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PTI design equations for expansive soil moment and shear in grade beam

PTI design equations for expansive soil moment and shear in grade beam

PTI design equations for expansive soil moment and shear in grade beam

(OP)
Hello,

I am designing to footings for a two-story wood framed house.

The house has a rectangular footprint 20' x 60'.

The soil is expansive clay.

I am trying to calculate the maximum moment and shear in the grade beams during edge-lift and center-lift

I have a soils report that recommends I use the PTI DC10.1-08 design method for calculating the moment and shear to use conventionally reinforced grade beams, with a structural slab (with voidspace).  There are not going to be any deep piers, just the grade beams and structural slab.

I am trying to use the PTI equations 6-13, 6-18, 6-24, and 6-25 to calculate the max moment and shear in my grade beams.

Question:
What are the units and values of the variables in these equations?

The units: some I think I know, and have filled in.
 L = lenght (feet)
 S = ?
 h = ?
 P = perimeter load (lbs/foot)
 y_m = max differential soil movment (inches)
 e_m = edge moisture variation (feet)

Does anyone know the ones I have missing, or if I have the others correct?  Is this a good method to calculate the moment and shear in a gradebeam supported on expansive soil?

Thank you!
--Luke

These are the values from the soil report, for reference:
Allowable Net Bearing Capacity (dead): 1000 psf
Allowable Net Bearing Capacity (dead + live): 1500 psf
Slab subgrade coefficient: 0.75
Edge moisture variation, e_m, feet:
    Edge lift: 4.8
    Center lift: 8.3
Differential Swell, y_m, inches
    Edge lift: 1.0
    Center lift: 1.2
Effective Plasticity Index (PI): 27
Support Index: 0.91
Minimum unconfined compressive strength: 2480 psf
climatic rating: 26
 

--Luke
 

RE: PTI design equations for expansive soil moment and shear in grade beam

Appendix A.1 of PTI "Design of PT SOG" Version 3 shows

h = Total depth of stiffening rib, measured from top surface of slab to bottom of rib, in.

S = Interior stiffening rib spacing, ft.

All others look good to me.

 

RE: PTI design equations for expansive soil moment and shear in grade beam

(OP)
Thank you Ericca!

So it is correct that I don't need to convert any units?

My Total load is 1165 plf on the perimeter grade beams.

I am planning to do the grade beams 12"x24", so I use 24" for "h"

The spacing across of the ribs is 12'.

Using the PTI equation for center lift, I get 7.9 kip-feet
and 5.3 kip-feet for edge lift.

But using the BRAB method, I get 22 kip feet.  

Are my values for the PTI method correct?

I have attached the excel spread sheet I made using the formulas.

Does anyone have software to check my result?

--Luke
 

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