AS 2870 Slab on Grade Design Calculator

The first step of almost any construction project is to build the foundation slab, typically referred to as a slab on grade. There are numerous methods of slab on grade construction, including flat slabs, stiffened raft slabs, waffle slabs and slabs with pile foundations. The optimal method of construction is dependent on geotechnical conditions, material/labour costs and access constraints.

The design of ground slabs is typically governed by the stiffness and reactivity (i.e. swell/shrink due to moisture changes) of the founding material. A slab on grade needs to be stiff enough to not experience large movements due to soil settlement, and strong enough to limit cracking due to differences in the stiffness of supporting material.

Ground slabs can be designed using 3D structural analysis software (such as SkyCiv S3D), or prescriptive "deemed-to-comply" methods created by national standards bodies. SkyCiv have a range of design tools available for both design approaches.

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About the AS 2870 Residential Slab Design Module

What does the Residential Slab Design Calculator do?

This tool carries out design and compliance checks for stiffened raft slabs in accordance with AS 2870:2011. Calculations are based on the Simplified Method for Raft Designs outlined in AS 2870 Clause 4.5, which is an extension of the deemed-to-comply values provided in Clause 3.2. The simplified method allows for extension and modification of the deemed-to-comply values to suit changes required by the design engineer, avoiding the need for complex 3D analysis of structure/soil interaction.

This method is compliant for slabs with a design distributed load up to 10kPa, and a design edge line load up to 25kN/m. Clause 4.5 has a set of strict conditions that must be satisfied for use, including maximum beam spacing, minimum beam depths and minimum slab reinforcement. This tool initially carries out a Clause 4.5 suitability check, followed by stiffness and strength checks in accordance with Clause 4.5.2 and Clause 4.4(f).

This calculator can also be used to export an engineering plan drawing with a detailed beam reinforcement schedule.

How to design a slab on grade to AS 2870?

AS 2870 has an extensive range of deemed-to-comply designs for stiffened rafts, footing slabs, waffle rafts and stiffened slabs with deep edge beams, outlined in Section 3 of the standard. These tabulated values allow designers to specify compliant designs without carrying out structural stiffness/capacity calculations. However, AS 2870 also provides guidance for designs which are outside the scope of the deemed-to-comply values for stiffened raft slabs, allowing designers to customise their designs to suit desired site geometry and beam dimensions. The method for modifying deemed-to-comply raft designs is outlined in AS 2870 Clause 4.5.

The Simplified Method for Raft Designs comprises the following:

  1. Suitability Check: Clause 4.5 has a series of criteria that a slab must meet to be suitable for the Simplified Method. These criteria include maximum beam spacing, minimum beam depths and minimum slab reinforcement.
  2. Soil & Settlement Parameters: The soil classification and maximum differential deflection must be determined based on the expected surface movement (ys) and method of wall construction.
  3. Stiffness Check: The stiffness of any thickened beams in the raft slab must be checked. The required stiffness is obtained from Figure 4.1, based on the soil classification and expected surface movement.
  4. Strength & Ductility Check: Clause 4.4 requires all beam sections to have sufficient reinforcement so that the ultimate strength (Mu) is 20% greater than the cracking moment calculated using AS 3600 Clause 8.1.6.1(1). The beams must also have sufficient reinforcement to satisfy the minimum reinforcement requirements of AS 3600 Clause 8.1.6.1(2).

Slab on Grade Calculator FAQs

What inputs are required?

The AS 2870 Residential Slab Design Calculator has the following inputs:

  • Slab / Beam Geometry: Length, width and depth of all structural elements within the slab.
  • Reinforcement Details: Slab and beam reinforcement. Inputs can be mesh reinforcement or deformed rebar.
  • Geotechnical Parameters: Moisture change properties of the subgrade material and expected surface movement, obtained from geotechnical investigation.
  • Loading Inputs: Design distributed and edge line loading around slab perimeter.
  • Material Properties: Concrete strength and reinforcement yield stress.

What results are provided?

The AS 2870 Residential Slab Design Calculator provides the following outputs:

  • Soil classification and maximum differential deflection.
  • Clause 4.5 suitability checks.
  • Stiffness check of edge / internal beams.
  • Ductility and strength check of edge / internal beams.

What assumptions / limitations does the calculator have?

The AS 2870 Residential Slab Design Calculator has the following assumptions and limitations:

  • This calculator is only intended for rectangular slabs (in plan). For non-rectangular plans, design for strength/ductility should be based on the overlapping rectangles method. Refer AS 2870 and associated commentary for further guidance.

  • Calculations for minimum reinforcement requirements are based on AS 3600 Clause 8.1.6.1. This tool includes the contribution of the slab as a flange for minimum reinforcement requirements but does not include contribution of the flange for section moment capacity.
  • This tool ignores the contribution of compression reinforcement for section moment capacity.
  • This tool uses AS 3600 for calculation of concrete flexural tensile strength, rather than the prescribed values in AS 2870.
  • The curve equations for Figure 4.1 graphs were developed using regression analysis of known values, as no official equations are available.

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