Abstract

This thesis explores the structural design and feasibility of a vertical expansion project involving an existing three-story Reinforced Cement Concrete (RCC) building constructed in 1968, currently in continuous use by a Civil Engineering department. The aim was to design a structurally independent seven-story composite steel extension directly above and around the existing structure, resulting in a ten-story building. A key constraint was to ensure that the original RCC structure remained untouched—without retrofitting, added loads, or functional interruption—throughout the construction and service life of the extended facility.

To achieve this, a composite steel structural system was selected for its favourable strength-to-weight ratio, reduced construction time, and adaptability to complex geometries. The design incorporated new steel columns placed within open internal spaces and along the external perimeter of the existing building. These columns were configured to completely bypass the original structure and transfer all vertical and lateral loads directly to a newly constructed matt foundation, forming an entirely independent load-bearing system.

The structural design followed the Load and Resistance Factor Design (LRFD) methodology, adhering to the Bangladesh National Building Code (BNBC 2020). The building was analysed under dead, live, wind, and seismic load combinations using ETABS 2018. Steel connection design was verified using IDEA StatiCa, and all construction drawings were produced with AutoCAD. A comprehensive seismic evaluation of the combined ten-story structure was conducted using the Seismic Evaluation Manual (2nd Edition) published under the Building Safety for Disaster Risk Reduction (BSPP) project by the Public Works Department (PWD), Bangladesh.

The results confirmed that the new structure could be safely constructed without affecting the original building, and that the entire ten-story assembly meets current seismic safety standards. The independent load path and dedicated foundation effectively isolate the new structure, ensuring both operational continuity and structural integrity.

In conclusion, this study provides a validated engineering solution for vertical expansion over operational structures, offering a model for sustainable urban densification where structural intervention is not permissible.