DFI Magazine March/April 2020 - 104
Foundation System Details
254
mm
254 mm (10 in)
19 mm
(0.75 in)
thick
160 mm
(6.3 in)
340 mm
(13.4 in)
he= 140 or 300 mm
(5.5 or 11.8 in)
51x51 mm
(2x2 in)
ρx= 0.2%, 0.4% or 0.8%
Tension
Studded Plate Bracket
Anchor bolts
Column
Base Plate
Pile
Cap
600 mm
(2 ft)
A concrete foundation system, representative of a pile cap, was designed using
two helical piles. This configuration creates
a one-way stress flow and helps better
isolate the connection response. The
helical piles are terminated with one of
three bracket types, as shown in Figure 1: a
single plate, a studded plate or a double
plate. The influencing parameters investigated include three bracket embedment
depths ( h e ); three longitudinal reinforcement percentages (also called reinforcement ratios, or ρx); three shear spanto-effective depth ratios (a/d ratios of 1.68,
1.42, and 1.11), which represent how thick
a concrete pile cap or footing is; and three
loading conditions: monotonic tension,
monotonic compression and reversed
cyclic.
d = 530 mm
(20.9 in)
2100 mm (6.9 ft)
he = 140, 300 or 460 mm
(5.5, 12 or 18.4 in)
Single Plate Bracket
CL
Double Plate Bracket
a = 890, 750 or 590 mm
(35, 30 or 23 in)
Experimental Approach
Computational Modeling
A two-dimensional (2D), continuum-type,
nonlinear plane-stress element was used
for the computational models through the
computer program VecTor2. The formulation employed is based on the Modified
Compression Field Theory (MCFT), which
has been adopted by several design
standards worldwide. The MCFT employs
a smeared, rotating crack approach within
a total-load, secant-stiffness solution
algorithm, and allows for the consideration
of the coupled flexure, axial and shear
effects. Although a 2D model is involved,
the triaxial concrete confinement is
accounted for by in- and out-of-plane
reinforcement components. Additionally,
VecTor2 incorporates a large number of
advanced material behavior models.
Although 2D numerical models are
computationally more efficient and faster
than 3D counterparts, they commonly
assume that stresses and strains are
constant through the out-of-plane
thickness of the elements. This assumption
is not valid when modeling helical pile
connections where the stresses propagate
in a conical shape from the edges of the
termination plate towards the surface of the
concrete, as shown in Figure 2a. This stress
pattern results in a conical failure termed
104 * DEEP FOUNDATIONS * MAR/APR 2020
Figure 1. Pile termination bracket types studied
Figure 2. The equivalent cone method
DFI Magazine March/April 2020
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DFI Magazine March/April 2020 - Intro
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