DFI Magazine March/April 2020 - 106

ratio (ρ x ) percentages where the
increased system load capacity resulted
in connection zone cracking in the
bottom h e position, such that the
connection zone starts to influence the
response (see the deviation in dashed
and dotted lines of Figure 4b).
* The a/d ratio, which represents how
thick a concrete pile cap or footing is,
affects the system capacity significantly,
with lower ratios (i.e., thicker members)
providing exponentially higher system
capacities (see bi-linear nature of lines
in Figures 4a to 4c).
* The double plate bracket has only one
he position, which exhibits a good
performance since the connection zone
does not influence the system capacity
(see Figure 4c).
* The tensile capacity of all bracket types
increased significantly with higher ρx,
except for the bottom he, where the
connection zone failure prevents any
significant capacity increase with
higher ρx.
* The compression load results are
presented in Figures 4d to 4f, where the
overlapping, colored lines demonstrate
that the compressive load resistance is
independent of h e changes for all
bracket types examined.
Crack Patterns and Failure Modes
The simulated deflected shapes, crack
patterns and failure modes are presented in
Figure 5 for the selected, representative
specimens. Major connection zone
cracking and failures are predicted for

some of the specimens subjected to tensile
loads. The bottom he of the single plate
bracket exhibited the least favorable
performance by sustaining connection
zone failures, as shown in Figure 5a. While
performing better, the bottom he of the
studded bracket exhibited major
connection zone cracking (see Figure 5b),
which reduced, but did not govern, the
system capacity. This type of cracking may
be detrimental to the long-term durability
of helical pile foundations. The double
plate bracket (Figure 5c), the middle he of
the single plate bracket (Figure 5d) and the
middle he of the studded bracket (Figure
5e) performed satisfactorily with no major
connection zone cracking. The double
plate bracket may provide additional
advantages, such as resilience to other
types of loads and long-term durability, due
to the top and bottom plates confining the
entire depth of the pile cap. Subjected to
pure compression loads, all specimens
with all bracket types exhibited global
failure modes of either flexure or shear,
without exhibiting any major connection
zone cracking, as shown in Figure 5f.

Comparison with Sectional
Analysis Methods
Both the numerical simulations and the
experimental tests include the influence
and failure modes of connection zones. The
traditional sectional analysis methods, on
the other hand, consider the concrete
foundation globally while neglecting the
local influences such as how the load is
introduced or how the supports and

Figure 5. Representative failure modes and crack patterns

106 * DEEP FOUNDATIONS * MAR/APR 2020

connection zones are detailed. To assess the
significance of considering or neglecting
the connection zone behavior, the experimental specimens were also analyzed with
the sectional methods contained in the ACI
318-19 standard. It is clear from Figure 6
that the experimental capacities are much
smaller than those predicted by the
sectional analysis method. This result
confirms that the connection capacity may
govern the entire system response and that
the use of the sectional analysis may
overestimate the system capacity (by a
factor of 2.2 on average in this study).

Conclusions and Recommendations
* The results demonstrate that helical
pile-to-foundation connections may
govern the entire system capacity for
the load conditions incorporating net
tension components. The traditional
global analysis methods, which are not
intended for the connection zones, may
significantly overestimate the capacity
of the helical pile systems, especially for
the design configurations involving
single plate terminations at the bottom
embedment (he) position.
* The tension load capacities of the
concrete pile caps (all of which are
doubly and symmetrically reinforced)
are found to be 54% of their compression load capacities. If analyzed with the
traditional sectional analysis methods,
which neglect the influence of the
connection zones, their load capacities
in tension and compression would be
incorrectly calculated as equal.
DFI Magazine March/April 2020

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