—Applicable design procedures

DESIGN OF SLABS-ON-GROUND 360R-39
9.4.2 Crack-control design—This design is normally used
for slabs with light loading, usually with no rack post
loading. Post-tensioning is used instead of reinforcing steel
or close jointing to compensate for concrete shrinkage and
temperature effects. The minimum required post-tensioning
force is calculated to provide some residual compression
over tension resulting from subgrade drag.
9.4.3 Industrial floor design—This design is normally
used for slabs with higher loads, especially concentrated
loading. The design of a typical post-tensioned industrial
floor can be accomplished following these steps:
• Determine slab geometry, placement sizes, estimate
slab thickness (normally 5 to 10 in. [125 to 250 mm]);
• Calculate the subgrade drag and the friction losses in
the post-tensioning tendons;
• Estimate long-term losses to arrive at the final effective
prestress force. For floors subjected to large temperature
changes, the effects of temperature on the concrete
should also be considered in determining the final
effective post-tensioning force;
• Analyze the loading effects using the Westergaard
equations or similar analysis yielding the stresses under
the concentrated, uniform, lift truck or line loads.
Different formulas are available for loads in the middle
of a slab and at the edge;
• Verify that the actual total superimposed stresses and
deflections do not exceed the allowable values.
Depending on the results a modification of the slab thick-
ness and or slab placement, layout may be necessary.
9.4.4 Post-Tensioning Institute (PTI) method—In 2004,
the PTI published a third edition of a document (PTI 2004)
containing recommendations for establishing the strength
and serviceability requirements primarily for residential
post-tensioned concrete slabs on either stable, expansive, or
compressible soils. These strength and deflection require-
ments are based on the assumption of an uncracked section.
The PTI design procedure uses the unique advantages of
post-tensioning as the primary reinforcement for a ribbed
and stiffened slab. A stiffened slab is reinforced to provide
sufficient strength and deflection control in swelling and
compressible soil conditions. The uncracked section
modulus in a post-tensioned analysis enhances stiffness and
flexural stress control—two of the most important factors
associated with slab-on-ground design.
Sections 9.7
, 
9.8
, and 
9.9 
present PTI equations for the
determination of the moment, deflection, and shear require-
ments for slabs cast on expansive or compressible soils.
These equations were developed by a log-linear regression
analysis based on the results of 768 separate analyses that
represented full consideration of both center lift and edge lift
conditions using a finite element plate-on-elastic-half-space
foundation (Wray 1978). The results of each analysis were
screened for the maximum values of moment, shear, and
differential deflection in both the long and short direction.
These values were then used in the regression analysis that
developed the design equations (PTI 2004).
Some of the equations contain variables with an exponent
carried out to three digits, which result from the analysis
described previously. It should not imply an accuracy of the
results, considering that this method provides theoretical
results based on ideal conditions. Any site-specific circum-
stances like climate, trees, slopes with cut and fill, and
surface water drainage conditions should be taken into account
by the engineer, and can change the results considerably.
Moisture-sensitive soils should be stabilized by minimizing
exchanges in moisture content.

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