Topic 9 Axial Load Capacity - Static Load Test [PDF]

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ECE 5413 SHALLOW & DEEP FOUNDATION DESIGN TOPIC 9 LOAD TRANSFER AND LIMIT STATE 0. Objective ASD:



LRFD:



1. Conventional Static Pile Load Test 1.1. Test Setup



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ECE 5413 SHALLOW & DEEP FOUNDATION DESIGN 1.2. Interpretation of Test Results •



Modulus of Elasticity a) Steel: 𝐸𝐸𝑠𝑠 = 200 GPa or 29,000,000 psi



b) Concrete:



𝐸𝐸𝑐𝑐 = 4700�𝑓𝑓𝑐𝑐′ (SI) or 𝐸𝐸𝑐𝑐 = 57000�𝑓𝑓𝑐𝑐′ (English)



c) Reinforced concrete or concrete filled steel pipe: 𝐸𝐸 = 𝐸𝐸𝑐𝑐 (1 − 𝜌𝜌) + 𝐸𝐸𝑠𝑠 𝜌𝜌



d) Timber: varies between 7 GPa – 10 GPa (Species dependent) •



Load Capacity a) Davisson Method The nominal axial load capacity is defined at the intersection point of the loadsettlement curve from static test and the following straight line, S=4 mm + B/120 + PD/(AE)



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ECE 5413 SHALLOW & DEEP FOUNDATION DESIGN Example 1: The load-settlement curve shown below is obtained from a static load test on a 400 mm diameter, 17 m long drilled shaft with 𝑓𝑓𝑐𝑐′ = 40 MPa and 𝜌𝜌 = 0.055. Use Davisson method



to compute the nominal axial downward load capacity.



b) Brinch Hansen Method Brinch Hansen (1963) proposed a parabolic relationship for stress-strain properties of soil near failure. He noted that the strain in soil at failure is four times the strain that corresponds to a stress equal to 80% of failure, and twice the strain that corresponds to a stress equal to 90% of failure. (Based on cohesive soils) Others have since extrapolated this concept to the interpretation of static load tests, and thus producing the Brinch Hansen 80% criterion and Brinch Hansen 90% criterion.



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ECE 5413 SHALLOW & DEEP FOUNDATION DESIGN Example 2: A 16 in diameter, 77.5 ft long auger pile has been designed using ASD to support a downward load of 350 kips. A prototype pile was constructed using concrete with 𝑓𝑓𝑐𝑐′ =



5,000 lb/in2 and a steel ratio of 𝜌𝜌 = 2%. The static load results are shown in the figure below. Intercept the results using both Davisson and Britch Hansen 90% methods.



𝑃𝑃, kips 745 746 747 748 749 750 751 752 753 754 755 756 757 758 759 760 761



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𝛿𝛿, in 1.62 1.64 1.67 1.69 1.72 1.75 1.78 1.81 1.84 1.87 1.90 1.94 1.98 2.02 2.06 2.10 2.15



0.9𝑃𝑃, kips



𝛿𝛿 at 0.9𝑃𝑃



𝛿𝛿 Ratio



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ECE 5413 SHALLOW & DEEP FOUNDATION DESIGN 2. Instrumented Static Pile Load Test 2.1. Strain Gages



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ECE 5413 SHALLOW & DEEP FOUNDATION DESIGN Example 3: A strain gage was embedded in the auger pile as in Example 2. This gage was located 5 ft above the toe, and the test results are shown in the following figure. Using this data, and the Brinch Hansen 90% load capacity, compute the average 𝑓𝑓𝑛𝑛 and 𝑞𝑞𝑛𝑛′ for this pile.



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ECE 5413 SHALLOW & DEEP FOUNDATION DESIGN



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ECE 5413 SHALLOW & DEEP FOUNDATION DESIGN 2.2. Telltale Rods



Example 4: Two telltale rods have been installed in a 45 ft long closed-end PP20×0.75 pile. This pile was subjected to a static load test, which produces the following results: Test load at failure:



250 kips



Settlement reading at failure: At pile head (gage #1)



0.570 in



Telltale anchored at 20 ft depth (gage #2)



0.530 in



Telltale anchored at 45 ft depth (gage #2)



0.503 in



Compute the force in the pile at 20 and 45 ft, and compute the average 𝑓𝑓𝑛𝑛 values on the pile and



the 𝑞𝑞𝑛𝑛′ value.



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ECE 5413 SHALLOW & DEEP FOUNDATION DESIGN



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ECE 5413 SHALLOW & DEEP FOUNDATION DESIGN 2.3. Osterberg Load Test (O-Cell Test)



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