Soft-storey risk after shop conversion in a Gyumri panel building
A 1983 nine-storey precast concrete panel apartment building in Gyumri contains 108 flats. The ground floor originally held storage rooms and a small lobby. Over fifteen years, owners removed infill walls and a stair enclosure to create shops, leaving only the precast columns and a 4.5 m clear height. After a magnitude 4.8 earthquake 40 km away, X-shaped cracks appear at the base of six ground-floor columns, and a plumb survey shows a 22 mm lateral offset between the ground and first floors. The building is in a high seismic zone. The owners' association wants to know whether the shops can remain open and whether cosmetic crack repair is enough.
1. Assess the structural significance of the ground-floor alterations. What calculations and inspections would you require before allowing occupancy?
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The removal of infill and the stair enclosure has probably created a soft and weak ground storey. The first check is a storey-stiffness comparison: estimate lateral stiffness of the bare-column ground floor against the panelled upper storeys. A ground-storey stiffness below about 70% of the storey above, or below 80% of the average of the three storeys above, indicates a soft storey; a strength ratio below about 80% indicates a weak storey. The 22 mm offset must be compared with the code drift limit and with column distortion, not treated as a cosmetic survey item. X-cracks at column bases suggest shear distress and possibly inadequate confinement or lap splices. Require a nonlinear static or dynamic model that includes the actual openings, torsion from asymmetric shop fronts, and column shear demand versus capacity. Inspect column dimensions, reinforcement, concrete strength and foundations by opening up. Until that analysis is complete, the shops should not be treated as safe merely because the cracks are patched.
2. Recommend immediate measures and retrofit options, stating decision criteria for re-occupancy.
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Immediate measures are temporary shoring of the affected columns, closure of the shops and restriction of the ground floor, installation of crack monitors and plumb readings, and a ban on further removal of walls. For retrofit, compare three schemes: adding reinforced-concrete shear walls or steel braced frames at the perimeter, jacketing the ground-floor columns, and a combination of both. Each scheme must restore stiffness and strength, control torsion, and be checked against the foundation capacity. Column jacketing alone may not correct the stiffness imbalance. Re-occupancy should require calculated drift below the code limit, column shear demand below capacity with a suitable margin, no progressive collapse mechanism, and completion of the works with inspection sign-off. If the analysis shows that the ground storey cannot be brought within acceptable limits at reasonable cost, partial or full evacuation and demolition must be recommended in writing.