Soft-storey and torsional retrofit of a 1970s RC frame
A six-storey reinforced-concrete frame office building was designed in the 1970s and is located in a seismic region with a design peak ground acceleration of 0.30g and a flexible soil profile. The plan is 30 m by 18 m with a 6 m by 6 m re-entrant corner. The ground storey is open for parking and is 4.2 m high; the upper storeys are 3.0 m high and are infilled with brick masonry. Columns are 400 mm square with 8-φ16 longitudinal bars and φ8 ties at 200 mm centres. The building is nearly symmetric in plan, but the only stair core is at one corner. A linear analysis shows an average ground-storey drift ratio of 2.4% under the design earthquake, compared with 0.9% in the upper storeys; the code drift limit is 2.0%. The client wants a low-disruption retrofit and asks whether adding masonry infill to the ground storey would solve the problem.
1. Identify the seismic irregularities and explain why adding masonry infill to the ground storey is unlikely to be a satisfactory solution.
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The building has a soft/weak storey at the open ground floor, a torsional irregularity because the only stair core is at one corner, and a re-entrant corner that creates a plan irregularity. The upper brick infills are much stiffer than the bare frame and act as diagonal struts, attracting force and leaving the ground-storey columns to carry most of the base shear with limited ductility. The 2.4% ground-storey drift, compared with 0.9% above and a 2.0% limit, confirms the concentration. Adding unreinforced masonry infill to the ground storey might reduce drift, but it introduces brittle behaviour, possible short-column effects, additional mass, and new torsion; if the infill is not engineered and tied to the frame, it can fail out-of-plane. It does not provide a reliable, ductile seismic-force-resisting system. A proper retrofit should add a continuous, engineered lateral system and correct the torsion, not rely on incidental infill.
2. Compare reinforced-concrete shear walls with steel buckling-restrained braces for this building. Recommend a scheme and explain the placement.
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RC shear walls add substantial stiffness and strength, reduce drift efficiently and are robust, but they add mass and overturning moments to the foundations and cause more construction disruption. BRBs add strength and energy dissipation with less added mass and can be installed in selected bays, but they need fire protection, substantial connections and a foundation that can take the brace forces. For this building, the drift and torsion demand a stiff system. Recommend two RC shear walls in each principal direction, placed symmetrically on the perimeter and near the stair core, with column jacketing at the ground storey. This corrects the torsional irregularity, reduces the soft-storey drift and provides a clear load path. If foundation capacity or disruption rules out walls, a BRB scheme in perimeter bays is the alternative, but it must be arranged to reduce, not increase, torsion and must be checked for the full base shear.
3. What analysis and acceptance criteria should be required for the retrofit?
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A 3D model should include the frame, the shear walls or braces, and the stiffness of the upper-storey infills, because ignoring the infills underestimates the soft-storey effect. Use response-spectrum analysis for the design earthquake and nonlinear static pushover or time-history analysis for the retrofitted system. Check ground-storey drift within the code limit (2.0% here, or a stricter performance target if the client requires); torsional ratio and diaphragm demands; shear and ductility of the existing columns and joints, including the short-column effect; foundation overturning and sliding; and the connection of new walls or braces to the existing frame. The retrofit should be capacity-designed so that the new system yields before the existing columns fail. Acceptance should be based on both global drift and local element checks, with a peer review of the modelled infill stiffness and the foundation assumptions.