Vertical Extension of a 1980s Concrete Frame
A Karachi client owns a four-storey reinforced-concrete frame building completed in the early 1980s and wants to add two office floors. The original structural drawings are missing; only an unstamped architectural set exists. A walkover survey shows 0.3–0.5 mm diagonal cracks in beams on the first and second floors, spalling with exposed corroded reinforcement at two column bases, and cover varying from 20 to 30 mm. Three cores from columns give equivalent cube strengths of 16, 19 and 21 MPa. The client's engineer says the original specification was 3000 psi (20.7 MPa) concrete and 60-grade steel, so 'it should be fine'. The building is in a seismic zone, and the client wants to start piling for a new lift shaft next month. He asks you to produce the extension design using the assumed original values and to issue a structural stability certificate.
1. Set out the investigation and verification programme you would require before designing the extension, explaining what each test or check establishes.
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I would start with a desk study and measured survey to establish the as-built frame, bay sizes, floor loads and any undocumented alterations. The condition survey should map crack widths, spalling and corrosion. Non-destructive tests - rebound hammer and ultrasonic pulse velocity - give uniformity and help locate weak zones, but they do not replace cores. I would require cores from columns and beams on several floors, tested in compression, because the 16–21 MPa results suggest the assumed 20.7 MPa may not be reliable. Carbonation depth by phenolphthalein, acid-soluble chloride content and half-cell potential would establish corrosion risk. A cover meter and sample openings would confirm bar diameter, spacing and lap lengths. Foundation exposure, trial pits and a soil-bearing check are needed because two extra floors add load. Finally, I would analyse the as-built frame for gravity and seismic loads under the Building Code of Pakistan, including column-beam capacity and drift, before any extension design.
2. Evaluate the client's instruction to use assumed 3000 psi concrete and 60-grade steel, and state what you would and would not certify at this stage.
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I would not accept the assumed values as a design basis. Original drawings, even when available, record what was specified, not what was built; here the drawings are missing. The cracking, spalling and corroded column bases are symptoms that may indicate inadequate cover, carbonation or overloading, and the core results are below the assumed strength on two of three samples. Using 20.7 MPa and 60-grade steel without verification would understate the risk, especially for seismic drift and column axial capacity. I would tell the client that the extension design can proceed only after the investigation establishes characteristic strength, reinforcement details and foundation capacity. If the frame is adequate after analysis, I can design strengthening and the extension. If not, I can design strengthening first. At this stage I would not issue a structural stability certificate; I would issue an interim report setting out the observed defects, the testing plan and the need to suspend piling until the foundation and frame assessment is complete.