A structural member's fire resistance is measured by the time it can support its design load during a fire. An exposed beam or column sized for a minimum one-hour fire resistance will support its full design load for at least one hour during standard fire test conditions which simulate an actual fire.

Beams. Glulam beams 6-3/4" and 8-3/4" wide can be adapted to a one-hour fire rating in accordance with procedures recognized by the model codes. For these widths, there is a minimum depth for a one-hour fire rating. the minimum depth increases when the design calls for the beam to be exposed on four rather than three sides. See Table 33.

To adapt beams whose dimensions qualify them for one-hour fire rating, the basic layup is modified as shown in Figure 29. On core lamination form the center of the beam is required by an additional tension lamination.

Columns. Column length determines the minimum size for one-hour rating.s the column size needed for a one-hour fire rating is determined by calculation the l/d and then using the appropriate minimum dimensions; see Table 34.

Columns generally need no special layup to qualify for a one-hour fire rating. For both 8-3/4" and 10-3/4" widths, all columns meeting the minimum size standard satisfy the one-hour fire rating requirements.

As with all structural framing, final specifications of members designed to have one-hour fire resistant should be carefully checked by a professional engineer or architect to assure compliance with all local building codes.

Note:

(a) Substitution of 1-1/8" APA RATED STURD-I-FLOOR 48 oc for the
combination of subfloor, paper and underlayment is often allowed.
Check with local Building Official.

(b) Tests have shown that substitution of OSB or composite APA
RATED SHEATHING subfloor and APA RATED STURD-I-FLOOR
underlayment for the plywood panels in rated assemblies will not
jeopardize fire-resistance ratings. Substitution is based on equivalent
panel thickness, except that 7/16" OSB subfloor panels may be used
in place of 15/32" plywood subfloor panels in two-layer assemblies.
OSB panels are listed as alternates to plywood subflooring or finish
flooring in U.L. Design Nos. L501,L503,L505 (2hr),L508,L511(2hr) L513
,L514,L516,L521,L526,L528,L529,L539,L540,L543 and L544.

(c) Lightweight concrete or gypsum concrete floor topping permitted
over single-layer floor or as alternate to plywood underlayment in many
assemblies (check details).