Footing design


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Footing design

  1. 1. Welcome To Presentation on Footing Design
  2. 2. Presented by Name Lamia Sultana Efa ID No
  3. 3. FOOTINGS What is Footing ? • Footings are structural members used to support columns and walls and to transmit and distribute their loads to the soil.
  4. 4. Different types of Footings: 1. Spread or isolated footing: A spread footing is provided to support an individual column. It is circular, square or rectangular of uniform thickness.
  5. 5. 2. Continuous or strip footing: The footing beneath a wall is known as a strip or continuous footing. It is provided for a raw of columns which are so closely spaced that spread footings are overlap or nearly touch each other.
  6. 6. 3. Combined footing: A Combined footing supports two columns. It is provided when two columns are close together, causing overlap of adjacent isolated footings and where soil bearing capacity is low, causing overlap of adjacent isolated footings.
  7. 7. 4. Strap or cantilever footing A strap footing consists of two isolated footings connected with a structural strap or a lever. The strap connect two footings such that they behave as one unit.
  8. 8. The Design Procedure. • 1.Determine the structural loads and member sizes at the foundation level; • 2.Collect all the geotechnical data; set the proposed footings on the geotechnical profile; • 3. Determine the depth and location of all foundation elements, • 4.Determine the bearing capacity, • 5.Determine possible total and differential settlements; check effects at 2B depths; • 6.Select the concrete strength (and possibly the mix),
  9. 9. The Design Procedure: • • • • 7.Select the steel grade, 8.Determine the required footing dimensions, 9.Determine the footing thickness, T (or D in some textbooks), 10.Determine the size, number and spacing of the reinforcing bars, • 11.Design the connection between the superstructure and the foundation, and • 12.Check uplift and stability against sliding and overturning of the structure-soil system.
  10. 10. Ground investigation for footings The recommended minimum depth of investigation, za, for spread foundations supporting high-rise structures and civil engineering projects is the greater of: za ≥ 3bF and za ≥ 6m, where bF is the foundation’s breadth. For raft foundations: za≥ 1.5bB, where bB is the breadth of the raft. The depth za may be reduced to 2m if the foundation is built on competent strata† with ‘distinct’ (i.e. known) geology. With ‘indistinct’ geology, at least one borehole should go to at least 5m. If bedrock is encountered, it becomes the reference level for za. Fig : Recommended depth of investigation for spread foundations
  11. 11. Design situations and limit states Examples of some ultimate limit states that footings must be designed to withstand
  12. 12. Design considerations for footings Euro code 7 lists a number of things that must be considered when choosing the depth of a spread foundation
  13. 13. Footings subject to vertical actions • For a spread foundation subject to vertical actions, Euro code 7 requires the design vertical action Vd acting on the foundation to be less than or equal to the design bearing resistance Rd of the ground beneath it: Vd ≤ R d Vd should include the self-weight of the foundation and any backfill on it. • This equation is merely a re-statement of the inequality: • Ed ≤ Rd. • Rather than work in terms of forces, engineers more commonly consider pressures and stresses, so we will rewrite this equation as: • qEd ≤ qRd • where qEd is the design bearing pressure on the ground (an action effect), and qRd is the corresponding design resistance.
  14. 14. Figure 1 shows a footing carrying characteristic vertical actions VGK (permanent) and VQK (variable) imposed on it by the super-structure. The characteristic self weights of the footing and of the backfill upon it are both permanent actions (WGK). The following sub-sections explain how qEd and qRd are obtained from VGK, VQK, WGK, and ground Figure 1. Vertical actions on a spread foundation
  15. 15. Effects of actions The characteristic bearing pressure qEK shown in Figure 1 is given by: Where Vrep is a representative vertical action; VGK, VQK, and WGK are as defined above; A' is the footing’s effective area and ψi is the combination factor applicable to the ith variable action. If we assume that only one variable action is applied to the footing, this equation simplifies to: since ψ = 1.0 for the leading variable action (i = 1). The design bearing pressure qEd beneath the footing is then: Where γG and γQ are partial factors on permanent and variable actions, respectively.
  16. 16. Eccentric loading and effective foundation area • • The ability of a spread foundation to carry forces reduces dramatically when those forces are applied eccentrically from the centre of the foundation. To prevent contact with the ground being lost at the footing’s edges, it is customary to keep the total action within the foundation’s ‘middle-third’. In other words, the eccentricity of the action from the centre of the footing is kept within the following limits: where B and L are the footing’s breadth and length, respectively; and eB and eL are eccentricities in the direction of B and L (see Figure 2) Figure 137. Effective area of spread foundation
  17. 17. Different types of load: • • • • • • • Dead load Live load Earth pressure Water pressure Earthquake loads Wind loads Snow loads
  18. 18. PROCEDURE (ACCORDING TO SIGHT) Step 1 : Plan the design
  19. 19. Step 2 : Start digging
  20. 20. Step 3 : Frame the footing
  21. 21. Step 4 : Set the column
  22. 22. • • • • • Step Step Step Step Step 1 2 3 4 5 : : : : : Measure the footing Grading Concrete calculation Pouring a footing Setting column
  23. 23. MATERIALS A)Sand B)Cement C)Brick chips & stone chips D)Reinforcement bar