Pole Footing Embedded in Soil |
  
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Pole Footing Embedded in Soil |
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This module determines actual soil pressures and required depths for pole footings primarily supporting lateral loads.
Such footings are commonly called "flagpole footings".
Since applied top moment generates lateral soil pressures that usually govern the design, these footings typically have a depth/width ratio of 2:1 and greater.
Cases with and without lateral restraint at the ground surface are allowed. Evaluation of actual and allowable pressures is in accordance with the IBC Section entitled "Embedded posts and poles".
General
Pole Footing Shape
Use this section to specify whether the pole is round or rectangular (assumed square).
Footing Width/Diameter
Enter the width or diameter of the pole footing. Width is measured perpendicular to force direction. If the pole is specified as square, the module will multiply the value entered for footing width 1.41 to determine an equivalent width dimension for calculations.
Restraint at Ground Surface
Specify whether the footing is free at the ground surface or restrained and cannot translate. A restrained footing indicates that a concrete slab or other rigid element prevents translation of the pole footing at the ground surface, but does not prevent rotation. When specifying a restrained footing, you must assure yourself that the final force required to provide the restraint can actually be developed by the restraining construction.
When ground surface restraint is present, the lateral pressure value at the bottom of the pole will govern the design.
Operation Mode
This setting provides an option to select from two different modes of operation as follows:
Calculate Minimum Depth: In this mode, the module will iterate to determine the minimum embedment depth required to make the actual lateral soil pressure lower than the allowable soil pressure.
Find Lateral Pressure for Given Depth: In this mode, the module will calculate the lateral earth pressures caused by the specified pole size, embedment depth and applied loads. When this option is selected, a Pole Footing Embedment Depth input field will appear as shown below:
Pole Footing Embedment Depth
The actual embedment depth of the pole footing. (Visible for "Find Lateral Pressure for Given Depth" mode only).
Allowable Lateral Passive Pressure
The allowable lateral passive pressure that the soil can withstand. This value is entered as pounds per square foot per foot of embedment depth.
Maximum Lateral Pressure Limit
This value is used to specify an upper limit on the Allowable Lateral Passive Pressure, so that it does not increase in an uncontrolled manner as the embedment depth increases. This value is entered as pounds per square foot.
Solution Procedure
Assume:
•Allowable lateral passive pressure = 200 psf/ft
•Maximum lateral pressure limit = 3000 psf
•No restraint at ground surface.
The solution progresses as follows:
1.The program begins with a shallow assumed embedment depth.
2.It calculates one-third of the embedment depth.
3.It compares that value to 12 feet and uses the smaller of the two.
4.It multiplies the governing depth by the allowable lateral passive pressure (psf/ft).
5.It compares that result to the Maximum Lateral Pressure Limit and uses the smaller value.
6.The IBC formula is used to determine the actual passive pressure for the assumed embedment depth.
7.If the actual pressure exceeds the allowable pressure, the embedment depth is increased and the process repeats.
8.When the actual pressure is less than or equal to the allowable pressure, the program rounds the embedment depth up slightly and reports that value.
Example: If the embedment depth reaches 42 feet during iteration:
1.One-third embedment depth = 42 / 3 = 14 ft
2.Compare to 12 ft limit → use 12 ft
3.Allowable lateral passive pressure = 200 psf/ft * 12 ft = 2400 psf
4.Compare to maximum limit → 2400 psf < 3000 psf → use 2400 psf
5.The program checks whether the actual pressure at 42 feet is less than or equal to 2400 psf. If not, it increases the embedment and repeats.
Applied Loads
This module allows many types of loads to be applied to a pole footing embedded in soil.
Lateral Concentrated Loads
Module allows one concentrated load with various load types to be applied at a specified distance above the surface of the soil.
Lateral Distributed Loads
You can apply a uniform lateral load to the pole by specifying the magnitude of the load and the starting and ending locations.
Applied Moments (Only displayed when restraint at ground surface exists)
You can apply a concentrated moment. No "height" entry is required, because it is purely a rotational force.
Vertical Load
You can also apply a vertical load so that the module can calculate the vertical bearing load on the footing for each load combination.
Load Combinations
Use this tab to specify the load combinations you want the module to analyze.
Calculations
The calculations tab provides a summary of the results.
The table reports the resulting forces, moments and required depth for each load combination.
The Controlling Values area provides information for the most severe load combination.
2D Sketch
3D Rendering
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