Irregularity of non-parallel systems
Another type of irregularity in structures is the irregularity of non-parallel systems. In the design of the structure, the optimal case of X-banding is parallel X-axis because it causes the earthquake force to be supported by the frame which is in the same direction. If the axis is non-parallel in the structure, it means that it bears the force of the earthquake in both directions, which is not pleasant, that is why the structure is irregular. The map control unit of Tehran Engineering System has limited the border between this irregularity and regularity to 15 degrees.
Geometric irregularity in height
In a multi-storey building, the lower floors have to bear more shear force from the earthquake and for this purpose, the structural members in the lower floors are usually stronger than their higher floors. As a solution, civil engineers sometimes increase the number of bracing openings in the lower floors, which can be a factor in creating irregularities in the height of the structure. According to the fourth edition of the 2800 standard, if the horizontal dimensions of the lateral bearing on each floor are more than 130% of those on the adjacent floors (top and bottom), the building has a geometric irregularity in height.
Mass irregularity in height
The fourth edition of Standard 2800 states that in cases where the mass of each floor differs by more than 50% from the mass of adjacent floors (top and bottom), the building is irregular. In this control, roof and truss floors are excluded.
Irregularity of cutting the side carrier system at height
Irregularity of disconnection of lateral bearing systems is a concept that was added in the fourth edition of the 2800 standard and was not considered in the third edition of this standard. According to earthquake engineering references, in cases where load-bearing components are in the same frame of a structure, the structure has an irregular performance in height.
Lateral stiffness irregularity
Sometimes in structures, due to architectural or technical reasons, in a specific floor, one of several braces in the openings is removed or shear walls of a floor are created. This causes the difficulty of that floor to decrease significantly compared to the floors above it. Regulations are more sensitive to this issue and consider it one of the reasons for creating irregularities in the height of the structure. Practically, if the stiffness of all the floors of a building has a proper distribution, the displacement of the building under the effect of earthquake force is distributed almost equally among its floors, and in case of a local reduction of the lateral stiffness in one floor, a relatively large displacement is observed in that specific floor. . In this case, relatively large forces are created in the members and connections related to the mentioned floor, which can have destructive effects. According to the fourth edition of standard 2800, a control should be carried out on the degree of change in the stiffness of the structure floors, according to which, if the lateral stiffness of each floor is less than 70% of the lateral stiffness of the floor above it, or less than 80% of the average lateral stiffness of the three floors above it, it The floor causes irregularity in height and as a result the whole building is irregular in height. A floor that causes irregularity in height is called a soft floor. In general, a soft or very soft floor is one of the things that makes the structure very vulnerable during an earthquake. Also, another common case that causes the soft floor to be created on the ground floor of the building is that the surrounding walls of the building that existed in the upper floors and relied on the structure and increase the hardness of those floors, due to architectural reasons on the floor The ground floor is removed, causing a sudden decrease in its hardness compared to the upper floors.
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