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Licture_7 - Foundation Foundation Foundation • The lowest...

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Unformatted text preview: Foundation Foundation Foundation • The lowest part of the structure is generally referred to as the foundation. Shallow Foundations Function • The main function of the foundation is to transfer the superstructure load to the underneath soil. Characteristics • A properly designed foundation transfers the load throughout the soil layers without causing excessive settlement or shear failure of the soil. Foundation Foundation Excessive settlement • can result in structural damage to building frames, cracks in walls, tiles, and to sticking doors and windows. • In some cases it may lead structure tilting. Soil shear failure • can result in excessive building distortion and even structure collapse. Excessive Settlement wall cracks due to tilting due to soil bearing wall differential settlement capacity failure. 1 Soil Shear Failure Bearing Capacity Ultimate Bearing Capacity • Is the load per unite area of the foundation at which shear failure in soil occurs. Small footing P (ton) Larger footing P (ton) Storage silos tilting due to bearing capacity failure Large stress Small stress Foundation Types 1. Shallow Foundation Foundation Types 2. Deep Foundation Df Df B soil B soil soil soil a) Spread footing b) Mat foundation c) Pile foundation d) Drilled shaft foundation Df < 3B 2 Shallow Foundation Isolated (Spread) Footing Types Q (kN) (kN) Q (kN) (kN) Shallow Foundation Isolated (Spread) Footing Types q (kN/m) (kN/m) Q (kN) (kN) f f Modes of failure under foundations 1. General shear failure Dense sand or stiff clay soil. The failure surface in the soil extend to ground surface 2. Local shear failure Sand or clay soil of medium compaction. A considerable movement of the foundation is required for the soil failure surface to extend to the ground surface. 3 3. Punching shear failure Fairly loose soil. The failure surface will not extend to ground surface. Variation of qu with Dr For Dr < 0.35 failure is Punching shear failure For 0.35 < Dr < 0.7 failure is local shear failure For Dr > 0.7 failure is general shear failure Variation of foundation settlement, S with Dr Terzaghi’s Bearing Capacity Equation • • • Continuous or Strip foundation. The failure is general shear failure. The soil above the foundation level is replaced by equivalent surcharge q = γDf. 4 Ultimate bearing capacity equation Ultimate bearing capacity cohesion term Bearing capacity factors 1000 Bearing capacity factors Nc, Nq, Nγ 100 Nc = cot φ ' Nq − 1 Nc Nq Nγ ( ) qu = c' Nc + qN q + 1 γBNγ 2 below foundation level 10 Nq = Nγ = For φ = 0 Nc = 5.7 Nq = 1.0 Nγ = 0.0 e 2 (3 π / 4 −φ' / 2 ) tan φ' 2 cos 2 (45 + φ' / 2 ) ⎞ 1 ⎛ Kp ⎜ ⎜ cos 2 φ' − 1 ⎟ tan φ' ⎟ 2⎝ ⎠ above the foundation level 1 c’= effective soil cohesion, γ = soil density, and B = foundation width. Nc , Nq , and Nγ = bearing capacity factors, depends on soil friction angle, φ’ 0.1 0.01 0 5 10 15 20 25 30 35 40 45 50 Soil friction angle φ (deg.) Or from Table 3.1 p.87 Example For the shown continuous strip footing. It is required to determine the ultimate bearing capacity of soil. Use a factor of Safety F.S = 4, calculate the load, qall, the footing can carry. q a ll (Ib/ft) γ = 115 Ib/ft3 φ = 25º 25º ’ = 400 Ib/ft2 c 2 ft 2.5 ft 5 ...
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