A Study on Application of Mixed Girder in Steel Box Girder Bridge

A <strong>Study</strong> on Application of Mixed Girder in Steel Box
Girder Bridge
Hyosang HWANG 1 Kabsoo KYUNG 2 and Hyungi MOON 3
1 Dept. of Civil Eng., Korea Maritime University
(Dongsam 2dong, Busan City, Korea)
E-mail:hhs0259@naver.com
2 Professor, Inst., Structual machanics, Korea Maritime University University
(Dongsam 2dong, Busan City, Korea)
E-mail:kyungks@hhu.ac.kr
By the completion of the Cheongpung Bridge, in December 2010, having a main span length of 372m,
Korea has become the third country having a hybrid, cable-stayed bridge with main span length of longer
than 300m, following France and Japan. Starting with Cheongpung Bridge, hybrid cable-stayed bridge,
extradosed bridge, and mixed girder bridges which are under construction or in design phase.
In civil projects, steel and concrete are the most common materials used to construct hybrid structures. The
main purpose of hybrid structures is to create superior structural characteristics, which cannot be achieved
in the structures comprised of a single material or type of member, by combining dissimilar materials or
members. Hybrid structures can be coarsely classified into composite structure and mixed structure.
The core technology of the steel-concrete mixed girder structure is the design of the connection between
the steel girder of the main span and the concrete girder of the side span. Since the detailed design standard,
related studies, and reference data for the design of the connections are not sufficiently available yet, the
application and spread of mixed girder structure are facing difficulties.
To this end, the authors have conducted structural analysis with the model of a real bridge to acquire
technical data and evaluate feasibility, in addition to structural, behavioral characteristics. The focus was to
investigate the stress transfer mechanism in the connections of mixed girders.
Key Words : steel-concrete mixed girder, Connection, shear connector
1. INTRODUCTION
By the completion of the Cheongpung Bridge, in December 2010, having a main span length of 372m, Korea
has become the third country having a hybrid, cable-stayed bridge with main span length of longer than 300m,
following France and Japan. The Cheongpung Bridge is the first hybrid cable-stayed bridge in Korea.Starting
with Cheongpung Bridge, hybrid cable-stayed bridge, extradosed bridge, and mixed girder bridges which are
under construction or in design phase.
A hybrid structure implements its functionality by being combined with different types of materials in the
member level or structural system level. In civil projects, steel and concrete are the most common materials
used to construct hybrid structures. The main purpose of hybrid structures is to create superior structural
characteristics, which cannot be achieved in the structures comprised of a single material or type of member,
by combining dissimilar materials or members. Hybrid structures can be coarsely classified into composite
structure and mixed structure.
The core technology of the steel-concrete mixed girder structure is the design of the connection between the
steel girder of the main span and the concrete girder of the side span. Since the detailed design standard, related
studies, and reference data for the design of the connections are not sufficiently available yet, the application
and spread of mixed girder structure are facing difficulties. To this end, the authors have conducted structural
analysis with the model of a real bridge to acquire technical data and evaluate feasibility, in addition to
structural, behavioral characteristics. The focus was to investigate the stress transfer mechanism in the connections
of mixed girders.
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2. MIXED GIRDER
(1) Characteristics of mixed girder
A mixed structure refers to the structural system consists of dissimilar materials in combination. Similarly, a
mixed girder consists of steel girders and concretes girder mixed in the longitudinal direction. Generally, a
mixed girder system provides significant advantages by using light-weight steel girders in the main spans and
concrete or PDC girders in the side spans. In this structure, the bending moment in the main span is reduced by
the difference in the weights of the steel girder and concrete girder, and the problem of the upward reaction
force at end bearing points which can occur when the side span is shorter than the main span can be resolved. In
addition, the travel performance of vehicles is improved, resulting in reduced maintenance cost. Mixed girders
are mainly applied to hybrid cable-stayed bridges, extradosed bridges, and girder bridges.
(2) Classification by connection
The connections between the steel girders and concrete girders of mixed girder structures can be classified into the
three types of: bearing plate type; filling concrete type; and shear connector type. Fig. 1 is the schematic view of the
three type of connections.
(a) Bearing plate (b) Filling concerete (c) Shear connector
Fig.1 Schematic diagram of the connection.
3. STRUCTURAL ANALYSIS
(1) Bridge of the present <strong>Study</strong>
The Shiotsubo Bridge is a mixed girder type, two continuous span bridge at Takasatomura, Yama-gung,
Fukushima-ken, Japan. Due to the restriction of the topology, the bridge has approximately 2:1 of unbalanced
span lengths. About 70% of the longer span is built with steel girder. Fig. 2 and Fig. 3 show the plan view and
the cross-sectional view of the bridge, respectively.
Fig.2 Plan view of the bridge.
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(a) Steel Girder
Fig.3 Cross-sectional view of the Bridge.
(b) Pc Girder
The connection of the bridge is a filling-concrete, rear-side bearing plate type. With the bearing plate positioned
at the rear, the filling concrete and PSC component form a continuous structure to transfer force by the
rear plate and the shear connector of composite shape flange, and prestress is applied by PS steel member. Fig.
4 shows the structural detail of connection part in the Shiotsubo Bridge.
Fig.4 Strucrural detail of connection part in Shiotsubo bridge.
The steel shell which comprises the connection part consists of 15 cells. The steel shell is provided with
perforated steel sheet shear connector (perfobond rib) and filled up with high-flowing concrete. The
pre-stressing method is conducted by extending the steel wire from the PC girder to the rear plate.
(2) Analysis model
With the design drawings and structural calculation sheet of the Shiotsubo Bridge, 3D grid analysis was
conducted by linear analysis to analyze the complicated behavior in the connection.
a) Model A(Frame analysis model)
To verify the model by comparison with the structural calculation of the Shiotsubo Bridge, a frame model
was constructed having the same nodes and cross-section as those of existing structural model, as shown in Fig.
5.
Fig.5 Frame analysis model of the bridge.
b) Detailed analysis model of connection part
Detailed analysis model of the connection part was modelled to analyze the local behavior of the connection
part. Fig. 6 and Fig. 7 show the detailed analysis model .
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Fig.6 Detailed structual analysis model.
Fig.7 PC Wire modelled with rod elements.
The detailed analysis model represented 3.75m of the PC girder and steel girder including the connection part
of the bridge. The steel members, concrete members and the PC steel members were modelled by 4-node shell
elements, 8-node solid elements, and beam elements, respectively.
c) Model B(frame model + detailed analysis model of connection part)
For effective load transfer in the model, the beam elements of the frame model and the elements of the detailed
analysis model were combined using the rigid link. Fig. 8 shows the frame + detailed analysis model.
Fig.8 Model B combined frame model and detail model.
(2) Model validaion
a) Model A
In the validation process, only dead loads were applied and the live loads (wind load, snow load, temperature
load, creep and shrinkage) were not taken into consideration. Table 1 presents the load cases applied in the
structural analysis. In the validation process, only dead loads were applied and the live loads (wind load, snow
load, temperature load, creep and shrinkage) were not taken into consideration. Table 1 presents the load cases
applied in the structural analysis.
Table 1 Load Case.
NO.
Load Case
Self-weight of main girder
Self-weight of slab
Upper deck and other self-weights
Construction Step 1
Construction Step 8
Surface load (pavement, curbs)
Surface load (guide rail, median strip)
In the first stage validation of the present structural analysis model, the self-weight of the main girder classified
as deal load in the structural calculation sheet was compared with the reaction force obtained by structural
analysis. Here, the self-weight of the main girder comprised of the load cases of ~ in Table1. Table 2
presents the reaction forces at A1, P1, and A2 points obtained from the structural calculation sheet and the
structural analysis.
In Table 2, while the errors at the A1 and A2 points are relatively large by 4.7% and 3.1%, respectively, that
at the P1 was very accurate by 0.01%. Since the total error is accurate by 0.3%, it could be judged that the
structure model and the applied loads are appropriate.
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Table 2 Comparison of the reaction forces at bearing points.
Classification A1 Point P1 Point A2 Point Total ( )
Reaction forces
by main girder
weight
Sheet 3173.49 22817.64 1550.06 27541.19
Analysis 3032.29 22820.15 1597.75 27450.19
Error rate 4.70 % 0.01 % 3.1 % 0.3 %
In the second stage model validation, the section forces of the structural calculation sheet and structural
analysis were compared. Load case of before composition and after composition are summarized as follows,
location of elements is shown Fig. 9. The section forces of the structural calculation and sum of section forces
of before and after composition obtraned from structural analysis were compared in Table 3.
Fig.9 Location of elements.
- Before composition: self-weigh of the main girder (the load case; ++++ of Table1) before the
composition was taken into consideration
- After composition: only the dead loads (the load case; + of Table 1) applied to the main girder after the
composition were taken into consideration
Table 3 Comparison of the bending moments at the bearing points.
Classification
Dead load in the structural
calculation sheet
Sum of the bending moments
before and after composition
Moment-y (kNm)
El. No.[6] El. No.[ [13] El. No.[I[23] El. No.[I[33]
67485 11636 -221077 657
52541 1952 -229886 965
(a) Bending moment diagram of structural calculation.
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(b) Bending moment diagram of structural analysis.
Fig.10 Comparison of the bending moments of structural calculation and analysis.
The results of the structural analysis were somewhat different from those of the structural calculation sheet,
which was though to be caused by the auxiliary loads of the steel girder which were omitted in the calculation
and/or difference in positions which were not presented in the drawings of the Shiotsubo Bridge. However, as
shown in Fig. 10, the bending moments obtained by the structural calculation and the structural analysis show
the diagrams of similar profile, and the focus of the present study is on the behavioral characteristics of the
connection caused by the changes of the structural detail, it was judged that the structural analysis model of the
bridge was appropriate.
b) Model B
For the applied loads, only the fixed loads were applied excluding live loads. Fig. 11 shows the maximum
deflection of the two models.
(a) Model A.
(b) Model B.
Fig.11 Comparison of the model A and model B.
The deflections at the Element 6 which were the largest showed about 3% of difference by were 209.5mm and
202.4mm, respectively, which was thought to be caused by the error in the weight calculation of the detailed
model of the combined connection.
The stress calculated using the section force of the Element 13 of the fram analysis model, which is equivalent
to the detailed model section of the connection part, and the stress obtained by the structural analysis of the
detailed model of the connection were 3.2 MPa and 4.1 MPa, respectively. While the difference is about 28%,
it is not significant since the stress level is very low.
Summarizing the above discussion, since the deflections and stresses show insignificant difference, it could
be judged that the behavioral characteristics of the two models are the same, and thus, the model B is effective.
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(4) Detailed structural analysis of the connection part
a) Implementation of the perforbond rib effect of the stuctural analysis model
In this study, concrete and perforbond rib were presumed to be partially combine, and the effect of perforbond
rib was implemented, applied to the connection part, through detailed analysis.
The analysis model was model B, and the load condition applied to the detailed analysis model was the live
load of DB-24 of Korean design load. The live load of DB-24 was applied by simultaneous loading on all of the
three car lanes and as concentrated loads at the points where the bending moment was greatest. As shown in
Fig. 12, the dead loads applied as line loads and concentrated loads in the fram analysis were transformed into
distributed loads and applied in the detailed structural model. The prestressing applied at the connection part
was substituted by temperature load by referring to the actual structural calculation sheet.
(a) Transformed into and applied as uniform load.
(b) Prestressing substituted by temperature load.
Fig.12 Loading conditions of detailed structural analysis.
Table 4 Positions of the representative elements of the structure model.
Left bottom of the steel pier shell rear plate
was defined to be the origin point, and the
elements were expressed with (x, y, z) coordinates
system
Element No.: 24603 (solid)
Coordinates: (20, 2421, 2560)
Between the anchored bearing plates of the
PC steel wire close to rear plate and subject
to concentrated stress
Element No.: 3966 (plate)
Coordinates: (0, 2421, 2560)
Between the anchors of the PC steel wire
subject to concentrated stress
Element No.: 19407 (plate)
Coordinates: (1514, 2361, 290)
Between the elastic links where stress flows
smoothly and not concentrated
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Combination method
Table 5 Stress inside the steel pier shell.
Concrete in the steel pier
shell
Rear plate
PBL in the steel pier shell
Fully synthesized model -0.698 MPa 4.577 Mpa 16.823 Mpa
Partially synthesized
-0.630 MPa 3.966 MPa 14.676 MPa
model
Difference Approx. 10.7% Approx. 3.5% Approx. 8.9%
The points where local stress concentration may occur due to the geometrical shape or characteristics of the
structural analysis were excluded, and the representative elements of the rear plates, filling concrete part,
perforbond rib parts were presented in Table 4. Table 5 presents the results of the structural analysis comparing
the average stresses of the parts.
The result of the structural analysis showed that the stress of the fully composite model was somewhat greater
than that of the partially composite model. The stress reducing ratios of the latter compare with the former were
10.7%, 3.5%, and 8.9% at the concrete part, rear plate, and perforbond rib part, respectively. The reason of
which was thoght to be the finite stiffness of the partially composite model compared to the fully composite
model having infinite stiffness, and the effect of perforbond rib was implemented through the structural
analysis of the present study. When the effect of perforbond rib was applied, the change of the stress in the
filling concrete in the steel shell was the greatest, while that in the rear plate was the smallest. This was thought
to be because of the indirect effect through the concrete part, different from the concrete part and the perforbond
rib part applied to elastic link directly.
b) Load transfer in the connector element
The connection part should be designed with a structure that can transfer stress smoothly for the section
forces applied on the connection faces of the steel girder and PC girder which have different stiffness. In this
study, the filling concrete rear bearing pate was adopted due to wide application and provides smooth stress
transfer since filling concrete and PC girder are connected. The dominant section forces of the Shiotsubo
Bridge are bending moment and shear force, due to the continuous girder structure of the bridge. In this section,
the structural analysis was conducted using the conditions (analysis model, applied loads) of section a) to
analyse behavioral characteristics through the distribution of the shear force and axial force in the connection
elements in the steel shell.
At the connection part, load is transferred to concrete through the bearing plate and shear connector. Fig. 12
shows the longitudinal distribution of the shear force applied to perforbond rib.
(a) PBL in the steel pier shell.
(b) Shear force distribution of the upper PBL of upper flange.
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(c) Shear force distribution in the PBL of upper/lower flanges. (d) Shear force distribution in the PBL web.
Fig.13 Shear force distribution in longitudinal direction in perforbond rib.
Fig. 13(a) shows the position of the perforbond rib in the steel shell, which were numbered from to . The
perforbond ribs in Fig. 13(b) ( ~ ) were located on top of the steel shell, symmetrical to the center line of
the cross-section. As shown in the graph, the perforbond ribs located almost symmetrically show similar distribution
of shear forces. The greatest shear force was approximately 19 kN at the beginning point of the and
of perforbond ribs, which was thought to be the result of the shear delay effect by the steel webs on both
sides.
The graphs in Fig. 13(c) compare the distribution of the shear forces in the perforbond ribs of the upper and
lower flanges, where, ~ were on the upper flange and ~ were on the lower flange. The distributions of
the perforbond ribs on the lines vertical to the longitudinal axis show similar trends but in the opposite directions.
The forces of the perforbond rib on the upper flange showed positive (+) direction, which agrees with the
behavior of the connection located in the negative moment section.
The graph of Fig. 13(d) shows the stress distribution of the perforbond rib of web. While of perforbond rib
was in positive (+) direction with a magnitude of almost zero, and the shear forces of the and of perforbond
rib were the greatest by -50.9 kN. Therefore, it was judged that the shear force of the perforbond rib of
web is in the negative (-) direction.
Fig.14 Definition of the cross-sections of steel pier shell.
Three cross-sections were defines as shown in Fig. 14, to investigate the distribution of the axial forces in the
upper/lower flanges and webs of the steel shell. No. 1 and No. 3 of cross-sections were at the ends of the steel
shell and No. 2 was located at the center of the steel shell.
Fig. 15 shows the distribution of the axial forces of the cross-sections at various positions of the flange. The
axial forces were based on the concept of representative forces which were the products of the average
cross-section (3198 ) and average stress.
The axial forces of the upper flange in the No. 1, No. 2, and No. 3 of cross-sections were 64.4 kN. 10.6 kN,
and 5.8 kN, respectively. In the similar manner, those in the lower flange were (-)77.9 kN, (-)66.6 kN, and
(-)37.4 kN, respectively. The distribution shows the trend of decrease from No. 1 to No. 3of cross-sections. It
was judged that, regarding the force in the connection, the load transferred from the steel girder to the flange is
transferred to the perforbond rib and to concrete.
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Fig.15 Axial force distribution of the
cross-sections of the flange.
Fig.16 Axial force distribution of the
cross-sections of the concrete.
Fig. 16 show the distribution of the section force of the concrete on the checked cross-section. The axial
forces on the No. 1 close to the steel girder was 8.9 kN, No. 2 at the center of the steel shell was 9.36 kN, and
the No. 3 close to the PSC girder was 9.56 kN, showing a trend of increase from No. 1 to No. 3 cross-section,
opposite from the trend in the flange. It was judged that the load born by the concrete part increased as the axial
force in the flange decreased.
4. CONCLUSION
The behavioral characteristics of the Shiotsubo Bridge, Japan, was analyzed by full-system analysis and
detailed structural analysis to investigate the applicability of mixed girders in steel box girder bridges. For the
optimal design of the connections part which are the most important parts of mixed girder bridges, the behaviors
of the connection elements were analyzed from the changes of the elements. The transition of the axial
forces and shear forces in steel pier shell were analyzed to investigate the load transfer mechanism.
The conclusion obtained from the study is summarized as follows;
(1) The results of the structural analyses of the rigid model sharing nodes and the model using linear elastic
links were compared to implement the effect of partial synthesis. The result of the structural analysis showed
that the fully synthesized model provides somewhat higher stress than that of partially synthesized model,
which was thought to be caused by the stiffness value of the partially synthesized model compared with that of
the fully synthesized model which had infinite stiffness. It could be judged that PBL effect was implemented
by the structural analysis program of the present study.
(2) The behavioral characteristics of the steel pier shell were analyzed by defining the target PBL points and
the cross-section in the connection elements of the steel pier shell and comparing the distributions of shear
forces and axial forces. It could be judged that the load transmitted from the steel girder to the flange was
transferred to the PBL and then to the concrete in the steel pier shell.
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