Example of SLOG

How to use SLOG® to design a house footing

SLOG® can carry out 3 different types of footing design. Please select a below option to see a different aspect of Program SLOG®:
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Raft Footing Option

To illustrate the efficient way in which SLOG® carries out a footing design, take the example of an articulated masonry veneer single storey house on a highly reactive class H-D soil profile with the following parameters:

  • Ys = 70 mm;
  • Ym = 0.7Ys = 49 mm (AS2870 -2011 clause F4a)
  • Hs = 4 m.
  • For articulated masonry veneer, by AS 2870 -2011 Table 4.1 The  maximum design differential movement Δ = L/400, with a maximum of Δ = 30 mm
  • The above-ground part of the footing is to be 200 mm.
  • The mound stiffness by AS 2870-2011 clause F4(c) is k = 1000 kPa/m
  • The house is 8 m x 16 m in area, so that for a beam spacing of 4 m
  • Number of beams in the long direction = 3
  • Number of beams in the short direction = 5

The loads acting on the footing are taken as perimeter line loads 6 kN/m, no central internal line loads, and an internal uniform distributed load of 1.5 kPa. Note that these loads are the wall, roof and factored live loads. SLOG® automatically determines the self weight of the footing. These loads are labeled with the rectangular house orientated with the short length directed north-south.

The input data screen therefore looks like this:

The next step is to choose the type of footing (stiffened strip footing system, raft footing or waffle raft) to be designed. The input shown before is appropriate for a strip or raft footing.

If a waffle raft is to be used, the sub-beam spacing is smaller so that the number of beams in the long and short directions is greater, and the program ignores the input for the depth of footing not embedded.

In this example, a raft footing is selected. Two choices are then given to the designer.

In the first option, the design can be carried out to determine the footing depth with the minimum amount of reinforcing steel to meet the stiffness requirement of AS 2870 – 2011 clause 4.4 (e), the  strength requirement of clause 4.4 (d) and the ductility requirement of clause  4.4 (f).

In the second option, the designer can input the amount of steel reinforcement, and SLOG determines the required footing depth, with the warning given in the case of the input amount of steel being insufficient to meet strength and ductility requirements.

The calculation of the footing depth for the minimum amount of steel is selected in this example.

The next step is to input the geometry and material properties of the raft footing sub-beams. 

In this example:

  • A sub-beam width of 300 mm
  • Top and bottom concrete cover of 30 mm and 50 mm respectively
  • Slab thickness of 100 mm
  • Slab steel quantity 179 mm2/m
  • Concrete f’c = 20 MPa
  • Steel grade fsy =  500 MPa
  • The concrete tensile strength in hogging is 0.4√f’c and in sagging 0.6√f’c
  • The concrete Young’s Modulus for 20 MPa concrete is E = 15000 MPa
  • The Mu/Mcr ratios are specified, all these in accordance with AS 2870 – 2011 clauses 4.4 (e) and (f).

The input is as follows:

The analysis can now be carried out.

Note in the Input Data box above that the option is given to “Show Interactive Graph”. SLOG® uses the Mitchell (1988) method for the soil structure interaction of a loaded footing on a mound of expansive soil in centre heave and edge heave. This interaction process is shown in an animation when the “Show Interactive Graph” tab is ticked.

The following graph is a copy of the animation at one instance during the process of the calculation. This is during the calculation for short span (8 m) centre heave when the trial stiffness equivalent to a sub-beam depth of 480 mm is trialed. The soil mound shape for Ym = 49 mm is shown, together with the deflected shape of the footing giving Δ = 27.9 mm. As this is greater than the permissible Δ = 20 mm (Δ/L = 400), the footing must be deepened to give a higher footing stiffness.

Note: By not ticking the tab “Show Interactive Graph” hastens the subsequent analysis, as the analysis does not have to be paused for 1 sec to allow the user to view each interactive graph.

The footing stiffness is adjusted until the differential movement of the footing Δ is very close to the design differential movement (Δ=L/400 or maximum 30 mm for this house type), thus giving the required stiffness. For the deflected shape of the footing in both centre and edge heave, the soil reaction forces are calculated, thus enabling the bending moment and shear to be determined.

The footing design is then carried out by determining the appropriate sub-beam depth and quantity of reinforcing steel from the determined stiffness and bending moment.

Pressing “Calculate” gives the input summary and output summary shown on the following two pages.

It can be seen from the “Output – Raft Footing” summary on page 6, that the required sub-beam depth is 500 mm (note this includes the 100 mm thick floor slab) with sub-beam reinforcement being at least 269 mm2 top bars (say 3-N12) and 244 mm2 bottom bars (say 3-N12).

SLOG® also allows the option of the designer specifying the quantity and grade of the sub-beam reinforcement, rather than determining the minimum steel as above. This is advantageous if the designer wishes to obtain a shallower footing but with more steel, and in cases of the grade of the slab steel being different to the grade of sub-beam steel.

To illustrate this, in the previous example

  • Take the top and bottom bars being 400 mm2
  • (Say 2-N16 bars) of grade 300 MPa steel, with the other parameters unchanged.

The Input Data box is therefore as follows:

It can be seen from the “Output – Raft Footing” summary, that the required sub-beam depth for the lower grade but higher quantity steel is still 500 mm. However note that SLOG® warns the user that the Mu/M* ratio in centre heave is less than the minimum of 1.25 required by AS 2870-2011 clause 4.4(d) so that the designer should increase the amount of the top sub-beam steel, or use a heavier slab mesh.

Strip Footing option

If the designer requires a stiffened strip footing for the house and soil type of the previous example, the following shows the sequence of input boxes resulting in the two-page input and output summary.

For a more in depth look into how the input boxes were filled see the Raft Footing option.

It can be seen from the “Output – Strip Footing” summary , that the required footing depth is 680 mm (note this includes the200 mm above ground section) with beam reinforcement being at least 356 mm2 top bars (say 2-N16) and 247 mm2 bottom bars (say 2-N16).

Waffle Raft option

If the designer requires a waffle footing for the house and soil type of the previous examples, the following shows the sequence of input boxes resulting in the two-page input and output summary shown.

For a more in depth look into how the input boxes were filled see the Raft Footing option.

It can be seen from the “Output – Waffle Raft” summary sheet on page 17, that the required waffle raft depth is 385 mm (requiring a waffle box height of 300 mm to give a 85 mm floor slab), with 227 mm2/m slab steel (say SL82), no sub-beam top bars required, and 83 mm2 sub-beam bottom bars (say 1-N12).

Additional Features of SLOG®

Several additional features of SLOG® can be illustrated from a screen dump of the output from the previous waffle raft design as follows:

Note the following features:

  • The “Go Back to Input Screen” button allows the user to go back to conveniently and rapidly modify any input data.
  • The “Save Input Data” button allows the user to save the data for each job to re-loaded later.
  • The “Print” button allowing the various print options.
  • The tab at the bottom of the screen titled “Graph Waffle – LSCH”. This gives for the long span centre heave case, the graphs of (1) the soil heave and footing deflection along the footing; (2) the soil reaction pressure along the footing; (3) the bending moment distribution; and (4) the distribution of shear force. These are shown on pages here and here.
  • The tab “Graph Waffle – LSEH” refers to the equivalent graphs for the long span edge heave case. These are shown on pages here and here.
  • The tab “Graph Waffle – SSCH” refers to the equivalent graphs for the short span centre heave case. These are shown on pages here and here.
  • The tab “Graph Waffle – SSEH” refers to the equivalent graphs for the short span edge heave case. These are shown on pages here and here.

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