721.2 - Concrete assemblies
The provisions of this section contain procedures by which the fire-resistance ratings of concrete assemblies are established by calculations.
721.2.1
Concrete walls. Cast-in-place and precast concrete walls shall
comply with Section 721.2.1.1. Multiwythe concrete walls shall comply with
Section 721.2.1.2. Joints between precast panels shall comply with Section
721.2.1.3. Concrete walls with gypsum wallboard or plaster finish shall comply
with Section 721.2.1.4.
721.2.1.1
Cast-in-place or precast walls. The minimum equivalent thicknesses
of cast-in-place or precast concrete walls for fire-resistance
ratings of 1 hour to 4 hours are shown in Table 721.2.1.1. For solid
walls with flat vertical surfaces, the equivalent thickness is the same as the
actual thickness. The values in Table 721.2.1.1 apply to plain, reinforced or
prestressed concrete walls.
TABLE 721.2.1.1 MINIMUM EQUIVALENT THICKNESS OF CAST-IN-PLACE OR PRECAST CONCRETE WALLS, LOAD-BEARING OR NONLOAD-BEARING
| CONCRETE TYPE | MINIMUM SLAB THICKNESS (inches) FOR FIRE-RESISTANCE RATING OF | ||||
| 1-hour | 11/2-hour | 2-hour | 3-hour | 4-hour | |
| Siliceous | 3.5 | 4.3 | 5.0 | 6.2 | 7.0 |
| Carbonate | 3.2 | 4.0 | 4.6 | 5.7 | 6.6 |
| Sand-lightweight | 2.7 | 3.3 | 3.8 | 4.6 | 5.4 |
| Lightweight | 2.5 | 3.1 | 3.6 | 4.4 | 5.1 |
| For SI: 1 inch = 25.4 mm. |
721.2.1.1.1
Hollow-core precast wall panels. For hollow-core precast concrete
wall panels in which the cores are of constant cross section throughout the
length, calculation of the equivalent thickness by dividing the net
cross-sectional area (the gross cross section minus the area of the cores) of
the panel by its width shall be permitted.
721.2.1.1.2
Core spaces filled.
Where all of the core spaces of hollow-core wall panels are filled with
loose-fill material, such as expanded shale, clay, or slag, or vermiculite or
perlite, the fire-resistance rating of the wall is the same as
that of a solid wall of the same concrete type and of the same overall
thickness.
721.2.1.1.3
Tapered cross sections. The thickness of panels with tapered cross
sections shall be that determined at a distance 2t or 6 inches
(152 mm), whichever is less, from the point of minimum thickness, where
t is the minimum thickness.
721.2.1.1.4
Ribbed or undulating
surfaces. The equivalent thickness of panels with ribbed or undulating
surfaces shall be determined by one of the following expressions:
For s >= 4t, the thickness to be used shall be t
For s <= 2t, the thickness to be used shall be te
For 4t> s > 2t, the thickness to be used shall be
| Click to view image | (Equation 7-3) |
where:
s = Spacing of ribs or undulations.
t = Minimum thickness.
te = Equivalent thickness of the panel calculated as the net cross-sectional area of the panel divided by the width, in which the maximum thickness used in the calculation shall not exceed 2t.
721.2.1.2
Multiwythe walls. For
walls that consist of two wythes of different types of concrete, the
fire-resistance ratings shall be permitted to be determined
from Figure 721.2.1.2.
TABLE 721.2.1.2(1) VALUES OF Rn0.59 FOR USE IN EQUATION 7-4
| TYPE OF MATERIAL | THICKNESS OF MATERIAL (inches) | |||||||||||
| 11/2 | 2 | 21/2 | 3 | 31/2 | 4 | 41/2 | 5 | 51/2 | 6 | 61/2 | 7 | |
| Siliceous aggregate concrete | 5.3 | 6.5 | 8.1 | 9.5 | 11.3 | 13.0 | 14.9 | 16.9 | 18.8 | 20.7 | 22.8 | 25.1 |
| Carbonate aggregate concrete | 5.5 | 7.1 | 8.9 | 10.4 | 12.0 | 14.0 | 16.2 | 18.1 | 20.3 | 21.9 | 24.7 | 27.2c |
| Sand-lightweight concrete | 6.5 | 8.2 | 10.5 | 12.8 | 15.5 | 18.1 | 20.7 | 23.3 | 26.0c | Note c | Note c | Note c |
| Lightweight concrete | 6.6 | 8.8 | 11.2 | 13.7 | 16.5 | 19.1 | 21.9 | 24.7 | 27.8c | Note c | Note c | Note c |
| Insulating concretea | 9.3 | 13.3 | 16.6 | 18.3 | 23.1 | 26.5c | Note c | Note c | Note c | Note c | Note c | Note c |
| Airspaceb | - | - | - | - | - | - | - | - | - | - | - | - |
| For SI: 1 inch = 25.4 mm, 1 pound per cubic foot = 16.02 kg/m3. |
| a. Dry unit weight of 35 pcf or less and consisting of cellular, perlite or vermiculite concrete. |
| b. The Rn 0.59 value for one 1/2" to 3 1/2" airspace is 3.3. The Rn 0.59 value for two 1/2" to 3 1/2" airspaces is 6.7. |
| c. The fire-resistance rating for this thickness exceeds 4 hours. |
TABLE 721.2.1.2(2) FIRE-RESISTANCE RATINGS BASED ON RO.59
| Ra, MINUTES | R 0.59 |
| 60 | 11.20 |
| 120 | 16.85 |
| 180 | 21.41 |
| 240 | 25.37 |
| a. Based on Equation 7-4. |
| Click to view image |
| For SI: 1 inch = 25.4 mm. |
FIGURE 721.2.1.2 FIRE-RESISTANCE RATINGS OF TWO-WYTHE CONCRETE WALLS
721.2.1.2.1
Two or more wythes. The fire-resistance rating
for wall panels consisting of two or more wythes shall be permitted to be
determined by the formula:
| R = (R 10.59 + R 20.59 + ... + Rn 0.59)1.7 | (Equation 7-4) |
where:
R = The fire endurance of the assembly, minutes.
R 1, R 2, and Rn = The fire endurances of the individual wythes, minutes. Values of Rn0.59 for use in Equation 7-4 are given in Table 721.2.1.2(1). Calculated fire-resistance ratings are shown in Table 721.2.1.2(2).
721.2.1.2.2
Foam plastic insulation. The fire-resistance
ratings of precast concrete wall panels consisting of a layer of foam
plastic insulation sandwiched between two wythes of concrete shall be permitted
to be determined by use of Equation 7-4. Foam plastic insulation with a total
thickness of less than 1 inch (25 mm) shall be disregarded. The
Rn value for thickness of foam plastic
insulation of 1 inch (25 mm) or greater, for use in the calculation, is 5
minutes; therefore
Rn0.59 =
2.5.
721.2.1.3
Joints between precast wall panels. Joints between precast concrete wall
panels which are not insulated as required by this section shall be considered
as openings in walls. Uninsulated joints shall be included in determining the
percentage of openings permitted by Table 705.8. Where openings are not
permitted or are required by this code to be protected, the provisions of this
section shall be used to determine the amount of joint insulation required.
Insulated joints shall not be considered openings for purposes of determining
compliance with the allowable percentage of openings in Table 705.8.
721.2.1.3.1
Ceramic fiber joint
protection. Figure 721.2.1.3.1 shows thicknesses of ceramic fiber
blankets to be used to insulate joints between precast concrete wall panels for
various panel thicknesses and for joint widths of
3/8 inch (9.5 mm) and 1 inch
(25 mm) for fire-resistance ratings of 1 hour to 4 hours. For
joint widths between 3/8 inch
(9.5 mm) and 1 inch (25 mm), the thickness of ceramic fiber blanket is allowed
to be determined by direct interpolation. Other tested and labeled materials
are acceptable in place of ceramic fiber blankets.
| Click to view image |
| For SI: 1 inch = 25.4 mm. |
FIGURE 721.2.1.3.1 CERAMIC FIBER JOINT PROTECTION
721.2.1.4
Walls with gypsum wallboard
or plaster finishes. The fire-resistance rating of
cast-in-place or precast concrete walls with finishes of gypsum wallboard or
plaster applied to one or both sides shall be permitted to be calculated in
accordance with the provisions of this section.
TABLE 721.2.1.4(1) MULTIPLYING FACTOR FOR FINISHES ON NONFIRE-EXPOSED SIDE OF WALL
| TYPE OF FINISH APPLIED TO CONCRETE OR CONCRETE MASONRY WALL | TYPE OF AGGREGATE USED IN CONCRETE OR CONCRETE MASONRY | |||
| Concrete: siliceous or carbonate Concrete Masonry: siliceous or carbonate; solid clay brick | Concrete: sand-lightweight Concrete Masonry: clay tile; hollow clay brick; concrete masonry units of expanded shale and <20% sand | Concrete: lightweight Concrete Masonry: concrete masonry units of expanded shale, expanded clay, expanded slag, or pumice < 20% sand | Concrete Masonry: concrete masonry units of expanded slag, expanded clay, or pumice | |
| Portland cement-sand plaster | 1.00 | 0.75a | 0.75a | 0.50a |
| Gypsum-sand plaster | 1.25 | 1.00 | 1.00 | 1.00 |
| Gypsum-vermiculite or perlite plaster | 1.75 | 1.50 | 1.25 | 1.25 |
| Gypsum wallboard | 3.00 | 2.25 | 2.25 | 2.25 |
| For SI: 1 inch = 25.4 mm. |
| a. For portland cement-sand plaster 5/8 inch or less in thickness and applied directly to the concrete or concrete masonry on the nonfire-exposed side of the wall, the multiplying factor shall be 1.00. |
TABLE 721.2.1.4(2) TIME ASSIGNED TO FINISH MATERIALS ON FIRE-EXPOSED SIDE OF WALL
| FINISH DESCRIPTION | TIME (minute) |
| Gypsum wallboard | |
| 3/8 inch | 10 |
| 1/2 inch | 15 |
| 5/8 inch | 20 |
| 2 layers of 3/8 inch | 25 |
| 1 layer 3/8 inch, 1 layer 1/2 inch | 35 |
| 2 layers 1/2 inch | 40 |
| Type X gypsum wallboard | |
| 1/2 inch | 25 |
| 5/8 inch | 40 |
| Portland cement-sand plaster applied directly to concrete masonry | See Note a |
| Portland cement-sand plaster on metal lath | |
| 3/4 inch | 20 |
| 7/8 inch | 25 |
| 1 inch | 30 |
| Gypsum sand plaster on 3/8-inch gypsum lath | |
| 1/2 inch | 35 |
| 5/8 inch | 40 |
| 3/4 inch | 50 |
| Gypsum sand plaster on metal lath | |
| 3/4 inch | 50 |
| 7/8 inch | 60 |
| 1 inch | 80 |
| For SI: 1 inch = 25.4 mm. |
| a. The actual thickness of portland cement-sand plaster, provided it is 5/8 inch or less in thickness, shall be permitted to be included in determining the equivalent thickness of the masonry for use in Table 721.3.2. |
721.2.1.4.1
Nonfire-exposed side. Where the finish of gypsum wallboard or
plaster is applied to the side of the wall not exposed to fire, the
contribution of the finish to the total fire-resistance rating
shall be determined as follows: The thickness of the finish shall first be
corrected by multiplying the actual thickness of the finish by the applicable
factor determined from Table 721.2.1.4(1) based on the type of aggregate in the
concrete. The corrected thickness of finish shall then be added to the actual
or equivalent thickness of concrete and fire-resistance rating
of the concrete and finish determined from Table 721.2.1.1, Figure 721.2.1.2 or
Table 721.2.1.2(1).
721.2.1.4.2
Fire-exposed side. Where gypsum wallboard or plaster is applied to
the fire-exposed side of the wall, the contribution of the finish to the total
fire-resistance rating shall be determined as follows: The
time assigned to the finish as established by Table 721.2.1.4(2) shall be added
to the fire-resistance rating determined from Table 721.2.1.1
or Figure 721.2.1.2, or Table 721.2.1.2(1) for the concrete alone, or to the
rating determined in Section 721.2.1.4.1 for the concrete and finish on the
nonfire-exposed side.
721.2.1.4.3
Nonsymmetrical assemblies. For a wall having no finish on one side
or different types or thicknesses of finish on each side, the calculation
procedures of Sections 721.2.1.4.1 and 721.2.1.4.2 shall be performed twice,
assuming either side of the wall to be the fire-exposed side. The
fire-resistance rating of the wall shall not exceed the lower
of the two values.
Exception: For an exterior wall with a fire separation distance greater than 5 feet (1524 mm) the fire shall be assumed to occur on the interior side only.
721.2.1.4.4
Minimum concrete
fire-resistance rating. Where finishes applied to one or both sides of a
concrete wall contribute to the fire-resistance rating, the
concrete alone shall provide not less than one-half of the total required
fire-resistance rating. Additionally, the contribution to the
fire resistance of the finish on the nonfire-exposed side of a
load-bearing wall shall not exceed one-half the contribution
of the concrete alone.
721.2.1.4.5
Concrete finishes. Finishes on concrete walls that are assumed to
contribute to the total fire-resistance rating of the wall
shall comply with the installation requirements of Section 721.3.2.5.
721.2.2
Concrete
floor and roof slabs. Reinforced and prestressed floors and roofs shall
comply with Section 721.2.2.1. Multicourse floors and roofs shall comply with
Sections 721.2.2.2 and 721.2.2.3, respectively.
721.2.2.1
Reinforced and prestressed
floors and roofs. The minimum thicknesses of reinforced and prestressed
concrete floor or roof slabs for fire-resistance ratings of 1
hour to 4 hours are shown in Table 721.2.2.1.
TABLE 721.2.2.1 MINIMUM SLAB THICKNESS (inches)
| CONCRETE TYPE | FIRE-RESISTANCE RATING (hour) | ||||
| 1 | 11/2 | 2 | 3 | 4 | |
| Siliceous | 3.5 | 4.3 | 5.0 | 6.2 | 7.0 |
| Carbonate | 3.2 | 4.0 | 4.6 | 5.7 | 6.6 |
| Sand-lightweight | 2.7 | 3.3 | 3.8 | 4.6 | 5.4 |
| Lightweight | 2.5 | 3.1 | 3.6 | 4.4 | 5.1 |
| For SI: 1 inch = 25.4 mm. |
721.2.2.1.1
Hollow-core prestressed slabs. For hollow-core prestressed
concrete slabs in which the cores are of constant cross section throughout the
length, the equivalent thickness shall be permitted to be obtained by dividing
the net cross-sectional area of the slab including grout in the joints, by its
width.
721.2.2.1.2
Slabs with
sloping soffits. The thickness of slabs with sloping soffits (see Figure
721.2.2.1.2) shall be determined at a distance 2t or 6 inches
(152 mm), whichever is less, from the point of minimum thickness, where
t is the minimum thickness.
| Click to view image |
| For SI: 1 inch = 25.4 mm. |
FIGURE 721.2.2.1.2 DETERMINATION OF SLAB THICKNESS FOR SLOPING SOFFITS
721.2.2.1.3
Slabs with ribbed
soffits. The thickness of slabs with ribbed or undulating soffits (see
Figure 721.2.2.1.3) shall be determined by one of the following expressions,
whichever is applicable:
For s > 4t, the thickness to be used shall be t
For s <= 2t, the thickness to be used shall be te
For 4t > s> 2t, the thickness to be used shall be
| Click to view image | (Equation 7-5) |
where:
s = Spacing of ribs or undulations.
t = Minimum thickness.
te = Equivalent thickness of the slab calculated as the net area of the slab divided by the width, in which the maximum thickness used in the calculation shall not exceed 2t.
| Click to view image |
| For SI: 1 inch = 25.4 mm. |
FIGURE 721.2.2.1.3 SLABS WITH RIBBED OR UNDULATING SOFFITS
721.2.2.2
Multicourse floors.
The fire-resistance ratings of floors that consist of a base
slab of concrete with a topping (overlay) of a different type of concrete shall
comply with Figure 721.2.2.2.
| Click to view image |
| For SI: 1 inch = 25.4 mm. |
FIGURE 721.2.2.2 FIRE-RESISTANCE RATINGS FOR TWO-COURSE CONCRETE FLOORS
721.2.2.3
Multicourse roofs. The
fire-resistance ratings of roofs which consist of a base slab
of concrete with a topping (overlay) of an insulating concrete or with an
insulating board and built-up roofing shall comply with Figures 721.2.2.3(1)
and 721.2.2.3(2).
| Click to view image |
| For SI: 1 inch = 25.4 mm. |
FIGURE 721.2.2.3(1) FIRE-RESISTANCE RATINGS FOR CONCRETE ROOF ASSEMBLIES
| Click to view image |
| For SI: 1 inch = 25.4 mm. |
FIGURE 721.2.2.3(2) FIRE-RESISTANCE RATINGS FOR CONCRETE ROOF ASSEMBLIES
721.2.2.3.1
Heat transfer. For
the transfer of heat, three-ply built-up roofing contributes 10 minutes to the
fire-resistance rating. The fire-resistance
rating for concrete assemblies such as those shown in Figure
721.2.2.3(1) shall be increased by 10 minutes. This increase is not applicable
to those shown in Figure 721.2.2.3(2).
721.2.2.4
Joints in precast
slabs. Joints between adjacent precast concrete slabs need not be
considered in calculating the slab thickness provided that a concrete topping
at least 1 inch (25 mm) thick is used. Where no concrete topping is used,
joints must be grouted to a depth of at least one-third the slab thickness at
the joint, but not less than 1 inch (25 mm), or the joints must be made fire
resistant by other approved methods.
721.2.3
Concrete cover over
reinforcement. The minimum thickness of concrete cover over
reinforcement in concrete slabs, reinforced beams and prestressed beams shall
comply with this section.
TABLE 721.2.3(1) COVER THICKNESS FOR REINFORCED CONCRETE FLOOR OR ROOF SLABS (inches)
| CONCRETE AGGREGATE TYPE | FIRE-RESISTANCE RATING (hours) | |||||||||
| Restrained | Unrestrained | |||||||||
| 1 | 11/2 | 2 | 3 | 4 | 1 | 11/2 | 2 | 3 | 4 | |
| Siliceous | 3/4 | 3/4 | 3/4 | 3/4 | 3/4 | 3/4 | 3/4 | 1 | 11/4 | 15/8 |
| Carbonate | 3/4 | 3/4 | 3/4 | 3/4 | 3/4 | 3/4 | 3/4 | 3/4 | 11/4 | 11/4 |
| Sand-lightweight or lightweight | 3/4 | 3/4 | 3/4 | 3/4 | 3/4 | 3/4 | 3/4 | 3/4 | 11/4 | 11/4 |
| For SI: 1 inch = 25.4 mm. |
TABLE 721.2.3(2) COVER THICKNESS FOR PRESTRESSED CONCRETE FLOOR OR ROOF SLABS (inches)
| CONCRETE AGGREGATE TYPE | FIRE-RESISTANCE RATING (hours) | |||||||||
| Restrained | Unrestrained | |||||||||
| 1 | 11/2 | 2 | 3 | 4 | 1 | 11/2 | 2 | 3 | 4 | |
| Siliceous | 3/4 | 3/4 | 3/4 | 3/4 | 3/4 | 11/8 | 11/2 | 13/4 | 23/8 | 23/4 |
| Carbonate | 3/4 | 3/4 | 3/4 | 3/4 | 3/4 | 1 | 13/8 | 15/8 | 21/8 | 21/4 |
| Sand-lightweight or lightweight | 3/4 | 3/4 | 3/4 | 3/4 | 3/4 | 1 | 13/8 | 11/2 | 2 | 21/4 |
| For SI: 1 inch = 25.4 mm. |
TABLE 721.2.3(3) MINIMUM COVER FOR MAIN REINFORCING BARS OF REINFORCED CONCRETE BEAMSc (APPLICABLE TO ALL TYPES OF STRUCTURAL CONCRETE)
| RESTRAINED OR UNRESTRAINEDa | BEAM WIDTHb (inches) | FIRE-RESISTANCE RATING (hours) | ||||
| 1 | 11/2 | 2 | 3 | 4 | ||
| Restrained | 5 | 3/4 | 3/4 | 3/4 | 1a | 11/4a |
| 7 | 3/4 | 3/4 | 3/4 | 3/4 | 3/4 | |
| >= 10 | 3/4 | 3/4 | 3/4 | 3/4 | 3/4 | |
| Unrestrained | 5 | 3/4 | 1 | 11/4 | - | - |
| 7 | 3/4 | 3/4 | 3/4 | 13/4 | 3 | |
| >= 10 | 3/4 | 3/4 | 3/4 | 1 | 13/4 | |
| For SI: 1 inch = 25.4 mm, 1 foot = 304.8 mm. |
| a. Tabulated values for restrained assemblies apply to beams spaced more than 4 feet on center. For restrained beams spaced 4 feet or less on center, minimum cover of 3/4 inch is adequate for ratings of 4 hours or less. |
| b. For beam widths between the tabulated values, the minimum cover thickness can be determined by direct interpolation. |
| c. The cover for an individual reinforcing bar is the minimum thickness of concrete between the surface of the bar and the fire-exposed surface of the beam. For beams in which several bars are used, the cover for corner bars used in the calculation shall be reduced to one-half of the actual value. The cover for an individual bar must be not less than one-half of the value given in Table 721.2.3(3) nor less than 3/4 inch. |
TABLE 721.2.3(4) MINIMUM COVER FOR PRESTRESSED CONCRETE BEAMS 8 INCHES OR GREATER IN WIDTH
| RESTRAINED OR UNRESTRAINEDa | CONCRETE AGGREGATE TYPE | BEAM WIDTHb (inches) | FIRE-RESISTANCE RATING (hours) | ||||
| 1 | 11/2 | 2 | 3 | 4 | |||
| Restrained | Carbonate or siliceous | 8 | 11/2 | 11/2 | 11/2 | 13/4a | 21/2a |
| Carbonate or siliceous | >= 12 | 11/2 | 11/2 | 11/2 | 11/2 | 17/8a | |
| Sand lightweight | 8 | 11/2 | 11/2 | 11/2 | 11/2 | 2a | |
| Sand lightweight | >= 12 | 11/2 | 11/2 | 11/2 | 11/2 | 15/8a | |
| Unrestrained | Carbonate or siliceous | 8 | 11/2 | 13/4 | 21/2 | 5c | - |
| Carbonate or siliceous | >= 12 | 11/2 | 11/2 | 17/8a | 21/2 | 3 | |
| Sand lightweight | 8 | 11/2 | 11/2 | 2 | 31/4 | - | |
| Sand lightweight | >= 12 | 11/2 | 11/2 | 15/8 | 2 | 21/2 | |
| For SI: 1 inch = 25.4 mm, 1 foot = 304.8 mm. |
| a. Tabulated values for restrained assemblies apply to beams spaced more than 4 feet on center. For restrained beams spaced 4 feet or less on center, minimum cover of 3/4 inch is adequate for 4-hour ratings or less. |
| b. For beam widths between 8 inches and 12 inches, minimum cover thickness can be determined by direct interpolation. c. Not practical for 8-inch-wide beam but shown for purposes of interpolation. |
TABLE 721.2.3(5) MINIMUM COVER FOR PRESTRESSED CONCRETE BEAMS OF ALL WIDTHS
| RESTRAINED OR UNRESTRAINEDa | CONCRETE AGGREGATE TYPE | BEAM AREAb A (square inches) | FIRE-RESISTANCE RATING (hours) | ||||
| 1 | 11/2 | 2 | 3 | 4 | |||
| Restrained | All | 40 <= A <= 150 | 11/2 | 11/2 | 2 | 21/2 | - |
| Carbonate or | 150 < A <= 300 | 11/2 | 11/2 | 11/2 | 13/4 | 21/2 | |
| siliceous | 300 < A | 11/2 | 11/2 | 11/2 | 11/2 | 2 | |
| Sand lightweight | 150 < A | 11/2 | 11/2 | 11/2 | 11/2 | 2 | |
| Unrestrained | All | 40 <= A <= 150 | 2 | 21/2 | - | - | - |
| Carbonate or siliceous | 150 < A <= 300 | 11/2 | 13/4 | 21/2 | - | - | |
| 300 < A | 11/2 | 11/2 | 2 | 3c | 4c | ||
| Sand lightweight | 150 < A | 11/2 | 11/2 | 2 | 3c | 4c | |
| For SI: 1 inch = 25.4 mm, 1 foot = 304.8 mm, 1 square inch = 645 mm2. |
| a. Tabulated values for restrained assemblies apply to beams spaced more than 4 feet on center. For restrained beams spaced 4 feet or less on center, minimum cover of 3/4 inch is adequate for 4-hour ratings or less. |
| b. The cross-sectional area of a stem is permitted to include a portion of the area in the flange, provided the width of the flange used in the calculation does not exceed three times the average width of the stem. |
| c. U-shaped or hooped stirrups spaced not to exceed the depth of the member and having a minimum cover of 1 inch shall be provided. |
721.2.3.1
Slab cover. The minimum thickness of concrete cover to the
positive moment reinforcement shall comply with Table 721.2.3(1) for reinforced
concrete and Table 721.2.3(2) for prestressed concrete. These tables are
applicable for solid or hollow-core one-way or two-way slabs with flat
undersurfaces. These tables are applicable to slabs that are either cast in
place or precast. For precast prestressed concrete not covered elsewhere, the
procedures contained in PCI MNL 124 shall be acceptable.
721.2.3.2
Reinforced beam cover.
The minimum thickness of concrete cover to the positive moment reinforcement
(bottom steel) for reinforced concrete beams is shown in Table 721.2.3(3) for
fire-resistance ratings of 1 hour to 4 hours.
721.2.3.3
Prestressed beam
cover. The minimum thickness of concrete cover to the positive moment
prestressing tendons (bottom steel) for restrained and unrestrained prestressed
concrete beams and stemmed units shall comply with the values shown in Tables
721.2.3(4) and 721.2.3(5) for fire-resistance ratings of 1
hour to 4 hours. Values in Table 721.2.3(4) apply to beams 8 inches (203 mm) or
greater in width. Values in Table 721.2.3(5) apply to beams or stems of any
width, provided the cross-section area is not less than 40 square inches (25
806 mm2). In case of differences between the values
determined from Table 721.2.3(4) or 721.2.3(5), it is permitted to use the
smaller value. The concrete cover shall be calculated in accordance with
Section 721.2.3.3.1. The minimum concrete cover for nonprestressed
reinforcement in prestressed concrete beams shall comply with Section
721.2.3.2.
721.2.3.3.1
Calculating
concrete cover. The concrete cover for an individual tendon is the
minimum thickness of concrete between the surface of the tendon and the
fire-exposed surface of the beam, except that for ungrouted ducts, the assumed
cover thickness is the minimum thickness of concrete between the surface of the
duct and the fire-exposed surface of the beam. For beams in which two or more
tendons are used, the cover is assumed to be the average of the minimum cover
of the individual tendons. For corner tendons (tendons equal distance from the
bottom and side), the minimum cover used in the calculation shall be one-half
the actual value. For stemmed members with two or more prestressing tendons
located along the vertical centerline of the stem, the average cover shall be
the distance from the bottom of the member to the centroid of the tendons. The
actual cover for any individual tendon shall not be less than one-half the
smaller value shown in Tables 721.2.3(4) and 721.2.3(5), or 1 inch (25 mm),
whichever is greater.
721.2.4
Concrete columns.
Concrete columns shall comply with this section.
TABLE 721.2.4 MINIMUM DIMENSION OF CONCRETE COLUMNS (inches)
| TYPES OF CONCRETE | FIRE-RESISTANCE RATING (hours) | ||||
| 1 | 11/2 | 2a | 3a | 4b | |
| Siliceous | 8 | 9 | 10 | 12 | 14 |
| Carbonate | 8 | 9 | 10 | 11 | 12 |
| Sand-lightweight | 8 | 81/2 | 9 | 101/2 | 12 |
| For SI: 1 inch = 25 mm. |
| a. The minimum dimension is permitted to be reduced to 8 inches for rectangular columns with two parallel sides at least 36 inches in length. |
| b. The minimum dimension is permitted to be reduced to 10 inches for rectangular columns with two parallel sides at least 36 inches in length. |
721.2.4.1
Minimum size. The minimum overall dimensions of reinforced
concrete columns for fire-resistance ratings of 1 hour to 4
hours for exposure to fire on all sides shall comply with this section.
721.2.4.1.1
Concrete strength less than
or equal to 12,000 psi. For columns made with concrete having a
specified compressive strength, f c, of
less than or equal to 12,000 psi (82.7 MPa), the minimum dimension shall comply
with Table 721.2.4.
721.2.4.1.2
Concrete strength greater than 12,000 psi. For columns made with
concrete having a specified compressive strength, f
c, greater than 12,000 psi (82.7 MPa), for
fire-resistance ratings of 1 hour to 4 hours the minimum dimension shall be 24
inches (610 mm).
721.2.4.2
Minimum cover for R/C
columns. The minimum thickness of concrete cover to the main
longitudinal reinforcement in columns, regardless of the type of aggregate used
in the concrete and the specified compressive strength of concrete, f
c, shall not be less than 1 inch (25 mm) times
the number of hours of required fire resistance or 2 inches
(51 mm), whichever is less.
721.2.4.3
Tie and spiral
reinforcement. For concrete columns made with concrete having a
specified compressive strength, f c,
greater than 12,000 psi (82.7 MPa), tie and spiral reinforcement shall comply
with the following:
1. The free ends of
rectangular ties shall terminate with a 135-degree (2.4 rad) standard tie
hook.
2. The free ends of circular
ties shall terminate with a 90-degree (1.6 rad) standard tie hook.
3. The free ends of spirals, including at lap
splices, shall terminate with a 90-degree (1.6 rad) standard tie hook.
The hook extension at the free end of ties and spirals shall be the larger of six bar diameters and the extension required by Section 7.1.3 of ACI 318. Hooks shall project into the core of the column.
721.2.4.4
Columns built into
walls. The minimum dimensions of Table 721.2.4 do not apply to a
reinforced concrete column that is built into a concrete or masonry wall
provided all of the following are met:
1. The
fire-resistance rating for the wall is equal to or greater
than the required rating of the column;
2. The main longitudinal reinforcing in the
column has cover not less than that required by Section 721.2.4.2;
and
3. Openings in the wall are
protected in accordance with Table 715.4.
Where openings in the wall are not protected as required by Section 715.4, the minimum dimension of columns required to have a fire-resistance rating of 3 hours or less shall be 8 inches (203 mm), and 10 inches (254 mm) for columns required to have a fire-resistance rating of 4 hours, regardless of the type of aggregate used in the concrete.
721.2.4.5
Precast cover units for steel
columns. See Section 721.5.1.4.
Notes
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