Provided by Lee Shores with C and L Development

The post tension slab foundations are looking great at The Villas at Park West.  EVstudio was hired by C and L Development to design the PT slabs located in Pueblo.

The picture above is a great example on the interior smooth finish and the exterior broom finish.  The smooth finish at the interior works well for finished floors such as tile and hard wood.  However, I don't recommend a hard wood floor of a slab-on-grade unless proper care was taken to prevent moisture percolating up into the concrete slab.  The broom finish gives the surface a rough texture that helps prevent people from slipping.  This building has an exterior corridor through the center of the apartment building.  You can also see the pump truck used to help distribute the concrete over the entire building.

Provided by Lee Shores with C and L Development

Here is the same foundation at the corridor prior to concrete placement.  In the background you can see one of the other buildings already framed.

PT Slab-on-grade, concrete slab not shown to see tendons

A Post Tension (PT) slab-on-grade (SOG) foundation is based on a typical raft SOG foundation with one major difference; high strength steel tendons pulled into tension.  Tendons are typically ½” diameter made of 7 steel wire strands that is then greased and encased in a plastic coating.  The grease and plastic coating helps to allow the tendon slide inside the concrete.  Once the concrete has sufficient strength the tendons are stressed using hydraulic jacks to a force of 33kips. The tendons compress the concrete adding strength to the foundation when done correctly.

To help explain how post tension works many engineers describe the system using a row of wood blocks with a hole drilled in the middle and a rope.  Alone the wood blocks fall apart without anything holding them together.  But when a rope is placed through the holes, pulled tight and knotted at each end the blocks are now held together using compression.  Compression describes a force pushing inwards on an object, while tension is the force pulling out on an object.

Typical live end of tendon with pocket former

Concrete is very strong in compression, but week in tension.  For example you can support a car with a couple concrete blocks, this is compression.  But if you were to hang a car from a couple concrete blocks they would rip apart, this is tension.  So to help make concrete stronger in tension you add steel.  Typically, rebar is added and concrete is placed around it and bonds to become a composite material.  One step further is adding steel that is in tension which then places the concrete into compression from equal and opposite forces.  This takes advantage of the very high compression strength of concrete and increases the failing point of concrete in tension.  Now the added internal force of roughly 50psi to 100psi must be exceeded before the concrete endures any tensile forces.

Typical tendon dead end

An advantage of adding these internal forces is greater strength than conventionally reinforced concrete.  This means less concrete and steel to generate the same results as conventional concrete design.  Another advantage of post tensioned concrete is control of cracks.  ALL concrete cracks, but the width of the cracks can be controlled.  Concrete shrinks as it cures and this puts internal tensile stress into the concrete.  Post tension adds compressive forces that help to overcome the internal tensile forces from shrinkage.  This means fewer cracks, add what cracks may occur are limited in width.

PROS
Less Concrete and Steel than typical reinforced SOG
Reduces cracks in the slab
A more rigid foundation and can resist movement from high swelling soils

CONS
A bit more technical than a reinforced SOG
Access to plumbing below slab is more difficult
Damaging tendons is expensive and dangerous

Learn more about different foundation types.

The Post Tension Institute manual states that an engineer may consider using the rib width PLUS 16 times the slab thickness at interior ribs and 6 times the slab thickness at exterior walls.  For example; a typical slab thickness is 5”, so the engineer may increase the bearing width at interior ribs by 80” PLUS the rib width and exterior ribs by 30” PLUS the rib width.  This makes a significant difference.  This should allow the engineer to stay with a typical 10” to 12” rib width and provide the best value engineering.  The only time I need to increase rib widths or provide thickened slabs is at large point loads.    And, due to the base plate attachment at these large point loads you need to thicken the slab anyways.

Section 5.4.2.3 Rib Width from PTI 3rd Edition

However, I have decided to reduce these allowable bearing areas at significant point loads and other locations I’m concerned about exceeding the soil bearing.  One area to consider is corners of a slab with a large point load.  Since the slab is not continuous in each direction of the rib, I have decided to reduce the allowable factor of 6 to 3 times the slab thickness.  Another concern about the allowable increases is the assumption that the soil capacity adjacent to the ribs meets the geotechnical engineer’s specified bearing capacity.  In the field you can clearly identify loose soil to either side of most ribs due to the fact most ribs are cut into the soil using heavy machinery.  When the ribs are cut, the soil is disturbed at the top edges of the trenches.  So be smart about what you can do versus what you should do when designing!

Another caution to structural engineers designing foundations is to make sure you understand what the loads are if provided by another structural engineer designing the frame.  I typically receive a ground floor plan with the line loads and point loads indicated to use for the foundation design.  First, the loads are generally total loads with no indication about the percentage of live load versus dead load.  Generally, not an issue since we are only concerned about the total bearing force on the soil.  Second, the loads are usually rounded up, sometimes rounded up by a lot.  I recommend adding up a few tributary areas to make sure the loads provided make sense.  Finally, you may want to consider getting something from the other structural engineer stating you can use appropriate live load reductions per IBC 1607.9.1 and 1607.9.2.

If you have any other questions about post tension design or need a structural engineer who knows it, please contact us.