IBC Design 721.1(1) 1-6.4

4-Hour Fire Rated Steel columns and all of primary trusses — Perlite or vermiculite gypsum plaster over two layers of…

IBC Table 721.1(1), Item 1-6.4 (Steel columns and all of primary trusses): protection by Perlite or vermiculite gypsum plaster over two layers of 1/2" plain full-length gypsum lath applied tight to column flanges. Lath wrapped with 1" hexagonal mesh of No. 20-gage wire and tied with doubled 0.035" diameter (No. 18 B.W. gage) wire ties spaced 23" on center. For three-coat work, the plaster mix for the second coat shall not exceed 100 pounds of gypsum to 2-1/2 cubic feet of aggregate for the 3-hour system. Minimum insulating material thickness — 4-hr: 2-1/2" · 3-hr: 2". Prescriptive protection from the adopted building code — see table footnotes for conditions.