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RR 21.03.00-3.5 Soft Story Checks per IS 1893 2016

The check soft story option may be added to a floor diaphragm command set to check for soft stories per IS1893:2016. The previous (approximate) method for checking soft stories implemented for IS 1893 2002 has been deprecated in favor of a displacement-based approach using rigid diaphragms.

Soft story checking a process by which designers check stiffness of a story with that of the story above. If the stiffness of a story is lower than that of the story above, the story is considered as a Soft story. Having a soft story in the building makes that story venerable to Earthquake. As per the revised IS 1893 Part-1:2016, if a soft story is detected, structural configuration needs to be change with the introduction of additional lateral load resisting elements in the story detected as soft story. The previous version of IS 1893 Part-1, had provisions to increase design story shear in the soft story and increase percent of steel in the lateral load resisting elements in that soft story. This provision has been withheld in the present version of the code. This makes the situation a stringent one for the designer if a story is detected as a soft story.

Soft Story Checking

There was an approximate methodology available in STAAD.Pro. The program used to compute story stiffness of a story by summing up the lateral stiffness of columns and shear walls (modelled using surface elements). The stiffness of a column is calculated as 12EI / L3 where E is the Young’s modulus, I is the moment of inertia and L is the length of the column respectively and that for a shear wall (without opening) is calculated as Ph3/3EI + 1.2Ph/AG (i.e. summation of flexural stiffness and shear stiffness, obtained as deflection of a cantilever wall under a single lateral load P at its top) where h is the height, A is the cross-sectional area and G is the shear modulus of the wall (E and I carry usual meaning).

This method is approximate one and works in the presence of FLOOR HEIGHT commands. On studying the literatures available on this subject, this method proved to be approximate as it fails to capture actual story stiffness including the effect of bracings. Story stiffness should be computed using a displacement based approach. STAAD.Pro is currently equipped with the facility to consider the in-plane stiffness of slabs as Rigid Diaphragm. In STAAD.Pro lateral stiffness is calculated only when the floor is modeled as rigid floor diaphragm since it functions as transferring story shears and torsional moments to lateral force-resisting members during earthquake.

Story Stiffness Computation Example

Consider a multi-story building:

The points are master nodes. The story stiffness is it defined as the inverse of inter-story drift when a unit load is applied at that story only.

The unit load applied is along X, Z and θY.

Consider the displacement of 2nd story. The displacements are:
  • Story 1: ΔX1, ΔZ1, θY1;
  • Story 2: ΔX2, ΔZ2, θY2

Relative Displacement for story 2 is:

RX2 = ΔX2 - ΔX1; RZ2 = ΔZ2 – ΔZ1; RθY2 = θY2 - θY1

Story Stiffness X =1/((ΔX2 – ΔX1) )

Story Stiffness Z =1/((ΔZ2 – ΔZ1) );

Story Stiffness θY = 1/((θY2 - θY1))

The rest is the same for all the other stories.

Note: This story stiffness is not the same as column story stiffness which is calculated as (nCol × 12EI/L3), n= number of columns.

Command Input

The story stiffness can be printed using the PRINT STORY STIFNESS command.

If soft story check is required to be performed following commands are required to be defined immediately after rigid floor diaphragm is specified in the FLOOR DIAPHRAGM command.