Design and Construction of Precast Panels for the Composite Cable-Stayed Bridge

Transcription

1 Design and Construction of Precast Panels for the Composite Cable-Stayed Bridge Woo-Jong KIM President, PE DM Eng. Co. Seoul, Korea Woo-Jong Kim, born 1959, received his Ph.D. degree in civil engineering form Seoul National Univ., Seoul, Korea in Summary Chan-Min PARK Research Director Korea Highway Co. Sungnam, Korea Chan-Min Park, born 1963, received his Ph.D. degree in civil engineering form Seoul National Univ., Seoul, Korea in Since the design specifications for a composite section of the cable-stayed bridge is not clearly established yet, various design concepts are used in the superstructure design of cable-stayed bridges. To predict the actual response of the bridge, creep and shrinkage behaviors and the effective width of composite deck should be included in the analysis. The method of combining stresses resulting from the global and local forces is also important. Keywords: cable-stayed bridge; composite deck; precast slab; stage analysis; slab thickness; effective width 1. Introduction The 990m long cable stayed structure which crosses the Asan bay in the west sea of Korea is the focal point of the 7.31km express highway crossing between two provinces. The cable stayed bridge is symmetrical, with two concrete portal frame pylons with vertical legs from which radiate four planes of 36 cables in a modified fan formation, making a total 144 cables. The 34m wide deck is located some 62m above mean water level and consists of steel plate girders with a composite reinforced concrete deck slab. The main span of 470m was needed to provide navigational clearance. Cable stayed bridge for the SeoHae bridge is 5 span continuous composite bridge with the total length of 990m(60m+200m+470m+200m+60m). The length of main span of 470m is the longest in Korea and was also the longest in the world at the beginning time of design. The superstructure is steel composite with 2 unsymmetric I-shaped steel girders spaced 34m which is slightly wider than the viaducts, since the cable stays require some space from concrete barrier to avoid any damage from vehicle collision. A reinforced concrete slab acts compositely with two 2.8m deep longitudinal steel plate girders and transverse floor beams at 4.1m centers. There is a longitudinal stringer in the middle of the steel girders. The reinforced concrete slab varies in thickness between 260mm and 310mm over the main girders. The 40 Mpa (4,000 t/cm 2 )concrete deck was cast onto the steel work on land, leaving 1m wide over the main steel girders and 0.25m over the stringer per side in situ sections to be cast after erection and bolting up of each unit. Composite action is ensured by 22mm dia. Studs and cast-in-place joints concrete. High strength friction grip bolts were used for all site connections to avoid site welding. A precast panel made of high strength concrete is often found in a composite cable-stayed bridge as a deck slab. Since the composite deck of cablestayed bridges is under severe compression due to the horizontal component of cable forces, the precast panel should be designed as a member Fig. 1 Cross Section under compression and bending. The effective width of concrete slab

2 considering shear lag effect, the method of combining stresses from local analysis and global analysis, and the minimum thickness of deck slab considering durability should be considered. 2. Effective Width of Slabs To simulate the concept of effective width calculations in continuous beams, the idea of using the distance, the effective span, L eff between inflection points is a good approach. The inflection points are the zero moment points in a moment diagram got from influence line analysis. But strictly speaking, because the moment diagram is unknown before first having the effective widths that is a trial and error process. In actual design, practical simplifications are made to allow easier analysis. SeoHae bridge used following 4 values-50m, 60m, 80m, and 120m along the bridge axis. Generally BS5400 code is used where the effective width is calculated based on the ratio of flange width/effective span length(b/l). Fig. 2 The Effective Span 2.1 Effective Width in Stage Analysis Balanced cantilever method is generally used in the cable-stayed bridge construction. The propagation angle of the horizontal component of a cable force is assumed to 30 degree. The construction sequence of composite cable-stayed bridge is, - Installation of steel girder - Initial stressing of cables - Installation of precast panels - Restressing of cables And this sequence is repeated segment by segment. Consequently, the effective width of a segment varies according to the construction step. So, it is desired to apply different effective widths to the same segment in different steps of stage analysis. In the following figure, the segments Seg 0, Seg 1, Seg 2, and Seg 3 have effective width of 0m, 2.26m, 9.63m, and 17.0m separately for the compression. For bending, the effective width of a segment is computed according to BS5400 code. With the full width of the deck (17.0m) and the distance between two adjacent anchorages (12.3m), the flange width-span length ratio is calculated as Fig. 3 Cantilever Construction (= 17.0/12.3=1.382 > 1.0). From the width-length ratio and the tables in BS5400 code, the factor(ϕ) will be 0.16 for mid span and 0.12 for support. SeoHae bridge used 0.16 for the factor(ϕ) and 0.12b for the effective width(b eff ) with some safety margin. Consequently the software to cover the stage analysis should have the function to simulate different section properties of a segment by changing section properties as the construction stage progresses. 2.2 Effective Width in Global Analysis After completion of the deck construction, the bridge should be analyzed with the live load and the additional dead load such as pavement, barrier etc. Here, the effective width is also used. The same concept used in the stage analysis is applicable in the global analysis. But, the effective span length is used instead of 12.3m, the distance between two adjacent anchorages.

3 ) 20 m15 ( th id W10 e tiv c A I In the global analysis of SeoHae bridge, the simplified different effective widths (Fig. 4) was used along the bridge axis and for bending analysis and axial compression. 3. Method of Design 5 f e Since the design specifications for a E PY1 Mid Span PY2 composite section of the cable-stayed bridge is not clearly established yet, various design concepts are adopted for Fig. 4 Effective Width the superstructure design of SeoHae cable-stayed bridges. The client, Korea Highway Corporation (KHC) called several committee meeting to prepare design specifications for high strength precast slab of cable-stayed bridges. Also, KHC investigated the specifications used in similar bridges worldwide and prepared specifications with sufficient safety margin within the basic boundary of Korean Bridge Specifications. Deck slab suffers from the vehicle weight directly loaded on it and works not only as a slab itself but also as a part of composite girder. And these two functions may be called as the local behavior and the global behavior respectively. In the local behavior, the slab works as a plate supported on the main steel girder and floor beams. And it suffers only from its dead weight and vehicle load. In the global behavior, the slab works as a part of composite girder of global structure. So it suffers from the horizontal component of cable tension, dead weight of superstructure, and live load. When combining stresses resulting from those different behaviors, it is difficult to combine those forces directly because of following reasons. - condition of max. stress is different - location of max. stress is different - time of max. stress is different If necessary, the stresses can be combined by use of the principal stress criteria. However, this would be too severe for a concrete due to the above reasons. In practice, they are designed separately. Nevertheless, the engineer still must realize that these stress components exist. The stress in a composite girder is very complex which requires extensive calculations to ascertain its magnitude. The most important are the creep and shrinkage and temperature stresses. Working stress design method or strength design method that is similar to the new LRFD can be used to design a deck slab. Most composite cable-stayed bridges have been designed by strength design method. But some consideration on actual tensile stress is necessary. This depends more on engineering judgement and experience. During the design of the slab which spans the floor beams as a plate, the axial forces from the global action of the girder were considered in the strength design and checked with P-M curve, similar to a column action. The slab was assumed to a longitudinally continuous beam supported on main girders and floor beams which had stiffness corresponding to the vertical stiffness of composite section. And the slab was separated into two parts. One is the area of central part of slab where is controlled by local analysis. In this area, the slab was modeled as a beam supported on 2 edges(2 floor beams) and local bending moments was calculated. The other is near the main girder where global behavior governs. 3 edges(1 main girder and 2 floor beams) are the support lines of this part. Fig. 5 Separation of Deck for Design

4 In the SeoHae bridge, following force combination was used to design slabs. Table 1 Combination of Forces Pu Mu Near Girder Central Part Global 1.3DL+2.15(LL+I) 1.3DL Local - - Global - - Local 1.3DL+2.15(LL+I) 1.3DL+2.15(LL+I) For the working stresses, if we check stresses, which is quite complex in a composite bridge unless the appropriate computer program prepared, the following allowable compressive stresses were suggested: - for axial stresses due to global action alone, use 0.40fc - for combined action of global action and local action under the wheel load, use 0.50fc The allowable stress is increased based on the probability and frequency of combination of extreme stresses mentioned above. The compression stresses of the SeoHae bridge at the top of the deck from axial forces(p) and bending moment(m) from the global analysis only are shown in Fig. 1. And the combined stresses at the top of the deck are shown in Fig. 2. The condition of combining stress is same to that of strength design method. - Near the main girder, (DL+LL)glo. + (DL+LL)loc. - Central part, (DL)glo. + (DL+LL)loc. In addition to strength design criteria, the following criteria are employed to prevent cracks. - under dead load condition, no 0.0 PY1 - tensile stress, under global action under dead DL(P)+LL DL(P+M )+LL load and live load(1.0dl+1.0(ll+i)), 0.75 (f ck )=15kg/cm Prestress was recommended to achieve this. From the global stresses and struttie model analysis, prestress was introduced to the central part of the Fig. 6 Global Stresses main span and the end part of the side span. Longitudinal tendons were prepared to keep above criteria. Transversal tendons were calculated by strut-tie concepts PY 1 Near Central 4. Thickness of the slab Thickness of the slab in a composite cable-stayed bridge depends on the spacing of floor beams and main girders, the interval of the cable anchorages, etc. Table 2 shows the slab thickness of the cable-stayed bridges Fig. 7 Combined Stresses worldwide. Sometimes, the life cycle of the bridge may depend on the durability of the concrete slab that is exposed directly to the vehicles. Replacing and reinforcing the concrete slab in the cable-stayed bridge are more difficult than other bridges because the slab is under severe compression. So, it is important to consider durability of the slab in design and construction phase. Korean Bridge Code requires more strict design condition to secure complete composite action and modified its criteria about slab thickness considering the fact that it is the most effective way to increase slab thickness to increase fatigue resistance of the slab. This means that design method

5 accessed to empirical design used in Ontario code. Table 2 Thickness of the Slab Bridge Location Main Span (m) Width (m) Thickness (cm) Space of Floor Beams (m) Completion SeoHae Korea Annacis Canada Nampu China Yangpu China Baytown USA Weirton- USA Steubenville 2 nd Servern UK Ting Kau HongKong Vasco da Gama Portugal Experience shows that the failure of the bridge deck is due to the punching shear by iterative vehicle load rather than by insufficient bending capacity. But, increasing the thickness excessively is no more effective and not desirable to cable-stayed bridges because of the additional weight. In SeoHae bridge, the slab consists of precast panels: 6 panels per 12.3m long segment connected with a cast-in-place closure joint. The connection between steel girders and concrete panels for composite action under the live load is provided by shear connectors. To achieve sufficient composite action, increase durability, and reduce selfweight, SeoHae bridge selected a nonprismatic section for deck slabs. That is, the edges where the connection is provided by studs have 31cm and the other part has 26cm. The areas near the pylon where suffers severe Fig. 8 Cross Section of the slab compression have 31cm as a whole. 5. Construction Precise arrangement of re-bars and careful casting of concrete in the narrow and long area of castin-place concrete joints are required to make successful composition between steel girders and concrete deck. Also, concerted efforts should be made to minimize cracks in concrete joints. Non-shrink concrete should be used at in-situ joints with sufficient hook reinforcement to get feasible continuity. To minimize creep and shrinkage effects, it is desirable to cure concrete panels for 60 days before installation. Slab blister is used to minimize the difficulties in installing intermediate anchors for longitudinal and transversal slab tendons in the precast slabs. Fig. 9 King Post It is very important to control initial cracks in deck slabs. Initial crack can be occurred easily at the joint between precast panels and cast-in-situ concrete due to shrinkage.

6 Introducing Reverse King- Post with hydraulic jack can be a good solution to control such a crack. Generally the erection procedure of one segment consists of 4 steps: Erection of steel girders, initial stressing of cables, erection of precast panels and pouring joint concrete, and restressing of cables. With the hydraulic jacks mounted in Reverse King-Post, floor beams are bent upward in noncomposite state. If the jacks are released after composite state, compression in transversal direction can be introduced to the cast-in-place joint concrete. Of course, this pre-compression will be disappeared due to creep behavior of concrete. In SeoHae bridge, the floor beams are jacked up about 30mm with 637 kn jacking force before composition. When Reverse King-Post is released after composition, 12mm vertical displacement is recovered and 20 kg/cm 2 pre-compression is expected. During the construction by the cantilever method, temporary bending moments in the deck can be created. To overcome this, using a deep girder or temporary PT bars can be a solution. But it becomes critical if the deck has a limited inertia and the efficiency of cantilever tendons in a slab is limited. SeoHae bridge considered the distance between cables in design phase to reduce the length of the bent zone and developed a new derrick crane somewhat different to that used in the other sites which can hang the deck before tensioning the cables to reduce bending moment. Also, the cracks parallel to the joints should be minimized by the proper sequence of placing concrete at joints as well as curing. So, it is needed to keep the surfaces of precast panels wet before placing concrete. Joint concrete is placed from the far end to the near end of the newly installed segment. This helps to eliminate the crack along the Fig. 10 Appling of Derrick Crane joint between new and old segments. 6. Conclusion Since the design specifications for a composite section of the cable stayed bridge is not clearly established, wider investigation was carried out in the design of the SeoHae bridge. Strength design method was used with strength reduction factor of 0.65 in P-M diagram for combined action from global and local analysis. Additionally, stresses were checked to access the allowable stress method. Thickness of the deck was compared to the similar bridges and decided to use non-prismatic slab varying from 26cm to 31cm. To minimize the occurrence of cracks, reverse king post was used during segment erection. References [1] C.M. Park, Design of the precast panel for the composite cable stayed bridge, IABSE Korea 3 rd Workshop, pp. 2-28, [2] Ministry of Construction and Transportation, Standard Specifications for Road Bridges, Korea, 1996 [3] BS5400 Code [4] M. Virlogeux, Erection of cable stayed bridges, Cable-Stayed Bridges, Recent Developments and their Future, ELSEVIER, 1991, pp [5] D.D. Byers, S.T. Hague, S.L. McCabe, D.M. Rogowski, Comparison of slab Participation : Assumed for design vs FEA, IABSE Conference on Cable-Stayed Bridges-Past, Present and Future, Malmo Sweden, 1999