Add lateral loads to the Day 7 building frame. Calculate BNBC wind pressure and seismic base shear, apply them as story forces, and read lateral drift and base reactions.
Gravity is predictable — it always acts downward and scales with area and weight. Lateral loads depend on building height, shape, location, and soil type. Getting them wrong can mean a building that passes gravity checks but fails under a moderate earthquake.
p = qz · G · Cp · Iw
where qz = 0.613 Kz V² (Pa). Bangladesh basic wind speed V = 200–260 km/h depending on zone.
V = Sa · W / R
Sa from response spectrum, W = seismic weight, R = response modification factor. Bangladesh Zone II–IV.
BNBC uses the Equivalent Lateral Force (ELF) method: the total base shear V is distributed to each floor as a story force proportional to that floor's height and weight.
Open your Day 7 building frame. Add wind and seismic load cases.
For each story height z, compute qz = 0.613 Kz V². Apply as a uniform pressure on the windward face (tributary area per story). Wind zone: use Dhaka V = 210 km/h as default.
Convert face pressure × tributary height → kN per floor. Apply as joint loads at each floor level on the windward column grid. Apply in +X and −X directions (two separate load patterns: WINDX+, WINDX−).
Define → Mass Source → From Loads: 1.0×DEAD + 0.25×LIVE (BNBC). This tells ETABS what the building weighs for seismic calculations.
Define → Load Patterns → SEISMICX (type: Seismic). Set BNBC 2020: Zone factor Z, soil type, R factor, and I (importance). ETABS auto-calculates story forces.
Key combos: 1.2D + 1.0L + 1.0W (wind governs); 1.2D + 1.0L + 1.0E (seismic governs); 0.9D ± 1.0W (check uplift). Run and compare drift.
Display → Show Deformed Shape (wind combo). Story drift ratio = Δ/h must be ≤ H/400 (BNBC serviceability). If drift exceeds limit, columns/shear walls are too flexible.
Compute base shear and story forces before running ETABS — then verify the software matches.