Two-Way Slab Calculator, ACI 318-19

What it checks

The module designs a uniformly-loaded two-way flat plate or flat slab supported on columns, assuming no beams exist between interior supports. It applies the Direct Design Method to distribute moments to column and middle strips, then calculates the required flexural steel and checks for punching shear at the column. The design method utilizes strength-based load combinations. The engine evaluates project-specific load combinations if provided; otherwise, it falls back to the ASCE 7-22 §2.3.1 strength set. The module identifies the governing load combination, noting whether vertical seismic effects are included based on the provided spectral response acceleration parameter.

Inputs

Geometry - Slab thickness and span dimensions, used to determine minimum thickness requirements and effective depth.
Material Properties - Concrete compressive strength (ksi) and steel yield strength (ksi), with typical values of 4 ksi and 60 ksi noted for simplified thickness checks.
Loads - Dead and live loads, along with optional project load combinations defined by the user.
Seismic Parameters - Spectral response acceleration parameter (SDS) to enable the inclusion of vertical seismic effects in combinations.

What it reports

Minimum Thickness - The required slab thickness based on span-to-depth ratios, compared against the provided thickness.
Governing Load Combination - The specific load combination that controls the design, including the source of the load factors.
Flexural Reinforcement - The required reinforcement per foot of strip width, calculated using a simplified effective depth approximation.
Punching Shear - The shear demand at the column, checked against the slab's shear capacity.
Seismic Combination Status - An indication of whether vertical seismic effects were folded into the governing combination and the associated SDS value.

Code references

The engine cites ACI 318-19 §8.3.1.1 for minimum thickness requirements and the Direct Design Method coefficients per ACI 318-14 §8.10. When project load combinations are absent, the module defaults to the ASCE 7-22 §2.3.1 strength set.

Limits

The module is limited to uniformly-loaded two-way slabs supported on columns without beams between interior supports. It does not account for non-uniform loading or beam-supported systems. The simplified thickness check assumes typical material strengths of 60 ksi for steel and 4 ksi for concrete. Results are engineering aids; the engineer of record is responsible for the design.

This module ships in the desktop app

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