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How does ideCAD calculate seismic base shear according to ASCE 7-16?

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Tip
Seismic
  • The seismic base shear for each direction is calculated automatically

according to Equation 12.8-1
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Symbols

Cs = The seismic response coefficient
W = The effective seismic weight
SDS = The design spectral response acceleration parameter in the short period range
R = The response modification factor in Table 12.2-1
Ie = The Importance Factor
SD1 = the design spectral response acceleration parameter at a period of 1.0 s
T = The fundamental period of the structure(s)
TL = The long-period transition period(s)
S1 = The mapped maximum considered earthquake spectral response acceleration parameter

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Equivalent The equivalent Lateral Force Method considered in buildings with low height can be represented by the behavior of the load-bearing system in the dominant vibration mode, and the shape of this mode can be considered as approximately the inverted triangle. Also, the procedure is valid only for structures without significant discontinuities in mass and stiffness along the height, where the dominant response to ground motions is in the horizontal direction without significant torsion.

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The seismic base shear, V, in the earthquake direction considered, is determined by the following equation.

Mathinline
body$$ \normalsize V=C_sW \; \; \; \; \;\;\;\;\; \; \; \; \; \;\;\;\;\; \; \; \; \; \;\;\;\;\; \; \; \; \; \;\;\;\;\; (12.8-1) $$

The base shear force is calculated by the above equation. The mass and Cs used in this calculation and the base shear force determined as a result of the calculation are tabulated in the Dynamic Analysis report.

The seismic response coefficient, Cs, is determined by the following equation.

Mathinline
body--uriencoded--$$ \normalsize C_%7Bs%7D=\frac %7BS_%7BDS%7D%7D %7B \big( \frac%7BR%7D%7BI_e%7D \big) %7D \; \; \; \; \;\;\;\;\; \; \; \; \; \;\;\;\;\; \; \; \; \; \;\;\;\;\; \; \; \; \; \;\;\;\;\; (12.8-2) $$

The value of Cs computed with Eq. (12.8-2) need not exceed the following:

for T ≤ TL

Mathinline
body--uriencoded--$$ \normalsize C_%7Bs%7D=\frac %7BS_%7BD1%7D%7D %7B T \big( \frac%7BR%7D%7BI_e%7D \big) %7D \; \; \; \; \;\;\;\;\; \; \; \; \; \;\;\;\;\; \; \; \; \; \;\;\;\;\; \; \; \; \; \;\;\;\;\; (12.8-3) $$

for T > TL

Mathinline
body--uriencoded--$$ \normalsize C_%7Bs%7D=\frac %7BS_%7BD1%7DT_L%7D %7B T%5e2 \big( \frac%7BR%7D%7BI_e%7D \big) %7D \; \; \; \; \;\;\;\;\; \; \; \; \; \;\;\;\;\; \; \; \; \; \;\;\;\;\; \; \; \; \; \;\;\;\;\; (12.8-4) $$

Cs should not be less than

Mathinline
body--uriencoded--$$ \normalsize C_%7Bs%7D=0.044S_%7BDS%7DI_e \ge 0.01 \; \; \; \; \;\;\;\;\; \; \; \; \; \;\;\;\;\; \; \; \; \; \;\;\;\;\; \; \; \; \; \;\;\;\;\; (12.8-5) $$

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