Theoretical
Approach A slab system can be supported by walls and columns, or it can be
treated as a slab-on-ground (waffle slab or raft). When slab-on-ground option is selected,
there is no need for any other support (wall or column). This option can be selected on
[Mesh and Material] input panel. Also, the soil stiffness in the vertical direction should
be entered. The soil stiffness is an equivalent to the modulus of elasticity for the soil.
The results are not sensitive on the value of the soil stiffness.
If there is some tension in the soil (negative deflection) the program
will give an error message. On the Soil Pressure result screen, the zones with tension in
the soil will be indicated. If this happens the model should be modified, so the slab
panels in these problem zones are not in contact with the ground.
The slabs can be partially supported by walls and columns, and partially
on ground. This is epically useful for structures on slopes.
Each slab panel can be selected to be in-contact or not-in-contact with
the soil. This option may be used for reactive clays when part of the raft is
in-contact with soil and the rest is not-in-contact with soil.
In addition to the elastic springs, defining the soil reaction n the Winkler mode, some piles
may be used. The piles can be placed at any location in the slabs. This option
can be used to control extensive settlement by introducing piles at the locations where
the larges deflections.
Application Notes This is some additional explanation about usage of SLABS
software to analyse raft slabs (slab-on-ground). SLABS software models the soil as a
series of independent elastic spring. There is one spring at each corner of all finite
element triangle. The stiffness of the elastic spring is derived from the Modulus of
Elasticity of the soil. This parameter is entered in the Mesh and Material input panel.
The stiffness of the spring is calculated assuming uniformly stressed 1m depth block of
soil deposit. In order to consider the actual depth of the soil deposit, the actual
Modulus of Elasticity should be modified.
The calculation of corrected Modulus of Elasticity of the soul is based on equalling of
the settlement (d) of the soil deposit, assuming the actual stress distribution in the
soil (figure left), and the deflection of the uniformly stressed 1x1m block of soil with
some equivalent finite depth De (figure right). Both models should provide identical
settlements (d). The procedure for calculation of the Ec (corrected Modulus of Elasticity)
follows below: 1. Estimate the average soil pressure (q) for the raft slabs you are
analysing. 2. Select an appropriate Modulus of Elasticity (E) for the soil type:
3. Calculate the soil settlement (d), considering the actual
stress distribution in the soil and the depth of the deposit, by using the actual E and q,
and some equivalent size rectangular shape load. 4. The corrected soil Modulus of
Elasticity, which should be used in SLABS is calculated as:
Ec = q / d
q - kN/(m*m)
d - m
Ec - kN/(m*m)
Note that the right-hand-side of the above expression is
multiplied by 1m to balance the units. Generally, the corrected Modulus of Elasticity (Ec)
will be smaller then the actual Modulus of Elasticity (E). The calculation of the
equivalent Modulus of Elasticity is performed automatically by the software, base on the
mechanical properties of the layers entered by the user.
Note, that there are many other factors which will contribute to the slab-on-ground
analysis. This is only a simplified linear elastic approach, which should be used very
carefully in combination with some other alternative method to confirm the results. Also,
it is recommended to vary the Modulus of Elasticity within +/- 30%, or even 50%, in order
to examine its influence on the results.
Application Example SLABS model: 42m long, slab-on-ground, loaded with eight cylindrical
fuel tanks:
Detail of the loading pattern:
Deflection Contours:
Bending Moment Contours:
Bending Moment Distribution:
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South, NSW 1235, Australia
tel: (02) 9267 0114, international tel: +612 9267 0114
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