6-Storey Mixed Use Building in Karachi A Pilot Case Study of Seismic Assessment and R

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6-Storey Mixed Use Building in Karachi A Pilot Case Study of Seismic Assessment and Retrofit Design

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In order to learn how to analyze typical reinforced concrete buildings, understand their seismic
behavior and to learn how guidelines such as ASCE 41, ATC-40 and FEMA could apply to buildings in
Pakistan, the project team idealized a typical Karachi residential-commercial mixed use building as
the pilot case study building. For simplicity, the team investigated the behavior of two-dimensional
frame models with and without infill walls, and simplified certain structural details. A separate
report describes a study of the three dimensional model of the building.
The building upon which the idealized case study structure is based is located in Gulistan-e-Johar, a
densely populated area in Karachi. This building consists of reinforced concrete framed building with
five storeys including the ground floor. The building has shops located at the ground floor, while the
above floors have residential apartments. The building was constructed before the 2005 Kashmir
Earthquake. Project participants selected this building as the pilot case study because it has several
seismic vulnerabilities common to mixed-use residential buildings in Karachi: a weak story created by
open shop fronts at the ground floor, an eccentrically located reinforced concrete core, and heavy,
stiff unreinforced masonry infill walls that were not considered during the structural design of the
building.
The case study team assessed the building’s potential seismic vulnerabilities using the US Federal
Emergency Management Agency (FEMA) Prestandard 310 Tier 1 Checklist modified for Pakistan
conditions, as well as the American Society of Civil Engineers (ASCE) Standard 31 Tier 2 and 3
analyses and acceptance and modeling criteria from ASCE 41. The building was found to be
inadequate for seismic zone 4 and requires retrofitting to rectify the soft storey at the base and
provide lateral stability to the building.
The team examined a number of potential retrofit solutions for both seismic performance and
economic considerations. In order to provide a cost-effective and minimally intrusive retrofit, the
team selected a rocking spine retrofit solution. A spine of existing infill panels reinforced with
shotcrete above a reinforced concrete wall at the open ground storey prevents the building from
collapsing. The spine provides stability and strength without extensive foundation work. This retrofit
solution promises to be an innovative and cost-effective alternative for buildings in Pakistan.

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