Topic 14 - Foundation Design - Civil Engineering

1 Foundation Design 14-1 Instructional Materials ComplementingFEMA 451, Design ExamplesFOUNDATION DESIGNP roportioning elements for:Transfer of seismic forcesStrength and stiffnessShallow and deep foundationsElastic and plastic analysisFoundation Design 14-2 Instructional Materials ComplementingFEMA 451, Design ExamplesLoad Path and Transfer to SoilSoil PressureForce on a pileEQ on unloaded pilePile supporting structureInertial forceUnmoving soilEQ Motiondeflectedshapesoilpressuredeflecte dshapedeflectedshapesoilpressuresoilpres sureFoundation Design 14-3 Instructional Materials ComplementingFEMA 451, Design ExamplesLoad Path and Transfer to SoilSoil-to- Foundation Force TransferEQ motionPassive earthpressureFrictionShallowFoundation Design 14-4 Instructional Materials ComplementingFEMA 451, Design ExamplesLoad Path and Transfer to SoilSoil-to- Foundation Force TransferDeepEQ MotionMotionSoilpressureBendingmomentFou ndation Design 14-5 Instructional Materials ComplementingFEMA 451.

2 Design ExamplesLoad Path and Transfer to SoilVertical Pressures - ShallowEQ motionOverturning momentFoundation Design 14-6 Instructional Materials ComplementingFEMA 451, Design ExamplesLoad Path and Transfer to SoilVertical Pressures - DeepEQ MotionOverturningmomentFoundation Design 14-7 Instructional Materials ComplementingFEMA 451, Design ExamplesReinforced Concrete Footings: Basic Design Criteria (concentrically loaded)d/2 (all sides)(c) Critical sectionfor two-way shear(b)Critical sectionfor one-way shear(a)Critical sectionfor flexureOutside face of concretecolumn or line midwaybetween face of steelcolumn and edge ofsteel base plate (typical)extent of footing(typical)dFoundation Design 14-8 Instructional Materials ComplementingFEMA 451, Design ExamplesFooting Subject to Compression and Moment: Uplift Nonlinear(a)Loading(b)Elastic, no uplift(c)Elastic, at uplift(d)Elastic, after uplift(e)Some plastification(f)Plastic limitMPFoundation Design 14-9 Instructional Materials ComplementingFEMA 451, Design ExamplesExample7-story Building:Shallow foundations designed for perimeter frame and core Bays @ 25'-0" = 125'-0"1'-2"1'-2"7 Bays @ 25'-0" = 175'-0"1'-2"NFoundation Design 14-10 Instructional Materials ComplementingFEMA 451, Design ExamplesShallow Footing ExamplesSoil parameters: Medium dense sand (SPT) N = 20 Density = 120 pcf Friction angle = 33oGravity load allowables 4000 psf, B < 20 ft 2000 psf, B > 40 ftBearing capacity (EQ) 2000 Bconcentric sq.

3 3000 Beccentric = Design 14-11 Instructional Materials ComplementingFEMA 451, Design ExamplesFootings proportioned for gravity loads aloneCorner:6'x6'x1'-2" thickPerimeter:8'x8'x1'-6" thickInterior:11'x11'x2'-2" thickFoundation Design 14-12 Instructional Materials ComplementingFEMA 451, Design ExamplesDesign of Footings for Perimeter Moment Frame5 at 25'-0"7 at 25'-0"NFoundation Design 14-13 Instructional Materials ComplementingFEMA 451, Design Examples7-Story Frame, DeformedFoundation Design 14-14 Instructional Materials ComplementingFEMA 451, Design ExamplesCombining Loads Maximum downward + + E Minimum downward + E Definition of seismic load effect E:E = 1QE1+ 2QE2+ x= y= and SDS= Design 14-15 Instructional Materials ComplementingFEMA 451, Design kA-5 EyExLiveDeadGridFoundation Design 14-16 Instructional Materials ComplementingFEMA 451, Design ExamplesReduction of Overturning Moment NEHRP Recommended Provisionsallow base overturning moment to be reduced by 25% at the soil- Foundation interface.

4 For a moment frame, the column vertical loads are the resultants of base overturning moment, whereas column moments are resultants of story shear. Thus, use 75% of seismic vertical Design 14-17 Instructional Materials ComplementingFEMA 451, Design ExamplesAdditive Load w/ Largest Eccentricity At A5: P = ( ) + ( ) + ( ( ) + ( )) = 324 kMxx= ( ) + ( ) = -1106 k-ft At A6: P = ( ) + ( ) + ( ( ) + (-281)) = kMxx= ( ) + (-891) = -974 k-ft Sum Mxx= ( ) -1106 -974 = -7258 Foundation Design 14-18 Instructional Materials ComplementingFEMA 451, Design ExamplesCounteracting Load with Largest e At A-5:P = ( ) + ( ( ) + ( )) = kMxx= ( ) + ( ) = -1106 k-ft At A-6:P = ( ) + ( ( ) + (-281)) = kMxx= ( ) + (-891) = -974 k-ft Sum Mxx= 6240 k-ftFoundation Design 14-19 Instructional Materials ComplementingFEMA 451, Design ExamplesElastic Response Objective is to set L and Wto satisfy equilibrium and avoid overloading soil.

5 Successive trials usually Design 14-20 Instructional Materials ComplementingFEMA 451, Design ExamplesAdditive CombinationGiven P= 234 k, M=7258 k-ftTry 5 foot around, thus L= 35 ft, B= 10 ft Minimum W= M/(L/2) P= 181 k = 517 psfTry 2 foot soil cover & 3 foot thick footing W= 245 k; for additive combo use Qmax= (P+ )/(3(L/2 e)B/2) = ksf Qn= (3)Bmin= ksf, OK by ElasticFoundation Design 14-21 Instructional Materials ComplementingFEMA 451, Design ExamplesPlastic Response Same objective as for elastic response. Smaller footings can be shown OK thus:PMWRLeRFoundation Design 14-22 Instructional Materials ComplementingFEMA 451, Design ExamplesCounteracting CaseGiven P= k; M= 6240 Check prior trial; W= 245 k (use ) e= 6240/( ) = > 35/2 NGNew trial: L= 40 ft, 5 ft thick W= 400 k; e= ft; plastic Qmax= ksf Qn= (3)4 = ksf, close Solution is to add 5 k, then e= ft and Qmax= Qn= ksfFoundation Design 14-23 Instructional Materials ComplementingFEMA 451, Design ExamplesAdditional Checks Moments and shears for reinforcement should be checked for the overturning case.

6 Plastic soil stress gives upper bound on moments and shears in concrete. Horizontal equilibrium: Hmax< (P+W)in this case friction exceeds demand; passive could also be Design 14-24 Instructional Materials ComplementingFEMA 451, Design ExamplesResults for all SRS FootingsCorner: 10'x40'x5'-0" w/top of footing 2'-0" below gradeMiddle:5'x30'x4'-0" Side:8'x32'x4'-0" Foundation Design 14-25 Instructional Materials ComplementingFEMA 451, Design ExamplesDesign of Footings for Core-braced 7-story Building25 foot square bays at center of buildingFoundation Design 14-26 Instructional Materials ComplementingFEMA 451, Design ExamplesSolution for Central MatMat: 45'x95'x7'-0"with top of mat 3'-6" below gradeVery high uplifts at individual columns; mat is only practical shallow Design 14-27 Instructional Materials ComplementingFEMA 451, Design ExamplesBearing Pressure Solution(a)Plasticsolution(b)Elastic solutionpressures (ksf) ksf~Plastic solution is satisfactory; elastic is not.

7 See linked file for more Design 14-28 Instructional Materials ComplementingFEMA 451, Design ExamplesPile/Pier FoundationsPassive resistance(see Figure )p-y springs(see Figure )PilecapPileView of cap with column above and piles Design 14-29 Instructional Materials ComplementingFEMA 451, Design ExamplesPile/Pier FoundationsPile Stiffness: Short (rigid) Intermediate LongCap influenceGroup actionSoil Stiffness Linear springs nomographs NAVFAC Nonlinear springs LPILE or similar analysisFoundation Design 14-30 Instructional Materials ComplementingFEMA 451, Design ExamplesSite Class E, depth = 10 ftSite Class E, depth = 30 ftSite Class C, depth = 10 ftSite Class C, depth = 30 resistance, p (lb/in.)101001,00010, Pile deflection, y (in.) , p-yCurvesFoundation Design 14-31 Instructional Materials ComplementingFEMA 451, Design ExamplesPassive ult /H Design 14-32 Instructional Materials ComplementingFEMA 451, Design ExamplesGroup Effect1 2 Group effect 3 Group size (piles per side) 4s = Ds = 2 Ds = 3 Ds = 4 DFoundation Design 14-33 Instructional Materials ComplementingFEMA 451, Design ExamplesPile Shear.

8 Two Soil Stiffnesses302520151050-5051015 Shear, V (kip)Depth (ft)Site Class CSite Class EFoundation Design 14-34 Instructional Materials ComplementingFEMA 451, Design ExamplesPile Moment vs Depth302520151050-1000-5000500 Moment, M ( )Depth (ft)Site Class CSite Class EFoundation Design 14-35 Instructional Materials ComplementingFEMA 451, Design ExamplesPile Reinforcement(4) #5#4 spiral at11 inch pitch(6) #5#4 spiral inch pitch(6) #5#4 spiral inch pitch4" pile embedmentSection ASection BSection CCBA21'-0"23'-0"6'-4" Site Class C Larger amounts where moments and shears are high Minimum amounts must extend beyond theoretical cutoff points Half spiral for 3 DFoundation Design 14-36 Instructional Materials ComplementingFEMA 451, Design ExamplesPile Design (4) #7#4 spiral at11 inch pitch(6) #7#5 spiral inch pitch(8) #7#5 spiral inch pitch4" pile embedmentBA32'-0"20'-0"12'-4"Section ASection BSection CC Site Class E Substantially more reinforcement Full spiral for 7D Confinement at boundary of soft and firm soils (7D up and 3D down) Foundation Design 14-37 Instructional Materials ComplementingFEMA 451, Design ExamplesOther topics for Pile foundations Foundation Ties: F =PG(SDS/10) Pile Caps: high shears, rules of thumb; look for 3D strut and tie methods in future Liquefaction: another Topic Kinematic interaction of soil layersFoundation Design 14-38 Instructional Materials ComplementingFEMA 451, Design ExamplesTie Between Pile Caps(2) #6 top bars(3) #6 bottom bars#4 ties at 7" " clearat sides3" clear attop and bottom Designed for axial force (+/-) Pile cap axial load times SDS/10 Often times use grade beams or thickened slabs one gra