📐 IS 1893 Learning Hub
IS 1893 (Part 1): 2016 + Amendment No. 2 (Nov 2020)
Floating Columns
— Are They Permitted?
A complete, code-accurate learning guide covering the definition, seismic hazards, codal restrictions, and design requirements for floating columns under Indian practice.
Short Answer: PROHIBITED if part of lateral load resisting system
Permitted only as gravity-only members — with mandatory dynamic analysis
Clause 4.26 — Amendment No. 2
What is a Floating Column?
The official definition was first formally added to IS 1893 by Amendment No. 2 (November 2020).
IS 1893 (Part 1): 2016 — Clause 4.26 [Inserted by Amendment No. 2, November 2020]
“Floating Column — A vertical element (column) which at its lower level (termination level) rests on a beam.”
What it looks like
A column that does not go all the way down to the foundation or plinth level. Instead, it terminates midway and sits on top of a beam — so the beam must carry the entire column load and transfer it laterally to other columns.
Why architects use them
To create open ground floors (parking, lobbies), cantilever overhangs, setback floors, or to accommodate irregular floor plans where the upper columns cannot align directly with lower columns.
Related terms
Stub column, transfer beam structure, out-of-plane offset irregularity. A floating column always creates a discontinuity in the vertical load path — the primary structural concern in seismic design.
Left: Continuous columns with direct vertical load path | Right: Floating column — load must travel horizontally through a transfer beam, creating seismic vulnerability
Seismic Engineering Principle
Why Are Floating Columns Seismically Dangerous?
IS 1893 describes floating columns as structures “likely to cause concentrated damage” — here’s why.
Codal Language (IS 1893 Table 6 — Sl. No. vi, as amended)
“Such columns are likely to cause concentrated damage in the structure, and are undesirable.”
01
Disrupted Vertical Load Path
Gravity and seismic forces should travel straight down through columns to the foundation. A floating column forces loads to travel horizontally through a transfer beam — an inefficient and risky path during earthquakes.
02
Stiffness Discontinuity
Upper storeys with floating columns are typically much stiffer than the lower storey. During seismic shaking, the storey below the floating column acts like a soft storey, concentrating damage and inelastic deformation.
03
Overloaded Transfer Beam
The beam supporting the floating column must carry combined gravity, seismic, and torsional loads. Under dynamic loading, the forces are amplified compared to static calculations — often underestimated by designers.
04
Torsional Irregularity
Floating columns often create mass and stiffness eccentricities. The resulting torsional response amplifies the demand on columns and connections — especially dangerous in corners and plan-irregular buildings.
05
Progressive Collapse Risk
If the transfer beam or its connections fail, the entire zone of floating columns loses support simultaneously — leading to progressive collapse of multiple upper floors in a cascade.
06
Connection Vulnerability
The connection between the floating column base and the supporting beam is a critical node subjected to high bending, shear, and axial forces — requiring special ductile detailing as per IS 13920.
Historical Earthquake Evidence
- The 2001 Bhuj earthquake caused catastrophic failures in buildings with floating columns — entire upper portions collapsed over open ground floors.
- The 1999 Turkey earthquake showed that transfer beam structures consistently performed worse than continuous column structures of similar height and mass.
- These observations directly motivated the strict codal restrictions now in IS 1893.
Table 6, Sl. No. vi — Vertical Irregularity
Codal Provisions — IS 1893 (Part 1): 2016
Floating columns are listed under Vertical Irregularities in Table 6 of IS 1893. The rules were significantly tightened by Amendment No. 2.
IS 1893 (Part 1): 2016 — Table 6, Sl. No. vi [Substituted by Amendment No. 2, November 2020]
“vi) Floating Columns — Such columns are likely to cause concentrated damage in the structure, and are undesirable. A building with floating columns shall not be permitted, if the floating columns are part of or supporting the primary lateral load resisting system.”
What is the “Primary Lateral Load Resisting System” (LLRS)?
The LLRS consists of the structural elements that resist lateral (seismic or wind) forces and transfer them to the ground. In a typical RC building, this includes: columns, beams (moment frames), shear walls, and braced frames. If a floating column is part of or bears directly on any of these elements during lateral loading, the restriction applies.
Two Scenarios — Which Rule Applies?
| Scenario | Role of Floating Column | IS 1893 Status | Analysis Required | Additional Requirements |
|---|---|---|---|---|
| Scenario A | Part of or supporting the primary Lateral Load Resisting System (LLRS) | NOT PERMITTED | N/A — Prohibited | Eliminate or redesign to avoid floating column in LLRS |
| Scenario B | Gravity-only column; not part of LLRS; lateral forces carried by other continuous columns/walls | CONDITIONAL | Dynamic Analysis mandatory (Response Spectrum or Time History) | Design transfer beam + connections for 2.5× amplified forces; follow IS 13920 |
Critical Distinction
- A floating column can never be part of the LLRS — it must be isolated from lateral load transfer.
- Even if the column itself is gravity-only, the supporting beam and connections must be designed for amplified forces because the out-of-plane offset creates an irregularity.
- Out-of-plane offset irregularity (Table 6, Sl. No. iv) also applies whenever elements have such offsets — floating columns inherently create this condition.
Related Irregularities That Also Apply
| Irregularity | Table 6 Sl. No. | Why It Applies to Floating Column Buildings |
|---|---|---|
| Floating Columns | vi | Direct — the floating column itself |
| Out-of-plane Offset | iv | Upper column is offset from lower columns — always present with floating columns |
| Stiffness Irregularity (Soft Storey) | i | Storey below transfer level often much more flexible than upper storeys |
| Mass Irregularity | ii | Transfer beam adds concentrated mass at a particular level |
| Strength Irregularity (Weak Storey) | v | Storey below floating column may have reduced lateral strength |
Zone-by-Zone Requirements
Seismic Zone-Specific Rules
IS 1893 divides India into Zones II–V. The restrictions on irregular buildings (including floating columns) intensify with zone severity.
ZONE II
⚠️ CONDITIONAL
Lowest seismicity. Floating columns (gravity-only) may be used with dynamic analysis. Drift check required: lateral drift ≤ 0.2% of building height for in-plane discontinuity.
ZONE III
🚫 VERY RESTRICTED
Floating columns as LLRS: NOT permitted. Gravity-only: dynamic analysis mandatory. In-plane discontinuity not permitted at all in Zone III–V.
ZONE IV
⛔ HIGHLY RESTRICTED
Same as Zone III plus: vertical effects must be considered; out-of-plane offset forces must be enhanced ≥ 2.5× for connecting elements.
ZONE V
⛔ MOST RESTRICTED
Highest seismicity. All restrictions of Zone IV plus mandatory vertical earthquake effects. OBF (Ordinary RC Moment Frames) not permitted. Only SMF or DSMF allowed.
India’s Major Cities by Zone
- Zone II: Hyderabad, Bangalore, Mumbai (much of it), Chennai, Bhopal
- Zone III: Delhi, Kolkata, Jaipur, Agra, Ahmedabad, Surat, Pune
- Zone IV: Jammu, Dehradun, Nainital, parts of North-East
- Zone V: Kashmir, Assam, parts of North-East, Andaman & Nicobar
Force Enhancement Requirements (Out-of-Plane Offset)
Applicable when a building is in Seismic Zones III, IV or V and has out-of-plane offset (which all floating column buildings have):
IS 1893 Table 6 — Sl. No. iv [Amendment No. 2]
Fdesign ≥ 2.5 × Fearthquake
FdesignDesign force/moment in connecting elements, supporting column, and connections
2.5Minimum amplification factor (may be higher based on analysis)
FeqForces/moments due to earthquake effects from analysis
This 2.5× factor applies to:
- Elements connecting the two offset vertical elements (the transfer beam)
- The vertical element supporting the offset (column below the transfer beam)
- The connections between these elements
Design & Analysis Requirements
Design Requirements for Floating Column Buildings
When floating columns are used as gravity-only elements, a series of mandatory analysis and detailing requirements must be followed.
Step-by-Step Design Workflow
01
Confirm LLRS Independence
Verify that the floating column is completely decoupled from the lateral load path. Lateral forces must be resisted by continuous columns, shear walls, or braced frames that extend to the foundation.
02
Dynamic Analysis
Perform 3D dynamic analysis: Response Spectrum Method (Cl. 7.7.1) or Time History Method (Cl. 7.7.2). Static equivalent method (Cl. 7.6) is NOT permitted for buildings with this irregularity.
03
Model Transfer Level Carefully
The transfer beam and the floating column junction must be accurately modelled with correct stiffness and mass. Use shell elements or detailed frame elements. Rigid diaphragm assumption may not be valid.
04
Apply Force Amplification
For Zones III, IV, V: Amplify forces in transfer beam, supporting column, and connections by minimum 2.5× the earthquake design forces.
05
Design Transfer Beam
Design the transfer beam for combined gravity + amplified seismic loads. Provide deep beams if necessary. Check shear, bending, and torsion. Avoid plastic hinge formation under design-level earthquakes.
06
Ductile Detailing (IS 13920)
Follow IS 13920: 2016 for all ductile detailing. Special attention at: floating column base joint, transfer beam-to-column connection, confinement reinforcement at critical sections.
Analysis Method Requirements
| Analysis Method | IS 1893 Clause | Applicable to Floating Column Buildings? | Notes |
|---|---|---|---|
| Equivalent Static (Seismic Coefficient) | 7.6 | NOT APPLICABLE | Only for regular buildings in all zones. Irregular buildings must use dynamic methods. |
| Response Spectrum Method | 7.7.1 | PERMITTED | Minimum requirement for irregular buildings. At least 3 modes or 90% mass participation. |
| Linear Time History Analysis | 7.7.2 | PERMITTED | More detailed. Requires a set of ground motion records compatible with site seismicity. |
| Non-linear (Pushover / NLTHA) | 7.7.3 | RECOMMENDED | Not mandatory but highly recommended to capture inelastic behaviour of transfer elements. |
Vertical Earthquake Effects
IS 1893 Clause 6.3.3.1 (Amendment No. 2) — Vertical Earthquake Shaking
Buildings with vertical or plan irregularities must consider vertical earthquake effects. For floating column buildings:
- Add ±2/3 × design horizontal PGA in the vertical direction
- The transfer beam must be designed for gravity loads combined with vertical seismic amplification
- This often governs the design of deep transfer beams
Legislative History
How the Code Evolved — Amendment Timeline
IS 1893 has progressively tightened its stance on floating columns through two key amendments.
IS 1893 (Part 1): 2002
Floating Columns Mentioned, No Formal Definition
The 2002 edition listed floating columns under vertical irregularities in a table but provided no formal definition. Restrictions were vague — essentially requiring dynamic analysis but without the “not permitted” language for LLRS involvement.
IS 1893 (Part 1): 2016 (Sixth Revision)
Floating Columns Retained in Table 6 — Original Version
The 2016 revision updated irregularity provisions but floating columns were listed without a formal definition (Clause 4.26 was not yet included). The table entry for Sl. No. vi classified them as a vertical irregularity requiring dynamic analysis.
Amendment No. 1 — September 2017
Formula for Aw Corrected
This amendment primarily addressed the formula for effective wall area in the period formula (Clause 7.6.2). Floating column provisions were not changed.
Amendment No. 2 — November 2020 🔴 KEY
Floating Columns Formally Defined + Strict Prohibition Added
The landmark amendment that introduced: (1) Formal definition in Clause 4.26 — “a vertical element which at its lower level rests on a beam”; (2) Clear prohibition: “A building with floating columns shall not be permitted, if the floating columns are part of or supporting the primary lateral load resisting system”; (3) Enhanced force amplification (2.5×) for out-of-plane offsets in Zones III–V; (4) Mandatory vertical earthquake effect consideration for irregular buildings; (5) In-plane discontinuity prohibited in Zones III, IV, V; (6) Strength irregularity prohibited outright.
What Amendment No. 2 Changed for Floating Columns
- Added formal definition (Clause 4.26) — now legally unambiguous
- Changed language from “undesirable” to “shall not be permitted” for LLRS involvement — absolute prohibition
- Clarified that gravity-only floating columns may still be used with dynamic analysis
- Cross-linked with out-of-plane offset irregularity and the 2.5× force amplification requirement
- Strengthened vertical seismic effect requirements for irregular buildings including floating column buildings
Summary: Before vs. After Amendment No. 2
| Aspect | IS 1893: 2016 (Before Amd. 2) | IS 1893: 2016 + Amd. 2 (Current) |
|---|---|---|
| Definition | Not formally defined | Clause 4.26: explicit definition added |
| Floating column in LLRS | “Undesirable” — dynamic analysis required | “Shall NOT be permitted” — absolute prohibition |
| Gravity-only floating column | Dynamic analysis required | Dynamic analysis + 2.5× force amplification in Zones III–V |
| Vertical seismic effects | Limited cases specified | Mandatory for irregular buildings including floating column buildings |
| Force enhancement factor | Not explicitly specified | Minimum 2.5× for connecting elements in Zones III–V |
Interactive Tool
Floating Column Permissibility Checker
Enter your building parameters to get a codal assessment and design checklist as per IS 1893 (Part 1): 2016 with Amendment No. 2.
🏗️ Building Parameter Input
Based on IS 1893 (Part 1): 2016 + Amendment No. 2 (November 2020)
Summary
Key Takeaways for Students & Practitioners
Everything you need to remember about floating columns and IS 1893 in one place.
TAKEAWAY 01
Absolute Prohibition — If Part of LLRS
The moment a floating column forms part of or supports the primary lateral load resisting system, it is prohibited outright by IS 1893. No amount of analysis or reinforcement can make it acceptable in this configuration.
TAKEAWAY 02
Amendment No. 2 Is the Current Law
Amendment No. 2 (November 2020) introduced the formal definition (Cl. 4.26) and changed the language from “undesirable” to “shall not be permitted.” Any design must comply with this amended version.
TAKEAWAY 03
Dynamic Analysis — No Exceptions
Buildings with floating columns (even gravity-only) can never use the Equivalent Static Method. Dynamic analysis — Response Spectrum or Time History — is mandatory.
TAKEAWAY 04
2.5× Force Amplification — Zones III, IV, V
For out-of-plane offset (inherent in floating columns), forces in the transfer beam, supporting column, and connections must be multiplied by at least 2.5 in Seismic Zones III, IV, and V.
TAKEAWAY 05
Multiple Irregularities Stack Up
Floating column buildings typically trigger multiple irregularity provisions: Sl. No. iv (out-of-plane offset), Sl. No. vi (floating column), often Sl. No. i (soft storey). Each comes with additional requirements.
TAKEAWAY 06
Vertical Earthquake Effects Are Mandatory
As per Clause 6.3.3.1 (Amendment No. 2), buildings with vertical or plan irregularities must consider vertical earthquake shaking — this directly impacts transfer beam design.
TAKEAWAY 07
IS 13920 Detailing Is Non-Negotiable
All connections, joints, and critical members at the transfer level must comply with IS 13920: 2016 ductile detailing requirements. Special confinement reinforcement is required at floating column base joints.
TAKEAWAY 08
India Learned from Bhuj (2001)
The progressive tightening of floating column rules is directly traceable to the 2001 Bhuj earthquake, where open ground floor buildings and transfer structures failed catastrophically. Code provisions reflect this hard-won knowledge.
Reference Clauses Summary
- Cl. 4.26 — Definition of floating column (inserted by Amendment No. 2)
- Table 6, Sl. No. iv — Out-of-plane offset irregularity + 2.5× force amplification
- Table 6, Sl. No. vi — Floating column prohibition if part of LLRS
- Cl. 6.3.3.1 — Vertical earthquake shaking (mandatory for irregular buildings)
- Cl. 7.6 — Equivalent Static Method (NOT applicable to floating column buildings)
- Cl. 7.7 — Dynamic Analysis methods (mandatory)
- IS 13920: 2016 — Ductile detailing for all members
