Vulnerability Assessment of Arizona's Critical
Infrastructure
10th International Conference on Short and Medium Span
Bridges
Quebec City, Quebec, Canada,
July 31 – August 3, 2018
Improving Live Load Rating of Culverts Using Structural Health
Monitoring
Okeil, Ayman 1,3, Ulger, Tuna 2 and Elshoura, Ahmed 1
1 Louisiana State University, USA
2 Bulent Ecevit University, Turkey
3 [email protected]
Abstract: Older culverts with low fill heights are known to
produce lower load rating factors when simplified AASHTO-LRFD
procedures for live load distribution are followed. Nevertheless,
the performance of these culverts is typically acceptable, and they
rarely show signs of distress. The purpose of this paper is to
assess the load rating of representative CIP-RC box culverts from
the Louisiana DOTD inventory with low earth fill heights. In the
first phase of this research, field live load testing of the
culverts was conducted after instrumenting each culvert with a
structural health monitoring (SHM) system consisting of a total 48
sensors including displacement, strain, and tiltmeter sensors. In
the second phase of the research, refined three-dimensional (3D)
finite element (FE) models were built for each tested culvert and
were calibrated using measured field data. AASHTO’s Manual for
Bridge Evaluation (MBE) rating methodology was followed in this
research to distribute the live loads through the soil fill, and
project drawings were used to develop connection details in FE
models. AASHTO’s design truck, HL-93, and legal trucks were passed
on the calibrated culvert models, and the resulting straining
actions were used to estimate load rating factors. This paper
provides details about the methodology used in this study, field
test data and 3D FE models, and load rating factors for the 8
culverts covered in this study. Finally, recommendations and
research needs are presented.
INTRODUCTION
It is known that conventional AASHTO load rating methods (AASHTO
2011) often result in unacceptable low rating factors.
Consequently, action is typically required from bridge owners to
address the low rating factors; leading to additional strain on the
limited available resources. The inherent conservatism is mainly
attributed to simplified live load distribution formulas adopted in
AASHTO specifications (AASHTO 2014) and manuals. Furthermore,
reinforcement detailing of these culverts also has its implications
on RC box culvert rating. Multicell structural systems are highly
statically indeterminate, and therefore are capable of resisting
larger pressures than similar ones with lesser degrees of
indeterminacy. However, the degree of indeterminacy of multicell
box culverts may be reduced if the reinforcement detailing at the
corners is not capable of transferring internal forces produced in
such systems. Many of the are cast-in-place (CIP) reinforced
concrete (RC) box culverts in Louisiana’s were built with old
details that may fall under this category. Figure 1 shows one of
these details, which reveals that: (1) exterior wall reinforcement
is not adequately embedded in the top or bottom slabs of the box,
(2) a single layer of reinforcement in these walls and at the ends
of the top and bottom slabs make them incapable of resisting
negative moments that would generate in a fully rigid corner joint.
Engineers have typically considered these connections to behave
like hinges rather than rigid connections.
Figure 1: Old LA-DOTD standard detail for a three-cell CIP box
culvert.
According to the National Bridge Inventory (NBI), the State of
Louisiana Department of Transportation and Development (LA-DOTD)
has in its inventory roughly 2600 buried culverts. Many of these
CIP-RC box culverts were built using the aforementioned details.
Based on visual inspections and on discussions with engineers
involved in culvert load rating in Louisiana and elsewhere, their
real field performance does not show any signs of distress despite
their old age and contrary to what AASHTO load rating calculations
show. The goal of this paper is to provide a methodology for
dealing with CIP-RC box culverts in the LA-DOTD’s inventory that
were built using old standard details. The developed methodology is
complemented with field live load testing for a select few culverts
from LA-DOTD inventory to better understand the actual behavior of
CIP-RC box culverts in service.
Live Load Distribution and Live Load Rating of Buried CIP-RC Box
CulvertsLive Load Distribution on Buried Culverts
There are fundamental design differences between culverts and
other type bridges. Box culverts are highly statically
indeterminate structures and encased with multiple soil
interactions. The single axle loads or sometimes single wheel load
are controlling the section forces due to live load as opposed to
the gross vehicle weight (GVW), which controls the section forces
in a typical bridge span. The hydraulics and roadway geometry at
the culvert location have great effects on the fill height beneath
the roadway which has significant impact on the live load effects
on the buried culverts. As such, buried CIP-RC culverts are
three-dimensional (3D) structures. However, engineers often
simplify culvert models and use two-dimensional (2D) models
instead, which requires an approximate assessment of the live load
distribution on the culvert. Several researchers studied this topic
and a few of these studies are described next.
Abdel-Karim et al. (1990) conducted a live load test on culverts
that have fill heights ranging from 2 ft to 12 ft by
placing pressure cells on the top surface to investigate the live
load distribution. The pressure is generally distributed over a
square area in both transverse and longitudinal directions, and the
intensity of the pressure becomes more uniform and distributed over
a larger area. It was noted that the rigid pavement provides
additional load distribution especially for the lower fill heights,
and pavement thickness was represented with an equivalent fill
height. The flexural rigidity of the top slab itself also helps the
distribution of the wheel loads trough two-way action. According to
the McGrath et al. (2005), AASHTO LRFD specifications yield
relatively more conservative section forces than AASHTO Standard
specifications yield for the culverts that have 15 ft or less
box span length. Furthermore, different distribution widths were
proposed for different section forces. In another field test study
conducted by Orton et al. (2015) it was indicated that the AASHTO
LRFD specifications is overly conservative in predicting the
straining actions for culverts with fill heights smaller than
8 ft in comparison with field data, and suggested that the 6
ft can be a threshold to disregard the live load effects when the
live load effects less than 10 % of the dead loads`. Wood et
al. (2015) suggested depth-calibrated method which estimates the
live load pressure at each critical section by defining its own
out-of-plane load distribution width instead of using top slab
cover depth; therefore, the conservative section forces were
attenuated in 2D FE culvert modeling. In another study, Wood et al.
(2016) compared three different FE models, structural-frame model,
top-slab calibrated soil-structure model, and fully
depth-calibrated soil-structure model, to show that load rating
could be improved by inclusion of depth calibrated method. The
proposed method provides improved result for wall and bottom
sections; however, top slab, i.e. most significant section, was not
affected significantly due to negligible difference, i.e. half
thickness of top slab, between cover fill height and depth
calibrated fill height for extracting the section forces. Acharya
et al. (2016) conducted field tests considering the static and
moving live loads to study the effect of axle load, pavement type
and speed of the load on the top slab of culvert. It was concluded
that concrete pavement distributes the load over a larger area, and
reduced the vertical displacements nearly 2 – 3 times when compared
to unpaved road surface under static load. It was also observed
that the moving live load resulted more vertical deflection than
the static load.
Live Load Rating of Buried Culverts
In 2010, National Cooperative Highway Research Program (NCHRP)
Report 647 (Li et al. 2010), “Recommended Design Specifications for
Live Load Distribution to Buried Structures”, provided simplified
design equations (SDEs) for buried structures after conducting
three dimensional (3D) analysis of 830 buried culverts. In the
report, the presence of pavement in live load distribution was not
considered which can further distribute the axle load before
reaching culvert. For design purposes, this can be considered a
conservative approach; however, not taking advantage of this
additional distribution penalizes the in-service culverts buried
under rigid pavements. According to AASHTO-LRFD Commentary
C4.6.2.10.2, AASHTO provisions for live load distribution are based
on “distribution of shear forces.” Therefore, it can be said that
another source of conservatism is added because critical sections
for shear and the axle position causing maximum shear are typically
close to the supporting walls. Conversely, axle loads causing
maximum positive and negative bending moments are typically
positioned closer to mid-spans, which are known to distribute the
load over a wider area. Furthermore, experimental tests have shown
that failure of RC box culverts is highly unlikely (Abolmaali and
Garg 2008). According to AASHTO, a wheel load at the road level is
distributed over an area defined by two dimensions, and , as can be
seen in Figure 2.
Figure 2: AASHTO live load distrubution with soil depth
The dimensions of the distribution is given by the following
formulas
[1]
for
[2]
for
where = factor for distribution of live load with depth of
fill, = depth of fill from top of culvert to top of pavement
(in.), = clear span (ft), and = length of tire contact
area parallel to span (in.).
Field investigated culvertsCulvert Description
The number of culverts tested in the scope of this project was
eight and they were selected by LA DOTD from various parts of the
state. Only cast-in-place reinforced culverts were investigated and
the priority was given to culverts with lower fill height. Only two
of eight culverts had a fill height that exceeded 2 feet. Because
of space limitation, only three culverts are presented in this
paper. The characteristic of each culvert and their GPS locations
are given in Table 1. The final instrumentation plans were produced
and implemented in the field before the tests. The selected
culverts for presentation here are nonskewed. The roadway was paved
with asphalt for the presented culverts.
Table 1: Characteristics of Selected Culverts and GPS
Locations
Culvert
Location
Opening Size
(m x m [ft x ft])
Slab / Wall Thickness
(mm [in])
Number of Openings
Skew Angle (o)
Location
(LAT/LONG)
2
Crowley
1.83 x 1.83 [6 x 6]
203 / 152 [8 / 6]
5
90
30.348276
92.397601
8
Blanchard
2.44 x 2.44 [8 x 8]
229 / 203 [9 / 8]
3
90
32.635494
93.893828
4
Sulphur
3.66 x 3.66 [12 x 12]
305 / 305 [12 / 12]
2
90
30.236565
93.384425
The roadway profile was surveyed to estimate the earth fill
height for each culvert, and concrete core specimens were extracted
from the walls of each tested culvert and tested to have a better
assessment of the actual concrete strength. It was decided that one
exterior and one adjacent interior barrel were sensor instrumented.
Only one barrel was instrumented for the culvert with two barrels
(Sulphur, LA) due to symmetry. The thickness of concrete walls was
confirmed with the field inspections; however, the slab thickness
was not verified in field due to soil cover. All available data for
the culverts are provided in Table 2. Noting that, the concrete
finishing in site pouring creates variations from project
dimensions especially for the slab sections. The load pressures at
culvert level are also given in Table 2.
Table 2: Culvert Material Properties and Calculated Loads
Culvert
FH
(m [ft])
*
(MPa [psi])
(GPa [ksi])
DW
(kPa [ksf])
EV
(kPa [ksf])
EH (KPa [ksf])
ES
(kPa [ksf])
LS
(kPa [ksf])
Top
Bot.
2
2.13
[7.00]
46.1
[6.69]
199.9
[29000.00]
3.35
[0.070]
38.83
[0.811]
19.63
[0.410]
38.78
[0.810]
1.68
[0.035]
7.37
[0.154]
8
0.49
[1.60]
49.4
[7.16]
199.9
[29000.00]
3.35
[0.070]
6.37
[0.133]
4.93
[0.103]
34.28
[0.716]
1.68
[0.035]
8.28
[0.173]
4
0.68
[2.24]
61.9
[8.97]
199.9
[29000.00]
3.35
[0.070]
10.10
[0.211]
6.42
[0.134]
43.76
[0.914]
1.68
[0.035]
6.85
[0.143]
*cored specimens, FH = Fill Height, A = Asphalt, C =
Concrete
Instrumentation Plans
In this project, a wireless structural health monitoring system
was employed. Three different sensor types were instrumented at
critical locations inside the exterior and interior barrels. The
total number of sensors allocated to this project was 48; 4
tiltmeters, 8 Linear Variable Differential Transducers (LVDTs), and
36 strain gauges. A typical instrumentation plan is shown in Figure
3, where tiltmeters were bonded to the wall and top slab in the
exterior barrel to measure the relative rotations between them, the
LVDTs were placed at the mid-span of the exterior barrel to measure
relatively larger vertical deflections of the top slab. One LVDT
was also placed in the second barrel. The remaining 36 stain gauges
were bonded to top slab or wall sections as shown. It should be
noted that the instrumentation was repeated at three locations
across the culvert’s length where three load paths were planned as
will be seen later.
Figure 3: Typical instrumentation plan for a straight
culvert
Live Load Tests
The roadway was marked for each load path according to in the
instrumentation plans. Loaded dump trucks with two axles (single
front/double rear) crawled at a speed of about 5 miles per hour
which can be considered static loading. Each load path was repeated
for at least twice to ensure repeatability of the results. The
trucks used in these tests were measured and weighed. Table 3 lists
these properties. Readings from the sensors were recorded for all
passes at each of the three paths indicated on the instrumentation
plans. The readings were then used to calibrate 3D finite element
(FE) models for each culvert, which were then used in load rating
the tested culverts.
Table 3. Properties of Loaded Test Trucks
Culvert
Test Truck GVW
(kN [kip])
Front Axle (kN [kip])
Rear Axles (total)
(kN [kip])
Front Axle Spacing
(m [ft – in])
Rear Axle Spacing
(m [ft – in])
2
254.1 [57.1]
56.1 [12.6]
198.1 [44.5]
3.66 [12” – 0’]
1.37 [4” – 6’]
8
187.5 [42.2]
42.4 [9.5]
145.1 [32.6]
3.66 [12” – 0’]
1.35 [4” – 5’]
4
229.4 [51.6]
57.8 [13.0]
171.6 [38.6]
3.99 [13” – 1’]
1.37 [4” – 6’]
Finite Element Model DevelopmentFinite Element Model and
Calibration
Shell elements were used to model slab and wall sections. The
presence of haunches at top corner connections between the top slab
and walls allowed adding offsets in FE models to account for the
rigidity of this relatively larger concrete region. In addition to
half the member’s thickness, the wall and slab offsets were further
increased by two-thirds of the haunch length. The rigid concrete
regions that defines the shell element offsets are shown in Figure
4. The connection of the non-rigid (i.e. wall and slab) and rigid
shell elements (i.e. offset region) were connected with the
rotational springs. The continuity of the elements or the reduced
stiffness could be defined as a fixed-connection or with reduced
stiffness values, respectively. External loads were applied on the
model as pressure acting on affected areas. For example, the earth
fill weight was applied over the entire top slab surface, while
linearly varying horizontal earth pressure was applied on the
exterior walls. Truck axle loads were applied as a pressure acting
on the area affected by the wheel/axle loads according to AASHTO’s
formulas for load distribution (Eqs. 1 or 2). Figure 5 shows one of
the full 3D-FE models. More details about the model can be found
elsewhere (Okeil et al. 2018).
(a) Actual Structure
(b) FE Model
Figure 4: Rigid concrete regions and shell element offsets
Figure 5: Full 3D FE model
Each FE model was calibrated to minimize the error between the
field measured response and the numerically obtained response. The
error function that was implemented in post-processing is given in
Equation [3], where is the sensor data obtained from field test, is
the sensor data obtained from FE model, is the number of sensors
used in one load path, and is the number of load tested truck path.
Several iterations were needed to reduce this error below 20%,
which typically was much higher (over 100%) if nominal properties
were used in the model. Figure 6 shows the plots for the readings
of three sensors and three cases of model adjustment including the
initial nominal case (Case 1). As can be seen, nominal values
result in a response that is substantially higher than the measured
response. Calibrating the model improves the response such that the
error is within acceptable limits.
[3]
Figure 6: Sensor data for nominal case (Case 1) and modified
cases (Case 2 and 3)
Load Rating
Load rating was conducted for two design and 10 legal trucks
typically considered in Louisiana. Straining actions (bending
moments, shear forces, and axial forces) were extracted at the nine
critical sections (① through ⑨) shown in Figure 7.
Figure 7: Critical sections for rating factor calculations
The rating factor, , was calculated using the following equation
in which the nominal capacity, , is factored by condition and
system factor, and , in addition to the typical design factor, , to
obtain the member’s capacity, . AASHTO BEM (AASHTO 2011) load
factors, , were considered for: component dead loads, , wearing
surface, , verical and horizontal earth pressure, and , uniform
earth and live load surcharg, and , and live load plus dynamic load
allowance, .
[4]
Table 4 lists the results of the load rating calculations for
the three culverts presented in this study. As can be seen, the
rating factors for all culverts exceeded 1.0 under all truck loads
(inventory, operational, and legal). It should be noted that using
conventional methods following AASHTO LRFR procedures resulted in
rating factors less than 1.0 for all three culverts. For example,
the rating factor for Culvert #4 was 0.03 under HL-93 at the
inventory level. This shows that there is a lot of conservatism in
the conventional methods that unnecessarily produce such
results.
Table 4. Rating Factors for Inventory Level Design Truck at
Critical Sections
Culvert
Truck
Type
Rating Factor
Section Number
Section Location
2
HL-93
Inventory Level
4.57
3
Exterior Top Slab Corner Shear
8
1.36
2
Exterior Top Slab Mid-span Moment
4
1.39
3
Exterior Top Slab Corner Shear
2
HL-93
Operational Level
5.93
3
Exterior Top Slab Corner Shear
8
1.76
2
Exterior Top Slab Mid-span Moment
4
1.81
3
Exterior Top Slab Corner Shear
2
Type 3-3
7.42
3
Exterior Top Slab Corner Shear
8
LA Type 6
2.42
2
Exterior Top Slab Center Moment
4
NRL
2.14
3
Exterior Top Slab Corner Shear
CONCLUSIONS
In this research, structural health monitoring (SHM) was
utilized to assess the load carrying capacity of three
cast-in-place reinforced concrete (CIP-RC) culverts from LA DOTD
inventory. Field tests were conducted on each culvert after
installing a 48-sensor SHM system that recorded strains,
displacements and rotations under known truck loads. Three
dimensional (3D) FE models were developed and calibrated using the
field measured data. These models were then used to load rate the
culverts following AASHTO load distribution formulas and load
factors by considering critical sections for the design truck, HL
93, and legal trucks. All three culverts whose rating factors were
less than 1.0 according to conventional methods resulted in rating
factors that are substantially higher than 1.0 using calibrated 3D
models.
Based on the results it is possible to conclude that utilizing
2D models for load rating of culverts using the current formulas in
AASHTO documents adds another layer of conservatism, which can be
attributed to:
· AASHTO load distribution formulas, which are based on shear
sections despite the fact that bending moments control load rating
more often.
· The inability of traditional 2D models of modeling the rigid
behavior of slab-to-wall connections, especially for connections
with haunches.
Therefore, it is recommended that live load distribution
formulas be revisited in such a way that separates the soil’s
ability to redistribute wheel loads through the earth fill from the
structures ability to redistribute the load as a typical 3D
structure.
Acknowledgment
The work presented herein is part of a research project
sponsored by Louisiana Transportation Research Center (LTRC Project
No. 16-3ST). The support of Dr. Walid Alaywan, Project Manager, Dr.
Dana Feng, Dr. Ching Tsai, and Dr. Qiming Chen, Bridge Design, is
greatly appreciated. Mr. Marco Canales was instrumental in
conducting field testing with support from Bridge Diagnostics, Inc.
(BDI). Any opinions, findings, and conclusions or recommendations
expressed in this material are those of the authors and do not
necessarily reflect the views of the sponsoring agencies.
References
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WallsBottom SlabTop SlabEdge BeamsRigid Joints
