Mezzanine Addition in Existing Office Building

The mezzanine adds an assembly occupancy with 80 occupants, which changes the building's occupant load and egress demand. Because the mezzanine is open to the first floor, the official must evaluate whether the space is a mezzanine under the code or a separate story; if it exceeds the allowable mezzanine area or occupant load, it may be regulated as a story. The partial sprinkler system is a major issue: the first floor is sprinklered, but the upper floors are not. The IEBC allows alterations to existing buildings, but the official must determine whether the work triggers upgrades. If the mezzanine is an alteration, the official should apply the IEBC's work-area and occupancy-change provisions. Adding an A-3 occupancy may require the mezzanine and the floor below to be sprinklered, and the open stair may need to be enclosed or protected. The official should not accept the owner's preferred alternative until the applicant provides a code analysis showing equivalent life safety. Under the IEBC, compliance alternatives are allowed, but they must be approved by the building official and supported by a rational analysis. The official should require the applicant to address egress from the mezzanine, smoke separation, and the interaction between the open mezzanine and the existing open stair.

Hanging a 2,500-square-foot mezzanine from existing roof trusses raises several structural concerns. The roof trusses were likely designed for roof loads, not for additional dead and live loads from a floor. The engineer must demonstrate that the trusses have reserve capacity for the mezzanine's dead load, live load, and any concentrated loads from hangers. The hanger connections must be designed for tension, shear, and any eccentricity, and the truss bottom chords must be checked for local bending and web buckling. The mezzanine framing must also be checked for vibration and deflection, because a conference room with 80 occupants is a live-load-intensive use. The official should require a sealed structural analysis that includes the existing truss design, the new loads, the hanger details, and the connection to the mezzanine framing. If the trusses cannot safely support the load, the applicant may need to add new columns or transfer beams. The official should also consider seismic and wind loads, because adding mass at the roof level can change the building's lateral-force-resisting system. The official should not approve the hanger proposal based only on the engineer's conclusion; the calculations and details must be in the permit set.

The existing open stair is a nonconforming feature, but the mezzanine addition may trigger a requirement to enclose it. The applicant has several options. The most straightforward is to enclose the stair with 1-hour fire-rated construction and self-closing doors, which would bring it closer to current code. Another option is to provide an approved alternative under the IEBC, such as a smoke-control system, full sprinkler protection throughout the building, or a combination of sprinklers and fire-rated separation. A third option is to demonstrate that the open stair is acceptable because the building is fully sprinklered and the egress path is protected by other means, but that argument is weak because the upper floors are currently unsprinklered. The official should require the applicant to choose one option and support it with a code analysis. If the applicant proposes an alternative, the official should evaluate whether it provides equivalent life safety and should document the approval. The official should not allow the mezzanine to be occupied while the stair enclosure issue remains unresolved. The final decision should be in writing and should specify any ongoing maintenance requirements, such as keeping the stair doors closed.