PE-18-Shallow-Foundations

PE-18-Shallow-Foundations - 18 Shallow Foundations*01...

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18 - Shallow Foundations *01: Design a simple square footing. *02: The column adds a moment to the previous problem. *03: Further develop Problem #2. *04: Develop a rectangular footing to carry a moment. *05: Further develop Problem #4. *06: Design a continuous (wall) footing. **07: Design a footing for a large billboard sign. 375
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Properties of Reinforcing Steel (British and SI units). 376
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*Footings–01: What is the Factor of Safety against uplift? (Revision: June-09) What is the Factor of Safety against uplift of the footing shown below, if the uplift force Q = 24 kips and the footing is embedded in a clay soil? There is no water table. The square footing is a 5’ x 5’ and 2’ thick. The depth of the footing D f = 8 feet ( D f is measured from the ground surface to the footing’s invert). The weight of the soil upon the footing is 14,400 lbs and the weight of the footing concrete, including the buried column, is 8,400 lbs. The cohesion of the clay is c = 200 psf. Assume that the adhesion with vertical concrete surfaces and the footing base is c a = 0.6 c if the structure has been in contact with the soil for a long period of time. Otherwise, the adhesion can be as low as zero. (a) FS = 1.0 (b) FS = 1.5 (c) FS = 2.0 (d) FS = 2.3 427
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Solution: The Factor of Safety FS can be found from, () ( ) 2 24 8400 84 14 400 14 4 200 4 6 5 24 concrete backfill soil cohesion adhesion concrete backfill soil cohesion soil WW FF Downward (Resisting)Forces FS UpliftingForces kips where, W, l b . k i p s W , lb . kips lb Fc A f t f tk ft + ++ == ⎛⎞ = ⎜⎟ ⎝⎠ ( ) 2 120 4 2 5 5 5 7 8 144 24 78 533 23 24 24 adhesion a concrete a ips F c A c area of footing sides + area of footing invert lb ft ft ft ft . kips ft Therefore, . kips . kips kips . kips . kips FS . kips kips ⎡⎤ =+ = ⎣⎦ + = Therefore, (d) is the correct answer. Designing for uplift requires a FS 2. The uplift force Q uplift upon a footing can come from several sources, such as: (1) Wind flowing over a structure can cause negative pressures (suction); (2) Flooding waters raise the water table and uplift the foundations, etc. 428
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*Footings–02: Design a simple square footing with only an axial load. (Revision: June-09) Design a square reinforced concrete footing to support a 15”x15” column reinforced with 4 # 8 rebars, and the following parameters, ' 100 , 120 , 3,000 , 60,000 41 02 . 5 LLc y all ult D kips L kips f psi f psi q ksf from q ksf and a FS === = === 1) Determine the footing size B under service loads: 220 7.42 4 all Qk i p s B feet qk s f == = Therefore use B = 7.5 feet . 2) Now check the footing size B under ultimate loads (ACI 318-08-9.2.1), ( ) ( ) 1.2 1.6 1.2 100 1.6 120 192 312 ult L L L k i p s =+= + = + = QD () 2 2 312 5.5 7.5 ult O Q q B ft k s f = < 10 ksf = GOOD ult q 3) Compute the allowable concrete shear strength ca l l vv = ( ACI 11.5.2), ( ) 4 0.75 164 23.7 Ca l l p s i p s i k s = f 4) Find d, the effective depth . In this case, two-way (punching) shear governs. 15” B = 7.5 feet 15” + d For a square footing, 22 2 2 2 0 44 4 5.5 15 7.5 0 4 4 12 4 1.22 1.25 3.0 0 hich yield to two solutions for d = 2.47 oo o cc qq q dv d v w B w dd ft w ⎛⎞ ++ + −− = ⎜⎟ ⎝⎠ ⎡⎤ ⎞ ⎛ ⎛⎞⎛ ⎞ ++ + − − = ⎢⎥ ⎟ ⎜ ⎜⎟⎜ ⎟ ⎠ ⎝ ⎝⎠⎝ ⎠ ⎣⎦ + +− =
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This note was uploaded on 09/07/2011 for the course CEG 4012 taught by Professor Staff during the Spring '10 term at FIU.

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PE-18-Shallow-Foundations - 18 Shallow Foundations*01...

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