How IS 1893 Treats RC Frame Buildings with Masonry Infill Walls
Masonry infill walls are everywhere in Indian buildings — but for decades, engineers modelled them as if they didn’t exist. IS 1893:2016 finally says: you can’t ignore them anymore. Here’s everything you need to know.
What Are Masonry Infill Walls?
Masonry infill walls (also called URM infills — Unreinforced Masonry) are non-load-bearing partition walls built inside the grid of RC (Reinforced Concrete) beams and columns. They typically consist of brick, hollow concrete block, or AAC panels, plastered on both sides.
In India, nearly every multi-storey RC frame building uses masonry infill walls extensively — for rooms, staircases, boundary walls, and external enclosures. The lower floors (like stilt/pilotis for parking) are often left open, while upper floors are fully infilled.
Historically, designers treated infill walls as non-structural — they ignored them in structural models entirely. The RC bare frame was designed for seismic loads, while infills were assumed to just sit there harmlessly. In reality, infills interact significantly with the frame during earthquakes, completely changing the stiffness distribution, natural period, and failure pattern of the building. Many spectacular collapses — including Bhuj 2001, Nepal 2015 — were directly caused by soft-storey failure due to ignored infill effects.
Why Infills Matter Seismically
✅ Infilled Frame (IS 1893:2016)
- Shorter natural period → higher seismic force
- Infill stiffness explicitly modelled as diagonal strut
- More realistic distribution of storey forces
- Detects irregularities introduced by infills
- Required when SPD > 20%
🚨 Open Ground Storey (Worst Case)
- Sudden stiffness discontinuity at ground level
- Columns carry huge ductility demands
- Weak storey + soft storey combined
- 2.5× force multiplier required for OGS columns
- Banned in Zones IV & V unless remediated
| Parameter | Bare Frame | Fully Infilled | Open Ground Storey |
|---|---|---|---|
| Natural Period T | High (longer) | Short (lower) | Between the two |
| Base Shear VB | Lower (unsafe) | Higher (correct) | Medium but concentrated |
| Ground Floor Drift | Moderate | Low | 🔴 Very High |
| Stiffness Irregularity | None | None (if uniform) | 🔴 Severe |
| Column Axial Force | Moderate | Higher (strut forces) | 🔴 Critical concentration |
| Short Column Effect | Possible if partial infill | If restricted height | Possible at transitions |
Actual earthquake forces on structures are much higher than the design forces specified in the code. The code relies on inelastic behaviour (ductility) and overstrength to cover the deficit. When infills create a soft storey, this ductility demand concentrates brutally at the weak level — the building collapses before it can redistribute forces.
Structural Plan Density (SPD) — The Trigger Check
Structural Plan Density (SPD) is the ratio of the cross-sectional area of all unreinforced masonry (URM) walls in a given plan direction to the total plan area of the floor, expressed as a percentage. It is defined in IS 1893:2016 Cl. 7.9 and cross-referenced in Table 6.
The concept is simple: if your building has a lot of masonry walls (high SPD), ignoring them in analysis will produce significantly wrong results. So IS 1893:2016 sets a threshold.
When SPD of URM infill walls exceeds 20% in any principal plan direction, the effect of URM infills shall be considered by explicitly modelling them in structural analysis as per Clause 7.9 (equivalent diagonal strut method). This is mandatory, not optional.
- Identify all URM infill walls at a floor level in the direction under consideration (X or Y)
- Measure the thickness (t) and length (L) of each wall in that direction
- Compute Aw = t × L for each wall (cross-section perpendicular to that direction)
- Sum all Aw values: Σ Aw
- Divide by the total plan area of the floor: SPD = (Σ Aw / Afloor) × 100%
- Compare: if SPD > 20%, proceed with Clause 7.9 modelling
SPD is calculated separately for X and Y directions. A wall parallel to the X-direction resists forces in X. Check both directions independently — one may exceed 20% while the other does not.
Clause 7.9 — Modelling of URM Infill Walls
When SPD > 20%, the engineer must model URM infill panels as equivalent diagonal compression struts in the structural analysis. The infill is treated as a pin-jointed diagonal member that can only carry compression (no tension), connected between opposing beam-column joints.
The design forces for RC members (beams, columns, joints) shall be taken as the larger of:
- Results from analysis of the building with infills modelled (infilled frame model)
- Results from analysis of the building without infills (bare frame model)
Infills can increase force demand in some members (due to strut action) and create different load paths. Running both bare-frame and infilled-frame analyses and taking the envelope ensures nothing is under-designed.
- Infill panels are modelled as pin-jointed diagonal compression struts only — no moment transfer
- The strut is placed diagonally in the panel, and it can only carry compressive force
- Two struts (one for each loading direction) may be modelled per panel (X and Y)
- The strut is NOT rigidly connected at the frame joints — it simulates the way infill leans against the frame
- The modulus of elasticity of the masonry (Em) is taken as per IS 1905 provisions
- The thickness of the strut equals the actual thickness of the infill wall (tinf)
- Openings in infill walls significantly reduce effective strut width — special treatment needed
- Partial-height infills (creating short-column effect) must be separately identified and checked
The Equivalent Diagonal Strut — Theory & Formula
When a masonry infill panel is subjected to lateral load, it interacts with the surrounding RC frame. Instead of the full wall engaging, the infill develops a diagonal compression zone — essentially acting like a diagonal brace. Holmes (1961), Smith (1962), and subsequent researchers showed this can be represented by an equivalent diagonal strut.
The width of this equivalent strut (w) determines the stiffness contribution of the infill to the frame. IS 1893:2016 adopts the FEMA 356 / ASCE 41 formulation.
λ₁ is the relative stiffness of the infill to the frame. A higher λ₁ means the infill is much stiffer relative to the column, so the contact zone is narrower and the strut width is smaller. A lower λ₁ (flexible frame, stiff infill) gives a wider strut. The 0.175 factor and the −0.4 exponent come from curve-fitting to experimental and FE results.
| Masonry Type | fm (MPa) | Em = 550 fm (MPa) | Unit Weight (kN/m³) |
|---|---|---|---|
| Burnt brick — M1 mortar | 3.5 – 5.0 | 1925 – 2750 | 19.2 |
| Burnt brick — M2 mortar | 2.5 – 4.0 | 1375 – 2200 | 19.2 |
| Hollow concrete block | 2.0 – 3.5 | 1100 – 1925 | 12 – 15 |
| AAC block | 1.5 – 2.5 | 825 – 1375 | 6 – 8 |
| Stone masonry | 5.0 – 8.0 | 2750 – 4400 | 22 – 26 |
fm = compressive strength of masonry prism. Use IS 1905 for actual values. Em = 550·fm is the FEMA 356 empirical formula commonly adopted.
When infill walls are partially built (e.g., up to window sill level only), they restrict the deformation length of the column. The effective length is shortened to the gap above the infill. During an earthquake, shear demand is concentrated in this short column segment, often causing brittle shear failure. IS 1893:2016 requires these partial-height infill conditions to be explicitly identified and the affected columns designed for the increased shear demand.
Stiffness Irregularity Due to Infills — IS 1893 Table 6
IS 1893:2016 Table 6 defines vertical irregularity types. Masonry infills directly contribute to two of these:
A storey whose lateral stiffness is less than 70% of the stiffness of the storey above, OR less than 80% of the average stiffness of the three storeys above it.
A storey whose lateral stiffness is less than 60% of the storey above, OR less than 70% of the average stiffness of the three storeys above. This is the open-ground-storey condition for most buildings.
When infills are present only in upper floors and absent at ground level (typical stilt building), the ground storey stiffness drops dramatically, triggering both Type 1a and 1b irregularities simultaneously.
| Condition | Seismic Zone II | Zone III | Zones IV & V |
|---|---|---|---|
| Regular buildings — analysis method | ESM or RSM | ESM or RSM | RSM mandatory for >12m irregular |
| Soft storey (Type 1a) | ESM allowed | ESM allowed | 🔴 RSM mandatory; special detailing |
| Extreme soft storey (Type 1b) — OGS | 2.5× multiplier | 2.5× multiplier | 🔴 2.5× or explicit infill model; Zones IV/V: extremely discouraged |
| Weak storey | Special design required | 🔴 NOT PERMITTED in Zones III, IV, V | |
ESM = Equivalent Static Method; RSM = Response Spectrum Method.
- Model the building with infills as diagonal struts (if SPD > 20%)
- Extract the lateral storey stiffness Ki for each floor from the analysis (= storey shear / storey drift)
- Check if Ki ≥ 0.70 × Ki+1 (storey above)
- Check if Ki ≥ 0.80 × average(Ki+1, Ki+2, Ki+3)
- If either condition fails → Soft Storey (Type 1a); if Ki < 60% → Extreme (Type 1b)
- Apply required analysis method and design provisions
Clause 7.10 — Open Ground Storey (OGS) Design
Open Ground Storey (OGS) buildings — with parking at ground level — are everywhere in Indian cities. Bhuj 2001, Sikkim 2011, Nepal 2015 all showed catastrophic OGS collapses. Clause 7.10 is IS 1893’s direct response to this epidemic of unsafe stilt buildings.
IS 1893:2016 Clause 7.10 provides two approaches for OGS buildings:
Model the entire structure including infill struts in all storeys. The analysis will automatically capture the stiffness discontinuity at the open storey. Design the OGS columns and beams for the resulting forces. This is the more accurate approach required for buildings in Zones IV and V.
The columns and beams of the open ground storey shall be designed for 2.5 times the storey shears and moments calculated under seismic loads in the bare frame analysis. Additionally, the structural shear walls, braces, or additional columns shall be provided to stiffen the OGS. The 2.5 factor represents the concentration of ductility demand at the open storey.
- RC Shear Walls: Provide RC structural walls in the ground storey extending to the foundation — this directly stiffens the OGS
- Diagonal Bracing: Steel or RC diagonal braces in the open storey can compensate stiffness loss
- Infill the Ground Storey: Add masonry infill walls (at least partially) in the ground storey — the most cost-effective retrofit
- Increase Column Size: Larger columns with more ductile detailing per IS 13920:2016 to handle the extra ductility demand
- Base Isolation: For critical structures, though expensive
Ductile detailing of OGS columns MUST comply with IS 13920:2016 (special moment-resisting frame requirements). Seismic evaluation and retrofitting of existing OGS buildings is covered in IS 15988:2013.
Key Takeaways & Design Checklist
| # | Check | Clause | Action if Failed |
|---|---|---|---|
| 1 | Calculate SPD in X and Y directions | 7.9 / Table 6 | If SPD > 20%, model infills as struts |
| 2 | Compute equivalent strut width w for each panel | 7.9.1 | Use FEMA 356 formula; consider openings |
| 3 | Run both infilled and bare frame analysis | 7.9 | Design for envelope (larger forces) |
| 4 | Check stiffness irregularity: Ki ≥ 0.70 Ki+1 | Table 6 Type 1a | Classify as soft storey; use RSM |
| 5 | Check extreme irregularity: Ki ≥ 0.60 Ki+1 | Table 6 Type 1b | OGS provisions: 2.5× or explicit model |
| 6 | Check for partial-height infills (short column) | 7.9 | Design column for full short-column shear |
| 7 | OGS columns: apply 2.5× force multiplier OR model infills | 7.10 | Provide shear walls or bracing in OGS |
| 8 | Detail OGS columns per IS 13920:2016 (SMRF) | Cross-ref | Ductile detailing mandatory for Zones III–V |
| 9 | Check weak storey (lateral strength, not stiffness) | Table 6 Type 2 | NOT permitted in Zones III, IV, V |
| 10 | Verify natural period T with infills vs bare frame | 7.6 | Use shorter T for base shear calculation |
- Treating infill walls as non-structural (pure dead load) in seismic analysis — specifically prohibited when SPD > 20%
- Using bare frame natural period T for design spectrum when infills are present — this underestimates base shear
- Not identifying partial-height infills that create short column effects
- Designing OGS columns for regular frame forces without the 2.5× multiplier
- Ignoring the stiffness of infills when checking vertical irregularity
- Not running both infilled and bare frame models and taking the design envelope
- Assuming symmetric infill plan when infills are actually asymmetric — introduces torsion
- Forgetting that infill removal (renovation) changes the irregularity profile post-construction
IS 1893:2016’s treatment of masonry infills represents a major step toward realistic seismic assessment of Indian RC buildings. The key philosophy is: infills are structural whether you want them to be or not. When you model them, you find the real vulnerabilities — the soft storeys, the short columns, the torsion — before the earthquake does. When you don’t, the earthquake finds them for you.
