Removing a Corner Column in an Occupied School Frame
A three-storey school in Ulaanbaatar is being renovated during summer break. The structure is a cast-in-situ reinforced-concrete frame on a 6.6 m × 6.6 m grid: 400 × 400 mm columns, 300 × 600 mm beams and a 180 mm slab. The architect wants to remove ground-floor corner column C-1 to create an open entrance. The contractor proposes to cut the column and replace it with a 300 × 900 mm transfer beam spanning 6.6 m, arguing that the deeper beam will carry the load. No structural calculations have been provided. The column supports two upper floors and a roof with a specified snow load of 1.2 kPa; the building is in a seismic zone. The adjacent infill walls and ground slab are to remain. The school must reopen in eight weeks. The site engineer asks whether the work can proceed and what temporary measures are needed.
1. Should the engineer approve the contractor's transfer-beam proposal as submitted? Explain the load-path and code checks that must be satisfied.
Read the worked answer
No - the proposal cannot be approved as submitted. Removing C-1 changes the load path from a direct column line to a transfer system. The upper column reactions, including snow and seismic effects, must be collected by the transfer beam and delivered to the adjacent columns and foundations. A 300 × 900 mm beam is not evidence of adequacy: it may be overstressed in shear or torsion, may deflect enough to crack the infill walls, and may overload the adjacent columns and footings. Because the building is in a seismic zone, the change may create a vertical irregularity, alter stiffness and drift, and affect diaphragm action and progressive-collapse resistance. Required checks include: transfer-beam flexure, shear, torsion, deflection and crack control; beam-column joint and anchorage; adjacent column and foundation capacity; slab diaphragm; seismic drift and torsional response; fire resistance; and construction-stage stability. The engineer of record must issue stamped calculations and revised drawings. The school should not be reopened until these are reviewed and accepted.
2. What temporary works and construction sequence should be required before the column is cut?
Read the worked answer
Temporary works must be designed, not improvised. Before cutting, the contractor should submit a shoring design prepared by a qualified structural engineer. It should support the beams on both sides of C-1 with steel props or needles founded on a stable base, preloaded to take the dead load, and checked for the loads imposed during cutting. The existing slab and beams must be checked for the shoring reactions. Services and finishes should be removed or protected. The sequence should be: survey and mark monitoring points; install and preload shoring; verify no movement; cut the column in controlled sections without impact; construct the transfer beam with specified reinforcement, anchorage and concrete; cure and test; only after the engineer confirms strength should shoring be released in a controlled sequence. Hold points, inspection records and movement monitoring are required. Cutting before the transfer system is proven would risk collapse.
3. What approvals and documentation are needed before the school can reopen?
Read the worked answer
Approvals and records: revised structural design and calculations by the engineer of record; code-compliance review, including seismic and fire; client and competent-authority approval; a peer review if the authority requires it; as-built drawings; concrete and reinforcement certificates; test results; inspection records for rebar, connections and shoring; monitoring records; and a handover report. The school should reopen only after the engineer certifies that the transfer structure has reached the required strength and that no unacceptable movement has occurred.