1. Seismic Zonation Factors (Z) for Eastern Nepal

Under NBC 105:2025, the seismic zone factor ($Z$) corresponds to the Peak Ground Acceleration (PGA) with a 10% probability of exceedance in 50 years (475-year return period). In eastern Nepal, the values reflect pronounced seismic vulnerability:

2. Soil Stratification & Site Amplification (Types A through D)

NBC 105:2025 classifies subsoil into four primary categories based on weighted average shear wave velocity (V_s30) or standard penetration resistance ($N_60$):

• Type A (Rock / Hard Soil): $N_60 > 50$, minimal ground motion amplification.
• Type B (Medium / Stiff Soil): $N_60$ between 15 and 50, typical for older alluvial river terraces in Dharan.
• Type C (Soft Soil): $N_60$ between 7 and 15, common across Itahari and Morang alluvial plains; requires higher spectral amplification.
• Type D (Very Soft / Liquefiable): $N_60 < 7$, requires deep pile foundations or ground improvement before construction.

3. Dynamic Analysis & Ductile Rebar Detailing

For buildings exceeding three stories or 10 meters in height, NBC 105:2025 mandates Modal Response Spectrum Analysis. USEC engineers employ CSI ETABS with 3D rigid diaphragm assumptions, cracked section modifiers (0.35 $I_g$ for beams, 0.70 $I_g$ for columns), and strict P-Delta second-order checks.

Ductile detailing must conform strictly to IS 13920:2016. This entails special confining rebar hoops with 135° seismic hooks extending 10 times bar diameter into concrete cores at all beam-column joints, preventing brittle catastrophic collapse.