Creep and Shrinkage Analysis of Composite Truss Bridge with Double Decks

Transcription

1 Abstract Creep and Shrinkage Analysis of Composite Truss Bridge with Double Decks XIN Haohui; LIU Yuqing; Zheng Shuangjie Tongji University,Shanghai, China MA Biao Shanghai Municipal ngineering Design General Institute,Shanghai, China An innovative large span composite bridge with double concrete decks and steel truss webs was proposed to resolve the traffic jam in cities in this paper. Based on a practical application of simply supported composite truss bridge with a main span of 96.0m, the time-dependent structural behaviors due to creep and shrinkage effects were investigated, by using a hybrid finite element (F) model. One of the web-deck composite joint was modeled by shell and solid element and embedded into the whole bridge model to investigate the local behavior of such composite joint structure. The exponential model of concrete creep and the implicit solution of concrete equations were introduced into ANSYS, and the concrete strain at any time can be solved by the hybrid finite element model. With time passed by, the deflection at midspan and maximum steel stress increases rapidly in the first year and the variation trend takes a more gentle course since most of the creep and shrinkage effect has been finished. The concrete stress and shear forces of headed studs gradually decreased. The decreasing velocity slowed down after one year. All the analysis results in this study could provide references for the design and construction of such composite truss bridges with double decks. 1. Introduction Deshenglu Bridge in Hangzhou (Fig.1) is the first simply supported composite truss bridge with double-decks in China [1-2]. The bridge has a main span of 96.0m, with 8 lanes on the upper deck and 6 lanes on the lower deck (Fig.2 (a)). The cross section of the bridge consists of the upper deck, lower deck and steel truss web members. As shown in Fig.2 (b), a composite truss joint is composed of a shaped steel chord, a concrete chord and two steel diagonal web members. Headed studs are used to resist shear forces at the interface between concrete chord and other steel members. ngineers are aware of the time-dependent behavior of concrete structures [3-6].Concrete creep and shrinkage effect will lead to tendons relaxation during construction and service stage, and cause additional deflections, cracking of concrete, reduction of prestress and redistribution of internal forces, and finally affect the structure long-term performance [7-8]. Therefore, the time-dependent effects of concrete creep and shrinkage are significant to the design of steel-concrete composite structures. On the basis of one engineering example of a composite truss bridge with double decks, this paper discussed the increased deflection, stress distribution of each component caused by concrete creep and shrinkage effect under serviceability states. In addition, the local stress state of the steel gusset plate was derived and shear forces of headed studs at the composite

2 joint were obtained. All the analysis results could provide references for the design and construction of composite truss bridges with double decks or similar structures. 1 st 2 nd 3 rd 5 th 6 th 7 th 8 th 9 th 10 th Fig.1 Deshenglu Bridge in Hangzhou, China (mm) (a)cross section (mm) Fig.2 Details of Bridge (b)composite joint 2. Creep simulation In order to predict and evaluate bearing and deformation behavior of bridge structure in serviceability state, creep and shrinkage analysis need to be performed.based on finite element program ANSYS [9], shrinkage effect is applied to the bridge structure by decreasing the temperature, and concrete creep effect is introduced by using creep state equation: C2 C3 C4 / T C1 cr e (1) Where: C 1, C 2, C 3 and C 4 are parameter relating to material, they may be the function of strain, stress, time or temperature. If C 2 was assumed to be 1, C 3 and C 4 were assumed to be 0; the creep equation was expressed as follows: (2) The time-dependent strain variation can be calculated depended on q. (3). C 1 C1 t (3) Creep ratio φ(t,τ 0 ) was recommended to describe concrete creep behavior. The concrete creep strain and creep strain variation can be gotten based on q. (4) and q. (5). e ( t, 0) ( t, 0) (4) 0 0 ( t, 0) ( t, 0) (5) Thus, the creep parameter for F simulation was expressed as q. (6) by comparing q. (3) with q. (5). Besides, the creep ratio under different state can be calculated according to relative specifications. ( ti, ti 1) 1 C1 (6) t Two traditional examples [3], including short compressive reinforcing concrete column (Fig.3) and cantilever beam (Fig.4), were modeled based on the above methods.the comparison F

3 250mm and theoretical results was summarized in Table.1. It indicated that the F results agreed well with theoretical results. The method can be used for structure creep analysis. N q=1kn/m A 9m 9m B A B (a) layout 250mm (a)layout (b) concrete column (c) reinforcing bars Fig.3 Short compression reinforcing concrete column Compression short reinforcing concrete column Cantilever beam (b) F model Fig.4 Cantilever beam Table.1 Comparison F with theoretical results Item Stage/Location F results Theoretical results Ratio initial stage Concrete stress last stage Creep effect initial stage Reinforcing bars stress last stage Creep effect A Bending moment B A Shear force B Finite element model The hybrid three-dimensional FA model involving the whole bridge with a detailed joint segment (Fig.5) were built by using ANSYS program [9]. The concrete decks cross beams and chords were modeled with SOLID45. The steel truss web members were built with BAM188. The internal and external tendons were stimulated by LINK8 elements with initial strains. The detailed model of the composite joint was embedded into the whole Fig.5 Hybrid 3-D FA model bridge model, with steel members stimulated by SHLL63 and headed studs by COMBIN14. In this study, the stiffness of headed studs is obtained from push-out test results. The concrete had a modulus of elasticity of MPa and a density of kg/m 3. The steel truss web members were made up of Q370qD steel, with modulus of elasticity as MPa, Poisson's ratio 0.3 and density kg/m 3. High strength low relaxation steel strands were used for internal and external tendons with the ultimate tensile strength of 1860MPa, and the modulus of elasticity MPa. Considering prestressing loss, the controlled stretching stress of tendons was set as 1116MPa. The calculated load case only involved dead load. The creep coefficients ratio and shrinkage temperature value were calculated based on Code for design of highway RC and PC bridges and culverts [10], as shown in Table.1.

4 Stress(MPa) Stress(MPa) Max deflection(mm) Table1 Temperature value and creep ratio for simulating shrinkage and creep effect Item 30 day 90 day 182 day 365 day 1095day 365ay Initial temperature Structure temperature Creep ratio coefficient Analytical results and discussion 4.1 Deflection-time relationship The variation of the composite truss bridge deflection at midspan due to creep and shrinkage effect is shown in Fig.6. The deflection at midspan increases rapidly in the first year after construction is completed. Afterward, the deflection-time curve takes a more gentle course since most of the creep and shrinkage effect has been finished. The peak value of deflection after ten years is 77 mm. 4.2 Stress distribution of concrete deck The change of stress distribution of concrete decks due to creep and shrinkage effect is shown in Fig.7, with the transverse distance of concrete deck as X-axis and normal stress as Y-axis. The results showed stress distribution was not uniform in the upper and lower slabs along the transversal direction of the bridge. Stress concentration appeared at the interface between steel truss web and concrete members, and the anchorage zone for tendons. With time passed by, the stress of concrete gradually decreased. The effect in the position of the connection between concrete and steel truss web was obvious and the value decreasing was larger than others. The decreasing velocity was slow down after one year. Compared with the upper deck, the stress distribution of the lower deck was easier to be affected by creep and shrinkage tranverse distance of deck(mm) tranverse distance of deck(mm) (a) Upper deck (b) Lower deck Fig.7 Normal stress of concrete deck 4.3 Local stress of steel member The time-dependent maximum stress of steel truss web members is shown in Fig.8. With time passed by, the maximum Von Mises stress gradually increased. In the first year, the stress increased substantially. After one year, the stress-time curve became flat. The stress changed from 232MPa to 258MPa in the following nine years. The maximum stress from to 3 occurred at the crossing of the third and fourth (Fig.1) diagonal steel truss webs. Afterward, the maximum stress was found at the joint of the fourth and fifth (Fig.1) diagonal steel truss web members time(day) deflection-time curve Fig.6 Deflection-time curve

5 max von mises stress(mpa) Fig.9 presents a joint connecting steel truss members and concrete slabs. The Von Mises stress of steel gusset plate in different period is shown in Fig.10. The steel gusset plate of the composite joint presented local stress concentration, with the stress increased from 259 MPa to 292 MPa in the first year. In the next nine years, the stress changed from 291MPa to 295MPa. The position in black diamond was where max local stress concentration occurred, and the stress in black rectangle and triangle gradually increased as well. The stress of the position in black ellipse was small at beginning and the local stress concentration gradually occurred until 3650 th day stress-time curve time(day) Fig.8 Maximum Von Mises stress-time curve Fig.9 Position of headed studs (a) 0 day (b) 30 day (c) 90 day (d) 182day (e) 365 day (f) 3650 day Fig. 10 The Mises stress of steel gusset plate in different period 4.4 Shear force of stud connector The change of shear forces of headed stud connectors (Fig.9)installed at the composite joint is showed in Fig.11. In the first row of headed studs, the shear forces of both the exterior columns were greater than those of the interior columns by about 25%. In the first column, the shear forces of headed studs increased by nearly 50%. The maximum shear force of headed studs was about 125KN near the crossing of tensile and compressive steel web members. In terms of shrinkage and creep effect, the shear forces of headed studs gradually decreased over time. After one year of construction, the maximum shear force of headed studs changed from 120kN to about 100kN. In the next nine years, the decrease of maximum shear force was less than 10%. 5. Conclusions A study on the creep and shrinkage effect of a composite truss bridge with double decks has been conducted in this paper. The following conclusions can be drawn:

6 Shear force(kn) Shear force(kn) Column number (a) First row d 1095d d 1095d Row number (b) First column Fig.11 Shear force of headed studs (1) The deflection at midspan increases rapidly in the first year after construction is completed. Afterward, the deflection-time curve takes a more gentle course since most of the creep and shrinkage effect has been finished. (2) The stress of concrete gradually decreased and the effect in the position of the connection between concrete and steel truss web was obvious. The decreasing velocity slowed down after one year. (3) The maximum steel Von Mises stress gradually increased. In the first year, the stress increased substantially. After one year, the stress-time curve became flat. The peak value was calculated as 258MPa in the 10th year. (4) The shear forces of headed studs gradually decreased over time. After one year of construction, the maximum shear force of headed studs changed from 120kN to about 100kN. In the next nine years, the decrease of maximum shear force was less than 10%. All the results of time-dependent behaviors in this paper could provide references for the design and construction of composite truss bridges with double decks or other similar structures. References [1] He J, Xin HH, et al. Mechanical performance of composite truss bridge with double decks. In Proceedings of innovative infrastructures - toward Human Urbanism Preliminary Invitation Korea. pp. 9 [2] LIU YQ,XIN HH,H J, et al.(2013).xperimental and analytical study on fatigue behavior of composite truss joints. Journal of Constructional Steel Research. 83.pp [3] Zhou L, Chen Y. (1994). Shrinkage and Creep[M]. China Railway Publishing House, China. [4] Gilbert IR.(1989).Time effects in concrete structures. International Journal of Cement Composites and Lightweight Concrete. pp [5] Yang IH. (2007).Uncertainty and sensitivity analysis of time-dependent effects in concrete structures. Journal of ngineering Structures. pp [6] Bazant ZP. (2001).Prediction of concrete creep and shrinkage: past, present and future. Journal of Nuclear ngineering and Design. 203:1.pp [7] Dezi L, Leoni G, Tarantino AM. (1998). Creep and shrinkage analysis of composite beams. Journal of Progr Struct ng Mater.1:4. pp [8] Bradford MA. (2010). Generic modeling of composite steel concrete slabs subjected to shrinkage, creep and thermal strains including partial interaction. Journal of ng Struct, 32 :5. pp [9] ANSYS Release ANSYS University Advanced. ANSYS Inc., [10] Ministry of communications. Code for design of highway reinforced concrete and prestressed concrete bridges and culverts. Beijing: China Communication Press, 2004.