Stilt-Parking Soft Storey in a G+4 RC Apartment (Seismic Zone III): Pushover Analysis and Retrofit Comparison

Drive through any growing Indian town and you will see the typical apartment block: four or five floors of brick-infilled flats sitting on an open ground floor used for parking. That stilt floor has columns but almost no walls, while every floor above is stiffened by masonry infill. In an earthquake the ground storey becomes a soft and weak storey, deformation concentrates there and columns can fail. This failure mode was seen widely in the 2001 Bhuj earthquake and in many earthquakes since.

This B.Tech project studies a G+4 reinforced-concrete apartment with stilt parking assumed at a site in Seismic Zone III (zone factor 0.16 under IS 1893 (Part 1):2016). The team builds six analytical models: a bare frame; an infilled frame with an open ground storey (masonry modelled as equivalent diagonal struts as permitted by IS 1893:2016); the same building designed with the code's provisions for flexible storeys; and three retrofit options for the ground storey: RC jacketing of columns, steel X-bracing in selected bays, and RC infill/shear walls in selected bays.

Each model undergoes nonlinear static (pushover) analysis with ASCE 41 / FEMA 356 plastic hinges. Capacity curves, performance points, storey drifts, hinge states (IO, LS, CP) and an approximate cost per retrofit are compared to recommend a practical retrofit for existing stilt buildings. The work fits JNTUH R22 Project Stage-I (IV-1) for literature and modelling and Project Stage-II incl. Seminar (IV-2) for analysis and the final report.

Abstract

A G+4 reinforced-concrete residential building with an open ground storey for parking, located in Seismic Zone III, is analysed to understand soft-storey behaviour and compare retrofit strategies. Six models are developed: bare frame, infilled frame with open ground storey, code-magnified design, and retrofits with column jacketing, steel bracing and RC walls. Nonlinear static pushover analysis with lumped plastic hinges gives capacity curves, performance points, inter-storey drift profiles and hinge distributions. The study quantifies the stiffness and strength irregularity created by stilt parking and identifies the most effective and economical retrofit.

Introduction

Open-ground-storey buildings are popular because they provide parking within a small urban plot. Conventional design often ignores the stiffness of masonry infill above, treating the building as a bare frame, so the soft storey is not recognised. IS 1893 (Part 1):2016 classifies such buildings as vertically irregular and gives specific provisions, including modelling infill as equivalent diagonal struts and either magnifying design forces in the soft storey or adding walls or bracing. Pushover analysis shows how a building actually yields and where damage concentrates.

Literature gap

Many student and published studies analyse soft storeys using linear equivalent static or response spectrum methods, or compare bare frames with fully infilled frames without retrofits. Fewer combine a code-compliant infill model with pushover analysis and compare multiple retrofit options with cost for a typical Indian G+4 stilt building.

Note on code editions

BIS published IS 1893 (Part 1):2025 with a revised seismic zone map and probabilistic hazard basis. This project designs to the 2016 edition, which is the basis of most current teaching and approvals, and includes a short comparison chapter on how the 2025 revision would change the seismic demand for the chosen site. Confirm with your guide which edition your department requires.

Feasibility

• Technical: ETABS is available in many college CAD labs (industry-standard, not named in the JNTUH syllabus); OpenSees is free.
• Economic: software licences through the department or open-source tools; no laboratory costs.
• Schedule: modelling in Stage-I, analysis and comparison in Stage-II.

A large share of new urban housing in Indian towns uses stilt parking under masonry-infilled upper floors, often designed as bare frames that ignore the infill. Such buildings are vertically irregular, with a soft and weak ground storey that can fail in moderate earthquakes. Owners' associations of existing buildings want to know whether their buildings need strengthening and which retrofit gives the most benefit for the cost. The problem for this project is to quantify the soft-storey effect in a typical G+4 stilt-parking RC building in Seismic Zone III using pushover analysis, and to compare column jacketing, steel bracing and RC walls as retrofit options on strength, stiffness, drift, hinge formation and approximate cost.

The study considers one regular-plan G+4 residential building with stilt parking, medium soil, Seismic Zone III, importance factor 1.0 and response reduction factor for SMRF. Masonry infill is modelled as equivalent struts; soil-structure interaction, torsional irregularity from asymmetric walls and nonlinear time-history analysis are outside scope. Retrofit costs are approximate, based on schedule-of-rates items, not detailed estimates.

Research design

Analytical (numerical) comparative study with code-based design and nonlinear static analysis.

Building data (typical)

• Plan 18 m × 12 m, 4 × 3 bays, storey height 3.0 m (ground 3.0 m), G+4.
• Columns 300 × 450 mm (ground), beams 230 × 450 mm, slab 125 mm; M25 concrete, Fe 500 steel.
• 230 mm brick infill on all upper-floor external and party walls; ground storey open except staircase and lift core walls.
• Loads per IS 875 Parts 1 and 2; seismic per IS 1893 (Part 1):2016: Zone III (Z = 0.16), I = 1.0, R = 5 (SMRF), medium soil.

Models

Stage-I (XX704PC, IV-1)

Literature review (30+ sources), problem and objectives, building layout in AutoCAD, gravity and seismic load calculations, linear model of M1 and M2, irregularity check (lateral stiffness of ground storey compared with storey above), plan of action approved by the project review committee.

Stage-II (XX801PC, IV-2)

1. Design M1 and M3 to IS 456 and IS 13920 (ductile detailing) using equivalent static and response spectrum methods.
2. Assign plastic hinges: flexural (M3) hinges at beam ends, axial-flexural (P-M2-M3) hinges at column ends, axial hinges for struts and braces (ASCE 41 / FEMA 356 tables).
3. Pushover: gravity load case first, then lateral push in X and Y with a load pattern proportional to the first mode, displacement-controlled to a target roof drift.
4. Performance point by FEMA 440 equivalent linearisation (or ATC-40 capacity spectrum) using the IS 1893 design spectrum.
5. Record base shear vs roof displacement, performance point, inter-storey drift at performance point, hinge states (B, IO, LS, CP, collapse), ductility and over-strength.
6. Retrofit cost: quantities of concrete, steel and formwork for jacketing; steel sections and connections for bracing; walls; rates from the state schedule of rates.

Comparison metrics

Initial stiffness, yield and ultimate base shear, displacement ductility, drift concentration ratio (ground-storey drift ÷ average upper-storey drift), number of hinges beyond LS, cost per unit increase in base-shear capacity.

Timeline (Stage-II, 16 weeks)