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The fire-resistance ratings of structural steel beams and girders shall be based on the size of the element and the type of protection provided in accordance with this section.
FIGURE 722.5.2
DETERMINATION OF THE HEATED PERIMETER OF STRUCTURAL STEEL BEAMS AND GIRDERS
722.5.2.1 Determination of Fire Resistance
These procedures establish a basis for determining resistance of structural steel beams and girders that differ in size from that specified in approved fire-resistance-rated assemblies as a function of the thickness of fire-resistant material and the weight (W) and heated perimeter (D) of the beam or girder. As used in these sections, W is the average weight of a structural steel element in pounds per linear foot (plf). The heated perimeter, D, is the inside perimeter of the fire-resistant material in inches as illustrated in Figure 722.5.2.
722.5.2.1.1 Weight-to-Heated Perimeter
The weight-to-heated-perimeter ratios (W/D), for both contour and box fire-resistant protection profiles, for the wide flange shapes most often used as beams or girders are given in Table 722.5.1(4). For different shapes, the weight-to-heated-perimeter ratios (W/D) shall be determined in accordance with the definitions given in this section.
722.5.2.1.2 Beam and Girder Substitutions
Except as provided for in Section 722.5.2.2, structural steel beams in approved fire-resistance-rated assemblies shall be considered to be the minimum permissible size. Other beam or girder shapes shall be permitted to be substituted provided that the weight-to-heated-perimeter ratio (W/D) of the substitute beam is equal to or greater than that of the beam specified in the approved assembly.
722.5.2.2 Sprayed Fire-Resistant Materials
The provisions in this section apply to structural steel beams and girders protected with sprayed fire-resistant materials. Larger or smaller beam and girder shapes shall be permitted to be substituted for beams specified in approved unrestrained or restrained fire-resistance-rated assemblies, provided that the thickness of the fire-resistant material is adjusted in accordance with the following expression:
(Equation 7-17)
where:
h = Thickness of sprayed fire-resistant material in inches.
W = Weight of the structural steel beam or girder in pounds per linear foot.
D = Heated perimeter of the structural steel beam in inches.
Subscript 1 refers to the beam and fire-resistant material thickness in the approved assembly.
Subscript 2 refers to the substitute beam or girder and the required thickness of fire-resistant material.
The fire resistance of structural steel beams and girders protected with intumescent or mastic fire-resistant coatings shall be determined on the basis of fire-resistance tests in accordance with Section 703.2.
722.5.2.2.1 Minimum Thickness
The use of Equation 7-17 is subject to the following conditions:
  1. The weight-to-heated-perimeter ratio for the substitute beam or girder (W2/D2) shall be not less than 0.37.
  2. The thickness of fire protection materials calculated for the substitute beam or girder (T1) shall be not less than 3/8 inch (9.5 mm).
  3. The unrestrained or restrained beam rating shall be not less than 1 hour.
  4. Where used to adjust the material thickness for a restrained beam, the use of this procedure is limited to structural steel sections classified as compact in accordance with AISC 360.
722.5.2.3 Structural Steel Trusses
The fire resistance of structural steel trusses protected with fire-resistant materials sprayed to each of the individual truss elements shall be permitted to be determined in accordance with this section. The thickness of the fire-resistant material shall be determined in accordance with Section 722.5.1.3. The weight-to-heated-perimeter ratio (W/D) of truss elements that can be simultaneously exposed to fire on all sides shall be determined on the same basis as columns, as specified in Section 722.5.1.1. The weight-to-heated-perimeter ratio (W/D) of truss elements that directly support floor or roof assembly shall be determined on the same basis as beams and girders, as specified in Section 722.5.2.1.
The fire resistance of structural steel trusses protected with intumescent or mastic fire-resistant coatings shall be determined on the basis of fire resistance tests in accordance with Section 703.2.

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