4.5.10.1 - Additional eccentricity effect is defined in order to take into account possible uncertainties in the effect of earthquake ground motion on the building and in the stiffness and mass distribution of the structural system .
4.5.10.2 - In case floor slabs are modeled as rigid diaphragm in their own plane according to 4.5.6.4 ,
(a) According to 4.5.9.3 , an earthquake calculation will be made in the direction of each earthquake based on the floor mass defined in the floor mass center ( main node point ) .
(b) Horizontal seismic loads affecting the center of the storey mass ( main node ) will be shifted by + 5% and - 5% of the floor dimension perpendicular to the earthquake direction considered, and earthquake calculation will also be made for these cases.
(c) seismic analysis of 4/7 in case of 'according to instead of shifting the modeling For ease of seismic loading, the floor mass center ( master node ) effect the lateral force F i A (X) with Eq. (17.4) with the additional layers torsion ' It is appropriate to consider.
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Here e indicates 5% additional eccentricity.
(d) seismic analysis of 8.4 according to the case of modal method instead of shifting the seismic loads in terms of modeling simplicity, the floor mass center ( master node ) is defined times the mass of m i with, floor mass moment of inertia m iθ 'or Eq. (4.18) It is convenient to add the increment Δ m iθ given by .
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4.5.10.3 - In case earthquake calculation is made under the effect of unidirectional earthquake according to 4.7 or 4.8.2 , additional eccentricity is taken into consideration for each direction. If the calculation is made under the effect of two-directional earthquakes acting simultaneously according to 4.8.3 , the eccentricities for both directions will be applied separately.
4.5.10.4 - In case floor slabs are modeled with two-dimensional plate ( membrane ) finite elements to contain the degrees of freedom related to the displacements in their own planes according to 4.5.6.2 ,
(a) With this model created, earthquake calculations will be made without the effect of eccentricity, and internal forces and displacements will be obtained in floors and bearing system elements other than floors. The sizes obtained for flooring will be taken into consideration in flooring design.
(b) In order to consider the eccentricity effect, the rigid diaphragm assumption will be made for in-plane finite element degrees of freedom and the storey mass centers will be shifted as defined in 4.5.10.2 . A second earthquake calculation based on rigid diaphragm modeling will be made to determine the effect of additional eccentricity on structural system elements other than slabs and beams.
(c) For structural system elements other than slabs and beams, the design essential internal forces and displacements will be determined as the envelope (unfavorable) of those obtained in (a) and (b) .Where diaphragms are not flexible, the design shall include the inherent torsional moment (Mt) resulting from the location of the structure masses plus the accidental torsional moments (Mta) caused by assumed displacement of the center of mass each way from its actual location by a distance equal to 5% of the dimension of the structure perpendicular to the direction of the applied forces. Where earthquake forces are applied concurrently in two orthogonal directions, the required 5% displacement of the center of mass need not be applied in both of the orthogonal directions at the same time but shall be applied in the direction that produces the greater effect.
Accidental torsion shall be applied to all structures for determination if a horizontal irregularity exists as specified in Table 12.3-1. Accidental torsion moments (Mta) need not be included when determining the seismic forces E in the design of the structure and in the determination of the design story drift in Sections 12.8.6, 12.9.1.2, or Chapter 16, or limits of Section 12.12.1, except for the following structures:
Structures assigned to Seismic Category B with Type 1b horizontal structural irregularity.
Structures assigned to Seismic Category C, D, E, and F with Type 1a or Type 1b horizontal structural irregularity.
12.8.4.3 Amplification of Accidental Torsional Moment
Structures assigned to Seismic Design Category C, D, E, or F, where Type 1a or 1b torsional irregularity exists as defined in Table 12.3-1 shall have the effects accounted for by multiplying Mta at each level by a torsional amplification factor (Ax) as illustrated in Fig. 12.8-1 and determined from the following equation:
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where
δmax = maximum displacement at level x computed assuming
Ax = 1 [in. (mm)], and
δavg = average of the displacements at the extreme points of the structure at level x computed assuming Ax = 1 [in. (mm)].
The torsional amplification factor (Ax) shall not be less than 1 and is not required to exceed 3.0. The more severe loading for each element shall be considered for design.
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