Faculty of Applied Engineering and Urban Planning Civil Engineering Department 2nd Semester 2008/2009 ECGD 4122 – Foundation Engineering Lecture 6
Content • Eccentrically Loaded Foundations • Foundations with two-way eccentricity • Field Load Test
Effect of Eccentricity Q = total vertical load on the foundation M = moment on the foundation e = M/Q
Effect of Eccentricity Meyerhof’s Principle Of Effective Area Method
Ultimate Load with Eccentricity Step 1: Find Eccentricity, If e ≤ B/6 If e > B/6
Ultimate Load with Eccentricity Step 2: Determine the Effective Dimensions If eccentricity is in the direction of B B΄= B – 2e L΄ = L If eccentricity is in the direction of L B΄= B L΄ = L – 2e
Ultimate Load with Eccentricity Step 3:Use general equation to calculate the soil ultimate bearing capacity q'u= c΄ Nc Fcs Fcd Fci + q Nq Fqs Fqd Fqi + 0.5 γ B΄Fγs Fγd Fγi USE B΄ to calculate: Fcs , Fqs , Fγs DON’T use B΄ to calculate: Fcd , Fqd , Fγd
Ultimate Load with Eccentricity Step 4:Determine the total ultimate load: Qult = qult A΄ = qult B΄ L΄ Step 5:Factor of safety against bearing failure:
Example 1 For the footing shown below, determine the ultimate vertical loading with an 0.15 m eccentricity using the general formula. Take Nq = 18.4, N = 22.4.
Two Way Eccentricity USE B΄ and L΄ to calculate: Fcs , Fqs , Fγs DON’T use B΄ and L΄ to calculate: Fcd , Fqd , Fγd
Two Way Eccentricity Case I: eL/L ≥ 1/6 and eB/B ≥ 1/6
Two Way Eccentricity Case II: eL/L < 0.5 and 0 < eB/B < 1/6
Two Way Eccentricity Case III: eL/L < 1/6 and 0 < eB/B < 0.5
Two Way Eccentricity Case IV: eL/L < 1/6 and eB/B < 1/6
Example 2 Refer to Example 1 and the figure shown below to determine the ultimate vertical loading using the general formula.
Field Load Test Clayey Soil Sandy Soil qu(F) Ultimate capacity for proposed foundation qu(P) Ultimate bearing capacity for test plate B F Width of foundation B P Width of test plate
Example 3 Refer to the accompanied figure for the results of a plate load test on a sandy soil. Define the dimensions of a square footing to carry a load of 2500 kN with a permissible settlement of 25 mm. The test plate is 1’ in diameter.