Types of Foundations

Transcription

1 Shallow Foundations Types of Foundations Foundations can be classified to two major categories: Shallow. Deep. 1

2 Introduction If the soil stratum is suitable for supporting the structural loads from the superstructure is located at a relatively shallow depth, the foundation is called shallow. If the upper layer soil strata are too weak to carry the structural loads, the loads may be transferred to more suitable deeper layers using deep foundations. Requirements for Foundations The foundation must be placed at an adequate depth. Stresses should not exceed the soil strength and total/differential settlement should be within tolerable limits. The foundations must be designed based on the structural loads. Foundations should be constructed from materials that withstand any harmful chemicals (specially sulfates and chlorides) in the ground / groundwater. 2

3 Requirements for Foundations Foundations should be placed below the top organic soil, fill materials, old abandoned foundations, & debris. Foundations should be placed below surface layers affected by seasonal temperature or moisture changes or by erosion. Foundations should be placed sufficiently away from the edge of a sloping ground. Foundations should be placed at an acceptable level with respect to adjacent foundations. The difference in levels between adjacent foundations should not cause undesirable overlapping between stresses. Types of Shallow Foundations Wall (Strip) footing is provided to support a wall in case the structure is bearing wall type. 3

4 Types of Shallow Foundations Isolated footing is used to support one column as in skeleton type buildings. Types of Shallow Foundations Isolated footing is used to support two columns. 4

5 Types of Shallow Foundations Strap footing is used when isolated footings are subjected to large eccentric loading as in case of an edge or exterior footing where the property line limits the extension if the footing needed to make the footing concentric with the column it supports. The strap beam footing consists of a rigid beam connecting the exterior footing to an interior footing in order to transmit the unbalanced shear and moment. Types of Shallow Foundations 5

6 Types of Shallow Foundations The mat or raft covers the entire area of the superstructure as it supports all the columns of the structure. Flat Plate Flat Plate thickened under columns Slab and beam Flat Plate with Cairo pedestals University Slabs with basement wall Strip Footing In the design of footings, the soil pressure is assumed to be uniform. Compute width of footing (B) using the allowable bearing pressure: B = Pt / qall(gross) Or B = Pnet / qall(net) Pnet = load at ground surface Pt = Pnet + weight of footing + soil above foundation level = approximately Pnet 6

7 Strip (wall) footing Strip (wall) footing 7

8 Strip Footing Thickness of plain concrete base t (optional) is typically taken as cm. Determine the projection (x) of the plain concrete footing. Typically, (x) ranges between (t) and (0.8t). Carry out the structural design of the footing i.e. determine the depth and reinforcement. The cover for the reinforcing bars ranges between 50 to 70 mm to protect reinforcement against chemical attack. The minimum diameter of reinforcing bare used in foundations is 12 mm. Strip Footing Maximum bending moment: (Section I-I : for a footing with a wall beam) Where p n is the net contact stress between the reinforced concrete footing and the plain concrete base and is equal to Pn/B1. If a masonry wall rests directly on the reinforced concrete footing, the critical section lies at a distance (b/4) beyond the face of the wall, i.e., midway between the center of the wall and the face of the wall 8

9 Strip Footing Check of shear: (Section II-II) The shear stress should not exceed the allowable shear stress of concrete (typically 6 kg/cm2). Centrically Loaded Isolated Footing Determine the footing area AxB using the allowable bearing pressure. A x B = Pt / qall(gross) Or A x B = Pnet / qall(net) Pnet = load at ground surface Pt = Pnet + weight of footing + soil above foundation level = approximately Pnet 9

10 Centrically Loaded Isolated Footing Centrically Loaded Isolated Footing 10

11 Centrically Loaded Isolated Footing Determine the footing dimensions A and B following preferably the recommendations given below. (i) If the column is square or circular in section, take A equal to B. (ii) If the column is rectangular in section (a x b), determine A and B such that the projections from the column faces are equal in both directions Centrically Loaded Isolated Footing Select the thickness of the plain concrete base (typically between 0.25-m and 0.80-m). Determine the projection of the plain concrete footing which is typically taken in the range between (0.8 t) and (t). Carry out the structural design of the footing i.e. determine the depth and reinforcement of the reinforced concrete base. 11

12 Maximum Bending Moment p n = Net contact pressure = reduction factor equal to 0.85 which takes into account the effect of including the pressure acting over the common area aekg in calculating both moments M m1 and M n1. Check of Shear 12

13 Check of Punching Eccentrically Loaded Isolated Footing Determine the footing dimensions A and B following preferably the recommendations given below. (i) If the column is square or circular in section, take A equal to B. (ii) If the column is rectangular in section (a x b), determine A and B such that the projections from the column faces are equal in both directions 13

14 Eccentrically Loaded Isolated Footing Eccentrically Loaded Isolated Footing Consider a footing subjected to a vertical force P n, a horizontal force Q and a moment M. At the foundation level, the forces acting are : (i) Vertical force P t which includes the force P n and the weight of the footing and the soil above it W. (ii) Horizontal force Q. (iii) Moment = W + Q.h = M + Q (t+t 1 ) 14

15 Eccentrically Loaded Isolated Footing In order to determine the stress under the footing, the moment may be removed by shifting the vertical load to a fictitious location with an eccentricity e, where In the design of an eccentrically loaded footing, the stress distribution is assumed to be linear. Eccentrically Loaded Isolated Footing 15

16 Eccentrically Loaded Isolated Footing 1- e < A/6 (i.e. resultant inside middle third), under such condition the pressure distribution will trapezoidal with the maximum and minimum intensities of pressure obtained as follows Eccentrically Loaded Isolated Footing 2- e = A/6 (i.e. resultant on the edge of core) the pressure distribution will be triangular. 16

17 Eccentrically Loaded Isolated Footing 3- e > A/6 (resultant outside middle third) The entire base of the footing is not considered effective because soil cannot resist tension. The maximum pressure is, in this case, given by: Eccentrically Loaded Isolated Footing Special Considerations The maximum soil pressure q max for all load combinations ( dead and live loads and moments) must not exceed 1.2 q all where q all is the allowable pressure under axial load only. If the column loads include crane loads, the entire footing area should be effective and the pressure distribution should satisfy the following condition: q min 0.25 q max 17

18 Eccentrically Loaded Isolated Footing Special Considerations For footings subjected to permanent moments, column could be placed off the center such that the resultant passes through the centroid of the footing (i.e. e = 0) and the stress distribution becomes uniform. If there are no cranes, a triangular pressure distribution is permissible, but at least three quarters of the footing area is effective, y 0.75 A Eccentrically Loaded Isolated Footing Permanent Eccentricity Uniform Stresses 18

19 Eccentrically Loaded Isolated Footing Special Considerations Safety against sliding should be checked as follows: Where: P t 2 Q = coefficient of friction between the footing and soil. Sliding may be restricted by connecting the footings with ground beams. Design of Combined Footing 19

20 Design of Combined Footing 1- Determine the value and position of the resultant R (Rnet = P1 + P2), Rt = P1 + P2 +W) Where W = own weight of footing and soil above it which can be assumed 10-15% of R. 2- Determine the area of the footing using the allowable bearing capacity A.B = R net /q allnet OR A.B = R t /q allgross Design of Combined Footing 3- Determine the footing dimensions A and B such that the centroid of the footing and the center of gravity of the column loads coincide. 4- Select the thickness (t) of the plain concrete base. A value ranging between 0.50 m and 1.0 m is usually chosen. The projection (x) of the plain concrete base can be taken in the range between (0.8t) and (t). 20

21 Design of Combined Footing 5- Draw shear and moment diagrams for the footing in the longitudinal direction. The column loads may be taken as concentrated loads for computing shear and moment diagrams or more accurately as distributed loads. In the former case, the moments at the columns are calculated. Design of Combined Footing 6- The maximum positive moment takes place at the point of zero shear at a distance (x m ) which can be calculated as follows: p n = net contact stress 21

22 Design of Combined Footing 7- Determine the depth of the reinforced concrete footing necessary to resist the maximum bending moment and compute the area of steel A sl ; A s2 and A s3 ) required to satisfy bending in the longitudinal direction. Design of Combined Footing 8-Check punching shear for each of the two columns as for isolated footings. If the column is located at the property line, the punching shear force and stresses will be calculated us follows: 22

23 Design of Combined Footing Column located at the property line Design of Combined Footing 9- Determine the area of steel in the short direction considering each column to be supported by hidden beams of widths W 1 and W 2 respectively; If the column is located at the property line, 23

24 Design of Combined Footing 9- Calculate the bending moments at the faces of the columns Design of Combined Footing 10- Calculate the required areas of main steel for the five moments. Use 5 12 /m as secondary steel in the form of upper and lower meshes. Distribute the hidden beam main reinforcement over the beam width W 1 and W 2. 24

25 Strap Footings Frequently, isolated footings cannot be extended beyond the face of the supported columns as for example when columns are close to property line. In this case the isolated footing will be subjected to a uneven eccentricity which would most probably lead to excessive tilting. In order to avoid such situation, two alternatives may be used as illustrated in: Combined footing Strap footing Strap Footings 25

26 Strap Footings When the distance between the edge column and the adjacent interior column is large, it will be more economical to use a strap footing. The strap footing may be regarded as two isolated footings connected by a member termed a strap beam. Its function is to transmit the unbalanced moment from the unbalanced exterior footing to the interior footing in order to obtain uniform pressure distribution beneath both footings. Strap Footings In design, the strap beam is considered to be a pure flexural member and does not take soil reaction. The strap must be sufficiently rigid for the solution to be valid. Its inertia should be equal to or greater than twice the footing inertia to avoid exterior footing rotation. 26

27 Design of Strap Footing 1- Assume a reasonable value for A e and determine the corresponding eccentricity. Design of Strap Footing 27

28 Design of Strap Footing 2- Sum moments about the center of the interior column and obtain the soil reaction beneath the exterior footing. Where W e = Own weight of footing and soil above it (approximately 10-15% of P e ). The other dimension B e of the footing can thus be determined, Design of Strap Footing If B is too large or too small compared to A, steps 1 and 2 can be repeated until satisfactory dimensions are obtained. B e should not be greater than 2A e. 3-Sum moments about the center of the exterior footing and obtain the soil reaction beneath the other footing. 28

29 Design of Strap Footing The footing dimensions A i and B i can then be selected to satisfy the following equation: Design of Strap Footing In order to design the strap beam and the individual footings, determine the net contact reactions and pressure between the reinforced concrete footings and the plain concrete bases. 29

30 Design of Strap Footing The bending moment and shearing force diagrams can be drawn, for the strap beam and the structural design can then be performed. It may be noted that the own weight of the strap can be included in the calculations. The footings are assumed to act as double cantilevers and can be designed in the same way an for wall footings. Design of Strap Footing 30