# ECGD 4122 – Foundation Engineering

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

Design of Strap Footings
Content Design of Strap Footings

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

Design of Strap Footings

Design of Strap Footings

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

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

Example 1

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)

Types of Mat Foundations

Types of Mat Foundations

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

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.

Example 2 - Solution

Compensated Mat Foundations
For no increase in the net soil pressure

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

Example 3 - Solution

Example 3 - Solution

Pressure Under Mat Foundations

Pressure Under Mat Foundations

Pressure Under Mat Foundations

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