This program determines actual soil pressures and required depths for 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 1994 U.B.C. Section 1806.7.

 

 

Unique Features

 

Assumptions & Limitations

 

Example

The data entry for this example is shown in the screen captures that accompany the Data Entry Tabs and Results & Graphics Tabs sections to follow.

 

 

Data Entry Tabs

This set of tabs provides entries for all input in this calculation. While you are entering data and switching between these tabs you can view the desired resulting information on the tabs on the right-hand side of the screen (calculated values, sketches, diagrams, etc.). A recalculation is performed after any entry data is changed. After each data entry you can view the results on the right-hand set of tabs.

 

General Tab

This tab contains all the date entry items for this calculation.

 

 

 

Allowable Passive Pressure

The allowable lateral force the soil can withstand. This value will be increased per foot of vertical embedment depth. For example, at 4'-0" below the surface allowable lateral pressure entry of 150 psf/ft would be able to resist 150 * 4 = 600 psf.

 

 

Max Passive

Maximum allowable passive pressure regardless of depth, and load duration factor to be used to increase the allowable pressures. Per code

 

Load Duration Factor

The load duration factor will be applied to the allowable lateral passive pressures. This number will then be used as the allowable pressures used to determine footing embedment.

 

Pole Shape

Use this section to specify whether the pole is round or square. If the pole is specified as square, the dimension entered is multiplied by 1.41 to arrive at an equivalent width dimension for calculations.

 

Pole Diameter / Pole Width

Enter the width or diameter of the footing, and specify whether a round or rectangular footing is used. Width is measured perpendicular to force direction.

 

Pole Restraint

Specify whether the footing is free at the ground surface or restrained and cannot rotate. A Restrained footing indicates that a concrete slab or other rigid element only allows rotation of the top of the footing without translation. When specifying a restrained footing, you must assure yourself that the final force required to be restrained at the top can be provided.

 

When ground surface restraint is present, the lateral pressure value that will govern the design will be at the base of embedment.

 

The program will iterate (solving the indeterminate equations) until the minimum embedment depth is determined.

 

Applied Loads

Point and Distributed Loads. Two types of loads may be entered; concentrated load applied at some point above the ground, or a partial length uniform lateral load. The program will combine both loading's and use the resultant moment and shear force at the ground surface.

 

 

 

Results Tab

 

 

Moments @ Surface

This moment results from applying both point and concentrated loads to the pole (above soil) at their respective distances above the soil surface

 

Total Moment

 

Total Lateral Load

This force is simply the sum of point and distributed lateral loads applied to the pole.

 

Without Surface Restraint

Required Depth : Based on the 1988 UBC 1806.7 formulas shown on the next page, the minimum required embedment depth is calculated to satisfy allowable soil pressures.

 

For poles not restrained at surface:

       Depth      =       A/2 * [ 1 + ( 1 + (4.36*h/A))½ ]

       A            =            2.34 P / ( S1 * b)

       P            =      Applied lateral force, lbs

       S1            =      Allowable Lateral passive pressure at 1/3 embedment * LDF

       b            =      Diameter or width of footing or pole

       h            =      Height of point of load application

 

Actual @ 1/3 Embedment : This is the location of maximum lateral pressure for non-restrained footings. Considering the applied loads, pole footing dimensions, and calculated length of embedment, the actual lateral pressure at 1/3 of embedment depth is given here. It is calculated using a modified version of the formula used to calculate depth of embedment.

 

Allow. @ 1/3 Embedment : The allowable passive pressure (after application of the load duration factor) is multiplied by 1/3 the pole embedment, compared with the limiting value, and the smaller value used. This number is then multiplied by the load duration factor to get the final allowable pressure at 1/3 embedment.

 

With Surface Restraint

Required Depth : Based on the formulas to follow, the minimum required embedment depth is calculated to satisfy allowable soil pressures. Per U.B.C Section 1806.7 :

       Depth      =      (4.25 * P * h / ( S3 * b ) )½

       P      =      Applied lateral force, lbs

       h      =      Height of point of load application, ft.

       b      =      Diameter or width of footing or pole, ft.

       S3      =      Allowable lateral bearing pressure at base of embedment

 

Allow. @Base of Embedment : This represents the maximum allowable pressure at the base of embedment, and is given only for poles with lateral restraint at the ground surface. It equals the maximum allowable passive pressure multiplied by the load duration factor.

 

Actual @ Base of Embedment : The allowable passive pressure (after application of the load duration factor) is multiplied by the footing depth, compared with the limiting soil pressure value, and the smaller value used. This number is then multiplied by the load duration factor to get the final allowable pressure at the base.

 

Surface Restraint Force : When surface restraint has been specified, the lateral force acting at the surface is given. The user should verify that this force can be resisted.

 

 

Sketch Tab

This tab provides a sketch of the beam with loads and resulting values shown. Using the [Print Sketch] button will print the sketch in large scale on a single sheet of paper.

 

 

 

Sample Printout