Post-tensioned

(b) Preliminary design, The three designs for re-

inforced and stiffened thin mats presented in table

related with the Thornthwaite Moisture Index (TMI) in

6-1 differ in the beam depth and spacing depending

figure 6-7. The TMI, a climate related parameter

roughly estimated from figure 6-8, represents the

ings for each of the light, medium, and heavy slabs are

overall availability of water in the soil. The TMI can

vary 10 to 20 or more (dimensionless) units from year

servative in view of still undetermined fully acceptable

design criteria and relatively high repair cost of rein-

forced and stiffened slabs. Stirrups may be added, par-

exceed the range given in figure 6-7, depending on the

ticularly in the perimeter beams, to account for con-

activity of the soil or extreme changes in climatic con-

centrated and exterior wall loads.

ditions (e.g., long droughts and heavy rainfall), the

(2) Post-tensioned. Figure 6-6 illustrates an ex-

ample of a posttensioned slab. Properly designed and

constructed posttensioned mats are more resistant to

fracture than an equivalent section of a conventional

(b) The loading distribution depends on the

architectural arrangement of the building and often

rebar slab and use less steel. However, post-tensioned

cannot be significantly altered. Perimeter and concen-

slabs should still be designed with adequate stiffening

beams to resist flexure or distortion from differential

trated loads should be supported directly on the stiff-

heave of the foundation soil, Experienced personnel

ening beams.

are necessary to properly implement the posttension-

(c) The length and width of the slab are usually

ing.

fixed by the functional requirement. Beam spacing de-

(3) Assumptions of design parameters. Design

pends on the slab geometry and varies between 10 and

20 feet. The depth of stiffening beams is controlled by

parameters include effects of climate, center and edge

modes of differential swelling, perimeter and uniform

the moment and shear capacity. The beam depth is ad-

I

loads, and structural dimensions.

justed as needed to remain within the allowable limits.

The width of the stiffening beam is usually controlled

(a) The effects of climate and differential swell-

--

by the excavation equipment and soil bearing capacity.

ing are accounted for by predictions of the maximum

(4) Structural design procedure, The design proce-

differential heave AH and the maximum edge lift-off

6-7