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A Study on Application of Mixed Girder in Steel Box

Girder Bridge

Hyosang HWANG1 Kabsoo KYUNG2 and Hyungi MOON3

1Dept. of Civil Eng., Korea Maritime University (Dongsam 2dong, Busan City, Korea)

E-mail:hhs0259@naver.com 2Professor, 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 con-nections 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.

3. STRUCTURAL ANALYSIS (1) Bridge of the present Study 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.1 Schematic diagram of the connection. (a) Bearing plate (b) Filling concerete (c) Shear connector

Fig.2 Plan view of the bridge.

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The connection of the bridge is a filling-concrete, rear-side bearing plate type. With the bearing plate posi-

tioned 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.

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.

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 .

(a) Steel Girder (b) Pc Girder Fig.3 Cross-sectional view of the Bridge.

Fig.4 Strucrural detail of connection part in Shiotsubo bridge.

Fig.5 Frame analysis model of the bridge.

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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 de-

tailed analysis model were combined using the rigid link. Fig. 8 shows the frame + detailed analysis 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.

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 clas-

sified as deal load in the structural calculation sheet was compared with the reaction force obtained by struc-tural 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.

Fig.6 Detailed structual analysis model. Fig.7 PC Wire modelled with rod elements.

Fig.8 Model B combined frame model and detail model.

Table 1 Load Case.

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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.

- 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

Classification Moment-y (kN m)

El. No.[6] El. No.[ [13] El. No.[I[23] El. No.[I[33] Dead load in the structural

calculation sheet 67485 11636 -221077 657

Sum of the bending moments before and after composition 52541 1952 -229886 965

Table 2 Comparison of the reaction forces at bearing points.

Table 3 Comparison of the bending moments at the bearing points.

(a) Bending moment diagram of structural calculation.

Fig.9 Location of elements.

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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.

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.

Fig.10 Comparison of the bending moments of structural calculation and analysis. (b) Bending moment diagram of structural analysis.

(a) Model A.

(b) Model B. Fig.11 Comparison of the model A and model B.

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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.

<Full model coordinates> <Concrete part>

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) co-

ordinates 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

<Rear plate part> <PBL part>

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

(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.

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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 com-paring 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 perfor-bond 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.

Combination method 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

model -0.630 MPa 3.966 MPa 14.676 MPa

Difference Approx. 10.7% Approx. 3.5% Approx. 8.9%

Table 5 Stress inside the steel pier shell.

(a) PBL in the steel pier shell. (b) Shear force distribution of the upper PBL of upper flange.

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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 dis-tribution 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 direc-tions. 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 perfor-bond 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.

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.

(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.14 Definition of the cross-sections of steel pier shell.

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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 be-haviors 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. REFERENCES 1) G.H.Kang, : Experimental study on behavior of perfobond rib shear connectors : Yonsei University(2006). 2) Construction Engineering Institute. : Report on the connection behavior analysis and safety analysis of hybrid bridges : Yonsei University (2004). 3) D. H. Rhyu, J. Sang, Gho, T. G. Park, D. G. Kim,: Design and Construction of the Cheongpung Bridge (Hybrid, Cable-Stayed Bridge : Journal of COSEIC, Vol.23 ,No.1, 2010.1, pp. 31-36 (2010). 4) Hiroyuki Ikeda, M. Nakasu : experimental design and fatigue of the floor slab junction digit (2000) : Yasuyuki Koga, in Ibi, the Kiso River Bridge, Proceedings of Structural Engineering, Volume Vol46-A3, 2000, pp. 1655-1666

Fig.15 Axial force distribution of the

cross-sections of the flange.

Fig.16 Axial force distribution of the

cross-sections of the concrete.

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