Concrete Slab Designer

Reinforced concrete one-way slabs, two-way slabs on walls and flat plates on columns, to ACI 318-19.

Slab

Kind of slab

Clear span (ft)

Clear span the other way (ft)

Spans

All the same length.

Outside ends

Beam or wall width (in)

Square columns.

Thickness (in)

Concrete, f′_c

Normal weight, grade 60 bars.

Exposure

Sets the cover.

Loads

Dead load (psf)

Finishes, ceiling and partitions, besides the slab's own weight.

Live load (psf)

40 for homes, 50 for offices, 100 for corridors.

Finishes

Sets the deflection limit.

Results

Drag to turn, scroll or pinch to zoom. Red: bottom bars. Orange: top bars.

How thick should a concrete slab be?

ACI 318-19 sets a minimum thickness for each kind of slab. A slab at least this thick doesn't need its deflection worked out, unless it carries partitions or finishes that crack. These are the minimums for normal weight concrete and grade 60 bars, rounded up to the next 1/4 in.

One-way slabs (ACI Table 7.3.1.1)

Span, center to center	Simply supported (l/20)	One end continuous (l/24)	Both ends continuous (l/28)	Cantilever (l/10)
6 ft	3.75 in	3 in	2.75 in	7.25 in
8 ft	5 in	4 in	3.5 in	9.75 in
10 ft	6 in	5 in	4.5 in	12 in
12 ft	7.25 in	6 in	5.25 in	14.5 in
14 ft	8.5 in	7 in	6 in	17 in
16 ft	9.75 in	8 in	7 in	19.25 in
18 ft	11 in	9 in	7.75 in	21.75 in
20 ft	12 in	10 in	8.75 in	24 in

Flat plates (ACI Table 8.3.1.1)

Clear span, long way	Exterior panel, no edge beam (l_n/30)	Interior panel (l_n/33)
12 ft	5 in	5 in
14 ft	5.75 in	5.25 in
16 ft	6.5 in	6 in
18 ft	7.25 in	6.75 in
20 ft	8 in	7.5 in
22 ft	9 in	8 in
24 ft	9.75 in	8.75 in
26 ft	10.5 in	9.5 in
28 ft	11.25 in	10.25 in
30 ft	12 in	11 in

Flat plates are at least 5 in. The clear span is face to face of the columns.

How the Concrete Slab Designer checks a slab

Every slab is designed per foot of width with its own weight at 150 pcf plus the dead load, and the live load, factored by 1.2 and 1.6 (or 1.4 times the dead load alone, if that's more), with grade 60 bars by the American Concrete Institute's ACI 318-19. No finite element model is needed: each kind of slab has a closed-form method.

One-way slabs: one span by statics, or two or more equal spans by the ACI moment and shear coefficients (Table 6.5.2), which apply when the live load is no more than three times the dead load. The bars meet the minimum steel, spacing and crack control limits, with shrinkage and temperature bars across the span.
Two-way slabs on walls: elastic plate theory for a slab simply supported on four edges (Navier's series, as ACI 8.2.1 allows), giving the moment each way and the deflection at the center.
Flat plates: the direct design method (ACI 8.10) for at least three equal spans each way, with the moment shared between the supports and midspan, then between the column and middle strips. Punching shear is checked at an interior, an edge and a corner column, with the share of the slab's moment each column takes by shear.
Shear: one-way shear without stirrups by ACI 22.5.5.1(c), with the size effect and the steel ratio, at d from the support.
Deflection: for one-way slabs and slabs on walls, worked out with the effective moment of inertia of ACI 24.2.3.5, a long-term creep factor of 2.0 and the limits of Table 24.2.2. Flat plates must meet the minimum thickness instead.

Anyone can check the minimum thickness. Pro adds the bars top and bottom, every check worked out with its ACI section, saved designs in projects and folders, and PDF reports.

Not covered: slabs on the ground, cantilevers, drop panels and edge beams, openings, unequal spans, point loads, and wind or earthquake loads on the slab.

Concrete slab questions

Is my slab one-way or two-way?

A slab held on two opposite sides spans one way. One held on all four sides spans both ways, unless it's more than twice as long as it is wide: then nearly all its load goes the short way, and it acts like a one-way slab.

What is punching shear?

Around a column, a flat plate can fail by the column pushing through it, tearing out a cone of concrete. It usually decides how thick a flat plate must be, most often at the corner and edge columns. A thicker slab, a bigger column, drop panels or shear studs fix it.

What's the difference between the column strip and the middle strip?

In a flat plate the slab near the column lines is stiffer and takes more of the moment. ACI splits each panel into a column strip a quarter of the shorter span each side of the column line, and the middle strip between, and gives the column strip 60% to 100% of the moment.