Step 5 – Lateral Capacity - Foundation …

1 Copyright 2003 Hubbell, Screw Foundation System Design manual for New Construction Chance Company5-1 step 5 Lateral CapacityIntroductionThe primary function of a deep Foundation is to resist axial loads. In some cases, they willbe subjected to horizontal, or Lateral loads. Lateral loads may be from wind, seismicevents, live loads, water flow, etc. The resistance to Lateral loads is in part a function of thenear surface soil type and strength, and the effective projected area of the structurebearing against these soils. This section of the design manual presents a summarizeddescription of the methods and procedures available to determine the Lateral Capacity ofhelical screw foundations in soil.

2 The analysis of deep foundations under Lateral loading iscomplicated because the soil reaction (resistance) at any point along the shaft is a functionof the deflection, which in turn is dependent on the soil resistance. Therefore, solving forthe response of a deep Foundation under Lateral loading is one type of soil-structure-interaction problem best suited for numerical methods on a desktop Resistance Methods UsedIt is obvious that helical screw foundations have slender shafts which offer limitedresistance to Lateral loads that are applied perpendicular to its shaft.

3 However, a largenumber of load tests have validated the concept that vertical pile foundations are capableof resisting Lateral loads via shear and bending. Two commonly used methods to analyzethe Lateral Capacity of pile foundations in soil is the finite difference method and the Broms method as outlined by Bengt B. Broms (1963). Lateral resistance can also be provided by passive earth pressure against the structuralelements of the Foundation . The resisting elements of the structure include the pile cap,grade beams, and stem walls.

4 The passive earth pressure against the structural elementscan be calculated using the Rankine screw foundations can be used to resist Lateral loads by assuming that thehorizontal load on the structure is resisted by components of the axial load. The implicitassumption in this is the battered foundations do not deflect laterally, which is not , it is better practice to use vertically installed screw foundations to resist onlyvertical loads and battered screw foundations to resist only Lateral resistance along the bottom of a footing, especially in the case of a continuousstrip footing or large pile cap, can be significant.

5 The friction component in a sandy soil isFigure PilesFigure Earth Pressure ResistanceFigure ResistanceEnlarged ShaftSectionBending MomentDistribution in PileOptional Lateral TiebackGrade Beam & Pile CapPassive EarthPressure ResistanceBattered PilesPile Cap5-2 Copyright 2003 Hubbell, Screw Foundation System Design manual for New Construction Chance Companysimply the structure s dead weight multiplied by the tangent of the angle of internalfriction. In the case of clay, cohesion times the area of the footing may be used for thefriction component.

6 When battered piles are used to prevent Lateral movement, thefriction may be included in the computation. The designer is advised to use caution whenusing friction for Lateral resistance. Some building codes do not permit friction resistanceunder pile supported footings and pile caps due to the possibility the soil will settle awayfrom the footing or pile cap. Shrink-swell soils, compressible strata, and liquefiable soilcan result in a void under footings and pile AssistanceIf required, the application engineers at Hubbell Power Systems/Chance can provideproject specific Lateral load response analysis - given sufficient data relating to the appliedloads and the soil profile.

7 If you need engineering assistance, please contact the Hubbell/Chance CivilConstructionDistributor in your information forHubbell/Chance CivilConstructionDistributors can befound professionals willhelp you to collect thedata required to performlateral Plot of Deflection vs. Depth LPILE PLUS Output Copyright 2003 Hubbell, Screw Foundation System Design manual for New Construction Chance Company5-3 Lateral Analysis by Finite DifferencesSeveral computer programs, such as LPILEPLUS (ENSOFT, Austin, TX), are subsequentrevisions of the COM624 program (Matlock and Reese) and its predecessor Beam-Column28 (Matlock and Haliburton), which both use the p-y concept, soil resistance is a non-linear function of pile deflection, which was further developed by Poulos (1973).

8 Thismethod is versatile and provides a practical means for design. This method is madepossible with computers for solving the governing nonlinear, fourth-order differentialequation, which is explained in greater detail in step 6. Lateral load analysis softwaregives the designer the tools necessary to evaluate the force-deflection behavior of a helicalscrew Foundation embedded in and b are sample LPILEPLUS output plots of Lateral shaft deflection andbending moment vs. depth where the top of the pile is fixed against rotation.

9 From resultslike this, the designercan quickly determinethe Lateral response atvarious horizontal loadsup to the structurallimit of the pile whichis typically geotechnicalconsultants useLPILEPLUS or othersoil-structure-interaction programs topredict soil-pileresponse to Output of Bending Moment vs. Depth LPILEPLUS Output5-4 Copyright 2003 Hubbell, Screw Foundation System Design manual for New Construction Chance CompanyLateral Analysis by Brom s MethodThe Broms method is best suited for applications where the top section of the helicalscrew Foundation is a greater diameter than the bottom section.

10 Enlarged top sections arecommonly used to increase the Lateral Capacity of the Foundation shaft. Design gives an example of this. It uses the Broms method for short piers in cohesive soil. A short pier is one that is rigid enough that it will move in the direction the load is tendingby rotation or translation. A long pier is one that the top will rotate or translate withoutmoving the bottom of the Foundation , a plastic hinge will developed Lateral Capacity methods for both short and long piles in cohesive andnon-cohesive soil.