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
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
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
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