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ECGD 4122 – Foundation Engineering

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1 ECGD 4122 – Foundation Engineering
Faculty of Applied Engineering and Urban Planning Civil Engineering Department 2nd Semester 2008/2009 ECGD 4122 – Foundation Engineering Lecture 7

2 Design of Strap Footings
Content Design of Strap Footings

3 Design of Strap Footings
A strap footing is usually used to connect an eccentrically loaded column footing to an interior column footing.

4 Design of Strap Footings

5 Design of Strap Footings

6 Design of Strap Footings
Basic considerations for strap footing design: Strap must be rigid to control rotation of the exterior footing Footing dimensions should be proportioned for approximately equal soil pressures and avoid large differences in B to reduce differential settlements Strap should be out of contact with soil so that there are no soil reactions to modify the design assumptions

7 Design of Strap Footings
Design Procedure: Assume (e), i.e. assume L1 Compute S1 , R1, R2 4. Calculate L1 , B1 , L2, B2

8 Example 1

9 Example 1 - Solution Assume: e = 1.0 m S1 = S – e = 6.2 – 1.0 = 5.2 m
ΣMR2 = 0 5.2 R (580) = 0 R1 = kN ΣMR1 = 0 -1.0 (580) + 5.2(900) – R2(5.2)= 0 R2 = kN L1 = 2(e + x) = 2( ) = 2.4 m B1 = R1/(L1qult) = 691.5/[(2.4)(183)] = 1.57 (Use 1.60 m) B2 = L2 = (R2/qult)1/2 = (788.5/183)1/2 = 2.08 (Use 2.10 m)

10 Types of Mat Foundations

11 Types of Mat Foundations

12 Stress Analyses of Mat Foundations
For Saturated Clays with ϕ = 0 Applied Stress

13 Example 2 Determine the net ultimate bearing capacity of a mat foundation sized 13.0 × 9.0 m2 on a saturated clay deposit with cu = 94 kPa,  = 0, and Df = 2.0 m.

14 Example 2 - Solution

15 Compensated Mat Foundations
For no increase in the net soil pressure

16 Example 3 A mat foundation sized 30.0 × 40.0 m2 is to withstand a total dead and live load of 200 MN and located on a soft clay deposit with cu = 12.5 kPa and  = 18.8 kN/m3. Determine the depth of foundation for: A fully compensated case A factor of safety = 3.0

17 Example 3 - Solution

18 Example 3 - Solution

19 Pressure Under Mat Foundations

20 Pressure Under Mat Foundations

21 Pressure Under Mat Foundations


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