Adopts With Amendments:

ACI 318 2019

Part 1: General

Part 2: Loads & Analysis

Part 3: Members

Part 4: Joints/Connections/Anchors

Part 5: Earthquake Resistance

Part 6: Materials & Durability

Part 7: Strength & Serviceability

Part 8: Reinforcement

Part 9: Construction

Part 10: Evaluation

REFERENCES & Appendices

Heads up: There are no amended sections in this chapter.
This chapter shall apply to the design of nonprestressed and prestressed slabs reinforced for flexure in two directions, with or without beams between supports, including (a) through (d):
(a) Solid slabs
(b) Slabs cast on stay-in-place, noncomposite steel deck
(c) Composite slabs of concrete elements constructed in separate placements but connected so that all elements resist loads as a unit
(d) Two-way joist systems in accordance with 8.8
A slab system shall be permitted to be designed by any procedure satisfying equilibrium and geometric compatibility, provided that design strength at every section is at least equal to required strength, and all serviceability requirements are satisfied. The direct design method or the equivalent frame method is permitted.
The effects of concentrated loads, slab openings, and slab voids shall be considered in design.
Slabs prestressed with an average effective compressive stress less than 125 psi shall be designed as nonprestressed slabs.
A drop panel in a nonprestressed slab, where used to reduce the minimum required thickness in accordance with 8.3.1.1 or the quantity of deformed negative moment reinforcement at a support in accordance with 8.5.2.2, shall satisfy (a) and (b):
(a) The drop panel shall project below the slab at least one-fourth of the adjacent slab thickness.
(b) The drop panel shall extend in each direction from the centerline of support a distance not less than one-sixth the span length measured from center-to-center of supports in that direction.
A shear cap, where used to increase the critical section for shear at a slab-column joint, shall project below the slab soffit and extend horizontally from the face of the column a distance at least equal to the thickness of the projection below the slab soffit.
Design properties for concrete shall be selected to be in accordance with Chapter 19.
Design properties for steel reinforcement shall be selected to be in accordance with Chapter 20.
Materials, design, and detailing requirements for embedments in concrete shall be in accordance with 20.6.
Beam-column and slab-column joints shall satisfy Chapter 15.
For nonprestressed slabs without interior beams spanning between supports on all sides, having a maximum ratio of long-to-short span of 2, overall slab thickness h shall not be less than the limits in Table 8.3.1.1, and shall be at least the value in (a) or (b), unless the calculated deflection limits of 8.3.2 are satisfied:
(a) Slabs without drop panels as given in 8.2.4........5 in.
(b) Slabs with drop panels as given in 8.2.4.............4 in.
For fy exceeding 80,000 psi, the calculated deflection limits in 8.3.2 shall be satisfied.
Table 8.3.1.1—Minimum thickness of nonprestressed two-way slabs without interior beams (in.)[1]
fy, psi[2] Without drop panels[3] With drop panels[3]
Exterior panels Interior panels Exterior panels Interior panels
Without edge beams With edge beams[4] Without edge beams With edge beams[4]
40,000 n/33 n/36 n/36 n/36 n/40 n/40
60,000 n/30 n/33 n/33 n/33 n/36 n/36
80,000 n/27 n/30 n/30 n/30 n/33 n/33
[1]n is the clear span in the long direction, measured face-to-face of supports (in.).
[2]For fy between the values given in the table, minimum thickness shall be calculated by linear interpolation.
[3]Drop panels as given in 8.2.4.
[4]Slabs with beams between columns along exterior edges. Exterior panels shall be considered to be without edge beams if αf is less than 0.8.
For nonprestressed slabs with beams spanning between supports on all sides, overall slab thickness h shall satisfy the limits in Table 8.3.1.2, unless the calculated deflection limits of 8.3.2 are satisfied.
Table 8.3.1.2—Minimum thickness of nonprestressed two-way slabs with beams spanning between supports on all sides
αfm[1] Minimum h, in.
αfm ≤ 0.2 8.3.1.1 applies (a)
0.2 < αfm ≤ 2.0 Greater of: (b)[1],[2]
5.0 (c)
αfm > 0.2 Greater of: (d)
3.5 (e)
[1]αfm is the average value of αf for all beams on edges of a panel.
[2]n is the clear span in the long direction, measured face-to-face of beams (in.).
[3]β is the ratio of clear spans in long to short directions of slab.
At discontinuous edges of slabs conforming to 8.3.1.2, an edge beam with αf ≥ 0.80 shall be provided, or the minimum thickness required by (b) or (d) of Table 8.3.1.2 shall be increased by at least 10 percent in the panel with a discontinuous edge.
The thickness of a concrete floor finish shall be permitted to be included in h if it is placed monolithically with the floor slab, or if the floor finish is designed to be composite with the floor slab in accordance with 16.4.
If single- or multiple-leg stirrups are used as shear reinforcement, the slab thickness shall be sufficient to satisfy the requirements for d in 22.6.7.1.
Immediate and time-dependent deflections shall be calculated in accordance with 24.2 and shall not exceed the limits in 24.2.2 for two-way slabs given in (a) through (c):
(a) Nonprestressed slabs not satisfying 8.3.1
(b) Nonprestressed slabs without interior beams spanning between the supports on all sides and having a ratio of long-to-short span exceeding 2.0
(c) Prestressed slabs
For nonprestressed composite concrete slabs satisfying 8.3.1.1 or 8.3.1.2, deflections occurring after the member becomes composite need not be calculated. Deflections occurring before the member becomes composite shall be investigated, unless the precomposite thickness also satisfies 8.3.1.1 or 8.3.1.2.
Nonprestressed slabs shall be tension-controlled in accordance with Table 21.2.2.
Prestressed slabs shall be designed as Class U with . Other stresses in prestressed slabs immediately after transfer and at service loads shall not exceed the permissible stresses in 24.5.3 and 24.5.4.
Required strength shall be calculated in accordance with the factored load combinations in Chapter 5.
Required strength shall be calculated in accordance with the analysis procedures given in Chapter 6.
For prestressed slabs, effects of reactions induced by prestressing shall be considered in accordance with 5.3.11.
For a slab system supported by columns or walls, dimensions c1, c2, and n shall be based on an effective support area. The effective support area is the intersection of the bottom surface of the slab, or drop panel or shear cap if present, with the largest right circular cone, right pyramid, or tapered wedge whose surfaces are located within the column and the capital or bracket and are oriented no greater than 45 degrees to the axis of the column.
A column strip is a design strip with a width on each side of a column centerline equal to the lesser of 0.252 and 0.251. A column strip shall include beams within the strip, if present.
A middle strip is a design strip bounded by two column strips.
A panel is bounded by column, beam, or wall centerlines on all sides.
For monolithic or fully composite construction supporting two-way slabs, a beam includes that portion of slab, on each side of the beam extending a distance equal to the projection of the beam above or below the slab, whichever is greater, but not greater than four times the slab thickness.
Combining the results of a gravity load analysis with the results of a lateral load analysis shall be permitted.
For slabs built integrally with supports, Mu at the support shall be permitted to be calculated at the face of support.
If gravity, wind, earthquake, or other loads cause a transfer of moment between the slab and column, a fraction of Msc, the factored slab moment resisted by the column at a joint, shall be transferred by flexure in accordance with 8.4.2.2.2 through 8.4.2.2.5.
The fraction of factored slab moment resisted by the column, γfMsc, shall be assumed to be transferred by flexure, where γf shall be calculated by:
(8.4.2.2.2)
The effective slab width bslab for resisting γfMsc shall be the width of column or capital plus a distance on each side in accordance with Table 8.4.2.2.3.
Table 8.4.2.2.3—Dimensional limits for effective slab width
Distance on each side of column or capital
Without drop panel or shear cap Lesser 1.5h of slab
Distance to edge of slab
With drop panel or shear cap Lesser 1.5h of drop or cap
Distance to edge of the drop or cap plus 1.5h of slab
For nonprestressed slabs, where the limitations on νuv and εt in Table 8.4.2.2.4 are satisfied, γf shall be permitted to be increased to the maximum modified values provided in Table 8.4.2.2.4, where νc is calculated in accordance with 22.6.5.
Table 8.4.2.2.4—Maximum modified values of γf for nonprestressed two-way slabs
Column location Span direction νuv εt (within bslab) Maximum modified γf
Corner column Either direction ≤0.5ϕνc ≥εty + 0.003 1.0
Edge column Perpendicular to the edge ≤0.75ϕνc ≥εty + 0.003 1.0
Parallel to the edge ≤0.4ϕνc ≥εty + 0.008
Interior column Either direction ≤0.4ϕνc ≥εty + 0.008
Concentration of reinforcement over the column by closer spacing or additional reinforcement shall be used to resist moment on the effective slab width defined in 8.4.2.2.2 and 8.4.2.2.3.
The fraction of Msc not calculated to be resisted by flexure shall be assumed to be resisted by eccentricity of shear in accordance with 8.4.4.2.
For slabs built integrally with supports, Vu at the support shall be permitted to be calculated at the face of support.
Sections between the face of support and a critical section located d from the face of support for nonprestressed slabs and h/2 from the face of support for prestressed slabs shall be permitted to be designed for Vu at that critical section if (a) through (c) are satisfied:
(a) Support reaction, in direction of applied shear, introduces compression into the end regions of the slab.
(b) Loads are applied at or near the top surface of the slab.
(c) No concentrated load occurs between the face of support and critical section.
Slabs shall be evaluated for two-way shear in the vicinity of columns, concentrated loads, and reaction areas at critical sections in accordance with 22.6.4.
Slabs reinforced with stirrups or headed shear stud reinforcement shall be evaluated for two-way shear at critical sections in accordance with 22.6.4.2.
For two-way shear with factored slab moment resisted by the column, factored shear stress vu shall be calculated at critical sections in accordance with 8.4.4.1. Factored shear stress vu corresponds to a combination of vuv and the shear stress produced by γvMsc, where γv is given in 8.4.4.2.2 and Msc is given in 8.4.2.2.1.
The fraction of Msc transferred by eccentricity of shear, γvMsc, shall be applied at the centroid of the critical section in accordance with 8.4.4.1, where:
γv = 1 — γf (8.4.4.2.2)
The factored shear stress resulting from γvMsc shall be assumed to vary linearly about the centroid of the critical section in accordance with 8.4.4.1.
For each applicable factored load combination, design strength shall satisfy ϕSnU, including (a) through (d). Interaction between load effects shall be considered.
(a) ϕMnMu at all sections along the span in each direction
(b) ϕMn ≥ γfMsc within bslab as defined in 8.4.2.2.3
(c) ϕVnVu at all sections along the span in each direction for one-way shear
(d) ϕvnvu at the critical sections defined in 8.4.4.1 for two-way shear
ϕ shall be in accordance with 21.2.
Mn shall be calculated in accordance with 22.3.
In calculating Mn for nonprestressed slabs with a drop panel, the thickness of the drop panel below the slab shall not be assumed to be greater than one-fourth the distance from the edge of drop panel to the face of column or column capital.
In calculating Mn for prestressed slabs, external tendons shall be considered as unbonded unless the external tendons are effectively bonded to the slab along its entire length.
Design shear strength of slabs in the vicinity of columns, concentrated loads, or reaction areas shall be the more severe of 8.5.3.1.1 and 8.5.3.1.2.
For one-way shear, where each critical section to be investigated extends in a plane across the entire slab width, Vn shall be calculated in accordance with 22.5.
For two-way shear, vn shall be calculated in accordance with 22.6.
For composite concrete slabs, horizontal shear strength Vnh shall be calculated in accordance with 16.4.
Openings of any size shall be permitted in slab systems if shown by analysis that all strength and serviceability requirements, including the limits on deflections, are satisfied.
As an alternative to 8.5.4.1, openings shall be permitted in slab systems without beams in accordance with (a) through (d).
(a) Openings of any size shall be permitted in the area common to intersecting middle strips, but the total quantity of reinforcement in the panel shall be at least that required for the panel without the opening.
(b) At two intersecting column strips, not more than one-eighth the width of column strip in either span shall be interrupted by openings. A quantity of reinforcement at least equal to that interrupted by an opening shall be added on the sides of the opening.
(c) At the intersection of one column strip and one middle strip, not more than one-fourth of the reinforcement in either strip shall be interrupted by openings. A quantity of reinforcement at least equal to that interrupted by an opening shall be added on the sides of the opening.
(d) If an opening is located closer than 4h from the periphery of a column, concentrated load or reaction area, 22.6.4.3 shall be satisfied.
A minimum area of flexural reinforcement, As,min of 0.0018Ag, or as defined in 8.6.1.2, shall be provided near the tension face of the slab in the direction of the span under consideration.
If on the critical section for two-way shear surrounding a column, concentrated load, or reaction area, As,min, provided over the width bslab, shall satisfy Eq. (8.6.1.2)
(8.6.1.2)
where bslab is the width specified in 8.4.2.2.3, αs is given in 22.6.5.3, ϕ is the value for shear, and λs is given in 22.5.5.1.3.
For prestressed slabs, the effective prestress force Apsfse shall provide a minimum average compressive stress of 125 psi on the slab section tributary to the tendon or tendon group. For slabs with varying cross section along the slab span, either parallel or perpendicular to the tendon or tendon group, the minimum average effective prestress of 125 psi is required at every cross section tributary to the tendon or tendon group along the span.
For slabs with bonded prestressed reinforcement, total quantity of As and Aps shall be adequate to develop a factored load at least 1.2 times the cracking load calculated on the basis of fr defined in 19.2.3.
For slabs with both flexural and shear design strength at least twice the required strength, 8.6.2.2 need not be satisfied.
For prestressed slabs, a minimum area of bonded deformed longitudinal reinforcement, As,min, shall be provided in the precompressed tension zone in the direction of the span under consideration in accordance with Table 8.6.2.3.
Table 8.6.2.3—Minimum bonded deformed longitudinal reinforcement As,min in two-way slabs with bonded or unbonded tendons
Region Calculated ft after all losses, psi As,min, in.2
Positive moment Not required (a)
(b)[1],[2]
Negative moment at columns 0.00075Acf (c)[2]
[1]The value of fy shall not exceed 60,000 psi.
[2]For slabs with bonded tendons, it shall be permitted to reduce As,min by the area of the bonded prestressed reinforcement located within the area used to determine Nc for positive moment, or within the width of slab defined in 8.7.5.3(a) for negative moment.
Concrete cover for reinforcement shall be in accordance with 20.5.1.
Development lengths of deformed and prestressed reinforcement shall be in accordance with 25.4.
Splice lengths of deformed reinforcement shall be in accordance with 25.5.
Bundled bars shall be detailed in accordance with 25.6.
Minimum spacing s shall be in accordance with 25.2.
For nonprestressed solid slabs, maximum spacing s of deformed longitudinal reinforcement shall be the lesser of 2h and 18 in. at critical sections, and the lesser of 3h and 18 in. at other sections.
For prestressed slabs with uniformly distributed loads, maximum spacing s of tendons or groups of tendons in at least one direction shall be the lesser of 8h and 5 ft.
Concentrated loads and openings shall be considered in determining tendon spacing.
At exterior corners of slabs supported by edge walls or where one or more edge beams have a value of αf greater than 1.0, reinforcement at top and bottom of slab shall be designed to resist Mu per unit width due to corner effects equal to the maximum positive Mu per unit width in the slab panel.
Factored moment due to corner effects, Mu, shall be assumed to be about an axis perpendicular to the diagonal from the corner in the top of the slab and about an axis parallel to the diagonal from the corner in the bottom of the slab.
Reinforcement shall be provided for a distance in each direction from the corner equal to one-fifth the longer span.
Reinforcement shall be placed parallel to the diagonal in the top of the slab and perpendicular to the diagonal in the bottom of the slab. Alternatively, reinforcement shall be placed in two layers parallel to the sides of the slab in both the top and bottom of the slab.
Where a slab is supported on spandrel beams, columns, or walls, anchorage of reinforcement perpendicular to a discontinuous edge shall satisfy (a) and (b):
(a) Positive moment reinforcement shall extend to the edge of slab and have embedment, straight or hooked, at least 6 in. into spandrel beams, columns, or walls
(b) Negative moment reinforcement shall be bent, hooked, or otherwise anchored into spandrel beams, columns, or walls, and shall be developed at the face of support
Where a slab is not supported by a spandrel beam or wall at a discontinuous edge, or where a slab cantilevers beyond the support, anchorage of reinforcement shall be permitted within the slab.
For slabs without beams, reinforcement extensions shall be in accordance with (a) through (c):
(a) Reinforcement lengths shall be at least in accordance with Fig. 8.7.4.1.3, and if slabs act as primary members resisting lateral loads, reinforcement lengths shall be at least those required by analysis.
(b) If adjacent spans are unequal, extensions of negative moment reinforcement beyond the face of support in accordance with Fig. 8.7.4.1.3 shall be based on the longer span.
(c) Bent bars shall be permitted only where the depth-to-span ratio permits use of bends of 45 degrees or less.
Fig. 8.7.4.1.3—Minimum extensions for deformed reinforcement in two-way slabs without beams.
All bottom deformed bars or deformed wires within the column strip, in each direction, shall be continuous or spliced using mechanical or welded splices in accordance with 25.5.7 or Class B tension lap splices in accordance with 25.5.2. Splices shall be located in accordance with Fig. 8.7.4.1.3.
At least two of the column strip bottom bars or wires in each direction shall pass within the region bounded by the longitudinal reinforcement of the column and shall be anchored at exterior supports.
External tendons shall be attached to the slab in a manner that maintains the specified eccentricity between the tendons and the concrete centroid through the full range of anticipated member deflections.
If bonded deformed longitudinal reinforcement is required to satisfy flexural strength or for tensile stress conditions in accordance with Eq. (8.6.2.3(b)), the detailing requirements of 7.7.3 shall be satisfied.
Bonded longitudinal reinforcement required by Eq. (8.6.2.3(c)) shall be placed in the top of the slab, and shall be in accordance with (a) through (c):
(a) Reinforcement shall be distributed between lines that are 1.5h outside opposite faces of the column support.
(b) At least four deformed bars, deformed wires, or bonded strands shall be provided in each direction.
(c) Maximum spacing s between bonded longitudinal reinforcement shall not exceed 12 in.
Post-tensioned anchorage zones shall be designed and detailed in accordance with 25.9.
Post-tensioning anchorages and couplers shall be designed and detailed in accordance with 25.8.
Length of deformed reinforcement required by 8.6.2.3 shall be in accordance with (a) and (b):
(a) In positive moment areas, length of reinforcement shall be at least n/3 and be centered in those areas
(b) In negative moment areas, reinforcement shall extend at least n/6 on each side of the face of support
Except as permitted in 8.7.5.6.3, at least two tendons with 1/2 in. diameter or larger strand shall be placed in each direction at columns in accordance with (a) or (b):
(a) Tendons shall pass through the region bounded by the longitudinal reinforcement of the column.
(b) Tendons shall be anchored within the region bounded by the longitudinal reinforcement of the column, and the anchorage shall be located beyond the column centroid and away from the anchored span.
Outside of the column and shear cap faces, the two structural integrity tendons required by 8.7.5.6.1 shall pass under any orthogonal tendons in adjacent spans.
Slabs with tendons not satisfying 8.7.5.6.1 shall be permitted if bonded bottom deformed reinforcement is provided in each direction in accordance with 8.7.5.6.3.1 through 8.7.5.6.3.3.
Minimum bottom deformed reinforcement As in each direction shall be the larger of (a) and (b). The value of fy shall be limited to a maximum of 80,000 psi:
(a) (8.7.5.6.3.1a)
(b) (8.7.5.6.3.1b)
where c2 is measured at the column faces through which the reinforcement passes.
Bottom deformed reinforcement calculated in 8.7.5.6.3.1 shall pass within the region bounded by the longitudinal reinforcement of the column and shall be anchored at exterior supports.
Bottom deformed reinforcement shall be anchored to develop fy beyond the column or shear cap face.
Single-leg, simple-U, multiple-U, and closed stirrups shall be permitted as shear reinforcement.
Stirrup anchorage and geometry shall be in accordance with 25.7.1.
If stirrups are provided, location and spacing shall be in accordance with Table 8.7.6.3.
Table 8.7.6.3—First stirrup location and spacing limits
Direction of measurement Description of measurement Maximum distance or spacing, in.
Perpendicular to column face Distance from column face to first stirrup d/2
Spacing between stirrups d/2
Parallel to column face Spacing between vertical legs of stirrups 2d
Headed shear stud reinforcement shall be permitted if placed perpendicular to the plane of the slab.
The overall height of the shear stud assembly shall be at least the thickness of the slab minus the sum of (a) through (c):
(a) Concrete cover on the top flexural reinforcement
(b) Concrete cover on the base rail
(c) One-half the bar diameter of the flexural tension reinforcement
Headed shear stud reinforcement location and spacing shall be in accordance with Table 8.7.7.1.2.
Table 8.7.7.1.2—Shear stud location and spacing limits
Direction of measurement Description of measurement Condition Maximum distance or spacing, in.
Perpendicular to column face Distance from column face to first peripheral line of shear studs All d/2
Constant spacing between peripheral lines of shear studs Nonprestressed slab with 3d/2
Nonprestressed slab with d/2
Prestressed slabs conforming to 22.6.5.4 3d/4
Parallel to column face Spacing between adjacent shear studs on peripheral line nearest to column face All 2d
Nonprestressed two-way joist construction consists of a monolithic combination of regularly spaced ribs and a top slab designed to span in two orthogonal directions.
Width of ribs shall be at least 4 in. at any location along the depth.
Overall depth of ribs shall not exceed 3.5 times the minimum width.
Clear spacing between ribs shall not exceed 30 in.
Vc shall be permitted to be taken as 1.1 times the values calculated in 22.5.
For structural integrity, at least one bottom bar in each joist shall be continuous and shall be anchored to develop fy at the face of supports.
Reinforcement area perpendicular to the ribs shall satisfy slab moment strength requirements, considering load concentrations, and shall be at least the shrinkage and temperature reinforcement area in accordance with 24.4.
Two-way joist construction not satisfying the limitations of 8.8.1.1 through 8.8.1.4 shall be designed as slabs and beams.
If permanent burned clay or concrete tile fillers of material having a unit compressive strength at least equal to fc' in the joists are used, 8.8.2.1.1 and 8.8.2.1.2 shall apply.
Slab thickness over fillers shall be at least the greater of one-twelfth the clear distance between ribs and 1.5 in.
For calculation of shear and negative moment strength, it shall be permitted to include the vertical shells of fillers in contact with the ribs. Other portions of fillers shall not be included in strength calculations.
If fillers not complying with 8.8.2.1 or removable forms are used, slab thickness shall be at least the greater of one-twelfth the clear distance between ribs and 2 in.
In slabs constructed with lift-slab methods where it is impractical to pass the tendons required by 8.7.5.6.1 or the bottom bars required by 8.7.4.2 or 8.7.5.6.3 through the column, at least two post-tensioned tendons or two bonded bottom bars or wires in each direction shall pass through the lifting collar as close to the column as practicable, and be continuous or spliced using mechanical or welded splices in accordance with 25.5.7 or Class B tension lap splices in accordance with 25.5.2. At exterior columns, the reinforcement shall be anchored at the lifting collar.
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