This paper describes the design of a slab-on-grade foundation for a light frame building on Houston Black Clay. The design follows the Post-Tensioned Institute method to a solution, and then converts that solution to a conventional reinforced design. Conventional reinforcing bar in substitution for tensioned cables is a viable design and implementation alternative when field conditions prevent effective post-tensioning.

The design and fabrication of the foundation described in this paper is designed only for use at the author’s location and its peculiar conditions. The author of this report is not responsible for any other use or reliance on this paper, errors, omissions, or damages arising out of the use of this information.

The author desired to build a light frame building capable of housing two vehicles. The desired location was in an agricultural field in north Texas without hard surface road or electricity. The soil at the site was identified as Houston Black Clay. Time of construction was to be in the winter.

Soil condition observations made in the year previous to construction confirmed that the soil was highly expansive. During dry periods the soil would crack with deep crevices. In wet weather the soil would cling and adhere, exhibiting high clay content. The author observed a neighboring light residential structure which had concrete foundation piles to a depth of twenty feet. The piles had been lifted, indicating that the active zone could be quite deep.

Construction in the winter carried a risk of delays due to rain. Work had to be halted for a period of a week to ten days each time it rained. This was due to the inability to access the site and to work in the sticky mud.

Vehicle housing made concrete flooring attractive. It was customary in the area to use post-tensioned slabs. However, the author felt that adding a vendor (for tensioning) and the necessity of tensioning the slabs at a specific time added risk to the project.

A 24-foot by 24-foot building would be adequate for the intended use. With a small, light building a mat or ribbed foundation would work provided that it would resist shear and moments due to soil expansion and contraction. One of the main considerations was uneven moisture content in the soil, where exposed soil outside of the building and soil under the foundation do not maintain even moisture. This leads to either edge lifting or center lifting of the concrete slab. The local conditions of climate at the site tended to conditions of low rainfall for extended periods during high heat leading to the edge falling (center lifting).

The introduction in the last century of poured concrete slab-on-grade foundations in Texas and Louisiana made apparent the difficulties of expansive soils. Foundation cracking and failure mushroomed as new construction dropped pier and beam foundations in favor of less expensive concrete slab-on-grade. In the mid-1970’s the Post-Tensioning Institute (PTI) was organized and initiated research efforts at Texas A&M University towards design procedures. Rational design procedures and parametric analysis resulted in the standard design methodology known as the PTI method.

The PTI method has the advantage of an accepted engineering practice and a savings in concrete and steel reinforcement over a conventionally reinforced concrete slab. However, it does require proper placement of the reinforcing cables and a machine to tension the cables after placing the concrete.

The author followed the PTI method using Design and Construction of Post-Tensioned Slabs-on-Ground, Post-Tensioning Institute, Second Edition, 1996, as the initial design method. Based on that design solution, the author then converted to a conventional reinforced design, adding an analysis of a cracked section which the post-tensioned design does not consider.

Soil type was identified by direct observation, aided by a US Department of Agriculture county soil survey. Hand-dug pits showed that the soil was consistent through the depths of interest. Soil attributes were based on the identified Houston Black Clay.

Moisture was based on the geographic location and Thornwaite moisture index charts in the PTI publication. The site had no trees, springs, or other anomalies.

Slab thickness was set at six inches and beam width at 12 inches. The slab was designed as a monolithic pour with outside stem-wall; if the stem-wall had not been chosen, a slab depth of 5.5 inches to accommodate a nominal 2x6 form board would have been chosen. Beam width was set with regard to conserving concrete, practicality of shovel width, and room for reinforcement.

Four beams spaced at eight feet, running in both directions, were set as the initial design point. Initial response from the model led to setting beam depth at 16 inches depth for the iterative process.

Allowable deflection was set at L/480, due to the light wood-frame construction. Had brick or masonry been an architectural element, L/960 or better would have been considered.

Design procedure continued for shear and moments in both directions for both center lift and edge lift conditions. The procedure converged to a solution with post-tensioned cables and beam depths of 16 inches.

At this point the PTI slab was converted to a conventionally reinforced slab. The calculated deflections and moments of inertia were used to set a starting point for analysis of a conventional cracked section. After iteration, the new beam depths were calculated to be 24 inches deep, containing four #6 reinforcing bars. #6 was the largest bar readily available from vendors.

While the calculated deflection was greater with conventional reinforcing bar than with post-tensioned cables at the beam depths, calculated deflection was still within allowable limits using either solution.

A spreadsheet of calculation results is presented in the Appendix. Terminology is defined in the PTI reference. Detailed formulas are not shown; they are also the PTI standard equations and method. Section A soil numbers are taken from the PTI reference. Sections B through K follow the PTI equations and recommendations. Section L converts the PTI solution to a conventional reinforced concrete design; a traditional transformed cracked section and moment of inertia lead to deflection calculations. Section L follows methodology in Ringo and Anderson Designing Floor Slabs on Grade.

One bright spot about the clay soil was that its cohesiveness worked to advantage when excavating the foundation beams. Actual concrete placement quantity was almost exactly the calculated quantity.

Top slab reinforcement was #3 reinforcing bar, 12 inches on center in both directions. The bar was supported on plastic chairs. The author prefers this over wire mesh, as the reinforcing bar may be stepped on without problems while the concrete is placed. Stepping on mesh deforms it down into the beams and to the bottom of the slab which negates reinforcement and shrink control.

Placement was made on a cool day, but with high wind. While the author would have preferred not to place on a windy day, it was the only opening between rainy wet spells and an impending freeze. A double layer of plastic sheeting protected the curing concrete from freezing conditions which came a few days after placing.

In this case, the choice of conventional reinforcement turned out to be the right choice for this project. After concrete placement, wet weather prevented access to the site. Cable stressing could not have been accomplished between three and ten days after concrete placing, as is recommended by PTI.

The author would use this method again when constrained by field conditions. Although material costs for concrete and reinforcement were higher, and the beam forms were dug deeper, costs were saved on the lack of cable tensioning and schedule slip, and no trucks were stuck in the mud.

James A. Lacy is a registered professional engineer in Texas. His publications include Systems Engineering Management: Achieving Total Quality, McGraw-Hill, 1992.

Brown, Robert Wade, ed. Practical Foundation Engineering Handbook. McGraw-Hill. 1996.

Parker, Harry. Simplified Design of Reinforced Concrete. Wiley. Second Edition. 1960.

Post-Tensioning Institute. Design and Construction of Post-Tensioned Slabs-on-Ground. Post-Tensioning Institute. Second Edition. 1996.

Ringo, Boyd C., and Anderson, Robert B. Designing Floor Slabs on Grade. The Aberdeen Group. Second Edition. 1996.