Fayoum University. Dr.: Youssef Gomaa Youssef

Transcription

1 Fayoum University Faculty o Engineering Department o Civil Engineering CE 40: Part Shallow Foundation Design Lecture No. (6): Eccentric Footing Dr.: Yousse Gomaa Yousse

2 Eccentric Footing Eccentric ooting: spread or wall ooting that also must resist a moment in addition to the axial column load. e < /6

3 Eccentric Loads or oments P P P D. L L. L e ( PW ) e < /6

4 Eccentric Footing Combined axial and bending stresses increase the pressure on one edge or corner o a ooting. We assume again a linear distribution based on a constant relationship to settling. I the pressure combination is in tension, this eectively means the contact is gone between soil and ooting and the pressure is really zero. To avoid zero pressure, the eccentricity must stay within the kern. The maximum pressure must not exceed the net allowable soil pressure. CE 40: Foundation Engineering Design

5 The kern To avoid zero pressure, the eccentricity must stay within the kern. /3 CE 40: Foundation Engineering Design

6 Eccentric Loads or oments Cases e </6 Case (a) e =/6 Case (b) e >/6 Case (c) CE 40: Foundation Engineering Design

7 Eccentric Loads or oments For case and B (not case C) min P * B ( 6e ) max P * B ( 6e ) For case c C max * P 3* C * B 3C max 0 a e >/6 Case (c) CE 40: Foundation Engineering Design

8 Footing Subjected to Double oment P e eb ( 6 6 * B B ) For contact pressure to remain (+) ve everywhere, 6e 6e B B.0 CE 40: Foundation Engineering Design

9 Design o Eccentric Footing Plain concrete ooting (P.C.) P.50P. rea * B ssume a G S rea t P F.L H D H * D F. L P F. L.5* PG. s Check max a e 6 PF. L 6e ( ) * B e P F. L F. L B t max min ssume thickness o P.C.: Dim. o P.C. = * B * t t = (0.5 to 0.50) CE 40: Foundation Engineering Design

10 Design o Spread Footing Reinorced concrete ooting (R.C.) X ( 0.8.0)t X e, P G. S P 6e ( G. S * B ) B B X a P I 3 a ( ) / ( 3 a )( ) * 3 t II ( B b ) )*( 3 d C b * Fcu t d cover Dim. o R.C. = * B * t CE 40: Foundation Engineering Design

11 Design o Spread Footing Shear Stress: P ( 4 Q s ( II ) d ) a *( ) a ( Q s ( III ) d ) B b *( ) t (II) d s su Qs b* d cu 0.75 c su 4 (II) Notes: I s > su, Increase d No shear RFT in Footing. Q s : shear orce at critical sec. (II). s : shear stress. su : ultimate shear strength. CE 40: Foundation Engineering Design

12 Design o Spread Footing Punching Stress: a P Q p ( ).50PG. S [( a d)*( b d)] t d ** ( a d ) ( b d ) p p Q p p d/ a d/ 0.5 ( a / b) / / cup cu c cu c d/ b d/ I p > cup, Increase d CE 40: Foundation Engineering Design

13 Design o Spread Footing Footing Reinorcement: s y I * d * j a s y II * d * j t ain RFT s ain RFT s Notes: inimum number o bars per meter is ive. inimum diameter or main RFT is mm. Number o bars may be taken 5 to 8. Diameter o bars may be selected rom to 8mm. CE 40: Foundation Engineering Design

14 Example(): Design o Eccentric Footing ake a complete design or a ooting supporting a 30cm X 60cm column load o 0t at ground surace (.), 0m.t moment and 0t horizontal orce at. The oundation level is.00 m below. and the net allowable bearing capacity is 0.80kg/cm. ake the design considering the ollowing two cases: - with plain concrete base - without Plain concrete base a = 0.60m. B= 0.30m p = 0t =0m.t H=0t cu = 50kg/cm. y =3600kg/cm a = 0.80kg/cm = 8t/m. CE 40: Foundation Engineering Design

15 Design o Eccentric Footing Plain concrete ooting (P.C.) rea.50* P.50*0 8 G. S * B. 5m a ssume rea m F. L 0 0* mt t t P H=0 P F.L.00 PF. L.5* PG. s.5*0 38t e max P F. L ssume thickness o P.C.: t = F. L *0.90 ( ) 8 5* B 5.00 e 6 CE 40: Foundation Engineering Design B 4. 65m max min Dim. o P.C. = 5.00* 4.65 * 0.30

16 Design o Eccentric Footing Reinorced concrete ooting (R.C.) X t 0. 30m X 5.00 * m B B X 4.65 * m 0.60 e I P 0 0*.70 G. S m.50*0 6*0.308, ( ) * II ( ) / ( )*( ) ( )( ) CE 40: Foundation Engineering Design *0 d cm 00*50 t cm * t F =4.34 F 3 =0.67 =4.40 P.90 F =5.85 Dim. o R.C. = 4.40 * 4.05* 0.75

17 Design o Spread Footing Shear Stress: P ( ) *( ) Q s ( ) ( II ) *( ) 5.7 (II) d t Q s ( ) ( III ) *( 0.70).86 =5.85 s.7* kg/ cm *70 =4.34 (II) 4 =.84 =4.40 Notes: I s > su, Increase d No shear RFT in Footing. Q s : shear orce at critical sec. (II). s : shear stress. su : ultimate shear strength. CE 40: Foundation Engineering Design

18 Design o Spread Footing Punching Stress: a P Q p p ( ).50*0 [( )*( )] **[( ) ( )] 3. t p Q p p p 66.88* *0 5.8 d/ a d/ 0.5 ( a / b) / / cup cu c cu c d/ b d/ I p > cup, Increase d CE 40: Foundation Engineering Design

19 Design o Spread Footing Footing Reinorcement: *0 3600*70*0.86 s *0 3600*70*0.86 s ain RFT 68/m ain RFT 88/m Notes: inimum number o bars per meter is ive. inimum diameter or main RFT is mm. Number o bars may be taken 5 to 8. Diameter o bars may be selected rom to 8mm CE 40: Foundation Engineering Design

20 Crane Footing aximum stress on soil should be less than allowable bearing capacity max a The ratio between maximum and minimum stresses should be less than our max min 4 max min ( 6e / ( 6e / ) ) 4 0e CE 40: Foundation Engineering Design

21 Uniorm Stress below Footing Subjected to oment Uniorm stress reuired that the eccentricity at oundation level eual zero P F. L e 0.0 e 0. 0 G. s. e a/ P c c a e P F.L.5P * B G. S B a a CE 40: Foundation Engineering Design