Soft Storey Created by Removing Columns in a Mixed-Use Building
A four-storey reinforced concrete building in Mostar has a ground floor used as retail and three residential floors above. The owner wants an open commercial space and proposes removing four interior columns on the ground floor, leaving a clear span of about 9 m. The building was designed in the 1980s to older seismic provisions; Mostar is in a zone of significant seismic hazard. The contractor's proposal shows only a new steel beam under the first-floor slab, with no check of the columns, foundations, or lateral system. The owner argues that 'the slab will redistribute the load' and that the neighbouring building did the same thing without problems. A tenant has already signed a lease for the enlarged space.
1. Explain the structural and seismic concerns with the proposal as presented.
Read the worked answer
Removing the interior columns does more than change vertical load paths:
- Vertical capacity: A 9 m steel beam must carry the tributary slab and live load from three floors above; its deflection and vibration performance, and the concentrated reactions it delivers to the remaining columns and footings, must be checked. The existing footings were sized for the previous column layout.
- Soft/weak storey: The removed columns reduce the ground-floor lateral stiffness relative to the floors above, creating a soft storey. In an older building with limited ductility detailing, this concentrates seismic drift at the ground floor where it is most dangerous.
- Redundancy: The remaining frame loses redundancy; loss of one column becomes more critical.
- Global stability: The lateral system must be re-evaluated; the slab cannot be assumed to redistribute seismic demands.
The neighbour's precedent is not evidence of safety and is irrelevant to this building's analysis.
2. Outline the analysis and documentation you would require before approving any scheme.
Read the worked answer
Require a full structural assessment by a qualified engineer, not a beam-sizing exercise:
- As-built survey of geometry, reinforcement (where exposed or by non-destructive testing), concrete strength, and foundation type.
- Global model including the original seismic provisions and the proposed configuration; compare storey stiffness and drift, and check for soft-storey formation.
- Member checks of the new beam, its connections, the receiving columns, and the footings for the new reaction pattern, including seismic combinations.
- Lateral system assessment: if the frame is inadequate, propose shear walls, bracing, or jacketing, with detailing consistent with the building's ductility.
- Construction sequence and temporary support during column removal.
Document assumptions, and state in writing that approval is conditional on the verified design and on inspection during construction. Notify the owner that the signed lease does not create an entitlement to an unsafe alteration.