footing file 2.ppt
- 1. Reinforced Concrete 3 Dr. Abdul Razzaq Touqan Department of Civil Engineering 1
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- 3. Types of Footings Combined footings usually support two columns, when: 1. two columns are so close that single footings cannot be used 2. one column is located at or near a property line. 3
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- 5. • The combined footing can be modeled as a beam element. • The column loads are downward point loads and the line soil pressure is the uniformly distributed line load. • Shear reinforcement can be used. • The footing thickness must be determined or checked based on punching shear or moment- transfer strength. • A strip in the transverse direction under the column of width equals column side plus d/2 at each side of column, so strip width equals: c2 + d/2 for exterior column and c2 + d for interior column where c2 is the transverse dimension of column. 5
- 6. Go to sheets 6-10 المساكن تجربة 6
- 7. • Objective -Introduction to strap footings + continuous footings -Design of mat foundations 7
- 8. Cantilever or strap footings -consist of two single footings connected with a beam or a strap and support two single columns. This type replaces a combined footing and is more economical. -An example of this type will be explained in a general topic of tie beams and their usage. لها الفكرة 5 عالمات 8
- 9. Continuous footings support a row of three or more columns. They have limited width and continue under all columns. 5 Marks question: Can we analyze it as an indeterminate beam? 9
- 10. Rafted or mat foundation consists of one footing usually placed under the entire building area. They are used, when soil bearing capacity is low, column loads are heavy, single footings cannot be used, piles are not used and differential settlement must be reduced. 10
- 11. Mat foundation • A mat foundation is a combined footing that may cover the whole area under a structure supporting several columns and walls. • If spread footings cover more than half the building area, mat foundations may prove to be more economical. • Some of the common types of mat foundations are: -Flat plate: uniform thickness mat -Flat slab: mat with drop panels to resist punching shear upward or downward -Mat with beams: one way or two way -Slab (mat) with basement walls as part of the mat -Voided mat -Mat on piles 11
- 12. Mat on piles -Mats are sometimes supported on piles for the following reasons: 1. Total area of building is not enough to carry the loads. 2. The piles help in reducing the differential or total settlement of the structure located over highly compressive soil. 3. where the ground water table is high, mats are often placed over piles to control buoyancy. -Mat must be analyzed using finite element method and result of the finite element method must be compared with approximate methods. One of these methods is a conventional rigid method. 12
- 13. The conventional rigid method • 1. Determine area of mat: check stresses under the mat: • The maximum compression stress under the mat should be less than the soil allowable bearing capacity and there is no tension under the mat, this is the typical case. If tension exists, this should be taken into account and part of the area is excluded (avoid). • The stress under the mat is given by: • Where: • P= , A= , My= • Mx= , Iy= , Ix= • X= , Y= , • X and Y are the axes that pass through the centroid of the mat area. • 2. Determine thickness of mat: The thickness of mat • is controlled by wide beam shear and punching shear. • 13
- 14. 3. Design the mat for flexure: • The mat should be analyzed like flat plate in both directions. • The mat should be divided into strips (frames) in the two directions. • The pressure (stress) on soil shall be computed for the start and the end of the strip q1 and q2 and should be averaged. • Usually for a strip, the sum of the column loads are not equal to the soil reaction. Thus, modify column loads and pressure on soil to be equal. Then construct shear force and bending moment diagram. • Continue after this the same as a flat plate dividing moment between column strip and middle strip. Giving column strip for flat plate 67% of total moment is a wise decision. 14
- 15. Comparison of One-way and Two-way slab behavior One-way and two-way slab action carry load in two directions.
- 16. 22/3/2015 16
- 17. • Go to sheets 11-15 17