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MeMo to Designers March 20144-1Spread FootingSIntroductionThe scope of this Memo is to clarify the terms and design methodology used in the Loadand Resistance Factor Design (LRFD) of spread footings as specified in AASHTO LRFDBridge Design Specifications with California Amendments (LRFD-BDS), and to improvecommunication between Structure Designer (SD) and Geotechnical Designer (GD).Definitions and Notation“Contact Surface” refers to the bottom surface of the footing located at a specified elevation,and resultant structural forces and moments are calculated at that level. Refer to Figure 1for footing local coordinates and illustration of the following parameters used throughoutthis Memo:B Footing Width. This is the short plan dimension of the footing.L Footing Length. This is the long plan dimension of the footing.M Factored Bending MomentMX and MY are calculated at the contact surface in the local X and Y directions,respectively.Local X axis is defined to be parallel to the long plan dimension of the footing (L).V Factored Shear Force:VX and VY are calculated at the contact surface in the local X and Y directions, respectively.B' Effective Footing Width:Reduced footing width for an eccentrically loaded footing is calculated as the smallerof : (B - 2eY) and (L - 2eX ) , where eY and eX are the eccentricities (always positive)corresponding to MX and MY , respectively.L' Effective Footing Length:Reduced footing length for an eccentrically loaded footing is calculated as the largerof (B - 2eY) and (L - 2eX ).A' B' x L' . Effective Footing Area.4-1Spread FootingS1

MeMo to Designers March 2014PTotal Total factored axial forceTotal factored axial force is calculated at the contact surface as gross (for Strength,Construction and Extreme Event load combinations) or net (for Service loadcombinations).PPerm Permanent net axial forcePermanent net axial force is calculated for LRFD permanent loads acting at thecontact surface and is used to evaluate settlement under Service-I limit state loadcombination.Y MB2 YBYY PX MX VLXL2Figure 1 – Components of Moment and Shear in Local Coordinates of the Spread Footingqg,u Gross Uniform Bearing StressUsed for footings on soil, gross uniform bearing stress is the equivalent uniformvertical stress determined by applying the vertical factored load over the effectivefooting area. When determining the gross uniform bearing stress, the designer mustinclude the weight of the footing and all overburden soil from the top of the footingto finished grade with applicable load factors.24-1 Spread FootingS