Geotechnical Engineering Circular No. 9 Design, Analysis, and Testing of Laterally Loaded Deep Foundations that Support Transportation Facilities
Free Head vs. Fixed Head (Effect of Pile Cap)
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| 6.7.3 Free Head vs. Fixed Head (Effect of Pile Cap) The connection of the deep foundation element to the cap can be fixed such that the rotation of the pile head relative to the cap is not permitted. Fixing a steel pile head to the pile cap requires that the pile be imbedded about two to three times the pile diameter or pile width into the cap. If the pile is embedded only a nominal amount into the cap, it behaves as a pinned connection and is free to rotate relative to the cap. Most installations are somewhere between these two extremes. Drilled shafts and concrete piles generally are considered fixed to the substructure unit if the reinforcing from the shaft or pile is fully developed into the substructure unit. However, a pure fixed-head condition is seldom achievable in the field, even when a pile group is constrained by a stiff concrete pile cap, because the cap itself rotates. Pile group deflections increase as the degree of fixity at the pile head decreases thus, assuming complete fixity (zero rotation at the pile head) can result in underestimated values of pile head deflection, and incorrect magnitudes and locations of maximum pile bending moments. On the other hand, assuming a free-head condition will most likely result in an over-conservative design. The degree of rotational restraint at the top of a pile group usually falls somewhere between the limiting boundary conditions represented by fixed-head and free-head cases. At the Service Limit State, the pinned condition will generally result in a larger horizontal deflection. At the Strength Limit State, the fixed condition will generally result in a larger bending moment at the head of the pile but may result in a larger or smaller moment at depths below the head of the pile. If there is uncertainty whether a connection should be treated as pinned or fixed, it is recommended to perform the single pile analysis with both types of connections to evaluate how sensitive the results are to the head condition. If the results are sensitive, a more detailed analysis, such as a pile group analysis, should be performed. Some computer programs for lateral analysis of pile groups include the effects of pile cap rotation in the analysis. Group pile analysis is discussed further in Chapter 7. Download 6.03 Mb. Share with your friends:
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