Underpinning a Post-Tension Cable Reinforced Slab-on-Ground Foundation

Many post-tension cable reinforced concrete slab-on-ground foundations cannot be repaired with conventional underpinning practice, and the only provision for underpinning such a slab is impractical. This is because if these slabs were designed according to the older requirements of the Post-Tensioning Institute (PTI), then they have only unbonded reinforcement. Lifting the foundation off the soil and supporting it with piers, however, makes the foundation a flexural structural member, for which bonded reinforcement is required in addition to the unbonded prestressing tendons.

Article 1918.9 of the 1994 Uniform Building Code (UBC) requires that “A minimum area of bonded reinforcement shall be provided in all flexural members with unbonded prestressing tendons . . . ,” and the Code then specifies how to calculate the required reinforcement. Similar requirements are in previous editions of the UBC, in the American Concrete Institute Standard 318-83 (Article 18.9, Minimum Bonded Reinforcement), and in the Standard Building Code.

Instead, ACI 318.1-83, Building Code Requirements for Structural Plain Concrete, would apply if such a PTI slab were lifted off the ground. This is because the slab would have less than the minimum reinforcement specified in ACI 318. ACI 318.1 requires that the extreme fiber stress in tension be limited to  For plain concrete, the strength reduction factor . Assuming 3000-psi concrete, then the allowable extreme fiber stress in tension is limited to 178 psi.

To see how this makes Building Code-compliant underpinning impractical, consider the typical 4-inch thick residential slab. A 4-inch thick slab would have a self weight of 50-psf, and a dead load factor of 1.4. Assuming the Code-specified 40-psf uniform live load, and a live load factor of 1.7, then a simply supported one-way slab could span up to 63 inches. (Shear would not dominate.) Supporting a slab with pier caps spaced no farther apart than five or six feet is impractical, and partition loads would decrease these spans. Similar calculations would be needed for both the exterior and interior beams.

No allowance at all can be made for the prestressing forces imposed by the unbonded cables. ACI-318.1, article 6.3.3, is explicit about this, and states “No strength shall be assigned to metal reinforcement that may be present.” The prestressing forces thus cannot be used to reduce concrete tensile forces: slab spans must be calculated for unreinforced plain concrete, whatever the post-tension cable contribution. A simply-supported slab thus was assumed in our example, because without reinforcement, negative moment cannot be transferred at the slab boundaries.

For similar reasons, underpinning only the perimeter of a typical PTI slab-on-ground building could result in a violation of the Building Code. This is because the interior slab and beam portions would span too far without support if the interior soil shrinks or subsides.