Effect of deep piers on house slabs on reactive sites

[flyingdebris]

Need help here guys.....If I have a large concentrated uplift force, say 10t, in a house built on a reactive clay site, where one would need to allow for this amount of tiedown with a bored pier say, how will this pier influence the slabs ability to perform as a free floating ductile element? Wouldn't the deep pier effectively "lock" down the slab in the immediate area, therefore restricting the slabs free movement and hence creating unwanted stresses in the house elsewhere if say the surrounding ground swelled, causing cracking??? But if we leave the pier out, then the slab doesn't have close to the required tie down capacity needed to hold down 10t. What are my options???

 

[FixedEarth]

For something like this, you need to get the input of a local geotechnical engineer. When we analyze the uplift capacity of a pier, we need values for lateral swelling pressure, depth of active zone, skin friction below the active zone and to see if 0.15 factor is a reasonable multiplier for getting the adhesion. All of the above values can best be determined by your local soils firm. We often place a void box on top of the pier and below the grade beam to isolate any movements.

 

[flyingdebris]

thank you for your response fixed. I have no dramas with designing the pier for any sort of uplift, so thats no issue. soil conditions are known. By adding grade beams, do you propose I introduce a number of piers for the grade beams to span to and be supported by piers. Unless you do this to the entire job, and treat as a full suspended system, I cannot see the benfefit of doing this adjacent to and only under the 10t uplift, as that immediate area is still going to be locked from free movement while the rest of the house moves. sorry, but im still in the dark somewhat!!!!

 

[flyingdebris]

To make it clear, the uplift force I mention above is generated from cyclonic winds and not soil swell pressure.

 

[hokie66]

On reactive soils, you have two choices: 1. Let the whole structure move up and down with the volume change, or 2. Isolate the structure from the soil which moves by supporting it on a deep footing system and providing a void between the structure and soil.

 

[FixedEarth]

Correct, you do this pier design on the entire job. Piers are supporting the grade beams which is supporting the slab. Only the pier is touching the soil. If you are in expansive soils, you always get uplift forces from the upper 5 ft (in CA) or to the depth of active zone in your locale. The upper soils grabs the pier circumferentially. This grabing force is approxmately equal to 0.15 times the lateral swelling pressure (F.H. Chen, 1988). You need a local geotechnical engineer to test the site and come up with the vertical swelling pressure value. I usually multiply this vertical swelling pressure by 0.8 (ko value) to come up with the lateral swelling pressure. Then I do the uplift clalcs per Chen 1988 and see if my F.S. against uplift is 1.5 or grater. This uplift force is in addition to the "structural" uplift loads. I have attached a typical calculation.

Another alternative is to use PT slab and make an allowance for your 20 kip uplift load by using the tiedown you mentioned. You would still need to isolate any footing or grade beam that sits on this tiedown by using a void box. Then suspend your slab.

I have a patent pending system which eliminates piers and PT slabs for lightly loaded structures on expansive soils. It is applicable for swelling pressure up to 25 ksf. You would need to proportion the isolated footing to take the uplift load with our system.( for example a 10 x 10 x 1.5 footing) See here:

http://www.foundeng.com/patents.html

 

[flyingdebris]

Thanks for the adivce guys. Thank you for your effort in posting the calc Fixed. Very kind. Just to throw something else out there. Would it be feasible to have a deep pier in the ground for my wind uplift force, with a cast in steel post running up through the slab to support the roof, and constructing the house footing/slab in a way to allow the concrete to slide up and down along the post as the soil swells and shrinks??? Hence, the pier is "independant" from the concrete slab structure. Comments????

 

[hokie66]

That would be quite simple, but I assume that the roof is connected not just to this post, but to other vertical elements. You would have to consider how the relative displacements affected those supports and roof members.

 

[TXStructural]

We have expansive clay soils here. Slabs are frequently floated while the building is founded on piers. Piers/columns are isolated and a slip joint crearted to allow movement. Be forewarned, one building was 897,000 sq ft, and the slab had risen approximately 6-9 inches over 40 years, so the interior partitions had to be cut and movement re-accomodated every few years. (And do not put the HVAC vents around the perimeter of the cafeteria floor... a poor design.)