Foundation, Concrete and Earthquake Engineering

Soft and Weak Stories

Open Ground Storey 

An open ground storey building has both columns and masonry infill walls in the upper stories but only columns in the ground storey. It is the most common type of vertical irregularity occurs in building which is left open for the purpose of parking, i.e., columns in the ground storey do not have any partition walls (of either masonry or RC) between them.

Characteristics of Open Ground Storey

The infilled brick walls in the upper stories increase the lateral stiffness of the frame by a factor of three to four times than that of lower weak storey. The following two features are characteristics of open ground storey buildings:

a) Relatively flexible ground storey in comparison to the stories above, i.e., the relative horizontal movement at the ground storey level is much larger than the stories above. This flexible ground storey is called soft storey.

b) Relatively weak ground storey in comparison to the stories above, i.e., the total horizontal earthquake force (load) resisted at the ground storey level is significantly less than the stories above resulting a weak storey.

Earthquake Behavior
The much stiffer upper stories behaves like a rigid block, and most of the horizontal displacement of the building occurs locally in the soft storey alone. This makes the building behave like an inverted pendulum, with the ground storey columns acting as the pendulum rod while the rest of the building act as a rigid pendulum mass which swings back-and-forth during earthquake shaking, and the columns in the open ground storey are severely stressed.

In such building the dynamic ductility demand during the probable earthquake gets concentrated in the soft storey and the upper storeys tend to remain elastic. Hence whereas the ‘soft’ storey is severely strained causing its total collapse, much smaller damages occurs in the upper storeys, if it all.

Soft Storeys at Mid Height

Generally the soft or weak storey usually exists at the ground storey level, but it could be at any other storey level too. Sometimes a soft storey is created some where at mid-height of the multi-storey building to provide gathering place or restaurant. Such buildings also behave likes open ground building and also collapse in different earthquake over the world.

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