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APPENDIX 2H
DOCUMENTATION
Chapter 2
Helical Foundation Systems
ESR-3074
|
Most Widely Accepted and Trusted
Page 3 of 9
The ASD capacities of FSI helical foundation system
components are indicated in Tables 1, 2, 3, and 5. The
geotechnical analysis must address the suitability of the
helical foundation system for the specific project. It must
also address the center-to-center spacing of the helical
piles, considering both effects on the supported
foundation and structure and group effects on the pile-soil
capacity. The analysis must include estimates of the axial
tension and/or compression capacities of the helical piles,
whatever is relevant for the project, and the expected total
and differential foundation movements due to single pile
or pile group, as applicable.
A written report of the geotechnical investigation must
be submitted to the code official as one of the required
submittal documents, prescribed in Section 107 of the
2012 and 2009 IBC (Section 106 of the 2006 IBC), at the
time of the permit application. The geotechnical report
must include, but need not be limited to, the following
information:
1. A plot showing the location of the soil investigation.
2. A complete record of the soil boring and penetration
test logs and soil samples.
3. A record of soil profile.
4. Information on groundwater table, frost depth and
corrosion-related parameters, as described in Section
5.5 of this report.
5. Soil properties, including those affecting the design
such as support conditions for the piles.
6. Recommendations for design criteria, including but
not limited to mitigations of effects of differential
settlement and varying soil strength, and effects of
adjacent loads.
7. Field inspection and reporting procedures (to include
procedures for verification of the installed bearing
capacity when required).
8. Load test requirements.
9. Any questionable soil characteristics and special
design provisions, as necessary.
4.1.2 Bracket Capacity (P1):
Only the localized limit
state of concrete bearing strength in compression has
been evaluated for this evaluation report. All other limit
states related to the concrete foundation, such as those
limit states described in ACI 318 Appendix D, punching
(two-way) shear, beam (one-way) shear, and flexural
(bending) related limit states, have not been evaluated for
this evaluation report. The concrete foundation must be
designed and justified to the satisfaction of the code
official with due consideration to all applicable limit states,
and the direction and eccentricity of applied loads,
including reactions provided by the brackets acting on the
concrete foundation. (See Tables 1, 2 and 3.)
4.1.3 Shaft Capacity (P2):
The tops of shafts must be
braced as prescribed in Section 1810.2.2 of the 2012 and
2009 IBC (Section 1808.2.5 of the 2006 IBC). In
accordance with Section 1810.2.1 of the 2012 and 2009
IBC (Section 1808.2.9 of the 2006 IBC), any soil other
than fluid soil must be deemed to afford sufficient lateral
support to prevent buckling of systems that are braced.
When piles are standing in air, water or fluid soils, the
unbraced length is defined as the length of pile that is
standing in air, water or fluid soils plus an additional 5 feet
(1524 mm) when embedded into firm soil, or an additional
10 feet (3048 mm) when embedded into soft soil. Firm
soils are defined as any soil with a Standard Penetration
Test (SPT) blow count of five or greater. Soft soil is
defined as any soil with an SPT blow count greater than
zero and less than five. Fluid soil is defined as any soil
with an SPT blow count of zero [weight of hammer (WOH)
or weight of rods (WOR)]. The SPT blow counts must be
determined in accordance with ASTM D1586. For fully
braced conditions where the pile is installed in
accordance with Section 1810.2.2 of the 2012 and 2009
IBC (Section 1808.2.5 of the 2006 IBC) and piles do not
stand in air, water, or fluid soils, the allowable shaft
capacities must not exceed the maximum design loads
shown in Tables 1, 2 and 5. Shaft capacities of helical
foundation systems in air, water or fluid soils must be
determined by a registered design professional. The ASD
shaft tension capacities are shown in Tables 3 and 5, the
ASD shaft compression capacities are shown in Tables 1,
2 and 5, and the shaft torsional rating is shown in Table 5.
The elastic shortening/lengthening of the pile shaft will
be controlled by the applied loads and the mechanical
and geometrical properties of the 2
7
/
8
-inch-diameter
(73 mm) round structural tubing and the shaft coupling.
The shaft elastic shortening or lengthening can be
determined from the equation:
∆
shaft
=
P × L
A × E
(Eq. 1)
where:
∆
shaft
= change in shaft length due to elastic shortening
or lengthening (inches)
P
= applied axial compression or tension load (lbf)
L
= pile shaft length (inches)
A
= shaft cross-sectional area (in
2
) (see Table 4)
E
= shaft steel modulus of elasticity (psi) (see
Table 4)
4.1.4 Helix Plate Capacity (P3):
The allowable axial
compression and tension load capacities (P3) for each
individual helical plate diameter (8, 10, 12 or 14 inches) is
55 kips (244.6 kN). (See Tables 1, 2, 3 and 5.) For helical
piles with more than one helix, the allowable helix
capacity (P3) for the helical foundation system may be
taken as the sum of the allowable capacity of each
individual helix.
4.1.5 Soil Capacity (P4):
The allowable axial
compressive or tensile soil capacity (P4) can be
estimated by a registered design professional in
accordance with a site-specific geotechnical report, as
described in Section 4.1.1, combined with the individual
helix bearing method (Method 1), or from field loading
tests conducted under the supervision of a registered
design professional (Method 2). For either Method 1 or
Method 2, the predicted axial load capacities must be
confirmed during the site-specific production installation,
such that the axial load capacities predicted by the torque
correlation method are equal to or greater than those
predicted by Method 1 or 2, described above.
With the individual helix bearing method, the total
nominal axial load capacity of the helical pile is
determined as the sum of the individual areas of the
helical bearing plates times the ultimate bearing
capacities of the soil or rock comprising the respective
bearing strata for the plates.
The design allowable axial load must be determined
by dividing the total ultimate axial load capacity predicted
by either Method 1 or 2, above, by a safety factor of at
least 2.0.
With the torque correlation method, the total ultimate
and allowable axial load capacities are predicted as
follows:
