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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

Memo to Designers March 2014qg,max Gross Maximum Bearing StressGross maximum bearing stress is the maximum applied vertical stress at the contactsurface when applying factored loads. The gross maximum bearing stress mustinclude the weight of the footing and of all overburden soil from the top of the footingto finished grade with applicable load factors. Used for footings on rock, qg,max is basedon triangular or trapezoidal stress distribution, as appropriate, on the footing area.qn,u Net Uniform Bearing StressNet uniform bearing stress is the gross uniform bearing stress for footings on soil (qg,u)minus the overburden or vertical effective stress at the contact surface calculated basedon grade elevation at the time of excavation for the footing. The net uniform bearingstress due to LRFD Service-I load combination is used to evaluate footing settlementon soil. The net uniform bearing stress is calculated for total factored axial forces.qn,max Net Maximum Bearing StressNet maximum bearing stress is the gross maximum bearing stress for footings onrock (qg.max), minus the overburden or vertical effective stress at the contact surfacecalculated based on finished grade for abutments and original grade for bents. Thenet maximum bearing stress due to LRFD Service-I load combination is used toevaluate footing settlement on rock. The net maximum bearing stress is calculatedfor total factored axial forces.qn Gross Nominal Bearing ResistanceUsed in Strength and Extreme Event Limit States, this is the gross uniform bearingstress for footings on soil, or the gross maximum bearing stress for footings on rockthat will fail the soil or rock, respectively, in shear based on a strength criterion.qR Factored Gross Nominal Bearing ResistanceUsed in Strength and Extreme Event Limit States, qR φb qn where φb is resistancefactor from Section 10.5 of the LRFD BDS.qpn Permissible Net Contact StressThis is the net bearing stress resulting in an estimated settlement that is structurallysafe and without adverse impact on serviceability. This value is calculated using totalService-I loads, irrespective of whether or not consolidation contributes to the totalsettlement.Design ProcessThe shallow foundation structural design process includes the following steps:4-1Spread FootingS3

MeMo to Designers March 20141. Contact Bearing Stress Check (Sizing the footing)Spread footings used to support abutments or bent columns or piers must satisfy the followingrequirements:For Service Limit State (Settlement):1. qn,u qpnfor footings on soil2. qn,max qpnfor footings on rockFor Strength, Construction and Extreme Event Limit States: qR1. qg,ufor footings on soil2. qg,max qRfor footings on rock2. Eccentricity LimitsAll footings must be checked for tilting/rotation by SD. On competent foundation materials,the tilting/rotation is considered within acceptable limits, when the maximum eccentricitycalculated based on gross axial force under Service-I Limit State load combination is limitedto B/6 and L/6 for footings on soil, or to B/4 and L/4 for footings on rock. Refer to LRFDBDS 10.6.4.2 for eccentricity limits under Extreme Event Limit State load combinations.When founded on poor or soft soils, a detailed tilting/rotation deformation analysis by GDis required in lieu of this check. SD and GD must communicate especially if non-standardor special design conditions exist.3. Sliding FailureFor foundation material at the contact surface the internal friction angle of drained soil(φf) for footings founded on rock or cohesionless soil, and the undrained shear strength forfootings founded on cohesive soil will be provided by the GD. SD will check sliding failurefor Strength and Extreme Event Limit States load combinations per LRFD BDS 10.6.3.4requirements.Structural design of the footing shall start after finalizing the size of the footing. The SDwill determine the required depth of the footing and also the amount and distribution ofsteel reinforcement.Communication with Geotechnical DesignerSizing a spread footing is an iterative process because the calculated Permissible Net Contact44-1 Spread FootingS

Memo to Designers March 2014Stress (qpn) and the Factored Gross Nominal Bearing Resistance (qR) depend on the location,dimensions and depth of the footing. To facilitate design, GD will provide these parametersas a function of the effective footing width, (B'), for a range of effective footing length toeffective footing width ratios, (L'/B'), and a given footing embedment depth specified byStructural Designer.If the support location or footing embedment depth changes during the design process, GDshall be contacted so that the geotechnical design data can be revised.To prepare foundation recommendations and reports, GD needs foundation location,geometry, and load data from SD. Attachment No. 1 shows examples of Foundation DesignData Sheets to be included in the request for Preliminary Foundation Report (PFR) .Attachment No. 2 (Foundation Geotechnical Data Tables) indicates the format that will beused for the transmittal of geotechnical foundation design data from GD. The communicationof this data is an intermediate step in the design process. This table will not be included inthe Foundation Report.Attachment No. 3 (Bridge Foundation Loads) is an example of the detailed load data whichmust be provided by SD. If requested, the GD must receive the data shown in AttachmentNo.3 prior to the completion of the Foundation Report (FR). Attachment No. 4 shows theFoundation Design Data Sheet to be included in the request for Foundation Report.Spread Footing Data TableSpread Footing Data Tables shall be included in the FR and on the Contract Plans in theformats shown in Attachment No. 5.Although Spread Footing Data Table is not necessary for contract administration purposes, itsinclusion is a useful addition to the plans. With the foundation design parameters includedin the table, the engineer can perform an informed inspection of the bearing material. Thesedesign parameters are also a starting point for the future design of widening and emergencyprojects.Originalsigned by Barton J. NewtonBarton J. NewtonState Bridge EngineerDeputy Division Chief, Structure Policy & InnovationDivision of Engineering Services4-1Spread FootingS5