Earthquake loads in RCC design

Earthquake loads are considered as equivalent static lateral forces (or by response spectrum analysis for tall or irregular buildings), as per IS 1893 (Part 1) or, in Nepal, NBC 105:2020.

Steps

1. Seismic weight $W$ = dead load + a percentage of the live load (25% if the live load is $\le 3$ kN/m$^2$, 50% if it is more; roof live load is neglected).
2. Time period of a bare RC frame: $T = 0.075\,h^{0.75}$ (s), with $h$ in m.
3. Design base shear:

$$V_B = A_h W,\qquad A_h = \frac{Z}{2}\frac{I}{R}\frac{S_a}{g}$$

with $Z$ from the zone map, $I$ the importance factor, $R$ the response reduction factor (5 for special moment resisting RC frames, 3 for ordinary ones) and $S_a/g$ from the spectrum for the soil type. 4. Distribute over the height: $Q_i = V_B \dfrac{W_i h_i^2}{\sum W_j h_j^2}$, and apply at each floor in both directions (plus 30% of the orthogonal force). 5. Load combinations (IS 1893 cl. 6.3; partial safety factors from IS 456 Table 18): $1.5(DL+LL)$, $1.2(DL+LL\pm EL)$, $1.5(DL\pm EL)$, $0.9DL\pm 1.5EL$. 6. Design and detail the members for the worst combination, and provide ductile detailing as per IS 13920.