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# PTI Design Method for Slabs on Ground

• To: Seaint(--nospam--at)seaint.org
• Subject: PTI Design Method for Slabs on Ground
• From: GSKWY(--nospam--at)aol.com
• Date: Mon, 18 Nov 2002 21:31:25 EST

Re: "The PTI design method is wildly unscientific".

It is always easier to criticize than to provide useful information;  however, there are a few things that can be noted:

The PTI method is based on two parameters,  em and ym. "em" is the edge moisture or edge penetration distance - the distance inward from the edge of the slab that the moisture varies. "ym" is the amount of differential soil movement - how much the soil swells or shrinks.

It has long been acknowledged that the  methods currently used to determine these parameters is probably not accurate.  Also,  these parameters depend on things like Atterberg limits which are not exactly high science and rely heavily on operator skill.  I spent a summer working in a testing lab - despite the fact I did a fair amount of soil testing, I would have to admit that my skill was (and still is) close to nonexistent.

The flow charts for determining these values use the word "estimate" quite often.   Revised equations are being proposed; however, it is my understanding that these revised parameters also use Atterberg limits,  plus they include a lot of other subjective decisions about the condition of the soil.

In addition, the PTI design method was developed in the late 70s when computer capabilities were orders of magnitude less than what is currently available.  In one of the appendices in the PTI Slab-on-Ground Design and Construction manual there is a note to the effect that the stresses determined from their analyses were approximately twice as large as they should be from statics so they divided everything by two.  That kind of makes me wonder what exactly they were calculating.

The PTI method makes a lot of  assumptions, such as the tendons in the beams are straight, not profiled and the soil has a modulus of elasticity of 1000 psi.  It also only allows for a load around the perimeter of the slab, it does not allow for post loads.

I don't think the PTI method of slab design is any worse than the other two equations, I am just not sure they are any better.  The PTI equations for  calculating various design parameters all have a lot of variables with exponents in them - I think this tends to give people an unreasonable impression of their precision.

It doesn't help that the PTI manual for Design of Post-tensioned Slabs leaves a lot to be desired as far as the quality of the writing.  Further adding to the confusion is that if people repeat "what they have heard" often enough, it gets taken as fact, whether or not there is any basis for it.

Gail S. Kelley,  P.E.