempt (shear, bending, compression, and gravitation), which all cords of it should reach the yield stress. One of the most important and influential steps are doing nonlinear analysis of the structural modeling steps and defining plastic hinges features. Considering to the importance of this subject, regulations which suggested nonlinear analysis methods allocated the large parts of their criteria to how to do modeling and definition of pasty joints. According to the fourth edition of 2800 standard, the plastic hinges modeling which has features of nonlinear structure should be adaptable with vitro data or reliable analysis model based on strength, stiffness and ductility. Therefore, we can do the plastic hinges modeling with two main approaches including:
General approach on plastic hinges modeling: that can be the two or three lines behavior. Two lines model is good for steel structures and three lines model is suitable for concrete and masonry structures. These models, actually is the relationship between deformation of members force and pasty joints. The approach of seismic rehabilitation criteria in plastic hinges modeling: that can use the proposed criteria in seismic rehabilitation instructions of current buildings (issue 360). How structural behavior done according to the criteria of this issue can divide their deformation based on types of internal effort and the shape of effort curve into two main groups: “Controlled by forces” and “Controlled by deformation”

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