Development of Preflex Composite Beam-Stub Abutment Integral Bridge System

Transcription

1 Steel Structures 6 (2006) Development of Preflex Composite Beam-Stub Abutment Integral Bridge System Jae-Ho Jung 1, Won-Sup Jang 1, Sung-Kun You 2, Young-Ho Kim 2 and Soon-Jong Yoon 3, * 1 Chun Il Eng. Consultants, Yeoksam-dong, Gangnam-gu, Seoul , Korea 2 Sinil CNI Co., Ltd., Rm. 701 Baekgung Plaza III, Jeongja-dong, Bundang-gu, Seongnam , Korea 3 Department of Civil Eng., Hongik University, 72-1 Sangsu-dong, Mapo-gu, Seoul , Korea Abstract In this paper, we present the results of an analytical study pertaining to the structural behavior of pre-deflected (so-called preflex) beam-stub abutment integral bridge system. In this bridge system, pre-deflected composite beam is connected with stub-type abutment resting on the H-piles driven in a row in the perpendicular direction of bridge span. To investigate the flexural behavior of preflex composite beam-abutment integral bridge, parametric studies are performed by changing the span length of bridge and the height of abutment. The results of an analytical study on the preflex composite beam-stub abutment integral bridge system were compared with those of the conventional joint bridge system, and the design of preflex composite beam, stub abutment, and pile foundation is also briefly discussed. Keywords: Earth pressure, Integral bridge, Preflex composite beam, Stub abutment, Thermal behavior 1. Introduction Traditionally, several types of expansion joints and bearings have been used to accommodate the seasonal thermal expansion and contraction of bridge decks. Because these joints and bearings are prone to be leakage and are affected by de-icing salts and other sources of corrosion, the cost of maintenance is an ever-growing problem for road administrations around the world, and bridges are no exception to the rule. One of the methods to reduce the need for future maintenance as well as the investment cost is to make bridges without transition joints and bearings, which is usually called as an integral abutment bridge. Integral abutment bridges are one of the jointless bridges and are generally referred to single-span or multiple-span continuous jointless bridge structures with capped-pile stub-type abutments as shown in Fig. 1. The examples of stub-type abutment details for different departments of transportation in USA are shown in Fig. 2 (Burke, 1990). It should be realized that these sketches are bare bones presentations. They do not reflect other important design aspects such as skew angle, construction procedures, etc. The major advantages of integral abutment bridges include the reduction of initial construction costs and long-term maintenance expenses, the elimination of cost expansive *Corresponding author Tel: , Fax: sjyoon@hongik.ac.kr joints and bearings, the decrease of impact loads, the improvement of riding quality (because of no expansion joint), and simple construction procedures (Burke, 1993). This type of bridges has been used successfully in many countries, particularly in USA and Canada, where overall deck span length up to m has been constructed (Kunin, 2000). In U.K., it has been proposed that all bridges with length up to 60 m and skew angle of less than 30 o be constructed integrally (Lock, 2002). Although most of the integral abutment bridges in service perform satisfactorily, design engineers in Korea are reluctant to design the integral abutment bridge system due to the lack of experience and the absence of reliable design guidelines for the design of integral abutment bridge. In accordance with the technical survey conducted in USA (Kunin, 2000), design limits and analysis of these bridges vary from state to state. Because the main structural behavior of integral abutment bridge is affected by the environmental conditions such as temperature variation, humidity, and the coefficient of thermal expansion of the material, the allowable limit of bridge length varies depending on the type of superstructure and the variation of temperature at the construction site. Therefore, in order to expedite a practical use of those bridges in Korea, it is necessary to understand the behavior of integral bridges with various types of superstructure. The integral abutment bridge considered in the study is a simple-span preflex composite beam bridge. The preflex composite beam is a pre-deflected steel beam

2 176 Jae-Ho Jung et al. Figure 1. Integral abutment bridge. Figure 2. Stub-type abutment (Burke, 1990). Figure 3. Fabrication process of pre-deflected beam. with concrete casing in its tension flange, and its fabrication process is shown in Fig. 3. The steel beam is built up with initial curvature, which is analytically predetermined camber, by shop welding and the concentric loads is applied to form pre-deflected steel beam (stated as preflexion). The preflexion load is a four point bending load and generally applied at one fourth of the beam span length. Under the preflexion load, the high strength concrete (the compressive strength is more than 40 MPa at the age of 28 days) is cast at the lower flange which is in tension. When the casing concrete is hardened, the preflexion load is released to obtain the prestress on the casing concrete. The preflex composite beam bridge has been widely constructed in Korea for the relatively short span-bridge, less than 50 m because of its low beam depth. The preflex composite beam bridge has many advantages in comparison with other types of bridge but one of the disadvantages is that low riding quality due to its excessive vibration. This somewhat over vibration problem can be mitigated by restraining the ends of bridge superstructure using integral abutment bridge system. In this study, we address the analytical investigation on the behavior of simple-span preflex composite beam-stub

3 Development of Preflex Composite Beam-Stub Abutment Integral Bridge System 177 abutment integral bridge. Upon brief review of existing literature, the preflex composite beam-stub abutment integral bridge is modeled as a plane frame and the parametric studies are performed considering the effect of backfill soil pressure and temperature changes. The result of preflex composite beam-stub abutment integral bridge is compared with that of the preflex composite beam joint bridge. 2. Analytical Model for the Soil-pile Interaction and Earth Pressure of Backfill In the analysis of integral bridge, the construction sequence must be considered. The construction of integral bridge is divided into two stages. The first stage includes the casting of fresh concrete on the deck slab and abutment. After the flexible piles are driven in a row to accommodate thermally induced bridge deck movements, fresh concrete is cast on the part of abutment, the precast beam or girder is launched, and the fresh concrete is cast on the deck slab and the rest part of abutment. The second stage is the completion of bridge structure after connecting between the deck slab and abutment. Therefore, the structural behavior of bridge up to the first stage of construction is the same as that of joint bridge. On the other hand, since the restraining effects induced by the surrounding soil and substructures are provided at the second stage, the behavior of integral bridges is affected by the creep and shrinkage of concrete, thermal expansion and contraction of superstructure, uneven settlement, earth pressure from backfill, and soil-pile interaction, in addition to the primary effects due to dead load, live load, etc. Of those influencing factors, the thermal expansion and contraction is the major factors inducing backfill pressure as well as soil-pile interaction. In this study, the literatures about the backfill soil pressure and soil-pile interaction in the second construction stage were briefly reviewed and one of those methods was applied for the parametric study Soil-pile interaction The closed-form solution for the laterally loaded pile is very limited because of the nonlinearity of soil property, therefore, approximate solutions and empirical methods are usually used for practice. The typical empirical methods are the method of p-y curve, Evans and Duncan Method (Evans and Duncan, 1982), SALLOP (Simple Approach for Lateral Loads on Piles, Briaud, 1997), and the equivalent cantilever method (Greimann et al., 1988, Abendroth et al., 1989). Of those methods, the second and third methods are based on the p-y curves concept which is the nonlinear relationship between the soil pressure (p) and the pile deflection (y) (Arsoy, 2000). The equivalent cantilever method appears to be widely accepted among the bridge engineers. In this method, the soil-pile system is reduced down to an equivalent cantilever beam-column whose length can be expressed as a function of surrounding soil properties and the rigidity of piles. This reduces the nonlinear soil-structure system to the equivalent linear structural system, which is valid only for the assumed loading and displacement level. Previous research results by the finite element simulation and experiment revealed that the results obtained using the equivalent cantilever model are conservative and sufficiently accurate (Abendroth et al., 1989; Girton et al., 1991) Backfill soil pressure According to the survey report conducted in USA, some agencies even do not consider soil pressure within a certain abutment size limit, and other agencies do not consider the earth pressure at all in their design. When the earth pressure is considered in the design, the earth pressure is assumed as a uniform or a triangular distribution, and it is recommended that two thirds of full passive earth pressures is used only for the design of integral abutment (Kunin et al., 2000; Burke, 1993). Because of the lack of a simple and reliable way of predicting the relationship between earth pressures and abutment movements, there has been several analytical and experimental studies as follows. To investigate the relations between earth pressure and wall displacement, Fang et al. (1994) conducted an experiment with a model. In his work, the modes of wall displacement were classified into horizontal translation (T mode), rotation about the wall top (RT mode), rotation about a point above the wall top (RTT mode), rotation about the wall base (RB mode), and rotation about a point below the wall base (RBT mode). It was shown that the distribution of earth pressure changes with respect to the mode of wall displacement. Thomson also conducted the experimental study for the passive earth pressures behind integral bridge abutment. Based on those modes of wall displacement proposed by Fang et al., Hong et al. (2003) analytically investigated the distribution of passive earth pressure of integral bridge using FLAC (Fast Lagrangian Analysis of Continua). From those experimental and analytical studies, it was shown that the distribution of passive earth pressure of integral abutment is not a triangular form and the resultant force of earth pressure, which is less than the maximum passive earth pressure, is located at about the half of the wall. In recent years, there have been efforts to develop new model of earth pressure that can take into account the displacement of the abutment. Dicleli (2000) suggested a simplified form of equation to find the coefficient of earth pressure and Hong et al. (2003) proposed the formulation of an elastic spring model for the earth pressure of backfill soil induced by the thermal expansion of deck in integral abutment bridges based on her analytical study and published experimental studies.

4 178 Jae-Ho Jung et al. Figure 4. Cross section of preflex composite integral abutment bridge (Unit: mm). 3. Parametric Studies In the design of integral abutment bridge, the maximum bridge length in the integral abutment bridge system needs to be limited by considering the thermal behavior. The thermal behavior of preflex composite integral bridge causes the backfill soil pressure and those forces induce the secondary internal forces which do not occur in the joint bridge system. Thus, the parametric studies are performed to investigate the response of preflex composite integral bridge due to temperature change including the main design load such as dead and live loads Bridges considered The preflex composite integral bridge as shown in Fig. 4 is considered in this study. The width of bridge is 20.9 m and the length of bridge varies from 30 m to 50 m by 10 m. Seventeen HP piles are driven under each abutment symmetrically and they are oriented to develop weak axis bending, i.e., perpendicular direction to the longitudinal axis of bridge Modeling Because the structural behavior considered in this study is the flexural behavior of preflex composite beam under length variation due to thermal loading, only the internal girder was considered to conduct parametric study. Fig. 5 shows the cross section of the internal girder and the dimension of the girder with respect to the length of bridge (L) is listed in Table 1. The internal girder was modeled as a plane frame member considering the effective width of the slab and the transformed section method was used to estimate the section properties of Figure 5. Cross-section of internal girder. composite section. The abutment was idealized to have a tributary width equal to the effective width of slab and the stiffness of piles within the effective width was lumped to a single pile element. The height of abutment was varied from 2 m to 4 m. To consider the soil-pile interaction, the equivalent cantilever method was used. The depth of pile is 18.8 m and the equivalent pile length estimated by following Abendroth et al. (1989) is 2.3 m. Schematic view of analytical model used in this study is shown in Fig. 6. In the analysis, general purpose structural analysis computer software, GTSTRUDL (2004) was used. For the analysis, the bending moment by the dead load (self-weight of girder) at the first construction stage was

5 Development of Preflex Composite Beam-Stub Abutment Integral Bridge System 179 Table 1. Dimension and material properties of composite girder L (m) H B fu T fu B cpu (1) T cpu (1) B fl T fl B cpl (1) T cpl (1) T ws B cs = 2400 mm T cs = 240 mm B cc = 1000 mm Slab F ck = 27 MPa E cs = 24.4 GPa T cc = 400 mm T wc = 300 mm H hun = 400 mm Casing Concrete F ck = 40 MPa E cc = 28.6 GPa V hun = 200 mm T u = 120 mm T l = 120 mm Steel F y = 360 MPa E s = 200 GPa Figure 6. Plane frame model for the analysis of integral abutment bridge. estimated by assuming that the both ends of girder are simply supported considering the construction situation and the results were added to the final results. At the second construction stage, the additional dead load (D), live load (L), temperature decrease ( T d ), temperature increase ( T i ), active earth pressure (P A ), and passive earth pressure (P P ) were applied by load combination listed in Table 2. The intensity of additional dead load 6.76 kn/m was applied uniformly and DB24 truck load (total load of 432 kn) was applied for live load according to the Korean Standard Specification for Highway Bridges (2005). For the temperature load, temperature changes of ±20 o C is uniformly applied for the superstructure. Creep and shrinkage of concrete were not considered in this study. For the earth pressure, Rankine s active earth pressure was applied in the triangular form and combined with the temperature decrease. For the passive earth pressure, the Table 2. Load combinations Load case Load combination 1 D 2 D + T i + P P 3 D + T d + P A 4 D + L + T i + P P 5 D + L + T d + P A half of Rankine s passive earth pressure was applied in accordance with the results of previous work by Kim (2004). The material properties of backfill soil are assumed to be those of the gravel or sandy gravel with few fines (i.e., density γ = kn/m 3, internal friction angle φ =35 o, cohesion c =0kN/m 2 ) Flexural behavior of preflex composite integral bridge Results of parametric studies are compared with those of joint bridges and the comparison results are shown in Figs. 7 and 8. Fig. 7 presents the ratio of maximum positive bending moment (M max ) between integral and joint bridge, and Fig. 8 shows the ratio of maximum displacement due to the live load. The maximum positive bending moment of integral bridge occurs due to the load case 5. As shown in those two figures, the maximum moment and displacement are reduced by replacing the joint system with the integral system. Especially, the deflection is reduced more than 20% of deflection at the joint system. From this result, it can be said that the serviceability of preflex composite beam bridge is improved significantly. Fig. 9 shows the bending moment at the superstructure and abutment junction. This bending moment is induced by the restraining effect provided by the substructure and surrounding soil behind the abutment. Since this moment does not occur in the conventional joint bridge, the magnitude of bending moment is compared with the

6 180 Jae-Ho Jung et al. Figure 7. Comparison of maximum bending moment. Figure 9. Moment at the deck-abutment junction. Figure 8. Comparison of maximum displacement. Figure 10. Moment at the pile. concrete cracking moment of preflex composite beam when the concrete crack occurs at the top of deck slab. The maximum negative moment at the junction is induced by the load combination 4 which is listed in Table 2. As shown in the graph, if the height of abutment is increased, then the negative bending moment is also increased. The reason of this tendency is that the magnitude of earth pressure increases as the height of abutment increases. It is noteworthy that the negative bending moment is smaller than the concrete cracking moment except for one case which is the bridge with 40m span length and 4m abutment height. Fig. 10 shows the maximum bending moment at the pile. The bending moment of pile is compared with the yield moment of pile. The response as shown in the graph is caused by the lateral movements due to the thermal expansion or contraction. This result indicates that the thermal movement must be taken into account for the design of piles. 4. Discussions and Conclusions In this study, the preflex composite beam-stub abutment integral bridge is introduced and the parametric studies are performed to investigate the global behavior of this

7 Development of Preflex Composite Beam-Stub Abutment Integral Bridge System 181 bridge system. The preflex composite integral bridge is modeled as a plane frame member and the flexural behavior of integral bridge is compared with that of joint bridge for the second construction stage. From the parametric studies, it was revealed that the maximum bending moment of integral bridge is reduced but the variation of bending moment is less than 10%. The negative bending moment at the junction of superstructure and abutment occurs due to the restraining effect of abutment but its magnitude is less than the concrete cracking moment of preflex composite beam. Therefore, the design method of preflex composite joint bridge may be used to design the superstructure of integral bridge system for the simplicity of design. When the integral abutment bridge system is applied to the preflex composite beam bridge, the maximum displacement due to live load is effectively reduced, thus, the serviceability may be improved in comparison with the joint bridge system. Preflex composite beam has been widely used to construct the bridge with span length less than 50 m in Korea. From the parametric studies performed in this study, the standard preflex composite beam section may be directly applicable to the integral abutment bridge system having the same span length of joint bridge system without considering the thermal behavior because the reduction of maximum bending moment in the integral system is less than 10% of the maximum bending moment in the joint bridge system. The thermal behavior of bridge should be taken into account for the design of abutment and pile foundation. The abutment of joint bridge is usually designed by applying the active earth pressure. The difference in the design of integral and joint bridge systems is that the integral bridge system must consider the passive earth pressure of backfill soil induced by the thermal expansion of the superstructure. According to the existing experimental and analytical studies for the passive earth pressure in integral abutment, the resultant of passive earth pressure is less than the Rankine s passive earth pressure and the distribution of passive earth pressure is not a triangular form. Despite several empirical methods for estimating the passive earth pressure of integral abutment have been suggested, the applicability of those methods still needs to be investigated by the field application. In the design of pile, the lateral movement due to the thermal expansion of superstructure should be taken into consideration in addition to the vertical force transferred from the superstructure. References Abendroth, R. E., Greimann, L. F., and Ebner, P. B. (1989). Abutment Pile Design for Jointless Bridges, Journal of Structural Engineering, ASCE, 115(11), pp Arsoy, S. (2000). Experimental and Analytical Investigations of Piles and Abutments of Integral Bridges, Ph.D. Thesis, Department of Civil Engineering, Virginia Polytechnic Institute and State University, Blacksburg, USA. Arsoy, S., Barker, R. M., and Duncan, J. M. (1999). The Behavior of Integral Abutment Bridges, Final Contract Report, VRTC 00-CR3, Virginia Transportation Research Council, Charlottesville, Virginia, USA. Burke, M. P. Jr. (1990). Integral Bridges, Transportation Research Board s 69th Annual Meeting, Washington, D.C., USA. Burke, M. P. Jr. (1993). Integral Bridges: Attributes and Limitations, Transportation Research Board s 72nd Annual Meeting, Washington, D.C., USA. Burke, M. P. Jr. (1993). The Design of Integral Concrete Bridges, Concrete International, 15, pp Dicleli, M. (2000). A Rational Design Approach for Prestressed Concrete Girder Integral Bridges, Engineering Structures, 22, pp Girton, D. D., Hawkinson, T. R., and Greimann L. F. (1991). Validation of Design Recommendations for Integral Abutment Piles, Journal of Structural Engineering, ASCE, 117(7), pp GTSTRUDL (2004). GTSTRUDL User's Manual, Latest Revision, GTICS Systems Laboratory, Georgia Institute of Technology, Atlanta, Georgia, USA. Hong, J. H., Jung, J. H., You, S. K., and Yoon, S. J. (2003). A Simplified Numerical Model for an Integral Abutment Bridge Considering the Restraining Effects Due to Backfill, KCI Concrete Journal, KCI, 15(5), pp Kim, S. S. (2004). A Study on the Design of Simple-Span Preflex Beam-Abutmemt Integral Bridges, Master Thesis, Department of Civil Engineering, Hongik University, Seoul. KRTA. (2005). Standard Specification for Highway Bridges, Seoul: Korean Road and Transportation Association. Kunin, J. and Alampalli, S. (2000). Integral Abutment Bridges: Current Practice in United States and Canada, Journal of Performance of Constructed Facilities, ASCE, 14(3), pp Lock, R. J. (2002). Integral Bridge Abutments, M. Eng. Project Report, CUED/D-SOILS/TR320, University of Cambridge, UK. You, S. K. and Park, J. M. (1998). A Study on Utilization and Application of Integral Abutment PC Beam Bridge, Proc. KCI Symposium-Fall, KCI, 10(3), pp