Mike Dolt Enterprises
Mike Dolt Enterprises
User: Grounded1, Location: Las Vegas
Asked on: Wednesday, April 6, 2011 at 2:26 pm EDT
Answered on: Tuesday, May 14, 2024 at 3:51 am EDT
Question: I am looking for an equation that can help me predict the resistance of a horizontal slab of concrete, on finished grade, with the rebar attached to some number of vertical ground rods in the earth. I have equations for ground grids, vertical rods, etc, but would like to calculate what kind of drop in resistance I would gain by using ufer grounds in a slab of concrete. Any help would be appreciated. I also have found one equation for a vertical concrete encased rod, but this is not the confuration I am looking for. Thanks in advance.
Equation for Resistance of Horizontal Slab of Concrete with Ufer GroundsAssumptions
Concrete slab is uniformly thick. Reinforcing steel is evenly distributed throughout the slab. Ground rods are evenly spaced and vertically aligned.
Equation
where: R = ρ L / (A n S) + R_g
R is the resistance of the slab (ohms) ρ is the resistivity of the concrete (ohm-meters) L is the length of the slab (meters) A is the cross-sectional area of the reinforcing steel (square meters) n is the number of ground rods S is the spacing between the ground rods (meters) R_g is the resistance of the ground rods (ohms)Calculating R_g
The resistance of the ground rods can be estimated using the following equation:
L_g) / (π r^2)where: R_g = (ρ_g
ρ_g is the resistivity of the soil (ohm-meters) L_g is the length of the ground rods (meters) r is the radius of the ground rods (meters)Using the Equation
To calculate the resistance of the slab, simply input the appropriate values into the equation. Note that the resistivity of concrete and soil can vary widely depending on the specific materials and conditions. It is recommended to obtain these values from reliable sources or through testing.
Example:
Consider a concrete slab with the following parameters: Length: 10 meters Thickness: 0.15 meters Resistivity of concrete: 150 ohm-meters Reinforcing steel: #4 bars spaced 0.3 meters apart Ground rods: 8 rods spaced 2 meters apart Resistivity of soil: 100 ohm-meters Length of ground rods: 3 meters Radius of ground rods: 0.0127 metersUsing the equations above: Cross-sectional area of reinforcing steel: 0.004 square meters Number of ground rods: 8 Spacing between ground rods: 2 meters Resistance of ground rods: 0.38 ohmsSubstituting these values into the main equation:
R = (150 10) / (0.004 8 * 2) + 0.38 = 11.75 ohms