IS 15988 : 2013Seismic Evaluation and Strengthening of Existing Reinforced Concrete Buildings - Guidelines

IS 15988 specifies the methodology for seismic evaluation and strengthening (retrofit) of existing reinforced concrete buildings — buildings constructed before modern seismic codes (IS 1893:2002 / 2016 and IS 13920:1993 / 2016) became enforceable, OR buildings that have suffered earthquake damage and need rehabilitation.

Use IS 15988 when: - Pre-1993 RCC buildings in seismic Zones III-V (most of them lack ductile detailing) - Buildings damaged by past earthquakes (Bhuj 2001, Sikkim 2011, Nepal 2015) needing rehabilitation - Change of building use (residential → school/hospital) that bumps Importance Factor (I) and requires re-evaluation - Heritage / institutional buildings undergoing structural review for continued use - Vertical extensions / change of structural system requiring capacity check of existing - Mandatory periodic safety review (some state DCRs require for buildings > 30 years old)

IS 15988 does NOT apply to: - New construction — use IS 1893 + IS 13920 directly - Masonry, steel, or composite buildings — use IS 4326 (masonry) or other codes - Major industrial structures — use specialised guidelines

IS 15988 follows a tiered approach: cheap screening first, detailed analysis only when screening fails.

Tier 1 — Rapid Visual Screening (Clause 5): - Walk-through assessment by experienced structural engineer - Identify gross deficiencies: soft-storey, weak storey, plan asymmetry, short column, pounding risk between adjacent buildings, inadequate cover, visible deterioration - Scoring system for each deficiency - Building either passes (no further work needed beyond minor repair) or proceeds to Tier 2

Tier 2 — Detailed Evaluation (Clause 6): - 3D structural modelling using as-built drawings (rebar from invasive testing if drawings unavailable) - Linear static or response spectrum analysis per current IS 1893 - Check capacity-to-demand ratios for all critical elements (columns, beams, walls, joints, foundations) - Identify deficient elements - Decide retrofit strategy (Tier 3 — design)

Tier 3 — Retrofit Design and Construction (Clause 7): - Strengthen specific elements identified in Tier 2 - Verify global response of strengthened structure - Construction supervision and post-retrofit testing

The progression is: Tier 1 cost ~ ₹50,000 (1 day) → Tier 2 cost ~ ₹2-10 lakh (3-6 weeks) → Tier 3 cost can run 30-60 % of new construction cost.

1. Lack of ductile detailing (the headline issue): - Insufficient confinement reinforcement at column ends - Joint cores without stirrups - Low column-to-beam moment capacity ratio (weak column / strong beam) - Lap splices in plastic-hinge regions - Hooks bent at 90° instead of seismic-grade 135° - Spacing of stirrups too wide (often 200 mm instead of 100 mm in plastic-hinge zones)

2. Configuration deficiencies: - Soft / weak storey — open ground floor parking under stiff masonry-infill upper floors (the classic 'Bhuj failure mode') - Plan asymmetry causing torsional response - Short captive column due to partial-height infill or deep beams - Pounding risk with adjacent buildings (insufficient gap)

3. Material deterioration: - Carbonation of cover concrete and rebar corrosion - Spalling exposing rebar - Concrete strength below specified (verify by core test per IS 516 Part 1) - Honeycombing, voids in original construction

4. Foundation issues: - Inadequate footing for current load (if seismic was not designed for) - Differential settlement - Liquefaction potential not assessed (for sandy founding strata in Zones III-V)

5. Non-structural hazards (often overlooked but life-safety critical): - Unanchored partition walls falling during shake - Suspended ceilings, light fixtures, water tanks dropping - Glass facades, parapets, chimneys

1. Member-level strengthening: - RC jacketing of columns / beams — add 75-150 mm RC sleeve around existing member; new longitudinal bars + ties; bonds via dowels and roughened surface. Increases capacity 30-100 %; the most common technique. - Steel jacketing — angle iron at corners, batten plates around. Adds confinement; faster than RC jacketing; less invasive. - FRP wrapping (carbon, glass, aramid fibre-reinforced polymer) — high-strength wrap glued onto member surface. Adds shear capacity, confinement, and limited flexural capacity. Lighter, faster, but expensive.

2. System-level strengthening: - New shear walls added to deficient frames — convert moment-frame to dual system - Bracing (steel braces) added in selected bays — converts to braced frame - Base isolation (high-end, for hospitals, heritage, institutional) — decouple superstructure from ground motion - Energy-dissipating dampers (viscous, friction, BRBs) — reduce drift and forces; expensive

3. Foundation strengthening: - Underpinning with new piles - Soil improvement (compaction grouting, jet grouting) for liquefaction mitigation - Tying together separate footings into a raft

4. Configuration fixes: - Infill of soft-storey openings with shear walls or bracing - Adding seismic separation joints between adjacent buildings - Anchoring water tanks, parapets, equipment

Strategy selection depends on cost vs benefit, occupied vs vacated during retrofit, downtime tolerance, and target performance level (life-safety vs immediate-occupancy).

• IS 1893 Part 1:2016 — earthquake resistant design (the standard against which 'capacity' is judged).
• IS 13920:2016 — ductile detailing of RC structures (the detailing standard for strengthening).
• IS 4326:1993 — earthquake resistant design and construction (older companion, masonry).
• IS 456:2000 — RCC design (used for retrofit element design).
• IS 875 Parts 1-5 — wind, dead, live loads (still apply to existing buildings).
• IS 13935:2009 — seismic evaluation, repair and strengthening of masonry (the masonry counterpart).
• IS 16700:2017 — tall building design (relevant if retrofit changes height or use).
• IS 13311 Part 1:1992 — non-destructive testing (UPV) for in-situ concrete strength assessment.
• IS 516 Part 1:2021 — compressive strength on cores extracted from existing structure.
• IS 1199 Part 1:2018 — fresh concrete tests for retrofit pours.
• IS 14591 — guidelines for buildings of seismic Zones IV and V.
• NBC 2016 Part 6 — National Building Code structural design framework.

1. Skipping Tier 1 and going straight to Tier 2 modelling. Tier 1 catches 60-70 % of buildings as 'OK' or 'unsafe-evacuate'. Skipping Tier 1 wastes engineering hours. 2. Modelling without as-built verification. Original drawings may not match the constructed building (rebar substitution, undocumented modifications, owner extensions). Demand invasive testing (rebar location by ferro-scan, core sampling) before trusting drawings. 3. Using design strength of concrete instead of in-situ measured strength. Old buildings have variable concrete; an unrepaired 30-year-old M20 might test as M15. Run cores per IS 516 Part 1. 4. Ignoring infill walls in lateral analysis. Brick / block infill increases stiffness 3-5× over the bare frame. If not modelled, period and forces are wrong; if soft-storey condition exists, infills concentrate damage. 5. Designing retrofit to current code seismic forces without Performance-Based Seismic Evaluation (PBSE). Existing buildings can rarely be brought to full new-build standard economically. Use PBSE to set realistic performance targets (life-safety vs immediate-occupancy) and design retrofit to meet that level. 6. Member jacketing without addressing joint deficiency. Beam-column joints often fail before members; jacketing the column without retrofitting the joint just shifts the failure point. 7. Steel jacket without epoxy injection of joint cracks. Jacket adds confinement but doesn't restore continuity. Inject existing cracks first. 8. Foundation untouched during superstructure retrofit. New shear walls and bracing dump higher loads on foundations originally not designed for them. Always check foundation capacity in Tier 2. 9. Skipping non-structural hazard mitigation. Most earthquake injuries come from falling debris (parapets, equipment, glass) — not collapse. Anchor, brace, and remove hazards as part of retrofit scope. 10. No post-retrofit testing. Specify verification: load tests on new piles, NDT on retrofit pours, instrumentation for vibration monitoring on critical buildings.

Triggers for IS 15988 evaluation: - Building age > 25 years AND located in Zones III-V - Visible cracks, settlement, tilt, or distress - Past earthquake damage (even if minor) - Change of use (residential → hospital / school) - Vertical extension / heavy MEP additions - Statutory periodic safety certificate (some states) - Insurance / banking renewal due-diligence

Typical project workflow: 1. Owner engagement — define scope, timeline, budget tolerance. 2. Tier 1 screening — 1-2 day visit, scored report. 3. If pass — minor maintenance / no action. 4. If proceed — Tier 2 detailed evaluation (4-8 weeks). 5. Strengthening recommendation — engineer's report with options + costs + downtime. 6. Owner decision — proceed / phase / vacate / demolish. 7. Tier 3 retrofit design — drawings, BOQ, peer review. 8. Construction — staged if occupied; full vacation if major. 9. Verification — load tests, NDT, instrumentation. 10. Re-classification — building issued new safety certificate / insurance review.

For large building portfolios (municipalities, institutional landlords), IS 15988 Tier 1 across the portfolio is the cost-effective starting point — it stratifies the inventory into 'safe', 'monitor', 'retrofit', 'demolish' bins for prioritised investment.