How does ideCAD define equivalent lateral load, according to ASCE 7-16, for two earthquake directions?

• Equivalent earthquake force procedure; it is used to control irregularities as indicated in Table 12.3-1 and 12.3-2.

• Seismic effects are calculated automatically with the Modal Response Spectrum Analysis.

Seismic loads acting on buildings; the floor level, assumed to accumulate the story masses, is accepted as the horizontal load acting on the floors. Seismic effects determined for two perpendicular directions are combined as specified in the regulation. The total equivalent lateral force is known as the base shear force and is used in the earthquake-resistant design of structures.

The equivalent lateral force method is a method that takes into account only the effect of the 1st mode. For this reason, there are limitations to using the method.

Vertical Structural Irregularities are given below.

Description

1a. Torsional Irregularity: The case where the Torsional Irregularity Factor, which is defined for any of the two orthogonal earthquake directions as the ratio of the maximum story drifts at any story to the average relative story drifts at the same story in the same direction, is greater than 1.2. Maximum story drift, computed including accidental torsion with Ax =1.0. Torsional irregularity requirements in the reference sections apply only to structures in which the diaphragms are rigid or semi-rigid.

1b. Extreme Torsional Irregularity: The case where the Torsional Irregularity Factor, which is defined for any of the two orthogonal earthquake directions as the ratio of the maximum story drifts at any story to the average relative story drifts at the same story in the same direction, is greater than 1.4. Maximum story drift, computed including accidental torsion with Ax =1.0. Torsional irregularity requirements in the reference sections apply only to structures in which the diaphragms are rigid or semi-rigid.

1. Reentrant Corner Irregularity:

The cases where the dimensions of projections in both of the two perpendicular directions in the plan exceed the total plan dimensions of that story of the building in the respective directions by more than 15%

1. Diaphragm Discontinuity Irregularity:

The cases where there is a diaphragm with an abrupt discontinuity or variation in stiffness, including those with a cut-off or open area greater than 50% of the gross closed diaphragm area, more than 50% effective diaphragm stiffness change from one story to the next.

1. Out-of-Plane Offset Irregularity:

The case involves a lateral force-resistance path discontinuity, such as an out-of-plane offset of the vertical elements.

1. Nonparallel System Irregularity:

The case where lateral force-resisting elements are not parallel to the major orthogonal axes of the seismic force-resisting system.

Horizontal Structural Irregularities are given below.

Description

1a. Stiffness–Soft Story Irregularity: The case exists where a story has a lateral stiffness is less than 70% of the above story or less than 80% of the average stiffness of the three floors above.

1b. Stiffness–Extreme Soft Story Irregularity: The case exists where a story has a lateral stiffness is less than 60% of the above story or less than 70% of the average stiffness of the three floors above.

1. Weight (Mass) Irregularity:

The case where the effective story mass is more than 150% of the effective mass of the above or below the story. Light roof story need not be considered.

1. Vertical Geometric Irregularity:

The case where seismic force-resisting system dimension is more than 130% of that in the above or below story.

1. In-Plane Discontinuity in Vertical Lateral Force-Resisting Element Irregularity:

The case where there is a discontinuity in a lateral force-resistance path, such as columns or structural walls are removed at some stories, or the structural walls of upper stories are supported by columns or beams underneath.

5a. Discontinuity in Lateral Strength–Weak Story Irregularity: The case where in each of the orthogonal earthquake directions, the story lateral strength is less than 80% of that in the story above.

5b. Discontinuity in Lateral Strength–Extreme Weak Story Irregularity: The case where in each of the orthogonal earthquake directions, the story lateral strength is less than 65% of that in the story above.

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