Torsional load transfer

The torsional load-transfer tool evaluates a straight, prismatic, solid-circular drilled shaft using the named Florida District 7 or CDOT equations summarized in FHWA-OR-RD-16-14. It separates shaft and toe ultimate resistance, then solves a monotonic torque–rotation and depth-transfer response using explicit project calibration and elastic shaft twist.

Calibrated scope, not a universal torsion model

FHWA-OR-RD-16-14 reports two full-scale shafts and states that generalized transfer curves, cyclic behavior, and combined-load models require further research. PileCalc therefore requires calibration and rejects those unsupported extensions instead of inventing defaults.

Named ultimate-capacity methods

Tᵤ = Tₛ,ᵤ + Tₜ,ᵤ

Total ultimate torque

Shaft torque integrates the selected method's tangential interface resistance at the shaft radius. Florida District 7 excludes the upper 1.5 m; CDOT excludes the upper 1.5D. Cohesive resistance uses αSᵤ for Florida District 7 and Sᵤ for CDOT. Sand usesKσ'v tanδ, with the method-specific definition of K.

Tₛ,ᵤ = ∫ rₛ(z) πD²/2 dz · Tₜ,ᵤ = πD³αSᵤ/12

Shaft and cohesive-toe components

A toe component is included only when requested. Cohesive toe torque follows the named strength expression. Cohesionless toe torque follows the selected Florida District 7 or CDOT normal-force expression. The old ΣQside·r result remains visible only as a comparison component; it is not the new total.

Calibrated depth transfer

Each soil layer requires θ50, the local rotation that mobilizes 50% of its ultimate tangential transfer. An included toe requires its own value. These are project calibration inputs from relevant testing or an approved project-specific basis—not FHWA defaults. The request also requires that evidence's reference and maximum supported head rotation; the analysis domain cannot exceed it.

m(θ) = mᵤ |θ| / (θ50 + |θ|) sign(θ)

User-calibrated hyperbolic mobilization

The solver enforces elastic compatibility and torque equilibrium along the shaft usingJ = πD⁴/32. It solves the coupled first-order equations with a shooting method and reports the internal torque and rotation from head to toe.

dθ/dz = −T/(GJ) · dT/dz = −m(θ)

Elastic twist and distributed transfer

Service rotation and stiffness

Supply a positive service torque that is bracketed by the declared maximum-rotation domain. The response returns the converged head rotation, shaft and toe shares, secant stiffness, and a centered local tangent stiffness. The tangent half-step must be positive and no more than 5% of the service torque.

Ksec = Ts/θs · Ktan = dT/dθ |s

Service-state definitions

The full analysis is repeated at the requested mesh and at half resolution. Ultimate capacity, maximum-rotation torque, service rotation, and tangent stiffness must all pass the declared convergence tolerance (no more than 3%), or no response is published.

Inputs and applicability

L, D, E, GDeclared shaft domain

The current method accepts a vertical, prismatic, solid-circular, non-displacement drilled shaft. Taper, batter, custom perimeter or toe area, rock, and displacement-pile construction are rejected because the registered equations do not cover them.

Su, φ, K, σ′vStrength and groundwater

Cohesive layers require exactly one of undrained shear strength or unconfined compression strength. Florida District 7 clay is limited to Su/Pa ≤ 2.5. Its sand branch requires a declared K from K0 through 1.75; CDOT computes Kfrom L/D and φ. A water table always requires explicit water unit weight, and the declared solid-shaft unit weight cannot be less than water unit weight in the submerged portion of this method's domain.

θ50Monotonic transfer calibration

Every layer and included toe needs a positive rotation50 tied to a documented source. PileCalc does not substitute a generic value. Declare the calibration reference and its maximum supported head rotation. The requested response limit cannot exceed that range, and the absolute software ceiling is 15°.

Interaction and cyclic limits

Nonzero axial compression is accepted only for the Florida District 7 cohesionless-toe normal-force term. That is a named toe equation, not a generalized axial–torsional interaction surface. Nonzero axial load with cohesive toe, excluded toe, or CDOT therefore fails validation.

Cyclic response is not modeled

The public report observed local softening and hardening but did not establish a general global degradation law. Cyclic loading, reversal, accumulated rotation, and generalized axial–torsional side interaction are not modeled; such requests are rejected explicitly.

Reading the results

capacity.shaft / toe / total — named ultimate components; total is their exact sum.
capacity.allowable — total divided by the declared factor of safety, not a factored resistance.
service — torque, rotation, transfer shares, secant and centered tangent stiffness.
curve — monotonic head torque versus head rotation through the declared limit.
depthTransfer — rotation and internal torque from head to toe at the service torque.
equilibrium / convergence — load residuals and fine-versus-half-mesh differences.

Validation status

Method ID method.torsion.shaft-transfer is validated: automated tests cover the Florida District 7 and CDOT cohesive equations, reproduce the published TDSFB shaft capacity in FHWA-OR-RD-16-14 Table 5.3, and check a rigid-shaft analytical hyperbolic limit, equilibrium, monotonicity, mesh refinement, and physical SI/US twins.

See the API contract for exact fields and units.