Mezzanine in a 1918 Warehouse
A 1918 warehouse is being converted to offices and a public market. The building is 30,000 sf with 24-ft-high unreinforced brick bearing walls, heavy-timber interior framing, and a 5-in slab-on-grade. The owner wants a new 6,000 sf steel mezzanine at 12 ft above grade, with columns carrying 45 kips each on a 20-ft grid. The owner’s contractor proposes bearing the new columns on the existing exterior brick walls and the slab-on-grade to save money. The building is in a moderate seismic region. The project is a change of occupancy, and the owner wants to keep the existing walls exposed. The architect must advise on structural feasibility before schematic design.
1. Evaluate the contractor’s proposal to bear the mezzanine columns on the existing brick walls and slab-on-grade.
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The proposal is not structurally credible without investigation and likely requires a different load path. A 5-in slab-on-grade is not designed to accept 45-kip column loads; it will crack and settle unless the load is carried on new isolated footings or a grade beam. The unreinforced brick bearing walls may have adequate capacity for their original roof and floor loads, but adding concentrated mezzanine columns introduces eccentric loads and local bearing stresses that the walls were not designed for. The existing foundations are unknown. In a moderate seismic region, the mezzanine must be tied into the lateral-force-resisting system or designed as an independent structure with a clear seismic gap. The architect should recommend an independent steel frame with new footings, or a transfer system that delivers loads to new foundations, and should require a structural engineer’s investigation of the existing walls and footings before design proceeds.
2. What code and seismic triggers apply to the mezzanine and the change of occupancy?
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The change of occupancy triggers the IEBC and likely a seismic evaluation of the existing building. A new mezzanine of 6,000 sf is a significant structural alteration and may also trigger upgrade requirements for the lateral-force-resisting system, especially in a moderate seismic region. The architect should determine whether the mezzanine is treated as a new story, whether it increases occupant load, and whether it creates a soft-story or irregularity. The exposed unreinforced brick walls may need anchors, pilasters, or reinforced bond beams. The architect should not assume that keeping the walls exposed means leaving them unreinforced. The code path should be confirmed with the building official early, and the structural engineer should provide a seismic evaluation and a phased upgrade strategy. The mezzanine’s connection to the existing framing must be detailed to accommodate differential movement.
3. What should the architect require in the structural specifications and construction sequence to protect the existing building?
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The specifications should require a structural investigation before demolition, including wall thickness, mortar condition, footing depth, and slab thickness. The mezzanine should be specified as an independent steel frame with new reinforced-concrete footings, anchor bolts, and a seismic gap where required. Connections to existing masonry should be limited to stabilization and should not rely on the brick for vertical load. The sequence should require temporary shoring, controlled demolition, and monitoring of crack widths and wall plumbness during construction. The architect should require shop drawings for connection details and special inspection of anchors and concrete. If the owner wants exposed brick, the architect should specify a mock-up of the anchor and repair strategy so the aesthetic goal does not compromise the structural upgrade. A preconstruction meeting with the structural engineer and contractor should establish hold points.