Non-Linear Finite Element Analysis and Parametric Study of Four-Pile Pile Caps

Abstract

Piles and pile caps are commonly used in the Netherlands due to the soft shallow subsurface soil that is predominant in the country which does not have sufficient bearing capacity to support heavy structures. Pile caps are currently designed analytically using the strut and tie model (STM). This is believed to be conservative and results in an over-reinforced structure with higher cost and unsustainable design due to inefficient use of materials. The main objective of this thesis is to investigate the application of Non Linear Finite Element Analysis (NLFEA) to design pile caps. Five experiments were selected from literature and modelled in DIANA. These pile caps had flexural, corner shear, flexure-induced punching and combined flexure and corner shear failure modes. Quarter of the pile caps were modelled using Finite Element Model (FEM) as it saves computational time and cost by making use of symmetry while still predicting the failure mechanism and failure load within 99% of the full model. The reinforcement was modelled using both embedded and Shima bond-slip. The FEM results were subsequently compared with the experiment to gain insight into how accurately FEM can capture the structural response of pile caps. The comparison shows that failure mechanism and crack pattern can be accurately predicted for all pile caps. However, the accuracy of the failure load depends on the failure modes of the pile cap as ductile failures are captured more accurately than those with brittle failure. The difference between the peak load in the FEM and the experiment is observed to be 5 - 7% for ductile failures while it varies between 25 - 42% for brittle failures. These differences are liberal estimates. Moreover, three pile caps that were designed using STM were modelled numerically to obtain the design resistance and compare the results. The comparison show that STM overestimates the stresses in the concrete by 40% – 70% as well as the crack width by 60 – 65%. This is because the effect flank reinforcement and post cracking contribution of concrete are not accounted in the STM. Numerical model results are also closer to the experimental results than analytical calculations by 50% on average. The comparison between STM and numerical model revealed that optimization of pile caps is possible. Subsequently, four parameters: pile cap geometry, bottom rebar percentage, number of flank rebar and concrete quality were reduced to evaluate the effect on the structural response of pile cap. These parameters were selected based on the interview with experts and results of the comparison between the FEM and experimental results. The parametric study was performed on a pile cap with punching failure. It was found that reducing the pile cap depth by 0.1m increases the rebar stress by 25 - 35% and reduces the failure load by 2 - 8%. Reduction of the bottom rebar percentage by 10% increases the crack width by 15 - 30% and lowers the failure load by 2 - 8%. A 50% decrease in the number of flanks is found to increase the stress in the bottom reinforcement by 20 - 25% but not affect the failure load significantly. Change in these three parameters does not change the failure mode and the failure load remained greater than the design load. However, decreasing concrete quality accelerates the onset of crack which decreases failure load and changes the failure mechanism from punching to corner shear. Cost analysis and environmental impact assessment also show that geometry optimization has more environmental and cost advantage than reducing the reinforcement. For every 0.1 meter reduction in depth, there is a 6% reduction in cost per pile cap and a 70 - 200 kg reduction in the CO2 footprint. Two sets of experiments were designed to validate the key findings of this thesis. The first set was designed to investigate if punching failure can be accurately predicted by FEM. This will be conducted on a scaled down pile cap with expected punching failure. A second set of experiment was designed to explore if the optimization observed in the numerical models can be achieved in reality. Two pile caps, with brittle and ductile failure were selected. Each will have a variable geometry, bottom rebar percentage, flank reinforcement and concrete quality. The current STM approach does not capture all the failure modes of pile caps since the unity check does not distinguish between certain failures such as concrete crushing and punching. It also does not account for the contribution of flank reinforcement and concrete contribution to the tensile strength post-cracking. Therefore, future designs of pile caps should take these parameters into account to obtain a safe design without underestimating the capacity of the pile cap. This would result in a more efficient design with lesser material and lower cost.