Types of irregularities in the structure
Irregularities in the structure are one of the important issues in the design of the structure, and every design must have a complete mastery over this issue. According to the fourth edition of Standard 2800, if any of the following items are present in the structure plan, the building has an irregularity in the plan:
1- Geometric irregularity
2- Torsional irregularity
3- Irregularity in the diaphragm
4- Irregularity outside the page
5- Irregularity of non-parallel systems
Geometric irregularity
The plan of a building should have a symmetrical or almost symmetrical shape compared to the frames along which the earthquake-resistant elements are located. According to this issue, if there is a depression (setback) in the building plan, if its size in one of the corners of the building in two directions simultaneously exceeds 20% of the length of the plan in that stretch, the building has a geometric irregularity. The presence of too much indentation or protrusion in the plan of a building can make it vulnerable during an earthquake, because when the building vibrates, different parts of the plan may tend to move uncoordinated with each other. In fact, in this case, due to the creation of sunken corners in the building plan, the structure may become vulnerable.
Torsional irregularity
Another type of irregularity in structures is torsional irregularity. If there is a distance between the center of mass and the center of stiffness in the floors of the building, the lateral force of the earthquake causes torsional anchoring in the floors and the structure can be considered as a type of torsional irregularity. According to the fourth edition of the 2800 standard, in an irregular structure with a lot of torsion, the maximum relative displacement at one end of the building (including random torsion and assuming Aj=1) is 20% of the average relative displacement at both ends. The building is bigger. Changing the relative location of a floor is actually changing the location of a floor relative to the floor below it. In cases where this difference is more than 40%, the building is considered to be of extreme twisting irregular type.
Irregularity in the diaphragm
In seismic standards, the roof of buildings is known as a diaphragm. The regulations classify the diaphragms into three categories: rigid diaphragm, semi-rigid diaphragm and flexible diaphragm. In general, the diaphragm plays the role of distributing the lateral forces on the building between the lateral load-bearing elements of the structure, and the inappropriate characteristics of the diaphragm, such as the large area of the openings or large changes in the stiffness of the diaphragm, can cause irregularities in the building plan. According to the fourth edition of the 2800 standard, the control of the characteristics of the aperture in the discussion of irregularity is done as follows:
If the total area of the opening in one floor is more than 50% of the total area of the diaphragm, the structure is irregular in plan. If the sudden change in the stiffness of the diaphragm of each floor is more than 50% of the stiffness of the diaphragm in the adjacent floors, the structure is irregular in plan. In the discussion of irregularity, the meaning of diaphragm stiffness is the in-plane stiffness of the diaphragm, which of course, in general, there is no explicit method to determine it.
Off-page irregularity
In cases where there is an interruption in the lateral force transmission path in the lateral bearing system (such as a change in the plane of at least one of the lateral bearing components of the floors), an out-of-plane irregularity has occurred in the plan. In this case, the diaphragm becomes vulnerable and will lead to unfavorable performance.
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