Transmission And Substation Foundations - Technical Design Manual

A Basic Guideline For Designers APPENDIX C: HELICAL PILES & ANCHORS

V. P1, P2 And P3 – Structural Strength

not listed as an option in Section 7 of the TDM since it is considered unrealistic and ensures the capacity of the helical pile will better match long term performance. Therefore, Section 7 details the nominal, LRFD design and ASD allowable compression strength of helical piles in terms of “firm soil” [5’- 0 depth to fixity] and ”soft soil” [10’-0 depth to fixity]. Once the soil strength is determined, the designer must now consider the end condition (K) at the pile head and how it affects the effective length of the pile shaft. The connection to the structure (and the effective length) greatly affects the structural capacity of the pile. A pinned condition means the pile head is restricted against lateral translation (side to side movement) but is free to rotate as shown in Table C-C2.2 (b) from AISC 360-05. A pinned condition uses a K of 0.7. A fixed condition mean the pile head is restricted against both lateral translation and rotation as shown in Table C-C2.2 (a). A fixed condition uses a K of 0.5. To achieve a fixed end condition, the pile head has to be embedded at least 7.5” from the bottom of a concrete pile cap/footing/grade beam. Anything less than that is typically considered pinned. A pile with a fixed end condition has a shorter effective length, thereby having a greater stability and higher axial compressive strength. The compressive strength of a “free” headed helical pile (Table C-C2.2 (e) is not provided in the TDM. It can be provided as needed using a K factor of 2.0.

The axial and lateral strength of the helical pile components (shaft, helix and connection to structure) is determined per the methods detailed in AISC 360-10 Steel Construction Manual and Chapter 18 of the International Building Code (IBC). The structural strength of Chance helical piles is detailed in Section 7 of the TDM [Method 4]. The factors required for structural design are soil strength (firm, soft, fluid), the strength of the concrete, end condition (pinned, fixed, free), Application (new construction, remedial repair, tiebacks), coupling strength, and load direction (tension, compression, or both). Soil strength is an important factor because it affects buckling & bracing of helical piles. It is important to categorize the project soils as either “fluid” (N60=0), “soft” (0

TABLE C-C2.2 - APPROXIMATE VALUES OF EFFECTIVE LENGTH FACTOR K, FROM AISC 360-05

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