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Proof Tests using Impact Load Method
To demonstrate and quantify Bravo 25 upgrade performance, NFESC
engineers applied proof loads in excess of 130 kips (580 kN) to
crucial areas of the deck. Measured pier responses included the
deflected shape of the pier deck and strain in the upgrade reinforcement
at critical locations. Strain response was measured at points of
maximum stress at the edge of pile girders, at the edge of transverse
girders, at midspan of center slabs, at midspan of the track slabs,
and at midspan of curb slabs. Test spans were selected that were
free of obstructions. The spans between the following bents were
tested:
181 – 182
184 - 185
185 –186
187 – 188
218 – 219
219 – 220
as well as Bent 23 adjacent to the curb.
NFESC used a falling weight deflectometer (FWD) and the Impact
Load Method (ILM) that was developed to quantify structural performance
for condition assessment. The FWD is a trailer-mounted, computer-controlled
load-testing device that applies an impact load whose magnitude
exceeds all wheel loads that are encountered on Navy piers (Figures
76 through 78). The FWD was specifically built to apply loads up
to 130 kips (580 kN) on a 12-inch (30 cm) diameter platten. The
length of the load pulse is approximately 30 milliseconds. The FWD
has built-in sensors to measure and record histories of the load
and displacement in the vicinity of the load.
The FWD data processor digitized (at 10,000 samples per second)
and converted the analog signals. A portable computer monitored
the test progress and stored the digitized load pulse and pier deflection
responses. Peak values were selected for comparison with finite
element model analysis. All data was stored in computer files compatible
with the spreadsheet program, EXCEL©. Each test generated over
3,000 data values.
Deflection data was recorded at 12 locations along two orthogonal
transducer beams that project from the load point. One transducer
beam was aligned with the longitudinal axis of the FWD trailer while
the other was transverse. Deflection sensors were positioned along
the longitudinal transducer beam at 13 inches (33 cm) aft of the
load point, at the load point, and at 16.5, 24, 36, 48.5, and 60
inches (42, 61, 91, 123, and 152 cm) forward of the load point.
Displacement sensors were positioned at 13, 24.5, 37, 48, and 60.5
inches (33, 62, 94, 122, and 159 cm) along the transverse transducer
beam. The longitudinal transducer beam was normally oriented along
the longitudinal direction of the wharf (perpendicular to the transverse
bent girders) except at curbside load points where the longitudinal
transducer beam orientation was rotated 90 degrees or parallel to
the transverse bent girders. Peak deflections from each sensor time
history determined the deflected basins that characterize the stiffness
of the structural members in the vicinity of the load application.
Strain gauges were positioned at crucial locations. NFESC attached
1/2-inch foil strain gauges to carbon rods prior to embedment at
the edges of transverse girders and outside rail girder. Five-inch
wire gauges were mounted in the epoxy saturate between laminate
layers at midspan. Strain response was measured and recorded using
a Campbell data logger. The strain under the load point, in neighboring
spans and adjacent structural elements was measured. The sample
rate was 2,000 data points per second for each gage. Many of the
laminate strain gauges were inoperative before the tests began.
The probable cause was not obtaining isolation of the wire strain
element from the carbon fibers during installation. Strain elements
must be electrically isolated from the carbon fibers by complete
epoxy encapsulation in future installations.
Each load test series consisted of at least four impact load applications
at increasing load levels preceded by a small load to set the load
platen. Peak loads were approximately 30 kips (135 kN) at Level
1; 55 kips (245 kN) at Level 2; 90 kips (400 kN) at Level 3; and
130 kips (580kN) at Level 4. Increasing load levels provide an opportunity
to check the linear load-deflection response of the structure.
Example load, deflection, and strain data is presented for the
curb slab area, rail slab, and center slab. Figures 79 through 98
have load-, deflection-, and strain-time histories, deck deflection
distributions, load-deflection responses, and load-strain responses.
The maximum measured response in the external composite reinforcing
was less than 600 microstrain (18 ksi (125 mPa) carbon fiber stress).
The maximum measured strain in the carbon composite rods was less
than 200 microstrain (9 ksi (62 mPa) carbon fiber stress). Except
at the center slab span, non-linear behavior was not encountered.
The concrete cracking strength of the center slab was exceeded so
there was a stiffness change from the first load level to the second,
however, the stiffness was constant from the second level to the
fourth. NFESC coupled ILM response with finite element analyses
(FEA). Analysts compared ILM deflections with those generated by
the FEA using the same loading. The finite element model results
are in the same order of magnitude (+/-100 percent). The actual
structural response is shaped by specific and localized crack networks
in the deck that were not modeled. The FEA models provide an accurate
representation of the service limits of the pier by precise modeling
of the support conditions of the piles, pilecaps and beams.
The center slab and the curb slab are crucial areas of the wharf
and limit the outrigger loads. The beam and column elements of the
utility loop below the curb slab at every bent add little strength
and stiffness to the curb slab that cantilevers out from the pile
girder. The structural responses are well below the service limit
of the composites and should be satisfactory for supporting outrigger
loads up to 125 kips (555 kN).
Figure 76. FWD positioned to apply
load to the midspan of the
“Outside Rail Slab”.
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Figure 77. FWD positioned on “Center Slab”
to apply load to the middle of the deck between bents
and between rail slabs. |
Figure 78. FWD positioned on “Curb Slab” (219-220)
to apply load to crucial point at the edge of the wharf.
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Figure 79. Load impulse applied to the midspan of rail
slab between Bents 184 and 185. |
Figure 80. Deflection response to impact load at midspan
of rail slab between Bents 184 and 185. |
Figure 81. Strain histories of carbon rods measured at
Bent 184 for load applied rail slab midspan. |
Figure 82. Strain history of carbon laminate measured
below the load point at midspan 184-185. |
Figure 83. Linear load-deflection response of midspan
rail slab. |
Figure 84. Linear load-strain response of midspan laminate
and carbon rod over Bent 184. |
Figure 85. Deflection distribution about load point at
rail slab midspan between Bents 184-185. |
Figure 86. Load impulse applied to the midspan of rail
slab between Bents 184 and 185. |
Figure 87. Deflection response of midspan of rail slab
to load applied between Bents 184 and 185. |
Figure 88. Strain history of carbon rod measured adjacent
to Bent 184 with load at midspan 184-185. |
Figure 89. Load-deflection response of curb slab between
Bents 219 and 220. |
Figure 90. Load-strain response of transverse carbon rod
between Bents 219 and 220. |
Figure 91. Deflection distribution of curb slab between
Bents 219 and 220. |
Figure 92. Load impulse applied to midspan of center deck
slab. |
Figure 93. Deflection response at the midspan of the center
deck slab between Bents 184 and 185. |
Figure 94. Deflection distribution of load applied midspan
of center Deck slab. |
Figure 95. Linear load-deflection response of center deck
slab between Bents 184 and 186. |
Figure 96. Center slab midspan laminate load-strain response
between Bents 184 and 185. |
Figure 97. Load-deflection response of the “Curb
Slab” at Bent 223 (above the utility loop). |
Figure 98. Deflection distribution about load point on
curb slab at Bent 223. |
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