India's seismic code, IS 1893:2016, divides the country into zones based on expected earthquake intensity. Sikkim falls in Zone IV, the second-highest category. In practical terms, buildings here must be designed and detailed to withstand strong shaking, and the 2011 Sikkim earthquake showed what happens when they are not.
Why many buildings are vulnerable
A large share of buildings in Sikkim were built without a structural engineer, or before earthquake-resistant detailing under IS 13920 became common practice. Others were sound when built, but have since had extra floors added without checking whether the columns and foundations could carry them.
- Columns that are too small or have too little reinforcement
- Widely spaced stirrups (ties), which let columns fail suddenly in shear
- Open ground floors used for parking or shops, creating a weak storey
- Extra floors added on foundations designed for fewer
- Construction on slopes without proper stepped foundations or retaining walls
What retrofitting does
Retrofitting strengthens an existing building so it behaves better in an earthquake. Common methods include jacketing columns and beams with new reinforced concrete or steel, adding shear walls, strengthening foundations and improving connections. The goal is a building that, even if damaged in a strong earthquake, does not collapse.
Where to start
The first step is always an assessment. An engineer inspects the building, reviews any drawings, and uses non-destructive testing to estimate concrete quality and locate reinforcement. From that, you get a clear picture of whether your building needs strengthening, how much, and what it will involve.