IS 4326 : 1993Earthquake Resistant Design and Construction of Buildings - Code of Practice

IS 4326:1993 is the code of practice for earthquake-resistant design and construction of buildings. It applies primarily to low-rise buildings, masonry structures, and traditional construction — the bulk of buildings in seismic zones III, IV, and V that are NOT designed as special moment-resisting RCC frames.

The scope that differentiates it from other seismic codes: - IS 1893 Part 1:2016 — determines seismic DEMAND (base shear) for structural analysis - IS 13920:2016 — ductile DETAILING of RCC frames with sophisticated analysis - IS 4326:1993 — DETAILING of buildings with simpler methods: load-bearing masonry, confined masonry, RCC buildings up to G+4, traditional low-rise construction - IS 13828:1993 — supplementary specifications for low-strength masonry

You reference IS 4326 for: - Residential G+1 to G+3 construction in Zones III-V where detailed IS 13920 design isn't economically justified - Load-bearing masonry buildings (schools, community centres, institutional structures) - Confined masonry (brick masonry with RCC tie columns and beams) - Traditional construction in hilly regions (timber, stone masonry buildings in Himachal, Uttarakhand, Northeast) - Retrofitting older buildings for seismic resistance - Boundary walls, compound walls, and non-load-bearing masonry in seismic zones

IS 4326:1993 introduces three key concepts that distinguish seismic-resistant from ordinary construction:

1. Seismic bands (Clause 8.4): Four continuous RCC bands around the building perimeter, tying walls together at key elevations: - Plinth band — at floor level, above foundation - Lintel band — at window/door head level - Roof band — at eave/roof level - Gable band — at gable roof apex (if applicable)

Each band is 75-150 mm deep × wall thickness, minimum 2 × 8 mm or 2 × 10 mm continuous longitudinal reinforcement, 6 mm stirrups @ 150 c/c. Bands tie the entire building into a box, preventing individual wall collapse.

2. Corner vertical reinforcement (Clause 8.5): At every building corner and door/window jamb, a vertical 10 or 12 mm bar embedded in the masonry or in an RCC tie column. Creates a frame of tie-ties that resists seismic shaking without relying on masonry alone.

3. Through-wall connection (Clause 8.3): Where two walls meet at a corner or T-junction, the bricks must interlock courses (not just butt against each other). For thicker walls, throughstones or RCC dowels connect the two wall panels.

Additional requirements: - Restriction on room size: max 6 m length, 4.5 m width for bearing-wall construction in Zone V - Maximum permissible floor height: 3.0 m for load-bearing masonry - No unreinforced masonry above lintel level in Zone V - Cross-wall spacing (interior bearing walls): maximum 6 m for Zone IV-V

Project: Single-storey school building, 12 × 20 m plan, 3 m wall height. Uttarakhand (mountainous, Zone V, Z = 0.36). Load-bearing brick masonry walls (230 mm thick), RCC slab roof.

Design approach — IS 4326 compliant construction (not a detailed frame analysis):

Step 1 — Establish cross-walls for stability: IS 4326 Clause 7.4: cross-wall spacing ≤ 4.5 m in Zone V. Plan has 20 m length → need at least 4 cross-walls at 4 m-5 m intervals. Position interior walls to subdivide the plan.

Step 2 — Plinth band (at floor level): Continuous RCC band, 230 mm × 75 mm depth, along entire building perimeter AND on all cross-walls. Reinforcement: 4 × 10 mm Fe 500 longitudinal, 6 mm stirrups @ 150 c/c. Total length for perimeter (2×12 + 2×20 = 64 m) plus cross-walls (4 × 12 m = 48 m) = 112 m of plinth band.

Step 3 — Lintel band (at 2.1 m above floor, window/door head level): Same cross-section as plinth band. 230 × 75 mm, 4 × 10 mm reinforcement. Total length: 112 m (perimeter + cross-walls).

Step 4 — Roof band / wall plate (at 3.0 m, under roof slab): Same cross-section. 112 m.

Step 5 — Corner and jamb reinforcement: At each corner (8 corners for a rectangular plan): vertical 12 mm bar embedded in masonry from plinth to roof, tied to plinth + lintel + roof bands. At each door jamb (2 doors × 2 jambs = 4 + window jambs 8 × 2 = 16) = 24 verticals total. Each vertical: 3 m length × 12 mm bar, tied to 3 bands.

Step 6 — Through-wall masonry interlocking: At every corner and T-junction, brickwork alternates between 'stretcher' and 'header' courses to interlock walls. Or use RCC dowels (10 mm L-shape, 3 per corner per metre height).

Step 7 — Roof slab (RCC): Standard RCC slab design per IS 456. Anchor slab into the roof band with ties.

Step 8 — Openings (IS 4326 Clause 8.7): Aggregate opening area ≤ 50% of wall length for Zone V. Maximum individual opening: 3 m wide. No opening within 600 mm of a corner.

Step 9 — Masonry specification: 1:1:6 mortar (cement:lime:sand) minimum for Zone V. Better: 1:1:4. First-class brickwork, no broken or damaged bricks.

Reinforcement summary for this school: Total band length: 3 × 112 = 336 m Band mass: 336 × (4 × 10² / 162.2 + 6 × stirrups per m × 0.222) ≈ 336 × 0.56 ≈ 188 kg Vertical bars: 24 × 3 m × 12 mm × 0.888 kg/m = 64 kg Total rebar for seismic detailing: ~250 kg

This is ~10-15% of what a full IS 13920 detailed frame would require, while providing substantially better seismic resilience than unreinforced masonry.

1. Skipping IS 4326 because 'it's just a small building'. Many rural residential and school buildings in seismic zones are built without any seismic detailing — load-bearing masonry with no bands, no corner reinforcement, thick walls but no ties. These are the buildings that collapse in earthquakes (Bhuj 2001, Sikkim 2011, Nepal 2015). IS 4326 detailing adds ~5-10% to construction cost and dramatically improves survival probability.

2. Only providing lintel band, missing plinth and roof bands. Lintel band alone is insufficient — the building becomes a 'hinged' structure that rotates at floor level during shaking. IS 4326 Clause 8.4 requires a continuous band system including plinth, lintel, and roof. All three must be continuous around the perimeter and cross-walls.

3. Interrupted bands at doors and windows. Critical error. Bands must continue through openings — either with increased depth above the opening OR with a pair of bars extending 600 mm each side into the wall. Many sites discontinue bands at openings; this defeats the entire seismic intent.

4. Using weak 1:6 mortar in Zone V. IS 4326 mandates 1:1:6 (cement:lime:sand) OR 1:1:4 (cement:lime:sand, stronger) for Zone V. Standard 1:6 mortar is for Zone II-III ordinary construction. Using 1:6 mortar in Zone V Himalayan construction has led to wall failures even in moderate tremors.

5. Ignoring the opening-size limits. Clause 8.7 restricts opening size and aggregate area. Modern architectural preferences for large windows and sliding doors often violate this in residential Zone IV-V. Either reduce opening size, add vertical reinforcement around openings, or design the building as RCC frame per IS 13920 (not bearing masonry).

• IS 1893 Part 1:2016 — seismic demand (base shear, zone factor)
• IS 13920:2016 — ductile detailing for RCC frames — supplement to IS 4326 for RCC buildings
• IS 13828:1993 — improving earthquake resistance of low-strength masonry (mud, stone)
• IS 13827:1993 — improving earthquake resistance of earthen buildings
• IS 1905:1987 — structural use of unreinforced masonry (non-seismic design base)
• IS 456:2000 — RCC design for bands, corner reinforcement, slabs
• IS 2212 — brickwork construction
• IS 2116 — sand for masonry mortar
• IS 1077 / IS 12894 — brick specifications
• IS 15988 — seismic evaluation and strengthening of existing RCC buildings

IS 4326:1993 is 33 years old — a pre-modern seismic code written before Bhuj 2001, Kashmir 2005, Nepal 2015 earthquakes revealed weaknesses in traditional South Asian construction. The 1993 revision replaced an even older 1976 code.

A revision is long overdue. Draft revision circulated within BIS CED 39 in 2019 and 2023, adding provisions for: confined masonry (currently supplementary), modern AAC block construction, retrofit guidelines, and performance-based design. Nothing published yet.

For current practice in seismic zones: - Zone II-III: IS 4326 seismic bands are recommended for all masonry buildings above G+0, mandatory for institutional / public buildings - Zone IV: IS 4326 bands + corner reinforcement + through-wall interlocking are mandatory - Zone V: Full IS 4326 compliance + restricted opening sizes + designer-approved architectural modifications + consider RCC frame as alternative for buildings G+1 and above

Retrofit applications: For existing buildings without seismic bands, retrofit approaches per IS 15988 include: external jacketing with shotcrete-reinforced layer, post-tensioned steel ties threaded through walls, external steel framing as 'seismic jacket'. These can add seismic resistance to buildings that would otherwise require demolition.

Cultural heritage buildings in seismic zones (temples, forts, heritage structures) are typically unreinforced masonry — IS 4326 doesn't directly apply but guides retrofit decisions. Archaeological Survey of India uses IS 4326 principles with heritage-appropriate materials (lime mortar, compatible stonework) for retrofit of national monuments.