Clause 7.2.1 defines the total design lateral force (base shear) VB as the product of the design horizontal seismic coefficient Ah and the seismic weight W of the building. The seismic weight includes the full dead load plus a percentage of live load depending on its intensity. This base shear is then distributed along the height of the building per Clause 7.6.3.
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Key Requirements
•Total design base shear VB = Ah × W along each principal direction
•Seismic weight W = total dead load + percentage of imposed load per Table 10
•Imposed load on roof need not be included in seismic weight
•For imposed load ≤ 3 kN/m², include 25% in seismic weight; for imposed load > 3 kN/m², include 50%
•Weight of permanent equipment, tanks, and materials stored shall be included at full value
•Base shear shall be computed independently in each principal direction
Reference Tables
Table 10 — Percentage of Imposed Load to be Considered in Seismic Weight (Clause 7.3.1)
Imposed Uniformly Distributed Floor Load (kN/m²)
Percentage of Imposed Load
Up to and including 3.0
25%
Above 3.0
50%
Imposed load on roof need not be considered. Weight of permanent equipment and stored material is included at 100%.
Formulas
VB = Ah × W
Design seismic base shear
VB = Total design lateral base shear in kNAh = Design horizontal seismic coefficient (from Cl. 7.2)W = Seismic weight of the building in kN
Practical Notes
✓Seismic weight typically adds 25-50% of live load to full dead load — do not include full live load
✓For storage buildings (warehouses), 50% live load applies even if design live load is ≤ 3 kN/m²
✓Water tanks should be considered at full capacity for seismic weight unless specifically justified
✓Base shear is computed in each principal direction independently — the critical direction governs member design via load combinations
Common Mistakes
⚠Including 100% of live load in seismic weight instead of the reduced percentage from Table 10
⚠Including imposed load on roof in seismic weight — roof live load is excluded
⚠Forgetting to include the weight of permanent equipment and water tanks
⚠Not computing base shear in both principal directions independently