UL Design G523
2-Hour Fire Rated Steel Joist Floor-Ceiling — concrete slab, 1/2" Type C suspended ceiling
UL Design G523 — 2-hour steel joist floor-ceiling per National Gypsum's published summary. Open-web steel bar joists at 24" o.c. support a 2-1/2" concrete slab on 3/8" rib lath; below, one layer of 1/2" Type C gypsum board is screw-attached to a drywall suspension system. Ceiling-grid literature (Rockfon/Chicago Metallic) lists G523 with 2-hr and 3-hr (restrained) ratings and a single-layer 1/2" or 5/8" taped gypsum option. STC ≈ 50 is a manufacturer ESTIMATE, not a test. Verify item-level options against the full UL text before specifying.
Bar joists @ 24" o.c., 2-1/2" concrete on rib lath, suspended 1/2" Type C ceiling
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- Layer-by-layer components with thicknesses
- Cross-section diagram
- Applicable code sections and test standard
- 6 related assemblies