Look at almost any apartment block in an Indian city and you'll see the same thing at street level: an open ground floor on columns, used for parking, with the solid, brick-walled flats stacked above it. It's practical, it's everywhere, and in an earthquake it's the single most dangerous configuration a building can have. Engineers call it a soft storey, and the open stilt-parking floor is its most common form.
At SM Structural Consultants, recognised as one of the top structural consultants in Hyderabad, the soft storey is one of the first things we look for when assessing an existing apartment building — because it turns an otherwise ordinary structure into one where all the earthquake damage concentrates in a single, weak level. This guide explains what a soft storey is, why stilt parking creates one, how it fails, and how it can be designed out or retrofitted.
Key Takeaways
- A soft storey is a level far more flexible (and often weaker) than the storey directly above it.
- Stilt parking is the classic cause: open columns below, brick-walled flats above.
- In an earthquake, the sway and damage concentrate almost entirely in that one open storey.
- The ground columns are forced to absorb the whole building's drift, hinge at their ends, and can collapse.
- It's extremely common in Indian apartments — and a known cause of collapse in past earthquakes.
- It can be designed out, or retrofitted, by adding stiffness and strength at the ground floor.
The convenience of an open ground floor is paid for, in an earthquake, by the columns holding it up.Section 01
What a Soft Storey Really Is
A soft storey is a level in a building that is significantly more flexible than the one above it — in code terms, its lateral stiffness is a fraction of the storey immediately above. In plain terms, it bends sideways far more easily than its neighbours. A closely related idea is the “weak storey”, where the level is not just more flexible but has less strength. In real buildings the two usually come together, and almost always at the same place: the open ground floor.
The reason is that stiffness in a typical building doesn't come only from the columns — it comes just as much from the brick infill walls packed between them. Take those walls away on one level, and you've made that level dramatically more flexible than the rest, whether you meant to or not.
Section 02Why Stilt Parking Creates One
Stilt parking needs the ground floor kept open — no walls, just columns, so cars can move between them. Every floor above, though, is full of brick partition and external walls that quietly stiffen the frame. The result is a sudden, drastic drop in stiffness at exactly one level: rigid, walled flats sitting on top of a bare, flexible frame. That abrupt change is the soft storey, and it's built in the moment you leave the ground floor open while filling the floors above with walls.
Walls Above
Upper floors are stiffened by their brick infill walls, so they hardly deform.
Open Below
The ground floor has only columns, so it's far more flexible than every floor above.
Stiffness Cliff
A sudden jump in flexibility at one level — the definition of a soft storey.
How It Fails in an Earthquake
When the ground shakes, a building sways side to side. In a regular building, that sway is shared across every storey, and each one bends a little. In a soft-storey building, the stiff upper floors barely bend — they ride along almost as a single rigid block — so the entire sway is dumped into the one flexible level below. The ground columns are forced to absorb the whole building's movement on their own. They bend into an S-shape, form plastic hinges at their tops and bottoms, and if the drift is large enough those columns fail and the block above drops straight down onto the parking level.
⚠ The Danger Is Invisible Until the Day It Matters
A soft-storey building looks completely normal — and behaves normally — every single day it isn't shaken. There's no crack, no sag, nothing a resident would notice. The weakness only reveals itself in an earthquake, which is exactly why it's so often left unaddressed, and why past earthquakes have seen rows of otherwise sound-looking apartment blocks fail at the stilt floor while neighbouring buildings stood.
The Structural Ideas Behind It
A handful of concepts explain why the soft storey behaves the way it does — and why codes single it out.
Stiffness Irregularity
An abrupt change in stiffness up the height concentrates demand at the weak level.
Storey Drift
The sideways movement of one storey relative to the next — huge at a soft storey.
Column Shear & Hinging
Ground columns take intense shear and bending, hinging at their ends.
P-Delta Effect
The weight above, acting through a large sway, worsens the demand on leaning columns.
Code Amplification
IS 1893 requires the soft-storey members to be designed for higher forces.
Redundancy Loss
With no walls to help, the columns alone must resist the earthquake.
How It's Prevented or Retrofitted
A soft storey can be designed out in a new building, or strengthened in an existing one. The principle is the same: reduce the stiffness cliff, and make the ground storey strong enough for what the earthquake asks of it.
Identify the Irregularity
Confirm the stiffness and strength drop at the open storey against code criteria.
Design Ground Members for Higher Forces
Apply the code's amplification so the soft-storey columns and beams are sized for the real demand.
Add Stiffness at the Ground Floor
Introduce shear walls or braced bays where they don't block parking, to share the drift.
Strengthen the Columns
Jacket existing ground columns with concrete or steel to add shear and bending capacity.
Check the Foundation
Make sure the new walls and strengthened columns carry their loads safely to the ground.
Review & Code Compliance
Independent review against IS 1893 and IS 13920 before the retrofit is finalised.
Myths & Facts
| Common Belief | What's Actually True |
|---|---|
| “Stilt parking is banned, so it must be safe.” | It's widely allowed — but only when the soft storey is specifically designed for, which is often skipped. |
| “The building looks fine, so it's fine.” | A soft storey shows no everyday symptoms; the weakness only appears in an earthquake. |
| “More columns at the ground floor fixes it.” | Stiffness, not just column count, is the issue — walls or bracing are usually what's needed. |
| “Nothing can be done to an existing building.” | Soft storeys are routinely retrofitted with added walls, bracing, and column jacketing. |
Related Reading
The soft storey is really a lateral-load problem — these cover the forces, the path, and the fix:
Frequently Asked Questions
Find out if your building has a soft storey — before an earthquake does.
Our team assesses apartment blocks for soft-storey and stilt-parking weakness, and designs retrofits — added shear walls, bracing, and column jacketing — to bring the ground floor up to code.