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DESIGN OF FOUNDATIONSDr. Izni Syahrizal bin IbrahimFaculty of Civil EngineeringUniversiti Teknologi MalaysiaEmail: [email protected]

Introduction Foundation – Part of structure whichtransmits load from the structure to theunderlying soil or rock All soils compress noticeably when loadedcausing structure to settle

Introduction Requirements in the design of foundations:(i) Total settlement of the structure to belimited to a tolerably small amount(ii) Differential settlement of various partsof structure shall be eliminated

Introduction To limit settlement, it is necessary to transmit thestructure load to a soil stratum of sufficientstrength Spread the structure load over a sufficiently largearea of stratum to minimize bearing pressure Satisfactory soil: Use footings Adequate soil: Use deep foundations i.e. piles

Introduction Pressure distribution under a footingUniformdistributedCohesive soilCohesionless soil

Types of FoundationPad Footings Transmit load from piers andcolumns Simplest and cheapest type Use when soil is relatively strong orwhen column loads are relativelylight Normally square or rectangularshape in plan Has uniform thickness

Types of FoundationCombine Footings Use when two columns are closedtogether Combine the footing to form acontinuous base Base to be arranged so that itscentreline coincides with the centreof gravity of the load – provideuniform pressure on the soil

Types of FoundationStrap Footings Use where the base for an exteriorcolumn must not project beyondthe property line Strap beam is constructed betweenexterior footing & adjacent interiorfooting Purpose of strap – to restrainoverturning forces due to loadeccentricity on the exterior footing

Types of FoundationStrap Footings (continued) Base area of the footings areproportioned to the bearingpressure Resultant of the loads on the twofootings should pass through thecentroid of the area of the twobases Strap beam between the twofootings should NOT bear againstthe soil

Types of FoundationStrip Footings Use for foundations to load-bearingwall Also use when pad footings fornumber of columns are closelyspaced Also use on weak ground toincrease foundation bearing area

Types of FoundationRaft Foundations Combine footing which covers thewhole building Support all walls & columns Useful where column loads areheavy or bearing capacity is low –need large base Also used where soil mass containscompressible layers or soil isvariable – differential settlementdifficult to control

Types of FoundationPile Foundations More economic to be used whensolid bearing stratum i.e. rock isdeeper than about 3 m Pile loads can either be transmittedto a stiff bearing layer (somedistance below surface) or byfriction along the length of pile Pile types – precast (driven into thesoil) or cast in-situ (bored) Soil survey is important to provideguide on the length of pile and safeload capacity of the pile

Types of FoundationPile FoundationsLoad fromStructurePile CapLower DensityMedium DensityHigh DensityPILEPILEPILEPILE

Design of Pad FootingThickness and Size of FootingArea of pad:𝑮𝒌 𝑸𝒌 𝑾𝑨 𝑺𝒐𝒊𝒍 𝒃𝒆𝒂𝒓𝒊𝒏𝒈 𝒄𝒂𝒑𝒄𝒊𝒕𝒚Minimum effective depth of pad:𝒅 𝑵𝑬𝒅𝒗𝒓𝒅,𝒎𝒂𝒙 𝒖𝒐NEd Ultimate vertical load 1.35Gk 1.5Qk𝑓vrd,max 0.5vfcd 0.5 0.6 1 𝑐𝑘250uo Column perimeter𝑓𝑐𝑘1.5

Design of Pad FootingDesign for Flexure Critical section for bending – At the face of the column Moment is taken on a section passing completely across thefooting and due to ultimate load on one side of the section Moment & shear is assessed using STR (Structure) combinationxyyxSTR Combination 1:𝑵 𝟏. 𝟑𝟓𝑮𝒌 𝟏. 𝟓𝑸𝒌

Design of Pad FootingCheck for Shear May fail in shear as vertical shear or punching shearVertical shearsectionsPunching shearperimeters2ddhBends may berequiredd

Design of Pad FootingCheck for Shear(i)Vertical Shear Critical section at distance d from the face of column Vertical shear force Load acting outside the section If VEd VRd,c No shear reinforcement is required

Design of Pad FootingCheck for Shear(ii) Punching ShearAxial Force Only Critical section at a perimeter 2d from the face of the columnPunching shear force Load outside the critical perimeter𝑽𝑬𝒅Shear stress, 𝒗𝑬𝒅 where u Critical perimeter𝒖 𝒅If vEd vRd,c No shear reinforcement is requiredAlso ensure that VEd VRd,max

Design of Pad FootingCheck for Shear(ii) Punching Shear (continued)Axial Force & Bending Moment Punching shear resistance can be significantly reduced of a coexisting bending, MEd However, adverse effect of the moment will give rise to a nonuniform shear distribution around the control perimeter Refer to Cl. 6.4.3(3) of EC2

Design of Pad FootingCheck for Shear(ii) Punching Shear (continued)Shear stress, 𝒗𝑬𝒅 𝜷𝑽𝑬𝒅𝒖𝟏 𝒅where; factor used to include effect of eccentric load & bending moment 1 𝑘𝑀𝐸𝑑𝑉𝐸𝑑𝑢1𝑊1k coefficient depending on the ratio between column dimension c1 & c2c1/c2 0.51.02.0 3.0k0.450.600.700.80u1 length of basic control perimeterW1 function of basic control perimeter corresponds to the distribution ofshear 0.5𝑐1 2 𝑐1 𝑐2 4𝑐2 𝑑 16𝑑 2 2𝜋𝑑𝑐1

Design of Pad FootingCheck for Shear(ii) Punching Shear (continued)

Design of Pad FootingCracking & Detailing Requirements All reinforcements should extend the full length of the footing If 𝐿𝑥 1.5 𝑐𝑥 3𝑑 , at least two-thirds of the reinforcement parallelto Ly should be concentrated in a band width 𝑐𝑥 3𝑑 centred atcolumn where Lx & Ly and cx & cy are the footing and column dimensionin x and y directions Reinforcements should be anchored each side of all critical sections forbending. Usually possible to achieve using straight bar Spacing between centre of reinforcements 20 mm for fyk 500N/mm2 Reinforcements normally not provided in the side face nor in the topface (except for balanced & combined foundation) Starter bar should terminate in a 90 bend tied to the bottomreinforcement, or in the case of unreinforced footing spaced 75 mmoff the building

Example 1PAD FOOTING(AXIAL LOAD ONLY)

Example 1: Pad Footing (Axial Load)Axial Force, N:Gk 600 kNQk 400 kNColumn size:300 300 mmhBH fck 25 N/mm2fyk 500 N/mm2 soil 150 N/mm2Unit weight of concrete 25 kN/m3Design life 50 yearsExposure Class XC2Assumed bar 12 mm

Example 1: Pad Footing (Axial Load)Durability & Bond RequirementsMin cover regards to bond, cmin,b 12 mmMin cover regards to durability, cmin,dur 25 mmAllowance in design for deviation, cdev 10 mmNominal cover, cnom cmin cdev 25 10 35 mm cnom 35 mmcmin 25 mm

Example 1: Pad Footing (Axial Load)SizeService load, NAssumed selfweight 10% of service load , WArea of footing required 𝑁 𝑊𝛾𝑠𝑜𝑖𝑙 1000 kN 100 kN1000 100150 7.33 𝑚2 Try footing size, B H h 3 m 3 m 0.45 mArea, A 9 m2Selfweight, W 9 0.45 25 101 kNCheck Service Soil Bearing Capacity 122 kN/m2 150 kN/m2 OK𝑵 𝑾𝑨 𝟏𝟎𝟎𝟎 𝟏𝟎𝟏𝟗

Example 1: Pad Footing (Axial Load)AnalysisUltimate axial force, NEd 1.35Gk 1.5Qk 1.35 (600) 1.5 (400) 1410 kN𝑁1410Soil pressure at ultimate load, P 𝐸𝑑 157 kN/m2𝐴9Soil pressure per m length, w 157 3 m 470 kN/m0.3 m1.35 mw 470 kN/m1.35 m

Example 1: Pad Footing (Axial Load)AnalysisUltimate axial force, NEd 1.35Gk 1.5Qk 1.35 (600) 1.5 (400) 1410 kN𝑁1410Soil pressure at ultimate load, P 𝐸𝑑 157 kN/m2𝐴9Soil pressure per m length, w 157 3 m 470 kN/mMEd0.3 m1.35 mw 470 kN/m1.35 m𝟏. 𝟑𝟓𝑴𝑬𝒅 𝟒𝟕𝟎 𝟏. 𝟑𝟓 𝟐 428 kNm

Example 1: Pad Footing (Axial Load)Main ReinforcementEffective depth, d h – c – 1.5 bar 450 – 35 – (1.5 12) 397 mm𝐾 𝑀𝐸𝑑𝑓𝑐𝑘 𝑏𝑑 2 428 10625 3000 3972 0.036 Kbal 0.167 Compression reinforcement is NOT required𝑧 𝑑 0.25 𝐴𝑠,𝑟𝑒𝑞 𝐾1.134𝑀𝐸𝑑0.87𝑓𝑦𝑘 𝑧 0.97𝑑 0.95d428 1060.87 500 0.95 397 𝟐𝟔𝟏𝟏 mm2

Example 1: Pad Footing (Axial Load)Minimum & Maximum Area of Reinforcement𝑓𝑐𝑡𝑚2.56𝑏𝑑 0.260.0013𝑏𝑑 0.0013𝑏𝑑𝑓𝑦𝑘500 0.0013bd 0.0013 3000 397 1589 mm2𝐴𝑠,𝑚𝑖𝑛 0.26 As,minAs,max 0.04Ac 0.04bh 0.04 3000 397 54000 mm2Provide 24H12 (As,prov 2715 mm2)

Example 1: Pad Footing (Axial Load)(i) Vertical Shear3m1.35 md 397 mmVEd953 mm3mw 470 kN/m0.953 mCritical shear at 1.0d from face of column: Design shear force, VEd 470 0.953 448 kN

Example 1: Pad Footing (Axial Load)(i) Vertical Shear𝑘 1 200𝑑 1 200397 1.71 2.0Note:Bar extend beyond critical section at 953 – 35 918 mm 𝑙𝑏𝑑 𝑑 40 𝑑 40 12 397 877 mm𝐴𝑠𝑙2715𝜌𝑙 0.0023 0.02𝑏𝑑 3000 397 Asl 2715 mm2

Example 1: Pad Footing (Axial Load)(i) Vertical Shear𝑉𝑅𝑑,𝑐 0.12𝑘 100𝜌𝑙 𝑓𝑐𝑘 1/3 𝑏𝑑 0.12 1.71 100 0.0023 251/33000 397 436463 N 436 kN𝑉𝑚𝑖𝑛 0.035𝑘 3/2 𝑓𝑐𝑘 𝑏𝑑 0.035 1.713/2 25 3000 397 465970 𝑁 466 kNVEd (448 kN) Vmin (466 kN) OK

Example 1: Pad Footing (Axial Load)(ii) Punching ShearCritical shear at 2.0d from face of column:Average d 450 – 35 – 12 403 mm 2d 806 mm13502d 806Control perimeter, u (4 300) (2 806) 6265 mm3002d 806300Area within perimeter, A (0.30 0.30) (4 0.30 0.806) ( 0.8062) 3.10 m2544 𝑙𝑏𝑑 𝑑 40 𝑑 40 12 397 877mm Reinforcement NOT contributed to punchingresistance

Example 1: Pad Footing (Axial Load)(ii) Punching ShearPunching shear force:VEd 157 (32 – 3.10) 925 kNAall 9 m2Aperimeter 3.10 m2Punching shear resistance:𝑉𝑅𝑑,𝑐 𝑉𝑚𝑖𝑛 0.035𝑘 3/2 𝑓𝑐𝑘 1/2 𝑢𝑑 0.035 1.71 3/2 25 1/2 6265 403 983199 N 983 kN VEd (925 kN) OKSoil pressure 157 kN/m2

Example 1: Pad Footing (Axial Load)(iii) Maximum Punching Shear at Column PerimeterMaximum punching shear force:VEd,max 157 (32 – 0.09) 1400 kNAall 9 m2Acolumn 0.09 m2Maximum shear resistance:𝑓𝑐𝑘 0.5𝑢𝑑 0.6 1 250𝑓𝑐𝑘𝑉𝑅𝑑,𝑚𝑎𝑥1.525 0.5 4 300 403 0.6 1 250 2176 kN VEd,max OK251.5Soil pressure 157 kN/m2

Example 1: Pad Footing (Axial Load)Crackingh 450 mm 200 mmMax bar spacing𝐴𝑠.𝑟𝑒𝑞𝐺𝑘 0.3𝑄𝑘Steel stress, 𝑓𝑠 1.35𝐺𝑘 1.5𝑄𝑘𝐴𝑠,𝑝𝑟𝑜𝑣600 0.3 4002611500 2131.35 600 1.5 40027151.15𝑓𝑦𝑘1.15N/mm2For design crack width 0.3 mm:Maximum allowable bar spacing 200 mmActual bar spacing 3000 2 35 1223 127 mm 200 mmCracking OK

Example 1: Pad Footing (Axial Load)Detailing24H12300024H123000450300024H12Plan ViewSection View

Example 2PAD FOOTING(AXIAL LOAD & MOMENT)

Example 2: Pad Footing (Axial Load &Moment)Axial Force, N 1500 kNMoment 50 kNmColumn size:250 350 mmhBH Design Life 50 years (Table 2.1: EN1990) Exposure Class XC3 fck 30 N/mm2 fyk 500 N/mm2 soil 150 N/mm2 Unit weight of concrete 25 kN/m3 Assumed bar 12 mm

Example 2: Pad Footing (Axial Load &Moment)Durability & Bond RequirementsMin cover regards to bond, cmin,b 12 mmMin cover regards to durability, cmin,dur 25 mmAllowance in design for deviation, cdev 10 mmNominal cover, cnom cmin cdev 25 10 35 mm cnom 35 mmcmin 25 mm

Example 2: Pad Footing (Axial Load &Moment)SizeService axial, NService moment, MAssumed selfweight 10% of service load , WArea of footing required 𝑁 𝑊𝛾𝑠𝑜𝑖𝑙 1071 107.1150 1500 kN / 1.40 1071 kN 50 kNm / 1.40 36.1 kNm 100 kN 7.85 𝑚2 Try footing size, B H h 2.80 m 3.50 m 0.65 mArea, A 9.80 m2Selfweight, W 9.80 0.65 25 159 kN

Example 2: Pad Footing (Axial Load &Moment)Size (Continued)𝐵𝐻 32.8 3.53𝐼𝑥𝑥 1212𝐻3.5𝑦 1.75 m2210.0 m4Maximum soil pressure, 𝑃 132 kN/m2 150 kN/m2𝑁 𝑊𝐴𝑀𝑦 𝐼 OK1071 1599.80 50 1.7510.0BxHx

Example 2: Pad Footing (Axial Load &Moment)AnalysisUltimate soil pressure, 𝑃 50 1.7510.0𝑁𝐴 𝑀𝑦𝐼 15009.80x 153 8.7 kN/m21.275 Pmin 144 kN/m2 and Pmax 162 kN/m2yyx0.35 m1.575 m1.575 m144162144154162

Example 2: Pad Footing (Axial Load &Moment)Analysis (Continued)𝑀𝑥𝑥x1.5752 154 21.2751.5752 1.575 23 197 kNm/m 2.80 m 553 kNm 162 154𝑀𝑦𝑦 𝟏𝟓𝟑1.2752 2yyx 124 kNm/m 3.50 m 0.35 m1.575 m435 kNm 144 16221441.575 m154162

Example 2: Pad Footing (Axial Load &Moment)Effective Depthdx h – c – 0.5 bar 650 – 35 – (0.5 12) 609 mmdy h – c – 1.5 bar 650 – 35 – (1.5 12) 597 mmMain Reinforcement – Longitudinal Bar𝐾 𝑀𝑥𝑥𝑓𝑐𝑘 𝑏𝑑 2 553 10630 2800 6092 0.018 Kbal 0.167 Compression reinforcement is NOT required𝑧 𝑑 0.25 𝐴𝑠,𝑟𝑒𝑞 𝐾1.134𝑀𝑥𝑥0.87𝑓𝑦𝑘 𝑧 0.98𝑑 0.95d553 1060.87 500 0.95 609 𝟐𝟏𝟗𝟕 mm2

Example 2: Pad Footing (Axial Load &Moment)Minimum & Maximum Area of Reinforcement𝑓𝑐𝑡𝑚2.90𝑏𝑑 0.260.0013𝑏𝑑 0.0013𝑏𝑑𝑓𝑦𝑘500 0.0013bd 0.0013 2800 609 2217 mm2𝐴𝑠,𝑚𝑖𝑛 0.26 As,minAs,max 0.04Ac 0.04bh 0.04 2800 609 72800 mm2Since As As,min, Use As,min 2217 mm2Provide 21H12 (As 2375 mm2)

Example 2: Pad Footing (Axial Load &Moment)Main Reinforcement – Transverse Bar𝐾 𝑀𝑦𝑦𝑓𝑐𝑘 𝑏𝑑 2 435 10630 3500 5972 0.018 Kbal 0.167 Compression reinforcement is NOT required𝑧 𝑑 0.25 𝐴𝑠,𝑟𝑒𝑞 𝐾1.134𝑀𝑦𝑦0.87𝑓𝑦𝑘 𝑧 0.99𝑑 0.95d435 1060.87 500 0.95 597 𝟏𝟕𝟔𝟓mm2

Example 2: Pad Footing (Axial Load &Moment)Minimum & Maximum Area of Reinforcement𝑓𝑐𝑡𝑚2.90𝑏𝑑 0.260.0013𝑏𝑑 0.0013𝑏𝑑𝑓𝑦𝑘500 0.0013bd 0.0013 3500 597 3147 mm2𝐴𝑠,𝑚𝑖𝑛 0.26 As,minAs,max 0.04Ac 0.04bh 0.04 3500 597 91000 mm2Since As As,min, Use As,min 3147 mm2Provide 28H12 (As 3167 mm2)

Example 2: Pad Footing (Axial Load &Moment)(i) Vertical Shear2.891Critical shear at 1.0d from face of column:2.8Average pressure at critical section:2.8913.50 18 159 kN/m2d 0.609 144 Design shear force, VEd 159 0.966 2.80 431 kN0.966144159Note:Bar extend beyond critical section at 966 – 35 931 mm 𝑙𝑏𝑑 𝑑 36 𝑑 36 12 609 1041 mm162 Asl 0 mm2

Example 2: Pad Footing (Axial Load &Moment)(i) Vertical Shear𝑘 1 𝜌𝑙 200𝑑 1 200609 1.57 2.0𝐴𝑠𝑙 0𝑏𝑑 𝑉𝑅𝑑,𝑐 0.12𝑘 100𝜌𝑙 𝑓𝑐𝑘 1/3 𝑏𝑑 0.12 1.57 100 0 30 1/3 2800 609 0 N 0 kN 𝑉𝑚𝑖𝑛 0.035𝑘 3/2 𝑓𝑐𝑘 𝑏𝑑 0.035 1.573/2 30 2800 609 644949 𝑁 645 kNVEd (430 kN) Vmin (645 kN)