Background

PTI analysis is performed using the methods presented in Design and Construction of Post-Tensioned Slabs-on-Ground published by the Post-Tensioning Institute and commonly referred to as the PTI Manual. This method in turn in based on the research published in Development of a Design Procedure for Residential and Light Commercial Slabs-on-Ground Constructed Over Expansive Soils written by W.K. Wray in 1978. This design method is based on a regression analysis of multiple foundation designs analyzed using a finite element analysis program.

We recommend that the user obtain and read the full text of the PTI manual as it contains additional limitations and design requirements not discussed here. The manual be obtained from the Post-Tensioning Institute at http://www.post-tensioning.org. To truly understand the limitations of the design method it is necessary to review Dr. Wray’s Dissertation. A copy of Dr. Wray’s dissertation can be obtained from ProQuest Dissertations & Theses at http://www.umi.com. The dissertation order number is 7909251.

Design Methodology

Similar to the other three design methods implemented in SlabWorks™, the PTI method is at it simplest level a method of predicting the maximum moment, shear and deflection occurring in a slab-on-grade. Once these values are established, the actual design of the beams is conventional and will not be unfamiliar to anyone who has performed post-tensioned concrete design.

The most important design values used in the PTI method are ym and em, which represent the expected differential movement and the expected distance over which the movement will occur. Design is very sensitive to these inputs.

The PTI manual present technique for designing slabs that are reinforced against temperature and shrinkage cracking (BRAB type II slabs) and slabs that are structurally reinforced and stiffened (BRAB type III slab). SlabWorks™ currently implements only the Type III design but considers both expansive and compressible soils, using post-tension or bonded reinforcing.

Changes from 2nd Edition to 3rd Edition

Several significant changes were made between from the 2nd to 3rd editions of the PTI Manual. These include:

1. Bearing Pressure – The 3rd edition allows bearing pressure to be calculated using a bearing width equal to the beam width plus 16 times the slab thickness for interior beams and 6 times the slab thickness of perimeter beams. This is the formula used by ACI-318 for flange contribution to bending strength.
2. Deflections – The 3rd edition calculates a minimum moment of inertia (stiffness) rather than calculating the actual deflection. In some cases there is a large difference between the two criteria.
3. Post-Cracking Behavior – The 3rd edition requires a minimum post-cracking strength of the section. Allowable post-cracking strength is can be provided by PT reinforcement or bonded reinforcement, and design moments are only 90% of the service moment (No load factors required). The examples in the manual use the full slab width rather than effective flange with when checking the positive moment capacity. SlabWorks™ used the same design method.