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Open Storey Buildings โ€” IS 1893 (Part 1): 2016 | Clause 7.10
IS 1893 (Part 1): 2016 ยท Clause 7.10

Open Storey Buildings:
Why Parking Floors Become Seismic Troublemakers

Every time an RC building skips masonry walls on the ground floor for parking space, it creates a fatal seismic trap. Learn how IS 1893 identifies this problem, quantifies it, and demands a structural fix.

๐Ÿ“– IS 1893 (Part 1): 2016
๐Ÿ›๏ธ Clause 7.10 + Table 6
๐Ÿ™๏ธ Seismic Zones IIโ€“V
๐ŸŽ“ Structural Engineering
Navigation

What You’ll Learn

01 What is an Open Storey? 02 The Seismic Problem 03 Key Definitions 04 Vertical Irregularities 05 Clause 7.10 in Full 06 Interactive Calculator 07 Remedies & Solutions 08 Historical Failures 09 Quiz Yourself
Module 01 โ€” The Big Picture

What is an Open Storey?

Walk through any Indian city and you will see it everywhere: a multi-storey apartment building with an open, column-only ground floor used for vehicle parking. Residents on upper floors enjoy walls, partitions, and masonry infill panels for privacy โ€” but the parking level is bare. This is an open storey.

IS 1893 (Part 1): 2016 specifically addresses this condition in Clause 7.10 โ€” RC Frame Buildings with Open Storeys. It is one of the most practically important clauses in Indian earthquake engineering.

OPEN STOREY (PROBLEM) Floor 4 โ€” Masonry Infills โœ“ Floor 3 โ€” Masonry Infills โœ“ Floor 2 โ€” Masonry Infills โœ“ OPEN STOREY (Parking โ€” No Masonry Walls) โšก Soft Storey Failure Zone! K high K low! V (EQ) WITH RC SHEAR WALLS (REMEDY) Floor 4 โ€” Masonry Infills โœ“ Floor 3 โ€” Masonry Infills โœ“ Floor 2 โ€” Masonry Infills โœ“ RC Shear Walls Added (Stiffness Restored) K balanced V (EQ) โœ“ Stiffness โ‰ฅ 80% of storey above โŒ Dangerous โ€” Prohibited in Zones IIIโ€“V โœ“ Compliant per Clause 7.10
Fig. 1 โ€” Left: Open storey at ground floor creates a soft storey / weak storey. Right: RC shear walls added to restore stiffness per Clause 7.10 of IS 1893 (Part 1): 2016.
Module 02 โ€” The Seismic Problem

Why Open Storeys Are Seismically Dangerous

<80%
Minimum lateral stiffness of open storey vs storey above (Cl. 7.10.3b)
<90%
Minimum lateral strength of open storey vs storey above (Cl. 7.10.3c)
โ‰ฅ2%
RC structural wall plan density required (Zones III, IV, V)
ฯw
Wall plan density = Awall / (b ร— h) ร— 100%

When an earthquake strikes, seismic forces travel as shear through every storey of the building. In a regular building with masonry infill walls at every floor, the lateral stiffness (resistance to sideways movement) is distributed relatively evenly. But when one floor โ€” typically the ground floor โ€” is left open, it becomes the weakest link in the chain.

The Physics in Plain Language

Think of stiffness like the spring constant of each floor. Upper floors with brick walls are very stiff โ€” like tight springs. The open ground floor without walls is very flexible โ€” like a loose spring. When earthquake shaking occurs, all the deformation concentrates in the softest (most flexible) storey. The building bends hugely at that one level while the rest stays relatively rigid. This is called a soft storey mechanism.

If this floor is also weaker in strength (lateral shear strength), it is called a weak storey, which leads to a storey mechanism collapse โ€” the building topples over at that single floor while the upper storeys remain intact above the rubble. This is exactly what happened in multiple earthquakes across India.

๐ŸŒ€

Soft Storey Failure

Lateral stiffness of the open storey is less than the storey above. All seismic deformation concentrates here, causing large interstorey drift and column failure.

๐Ÿ’€

Weak Storey Collapse

Lateral strength is less than 80% of storey above. The open storey columns cannot resist the shear force and fail simultaneously โ€” a “pancake” style collapse.

๐Ÿ”„

Torsional Amplification

If stiffness is asymmetric in plan, the open storey can also twist, combining bending and torsion โ€” further increasing column demands beyond design capacity.

๐Ÿ“

Masonry Contribution

Masonry infill walls, though not formally designed as structural elements, dramatically increase lateral stiffness. Their absence at one storey creates a dramatic stiffness discontinuity.

Module 03 โ€” Key Definitions

Terminology from IS 1893: 2016

IS 1893 provides precise definitions in Clause 4.20 and Clause 4.25. These are not interchangeable โ€” understanding the difference between a soft storey and a weak storey is critical.

๐Ÿ“

Storey (Cl. 4.20)

The space between two adjacent floor slabs. Stiffness and strength checks are performed at each storey level.

๐ŸŒŠ

Soft Storey (Cl. 4.20.1)

A storey whose lateral stiffness (Ki) is less than the lateral stiffness of the storey above it. The concern is flexibility โ€” it will drift too much.

๐Ÿ’ฅ

Weak Storey (Cl. 4.20.2)

A storey whose lateral shear strength (Si) is less than the lateral strength of the storey above. The concern is strength โ€” it cannot resist the force.

๐Ÿ“Š

RC Structural Wall Plan Density ฯw (Cl. 4.25)

Ratio of the cross-sectional area of RC structural walls at plinth level to the plan width of the building, expressed as a percentage. Must be โ‰ฅ 2% in Zones III, IV, V.

โšก

Storey Lateral Stiffness Ki (Cl. 4.24)

Total lateral translational stiffness of all lateral force-resisting elements in storey i, in a given principal plan direction.

๐Ÿ’ช

Storey Lateral Shear Strength Si (Cl. 4.23)

Total lateral strength of all seismic force-resisting elements sharing the lateral storey shear in the considered direction for storey i.

Module 04 โ€” Vertical Irregularities

Table 6 of IS 1893 โ€” Vertical Irregularity Definitions

IS 1893 Table 6 defines seven types of vertical irregularity. Open storey buildings most commonly trigger types (i) and (v) โ€” stiffness irregularity and strength irregularity. This table is directly referenced by Clause 7.10.

Sl No. Type IS 1893 Criterion Zone Consequence Relevance to Open Storey
i Stiffness Irregularity (Soft Storey) Ki < Ki+1 (storey above) Dynamic Analysis mandatory in Zones III-V โšก Directly triggered by open ground storey lacking masonry infills
v Strength Irregularity (Weak Storey) Si < 0.80 ร— Si+1 Must comply with Cl. 7.10 in Zones IIIโ€“V โšก Directly triggered โ€” bare RC columns far weaker than infilled upper storey
ii Mass Irregularity Wi > 1.5 ร— Wi+1 Dynamic Analysis in Zones IIIโ€“V May occur if heavy equipment on specific floor
iii Vertical Geometric Irregularity Dimension in storey > 1.25ร— storey below Dynamic Analysis in Zones IIIโ€“V Typical setback buildings
iv In-Plane Discontinuity Offset > 20% of element plan length Dynamic Analysis required Not typical in open storey context
vi Floating / Stub Columns Column doesn’t reach foundation Not permitted if part of LLRS Dangerous companion to open storeys
vii Irregular Modes of Oscillation MPF <65% or periods too close Dynamic Analysis required Can be worsened by soft storey effect
โš ๏ธ
Amendment Note: Per the official Amendment to IS 1893: 2016, the definition of Stiffness Irregularity (Soft Storey) has been updated. The inter-storey drift shall be limited to 0.2% in that storey and all storeys below in case of stiffness irregularity. Where the weak storey arises from URM infills, provisions of Clause 7.10 shall be followed.
Module 05 โ€” The Standard’s Direct Provisions

Clause 7.10 โ€” Full Breakdown

This is the most important section in IS 1893 for practising engineers dealing with residential and mixed-use RC frame buildings. Read each sub-clause carefully.

RC Frame Buildings with Open Storeys

What it says: RC moment resisting frame buildings which have open storey(s) at any level โ€” such as due to discontinuation of unreinforced masonry (URM) infill walls or of structural walls โ€” are known to have flexible and weak storeys as per Table 6.

What you must do: In such buildings, suitable measures shall be adopted to increase both stiffness and strength to the required level in the open storey and the storeys below. These measures shall be taken along both principal plan directions.

The fix options are:

  • a) RC structural walls (shear walls), or
  • b) Braced frames, in select bays of the building

Requirements for RC Structural Walls

When RC structural walls are provided, they shall be:

  • a) Founded on properly designed foundations
  • b) Continuous preferably over the full height of the building
  • c) Connected preferably to the moment resisting frame
๐Ÿ’ก
Why continuous walls? An RC wall that starts and stops mid-height creates its own irregularity. Continuity ensures the load path for seismic forces runs smoothly from roof to foundation.

โญ The Numerical Checks โ€” Most Important Provisions

When RC structural walls are provided, the design must ensure the building does not have:

Cl. 7.10.3b โ€” Stiffness Check
K_open โ‰ฅ 0.80 ร— K_above
Lateral stiffness of the open storey must be at least 80% of that in the storey above. If not achieved with walls, redesign!
Cl. 7.10.3c โ€” Strength Check
S_open โ‰ฅ 0.90 ร— S_above
Lateral strength of the open storey must be at least 90% of that in the storey above. Higher threshold than stiffness โ€” strength is more critical.
๐ŸŽฏ
Practical implication: Stiffness of bare RC columns alone is typically only 2โ€“5% of a fully infilled storey. You need to add substantial RC walls to meet 80% stiffness and 90% strength ratios. The amendment also reminds that torsional irregularity must NOT be worsened by the wall placement.

RC Structural Wall Plan Density (ฯw)

Definition (Cl. 4.25)
ฯ_w = (A_wall / b) ร— 100%
Where A_wall = cross-sectional area of RC walls at plinth level (in direction considered), b = plan width of building perpendicular to direction of earthquake.
Minimum Requirement
ฯ_w โ‰ฅ 2% (Zones III, IV, V)
Along each principal direction. Walls must be well distributed in plan. This requirement also applies to regular buildings as good seismic design practice.

Compliance with IS 13920

RC structural walls in buildings located in Seismic Zones III, IV and V shall be designed and detailed to comply with all requirements of IS 13920 (Ductile detailing of reinforced concrete structures subjected to seismic forces).

๐Ÿ“‹
IS 13920 specifies requirements for boundary elements, shear reinforcement, lap splices, and other ductility details for RC shear walls. Compliance is mandatory โ€” not optional โ€” in Seismic Zones III, IV, and V.

Summary Compliance Checklist for Cl. 7.10

  • M
    Open storey identified (discontinuation of URM infill or structural walls at any level) โ†’ provisions of Cl. 7.10 apply
  • M
    Increase stiffness AND strength in the open storey and all storeys below, along BOTH plan directions
  • M
    Provide RC structural walls or braced frames as the strengthening measure
  • M
    Verify: Lateral stiffness of open storey โ‰ฅ 80% of storey above (Cl. 7.10.3b)
  • M
    Verify: Lateral strength of open storey โ‰ฅ 90% of storey above (Cl. 7.10.3c)
  • M
    No additional torsional irregularity introduced by wall placement (Cl. 7.10.3a)
  • M
    Wall plan density ฯw โ‰ฅ 2% in each principal direction (Zones III, IV, V) (Cl. 7.10.4)
  • S
    RC walls continuous over full building height (preferred) (Cl. 7.10.2b)
  • S
    RC walls connected to moment-resisting frame (preferred) (Cl. 7.10.2c)
  • M
    RC walls detailed per IS 13920 for buildings in Zones III, IV, V (Cl. 7.10.5)
  • P
    Dynamic analysis used to capture actual lateral stiffness distribution (Amendment to Cl. 7.1)

M = Mandatory    S = Should (preferred)    P = Preferred but contextual

Module 06 โ€” Interactive Tool

Open Storey Compliance Calculator

Use this tool to check whether your building’s open storey complies with IS 1893 Clause 7.10. Enter the storey properties and the calculator performs the stiffness ratio, strength ratio, and wall plan density checks step by step.

๐Ÿ—๏ธ Building & Open Storey Data Input

Include RC columns + any walls added at this storey
Typically infilled storey with masonry walls
Sum of shear capacities of all columns + walls
Sum of shear capacities of all members
Total area of all RC walls running in X-direction
Plan dimension perpendicular to X-direction walls
Total area of all RC walls running in Y-direction
Plan dimension perpendicular to Y-direction walls

๐Ÿ“Š Compliance Analysis Results

๐Ÿ”ข Step-by-Step Calculations

    ๐Ÿ“ Assumptions & Reference

    • Stiffness and strength values are user-supplied from structural analysis model
    • IS 1893 (Part 1): 2016, Clause 7.10 thresholds applied
    • Wall plan density formula: ฯ_w = A_wall / b ร— 100% (Cl. 4.25)
    • Detailing per IS 13920 required for Zones III, IV, V (not checked here)
    • Dynamic analysis requirement for soft storey buildings noted
    Visualisation Aid

    Stiffness Ratio Visualizer

    Move the sliders to see how Kopen / Kabove relates to the 80% threshold from IS 1893 Cl. 7.10.3b.

    80% threshold

    Module 07 โ€” Solutions

    Remedies & Design Solutions

    IS 1893 Cl. 7.10 mandates that the open storey must be stiffened and strengthened. Here are the two code-prescribed approaches, plus practical considerations.

    Feature RC Structural Walls (Shear Walls) Braced Frames
    Prescribed by Cl. 7.10.1(a) + 7.10.2โ€“7.10.5 Cl. 7.10.1(b)
    Stiffness added Very high โ€” ideal for open storeys Moderate to high โ€” depends on section
    Continuity required Preferably full height (Cl. 7.10.2b) Typically within the open storey and below
    Detailing standard IS 13920 mandatory (Zones III, IV, V) IS 800 for steel braces
    Plan density check ฯw โ‰ฅ 2% (Cl. 7.10.4) Not directly, but strength check still applies
    Torsion risk Must be symmetric to avoid adding torsion (Cl. 7.10.3a) Same โ€” must be symmetric in plan
    Foundation Properly designed foundations required (Cl. 7.10.2a) Column base plates on ductile connections
    Practicality Most common in Indian RC construction Common in industrial / steel structures

    RC Structural Wall Plan Density โ€” How to Calculate

    X-Direction Density
    ฯ_wX = (A_wX / b_y) ร— 100%
    A_wX = total wall area in X-direction (mยฒ); b_y = building width in Y (m). Result must be โ‰ฅ 2%.
    Y-Direction Density
    ฯ_wY = (A_wY / b_x) ร— 100%
    A_wY = total wall area in Y-direction (mยฒ); b_x = building width in X (m). Result must be โ‰ฅ 2%.
    โœ…
    Example: For a 12m ร— 14m building in Zone IV, you need A_wX โ‰ฅ 2% ร— 14 = 0.28 mยฒ of wall cross-section in the X-direction and A_wY โ‰ฅ 2% ร— 12 = 0.24 mยฒ in the Y-direction, at the plinth level. This corresponds to e.g. 2 walls of 140mm ร— 1000mm each per direction.
    ๐Ÿ”
    Why Cl. 7.10.4 also applies to regular buildings: The standard explicitly notes that RC structural walls for improving seismic performance “can be adopted even in regular buildings that do not have open storey(s).” Good seismic design recommends shear walls regardless of infill configuration.
    Module 08 โ€” Real-World Context

    Historical Evidence โ€” Why This Matters

    Open storey failures have been documented in nearly every significant Indian earthquake. These are not theoretical risks โ€” they are well-documented failure modes that killed thousands of people.

    2001 โ€” BHUJ EARTHQUAKE (Mw 7.7)
    Gujarat, India โ€” Zone V
    Catastrophic failure of open ground storey RC buildings in Bhuj and Ahmedabad. The soft storey mechanism was the dominant failure mode in many residential buildings. Over 13,000 lives lost.
    1993 โ€” LATUR EARTHQUAKE (Mw 6.2)
    Maharashtra, India โ€” Zone III
    While primarily masonry failures, some RC buildings with open ground storeys also failed demonstrating the danger of stiffness discontinuities even in moderate seismic zones.
    1999 โ€” CHAMOLI EARTHQUAKE (Mw 6.8)
    Uttarakhand, India โ€” Zone V
    RC buildings with open storeys sustained severe damage. Soft storey columns failed by shear and bending, leading to partial collapse of the building while upper floors tilted.
    2015 โ€” NEPAL EARTHQUAKE (Mw 7.8)
    Kathmandu โ€” (Comparative)
    Extensive documentation of open storey building failures in Nepal confirmed the universal nature of this problem โ€” masonry-filled upper floors riding on bare-frame ground floors failed systematically.
    ๐Ÿ“Œ
    IS 1893’s Response: The 2016 edition of IS 1893 (which replaced the 2002 edition) introduced more stringent provisions for open storey buildings including the specific quantitative checks in Clause 7.10.3 (80% stiffness, 90% strength) and the mandatory wall plan density requirement โ€” all directly informed by post-earthquake reconnaissance findings.
    Module 09 โ€” Test Your Knowledge

    Self-Assessment Quiz

    Test your understanding of IS 1893 Clause 7.10 and open storey buildings.

    IS 1893 (Part 1): 2016 โ€” Open Storey Buildings Learning Module
    Based on Bureau of Indian Standards, IS 1893 (Part 1): 2016 โ€” Criteria for Earthquake Resistant Design of Structures.
    Clause 7.10 | Table 6 | Clauses 4.20, 4.21, 4.23, 4.24, 4.25 | Amendment No. 1.
    For educational purposes only. Always consult a qualified structural engineer for design decisions.
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