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Camber Variations in Precast Bridge Girders
Article · July 1999
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Is camber “growing” in your precast yard?
Camber Variation
in Precast Girders
by Nur Yazdani, Primus Mtenga, and Nigel Richardson
P
recast prestressed concrete members usually experience camber as a by-product of a prestress force application. The camber counteracts the deflections
caused by the application of dead load from the bridge deck.
The Florida Department of Transportation (FDOT) has experienced slab construction problems in the field that may be
associated with unexpected camber variations in prestressed
highway girders.
Previous experience has shown that the actual camber for
wide-flanged girders such as Types V, VI, modified Type VI,
and the Florida Bulb Tee are typically less than that predicted
by theoretical analysis at the FDOT at the time of casting the
slab. Such problems have been documented in unpublished
internal studies performed at the FDOT structural design office.1,2 As suggested in the FDOT structures design guidelines, the haunch between the slab and the girder can be
adjusted to compensate for camber and to maintain the required deck profile on bridges.3
The haunch provides a means of maintaining constant slab
thickness. The thickness of the haunch varies with the beam
camber, while the slab thickness remains constant. Another
option is to lower the vertical profile of the bridge deck. Both
options result in expensive modifications in the field.
Several factors may influence the variation in camber experienced in precast prestressed girders. One such explanation
could be a change in the cross section stiffness not accounted
for in the design. This change in stiffness could be a result of
the presence of unwanted and unnoticed microcracks.
Crack formation cannot always be directly linked to one
(a) Type III
factor and instead could be a result of high tensile stresses
generated by techniques used in site handling and vibrations experienced during transportation procedures. High
tensile stresses can also yield cracks if the tendons are released prior to the attainment of sufficient concrete compressive strength to resist the negative moment stresses resulting from the prestressing strands and their eccentricity. Onsite variations in the magnitude of prestressing forces imparted on the girders could also influence the camber. In
addition, the assumed values for prestress loss at various
intervals of the girder life may not be appropriate.
FDOT’s structural design office utilizes a prestressed beam
program to design and review simply supported pretensioned
bridge girders.4 Under the recommendations of ACI Committee 209, Creep and Shrinkage in Concrete, which studied the
effects of various parameters and environmental effects on
prestress loss and camber of precast girders, the FDOT inhouse prestress beam program was updated to incorporate
various modifications on material properties that might affect
camber output. In spite of these general modifications, the
problem of camber still remains evident in girders utilized in
the highways of Florida.
It has been argued that it is feasible to operate with simplified deflection and cracking rules in the design of partially
prestressed cracked beams.5 The use of span/depth limits to
control deflection is especially attractive. It has also been
contended that the presence of initial camber reduces the
critical buckling load of prestressed girders while they are
(b) Type IV
(c) Modified Type VI
Fig. 1 — Cross sections of monitored AASHTO girders, all dimensions in mm. (Note: 25 mm = 1 in.)
Monitoring per i od
Monitoring s it e
Ty pe o f g irder
N umber o f g irders
May-August 1995
Si te 1, Tampa
Modi fi ed AASHTO Type VI
20
September-November 1995
Si te 2, Jacksonvi l l e
AASHTO Type III
AASHTO Type IV
6
8
In this procedure illustrated in Fig. 2,
marks were placed at both ends and the
midspan along the girder centerline.
From this datum, a surveying rod was
held and the dumpy level was used to
measure the vertical elevation. Based
on the diagram in Fig. 2, the following
procedure was used to calculate camsuspended during the erection process.6 Therefore, this
effect should be considered in the design process. The application of the I-effective method of calculating deflections of
partially prestressed members has been explored in several
studies.7,8,9
In this study, the growth of actual camber in American Association of State Highway and Transportation Officials
(AASHTO) girders during storage in the precasting yard was
monitored. Measured cambers were also compared with theoretical cambers generated by the in-house program used by
the FDOT.
Research significance
Camber growth with time is an inherent property in precast
prestressed bridge girders. Accurate prediction of field camber is essential in avoiding subsequent problems with deck
construction. This study was focused on evaluating the actual growth of camber with time, and the variations with theoretical camber. The research results can be utilized in better
prediction of camber in AASHTO bridge girders during storage.
Field monitoring
To monitor the actual camber experienced in the field, camber
values were measured from various Florida prestressing yards,
including two permanent yards in north and central Florida.
At the first site, located in Tampa, a total of 20 modified
AASHTO Type VI girders were monitored for camber. Data
were obtained at different stages of the girder life, which
were: initial camber, immediate camber after removal from the
bed, and camber during storage at the yard. At the second
site in Jacksonville, a total of 14 standard AASHTO girders
were monitored, including six AASHTO Type III and eight
AASHTO Type IV girders. The cross
sections of the three girder types studied are shown in Fig. 1. Table 1 is a summary of the girders studied and the approximate time periods for which they
were monitored.
A number of experimental procedures
were tried to measure the field camber
of the prestressed girders. A method
involving a dumpy level and a surveying rod was chosen for reasons of accuracy, convenience, and practicality.
ber from field data:
δH3 = δH4 - δH1
(Eq. 1)
in which,
δH1 = δH2 / 2
δH2 = relative elevation of girder ends
δH3 = camber at midspan
δH4 = elevation at midspan
A taut string used as a datum is a common technique used
to measure camber, and was initially tried for this study. Even
when high-strength piano wire was used, it was impossible
to eliminate the sag in the string. This difficulty was especially pronounced for large span girders. The technique chosen was far more accurate than the procedure currently utilized by the precast yard.
Typical concrete average compressive strengths at prestress transfer and at 28 days for the monitored girders are
presented in Table 2. The AASHTO Type III girders were
prestressed with straight strand 1862 MPa (270 ksi) stress
relieved tendons. These girders spanned a distance of 21.34
m (70 ft) and were erected on a bridge over a waterway. The
Type IV girders were also prestressed using 1862 MPa straight
strand, stress relieved tendons. The span length for these
girders measured approximately 31 m (100 ft). The modified
AASHTO Type VI girders were designed for a span of 43.6 m
(143 ft) with 50 straight strand low relaxation 1862 MPa tendons and 14 post-tensioned strands. The post-tensioning
for these girders was completed during placement at the site.
Time int e rv a l
Ac t ua l ca mber,
mm
G ro w t h in
ca mber , %
F D O T p r e d i c te d
ca mber , mm
G ro wt h in F D O T
ca mber , %
D i f f e re nce
be t we e n f ie ld a nd
F D O T ca mber , %
I nit ial
32.8
—
32.0
—
6
1 month
50.0
52
46.7
46
7
2 months
54.5
60
50.0
56
9
an in-house check of girders designed for Florida’s
highways. In general, the FDOT beam program complies with AASHTO Specifications for Highway
Bridges, American Railway Engineering Association
Specifications, ACI Building Code Requirements for
Reinforced Concrete, FDOT Structures Design
Guidelines, and FDOT Standard Specifications.
A camber summary of the beam up to a period of
240 days and allowable stresses are included in the
output file. In addition, information regarding elastic and time-dependent shortening effects is included
in the output summary. Moments, shears, and
stresses experienced during service loads are also
provided. Prestress losses in the FDOT beam program are approximated using the AASHTO guideline 9.16.2.10 In the FDOT beam program, the theoretical camber for AASHTO girders is predicted by
the conjugate beam approach. The FDOT beam program was used in this study to predict theoretical
camber in AASHTO Types III, IV, and modified VI
girders. Design values of strengths and dimensions were
utilized for camber prediction purposes.
Camber results
The average camber results obtained from the field monitoring and the theoretical models are presented in Fig. 3, 4, and
5, and Tables 3, 4, and 5, for AASHTO Types III, IV, and
modified Type VI girders, respectively. The time dependent
growth rates of camber (based on initial camber) and the
differences between the actually measured and theoretically
predicted cambers are also included in Tables 3, 4, and 5. It is
observed from these tables that both the actual and the theoretical cambers grow significantly with time in precast prestressed bridge girders.
As seen from Table 3 and Fig. 3, measured average camber
at 1 month grew about 52% over the initial camber in the
AASHTO Type III girders. Subsequently, the rate of camber
growth slowed, and at 2 months, the actual camber grew by
an additional 8%. The camber growth predicted by the theoretical model lagged the actual camber growth at various time
intervals. For AASHTO Type III girders, camber recorded at
each time interval was greater than that derived from the
FDOT program. At the initial stage, differences between the
recorded and the predicted camber was about 6% on average. At the 1 and 2 month stages, camber variations between
4 to 12% were evident. A few Type III girders exhibited actual
camber variations of nearly 15% from the expected results of
the FDOT program.
Fig. 4 and Table 4 illustrate the average measured and theoretical camber variations in AASHTO Type IV girders. All
eight Type IV girders monitored reflected camber increase
over the time period of storage. Similar to the Type III girders,
the Type IV girders experienced significant camber growth
within a 1 month interval. The camber growth decreased for
subsequent time intervals. Measured camber values in the
Type IV girders showed 0 to 4% variation as compared to the
predicted initial camber from the FDOT program. At the 1
month and 2 month time intervals, camber variations of 1 to
5.5% and 2 to 9% were evident in these girders, respectively.
Fig. 5 and Table 5 show the average camber variations in
the modified AASHTO Type VI girders. After an initial surge
in camber growth during the first month, the AASHTO Type
VI girders demonstrated subsequent decreased camber
growth rates. Field camber values in these girders varied by 1
to 5.5%, 2 to 7.5%, 2 to 9%, and 3 to 20% from the theoretical
values for the initial, 1 month, 2 month, and 3 month intervals,
respectively. Camber in AASHTO Type VI girders did not
grow as quickly during the first month as in the smaller girders. But unlike the smaller girders, the camber growth did not
decrease substantially in subsequent months.
Statistical techniques were used to determine if the measured cambers varied significantly from the expected cambers. Assuming the predicted camber value from the FDOT
program as the population mean, the t statistic as shown
below was utilized to test the mean:11
t = [(X −µ ) n ]/ S
(Eq. 2)
in which,
X = sample mean
µ = population mean
S = sample standard deviation
n = sample size
Table 4 — Average variation in camber for AASHTO Type IV girders
Time int e rv a l
Ac t ua l ca mber,
mm
G ro w t h in
ca mber , %
F D O T p r e d i c te d
ca mber , mm
G ro wt h in F D O T
ca mber , %
D i f f e re nce
be t we e n f ie ld a nd
F D O T ca mber , %
I nit ial
58.1
—
56.6
—
3
1 month
81.6
40.4
79.5
40.5
3
2 months
87.1
49.9
84.3
48.9
3
Note: 25 mm = 1 in.
The following hypotheses were tested:
Ho : µ = µo
null hypothesis
HA : µ > µo
alternate hypothesis
The null hypothesis will be rejected if t > tα, where α is the
level of significance. Based on various statistical parameters
and the values of the t-statistics at 2.5% and 1.0% levels of
significance (which correspond to 97.5% and 99% confidence
levels, respectively), it was found that at 97.5% confidence
level, the test statistic is greater than the acceptable value for
all girders and all time intervals. Therefore, the null hypothesis must be rejected. As the girder becomes larger, the difference between the theoretical and actual camber also increases.
These observations are also valid at the 99% confidence
level, except for the isolated case of the Type III girder at
initial conditions. This means that the measured cambers are
indeed significantly greater than the predicted camber from
the FDOT beam program, although they are within acceptable engineering accuracy.
Discussion
The field data suggest that AASHTO precast girders experience increased camber due to creep, which takes place with
time as expected. Creep is impacted by the large prestressing
forces, eccentricity, change in concrete strength and the
modulus of elasticity as the concrete continues to cure and
increase in strength. It is unlikely that abnormal stresses were
imparted to the girders during handling, because they were
removed from the precast bed to storage and to trucks/barges
from storage according to industry recommendations.
The two test sites contained numerous potholes and ruts,
which may impart impact loads from the transport
vehicles to the tested girders. These vibrations may
have caused additional tensile stresses and cracking in the girders. Thus the possibility of microscopic cracking exists at this stage. The importance
of this stage might prove to be critical in design
procedure and should be explored. In general, actual camber increased by more than 50% in most of
the girders monitored in the 2 month period.
It is possible that the problem of additional field
camber is more involved than the inherent assumptions. The presence of unwanted microcracks causes
a reduction in the effective cross-sectional area of
the girder, leading to a decrease in the eccentricity
of the prestressing force. If tensile cracks are allowed, a modified approach must be implemented
that accounts for the changes in the cracked section. New and improved methods are available that
could be used to modify the existing techniques for
predicting camber.7 The platform for this modification is that of the Effective Moment of Inertia
method. Other factors such as temperature range,
humidity, mix proportions, and quality control in casting,
which are not accounted for in FDOT’s software, may cause
the differences observed between the actual and theoretical
cambers.
The measured cambers varied from the predicted cambers
by about 1 to 15% during storage. Statistical tests showed
that the FDOT beam program consistently predicted lower
camber values than the actual measured camber. It is interesting to note that the test t-statistics increased significantly
from initial conditions to the 1 and 2 month intervals. Therefore, it is possible that the difference between the actual camber values and the predicted camber may become greater at
later stages, such as under service conditions. Unfortunately,
because service camber data could not be measured, no comparisons could be made for that stage.
Conclusion
The following conclusions and recommendations are made
based on the theoretical modeling and results gathered:
1. This investigation has shown that the actual camber in
Florida’s precast prestressed bridge girders increases significantly during storage at precast yards. This increase is
due to creep effect and is expected. The camber increase is
maximum during the first month in storage. The camber growth
rate diminishes in subsequent months.
2. Camber in larger AASHTO girders such as the modified
Type VI does not grow as quickly as in the smaller girders
during the first month in storage. However, the camber growth
rate in the larger AASHTO girders does not decrease
Table 5 — Average variation in camber for modified AASHTO Type VI girders
Time int e rv a l
Ac t ua l ca mber,
mm
G ro w t h in
ca mber , %
F D O T p r e d i c te d
ca mber , mm
G ro wt h in F D O T
ca mber , %
D i f f e re nce
be t we e n f ie ld a nd
F D O T ca mber , %
I ni t i a l
76.4
—
74.1
—
3
1 month
100.0
30.9
96.0
29.6
4
2 months
123.8
62.0
115.4
55.7
7
3 months
133.3
74.5
120.3
62.3
11
substantially in subsequent months, as is the case
for smaller girders.
3. The variation between field camber and the theoretically predicted camber for AASHTO Types III,
IV, and modified VI girders is in the 3 to 11% range
on the average during storage in the precasting
yard. For individual girders, the difference can be
as high as 20% during storage in the precast yard.
The actual camber is significantly greater than theoretical values for the monitored girders.
4. The actual measured cambers were found to be
significantly larger than the corresponding FDOT
predicted values for all monitored girders up to 4
months after casting. Apparently there is loss of
girder stiffness in the field not properly accounted
for in the FDOT software. The stiffness loss could
be due to microcracks caused by vibrations induced
during transportation. Another reason could be
cracking due to low concrete tensile strength at initial stages.
5. Additional research is needed to determine actual camber in AASHTO girders under service conditions.
The effect of microcracks on the reduction of girder moment
of inertia also needs investigation.
Acknowledgment
The study reported herein was funded by a research grant from the
Florida Department of Transportation.
References
1. Nichols, B., and Ausley, M., “Camber Analysis for the Tampa
Grandy Bridge,” Presentation at the annual FDOT Structural Design
Meeting, Orlando, Fla., 1993.
2. Bowman, H., “Camber Analysis for Alligator Creek Bridge,” FDOT
Internal Report, 1993.
3. “Structures Design Guidelines,” FDOT Structures Design Office, July
1997.
4. “The Design and Analysis of Simple Span Prestressed Concrete
Beams Computer Program, Users Manual,” V. 2.5, FDOT Structures
Design Office, May 1994.
5. Scholz, H., “Simple Deflection and Cracking Rules for Partially Prestressed Members,” ACI Journal, V. 88, No. 2, March-April 1991, pp. 199203 .
6. Peart, W. L.; Rhomberg, E. J.; and James, R.W., “Buckling of
Suspended Cambered Girders,” ASCE Journal of Structural Engineering, V. 118, No. 2, February 1992, pp. 505-527.
7. Tadros, M. K.; Ghali, A.; and Meyer, A. W., “Prestress Loss and Deflection
of Precast Concrete Members,” PCI Journal, V. 30, No. 2, Jan.-Feb. 1985, pp.
114-141.
8. Branson, D. E., and Trent, H., “Application of the I-Effective
Method of Calculating Deflections of Partially Prestressed Members,”
PCI Journal, 62-77, 1982.
9. Teng, S., and Branson, D. E., “Initial and Time-Dependent Deformation of Progressively Cracking Non-Prestressed and Partially Prestressed Concrete Beams,” ACI Structural Journal, V. 90, No. 5, Sept.Oct. 1993, pp. 480-481.
10. American Association of State Highway and Transportation
Officials, Standard Specifications for Highway Bridges, Washington
D.C., 1995.
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ACI member Nur Yazdani is a professor of
civil engineering at the Florida A & M University-Florida State University College of
Engineering. He is a member of ACI Committees 343, Concrete Bridge Design —
Joint ACI-ASCE; 123, Research; and 201,
Durability. His research interests include
prestressed concrete members, bridge design, and rehabilitation.
Primus Mtenga is an assistant professor
of civil engineering at the Florida A & M
University-Florida State University College
of Engineering. His recent research in the
area of structural analysis and design and
construction materials has included structural system reliability and performance of
bio-composites.
Nigel Richardson is a PhD student at the
Florida A & M University-Florida State University College of Engineering. His past
research has involved prestressed girders
and moisture effects on cupping forces in
lumber. Current interests include engineering mechanics and numerical modeling of
the behavior of materials commonly used
in structural engineering applications.