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PDHonline Course S235 (1 PDH)Pole Foundation Design with SpreadsheetInstructor: John W. Andrew, PE2012PDH Online PDH Center5272 Meadow Estates DriveFairfax, VA 22030-6658Phone & Fax: 703-988-0088www.PDHonline.orgwww.PDHcenter.comAn Approved Continuing Education Provider

www.PDHcenter.comPDH Course S235www.PDHonline.orgPole Foundation Design with SpreadsheetJohn W. Andrew, PEINTRODUCTIONPoles foundations are commonly used for sign, flagpole and light pole foundations and poleframed buildings. A design criteria for pole foundations is indicated in Section 1804 andParagraph 1805.7 of the 2006 International Building Code. This criteria applies to verticalpoles considered columns embedded in either earth or in concrete footings in the earth andused to resist lateral loads. The backfill in the annular space around a column that is notembedded in a concrete footing shall be clean sand thoroughly compacted by tamping inlayers not exceeding 8 inches in depth.ALLOWABLE FOUNDATION AND LATERAL BEARING PRESSURESThe allowable lateral pressures for various classes of soil are indicated in Table 1804.2.The values for lateral bearing pressure are permitted to be increased by the tabular value foreach additional foot of depth to a maximum of 15 times the tabular value. Poles for usessuch as signs or flagpoles and poles used to support buildings that are not adverselyaffected by a 1/2-inch motion at the ground surface due to short-term loads are permitted tobe designed using lateral bearing values equal to two times the tabular values.MINIMUM EMBEDMENT DEPTH CALCULATIONSWhere no constraint is provided at the ground surface, such as rigid floor or rigid groundsurface pavement, and where no lateral constraint is provided above the ground surface,such as a structural diaphragm, the following equation is used to determine the requiredminimum embedment depth.d 0.5 A {1 [1 (4.36 h / A)]1/2 }where:A 2.34 P / S1 bb diameter of round post or footing or diagonal dimension of square post or footing in feetd depth of embedment in earth in feet, but not over 12 feet for the purpose of computinglateral pressureh distance in feet from ground surface to point of application of “P”P applied lateral force in pounds John W. AndrewPage 2 of 5

www.PDHcenter.comPDH Course S235www.PDHonline.orgS1 allowable soil-bearing pressure as set forth in Section 1804.3 based on a depth of onethird the depth of embedment in pounds per square footWhere constraint is provided at the ground surface, the following equation is used todetermine the required embedment depth.d2 4.25 (P h / S3 b)where:b diameter of round post or footing or diagonal dimension of square post or footing in feetd depth of embedment in earth in feet, but not over 12 feet for the purpose of computinglateral pressureh distance in feet from ground surface to point of application of “P”P applied lateral load in poundsS3 allowable soil-bearing pressure as set forth in Section 1804.3 based on a depth equalto the depth of embedment in pound per square footSince the solution of these equations to determine the required embedment depth requiresan estimated depth to determine a calculated depth for comparison with the estimateddepth, a trial and error approach is necessary. Additional trial depths must be used until thetrial depth and calculated depth converge.VERTICAL LOADSThe foundation pressure resulting from the applied vertical load may not exceed theallowable foundation pressure indicated in Table 1804.2 for the class of soil. The Codedoes not require a combined lateral and vertical load to be considered.EXAMPLE PROBLEMA sign is to be constructed at a site with sandy clay soil. The column for the sign will be asteel tube encased in a circular concrete footing with a diameter of 32 inches. There will beno lateral support at the ground surface and the assembly will not be adversely affected by a1/2-inch motion at the ground surface. The design lateral force is 1,200 pounds and theheight to the load is 16’-0” above the ground surface. The vertical load is 775 pounds.What is the depth of embedment required?From Table 1804.2, for sandy clay soil, the allowable foundation pressure is indicated to be1,500 pounds per square foot and the allowable lateral bearing pressure is indicated to be100 pounds per square foot per foot below natural grade. Since the assembly will not beadversely affected by a 1/2-inch motion at the ground surface, Paragraph 1804.3.1 permitsthe allowable lateral bearing to be two times the tabular value, or 200 pounds per square John W. AndrewPage 3 of 5

www.PDHcenter.comPDH Course S235www.PDHonline.orgfoot per foot below natural grade. Per Section 1804.3.1, the allowable lateral bearing maynot exceed 15 times the tabular value, or 1,500 pounds per square foot.Using a trial depth of 8.00 feet,S1 (200) (8.00) / 3S1 533 psfA (2.34) (1,200) / (533) (2.67)A 1.97d (0.5) (1.97) {1 [1 (4.36) (16.00) / 1.97]1/2}d 6.93 ftSince 6.93 feet is less than the trial depth of 8.00 feet, the new trial depth should bebetween 6.93 feet and 8.00 feet.Using a trial depth of 7.50 feet,S1 (200) (7.50) / 3S1 500 psfA (2.34) (1,200) / (500) (2.67)A 2.10d (0.5) (2.10) {1 [1 (4.36) (16.00) / 2.10]1/2}d 7.19 ftSince 7.19 feet is less than the trial depth of 7.50 feet, the new trial depth should bebetween 7.19 feet and 7.50 feet.Using a trial depth of 7.31 feet,S1 (200) (7.31) / 3S1 487 psfA (2.34) (1,200) / (487) (2.67) John W. AndrewPage 4 of 5

www.PDHcenter.comPDH Course S235www.PDHonline.orgA 2.16d (0.5) (2.16) {1 [1 (4.36) (16.00) / 2.16]1/2}d 7.31 ftThe calculated depth equals the trial depth. Therefore the depth of embedment required is7.31 feet.The foundation pressure resulting from the vertical load,Pf 775 / (3.14) (1.333)2Pf 139 psfThe example problem above is entered in the spreadsheet. The Excel spreadsheet hasthe appropriate formulas embedded. The formulas and calculated results are in protectedcells so that they may not be inadvertently changed. Information provided by the user isentered into yellow unprotected cells. Calculation results are provided in blue colored cells.With the data entered in the appropriate cells, the spreadsheet will calculate the requiredembedment depth and the foundation pressure resulting from the vertical load. For thecases where the embedment depth required or the lateral bearing stress exceed thelimitations required by the Code, the user will be alerted with a warning statement.REFERENCES1. 2006 International Building Code, International Code Council, Inc. 500 New Jersey Ave.,NW, Washington, DC 20001.2. 2006 IBC Handbook Structural Provisions, International Code Council, 4051 FlossmoorRd., Country Club Hills, IL 60478. John W. AndrewPage 5 of 5