Development of timber concrete composite bridge decks in Brazil

Transcription

1 Paper Development of timber concrete composite bridge decks in Brazil Julio Cesar Molina, DSc, CEng Department of Structural Engineering, University of São Paulo USP/EESC, São Carlos, SP, Brazil Carlito Calil Junior, DSc, CEng Department of Structural Engineering, University of São Paulo USP/EESC, São Carlos, SP, Brazil Keywords: Bridge decks, Brazil, Composite construction, Timber, Concrete, Shear connectors, Testing Received: 04/09: Modified: 09/09; Accepted: 10/09 Julio Cesar Molina & Carlito Calil Junior Synopsis This paper reports on an evaluation of the behavior of X-shaped shear connectors for timber-concrete composite bridge decks. Direct shear tests of the connectors, which were subjected to a total of load cycles, indicated that their ultimate capacity was not affected by the fatigue cycles and that accumulated fatigue damage was the result of an initial slip that tended to stabilize after cycles. Three composite girders with T- shaped cross section were loaded to failure after being subjected to cycles. The test results indicated that the X-shaped connection conferred high resistance and stiffness to the composite system, offering an excellent alternative for composite bridge decks. To conclude, details of the design and load testing of a 7m span composite bridge in Brazil are presented and the test results are compared with those of a timber bridge, confirming the advantages of the composite system. Notation E c,m = mean compressive modulus of elasticity of the concrete on the day of the test E c0,m = mean modulus of elasticity of the timber along the grain E fl,m = mean bending modulus of elasticity of the timber ρ = density of the wood K = slip modulus EI = stiffness of the composite section L = span f c,m = mean compressive strength of the concrete on the day of the test f c0,m = mean compressive strength of the wood along the grain U = moisture content (%) S max = maximum load fatigue level S min = minimum load fatigue level R = ratio between the minimum and maximum levels of load fatigue u = vertical displacements at midspan φ = diameter of the steel bar Introduction Timber bridges on secondary roads in Brazil are undergoing a process of deterioration, which is reducing the durability of these structures. For many years, bridge decks on secondary roads were generally made of sawn hardwood boards. The life of these bridges was limited both by the lack of a maintenance program and by not using timber preservative to prevent decay (due to the difficulty of impregnating the hardwoods). This type of deck presents several problems, such as loosening of the boards as a result of the dynamic movements caused by the passage of vehicles on the bridge. Asphalt paving is not common on this type of deck due to the major displacements of this system. Moreover, the direct exposure of timber to the elements and the use of untreated wood favour biological attack by insects and humidity. Since 1969, the Laboratory of Wood and Timber Structures (LaMEM) at the University of São Paulo s São Carlos School of Engineering has been promoting the development of the timber bridges. To continue these studies, LaMEM recently developed an Emergency Program for Wooden Bridges in the State of São Paulo: Technology for Society, an integrated research project. The main objective of this program is to investigate new technologies in order to create structures with low construction and maintenance costs, whose safety and durability are compatible with those of other structural materials. Timber concrete composite decks for bridges The combined use of concrete and timber offers an alternative to 1 1 Loosening of sawed boards 2 The Structural Engineer 87 (1) 8 January 2009

2 Concrete Timber Specimen Test f c,m E c,m f c0,m E c,0,m ρ (kgf/m 3 ) U (%) S1 static S2 static S3 static S4 fatigue S5 fatigue S6 fatigue Table 1 Mechanical properties of the materials of specimens S1 to S6 Specimen Number of cycles S S S S S S Capacity of connectors (kn) 2 Composite deck. Source: Hellmeister (1978) 3 Connection system. Source: Molina (2007) Table 2 Post-fatigue capacity of two X- shaped connectors solve the problems presented by timber bridges. A timber concrete composite deck consists of a reinforced concrete slab connected to structural timber members, which work together as a monolithic system. Log beams are laid side by side, alternating thick and thin ends to eliminate the effect of the conical shape of the logs. The natural irregularities among the log beams were filled out with aggregate and small diameter log beams, both with compatible dimensions, enough to impede leaks during the release of the concrete (see Fig 25). A layer of reinforced concrete, normally prepared in situ, is then spread over the logs. In Brazil concrete is commonly use in civil construction, even remote locations. Because it is batched on site it is a cost-effective material. The partial attachment is provided by a series of steel connectors, which not only prevent the materials from separating but also transfer shear forces. The use of glued X-shaped steel bar connectors is a simple and inexpensive alternative which is easy to apply. According to Matthiesen 11, the use of X-shaped connector present rigidity from two to ten times larger compared to the normal shear connector, depending on the considered diameter. Besides, Madsen 12, affirms that the use of steel bars inclined relative to the grain allows a better distribution of the stress. They are also less vulnerable to cracks of the wood in the area of the connection and increase the shear resistance of the timber. This structural system is completely suitable for these materials, because they are used in their ideal conditions, i.e. timber in tension and concrete in compression. This paper presents the results of a research program developed in Brazil to verify the behavior of X-shaped steel bar connectors that are frequently used in timber-concrete composite bridge decks. Also discussed here are the development and results of direct shear connector tests and flexural tests. A practical application is also presented, showing details of the construction and stiffness of a 7m span timber concrete composite bridge deck built in Brazil. Description of direct shear tests To evaluate the performance of the connectors to be used in the timber-concrete composite bridge deck, six specimens (S1-S6) were tested, in direct shear, in October Eucalyptus citriodora log beams treated with CCA (chromated copper arsenate) were used as direct shear specimens and the mechanical properties of the timber were determined according to the Brazilian NBR standard. Type 3 concrete with Portland cement was used, and its compressive strength was measured from 10cm diameter, 20cm high cylindrical specimens, which were tested at 28 days according to the Brazilian NBR , NBR and NBR standards. The connection system of specimens consisted of galvanized rebar segments (with tensile yield stress of 500MPa), glued with Sikadur 32 epoxy adhesive (60MPa strength) and fixed into holes drilled at a 45º angle in relation to the wood grain. This adhesive performed well in previous research at LaMEM (Pigozzo 1 and Molina 3 ). The steel connectors should be treated against corrosion (hot-dip galvanization). In addition, polyethylene plastic bags were placed at the interface of the materials to eliminate friction. Specimens S1, S2 and S3 were loaded quasi-statically at 0.10kN/mm until failure to determine the load capacity of the X- shaped connectors. Specimens S4, S5 and S6 were subjected to cycles at constant amplitude in sinusoidal fatigue cycles at a frequency of 3Hz, after which they were loaded quasi-statically to failure, as illustrated in Fig 6. The capacity of the connectors corresponded to the load required to reach a strain of 2, according to the NBR standard. The base measure of the displacements for the specimens was 33cm. The quasi-static test data of specimens S1, S2 and S3 were used to determine the maximum (S max ) and minimum (S min ) load fatigue levels in the tests of specimens S4, S5 and S6. Thus, the maximum load levels used in the cyclic loads were 30, 40 and 50% of the average capacity of the connectors, estimated from the quasi-static tests of specimens S1-S3. The minimum cyclic load applied here was 5% of the connectors average capacity of connectors. Specimens S1, S2 and S3 were loaded quasi-statically to failure in an H-frame hydraulic press with 480kN capacity, while the fatigue tests were performed with a DARTEC M1000/RC universal testing machine. At intervals of cycles, specimens S4, S5 and S6 were subjected to quasi-static loads of 17.46kN, 23.28kN and 29.11kN, respectively, using a DARTEC M1000/RC universal test machine. To evaluate the effect of fatigue on the service load The Structural Engineer 87 (1) 8 January

3 Direct shear test specimens 5 Reinforcement and mold for specimens 6 Direct shear fatigue test parameters 7 H-frame hydraulic press (480kN capacity) 8 DARTEC M1000/RC universal testing machine 9 Load versus slip for specimens S1, S2, and S response, slipping (relative displacements) between the concrete and the timber was recorded using two direct-current displacement transducers (DCDTs), one attached to each side of the specimen. The DCDTs had a sensitivity of mm and a maximum path of 5mm. Slipping was recorded at 1s intervals by a SYSTEM 5000 data acquisition system. Direct shear test results Figure 9 shows the load-slip curves for specimens S1, S2 and S3 loaded quasi-statically to failure. The average strength (corresponding to a strain of 2%) of two connectors of specimens S1, S2 and S3 was 58.22kN (29.11kN/connector) and the average stiffness K (load/slip) was 75.92kN/mm (37.96kN/mm per connector). The maximum load applied on each specimen up to failure was 59.97kN for S1, 56.11kN for S2, and 61.16kN for S3. After cycles, the average capacity (corresponding to a strain of 2%) of two X-shaped connectors was 58.96kN for specimens S4, S5 and S6 loaded quasi-statically to failure. The maximum load applied on each specimen up to failure was 64.26kN for S4, 59.87kN for S5, and 55.01kN for S6. Figure 11 shows the quasi-static test results for specimens S4, S5 and S6 conducted post-fatigue to failure, while Table 2 presents the post-failure test results for specimens S1 to S6. The fatigue-tested specimens, S4, S5 and S6, displayed no loss 4 The Structural Engineer 87 (1) 8 January 2009

4 Slipping of specimens S4, S5 and S6 recorded at intervals of cycles 11 Load versus slip for fatigue-tested specimens S4, S5 and S6 12 Failure of specimens after cycles 13 Configuration of girders G1 to G6 14 Details of steel bar connectors and concrete reinforcement 15 Flexural fatigue test parameters The Structural Engineer 87 (1) 8 January

5 Concrete Timber Girder Test f c,m E c,m f c0,m E fl,m E c,m U (%) ρ (kg/m 3 ) G1 static G2 static G3 static G4 fatigue G5 fatigue G6 fatigue Table 3 Mechanical properties of the materials of specimens G1-G6 Deflections (mm) Number of cycles G4 (S max = 39.61kN) G5 (S max = 52.81kN) G6 (S max = 66.02kN) Table 4 Effect of fatigue on measured midspan deflections Girder Number of cycles Ultimate strength (kn) G G G G G G Table 5 Capacity of girders after cycles Quasi-static flexural tests 17 Fatigue tests of ultimate capacity or ductility. However, an initial slip was observed and was attributed to accumulated damage by fatigue in the form of crushing of the wood around the connectors. After cycles, the X-shaped connectors showed a loss of stiffness K (load/slip) of 29.50% for specimen S4, 30.93% for S5, and 32.76% for S6. Figure 10 depicts slipping observed at loads of 29.11kN, 23.28kN and 17.46kN versus the number of cycles, indicating that most of the accumulated fatigue damage occurred during the first cycles. After fatigue cycles, the initial slip tended to stabilise, remaining relatively constant up to cycles. After cycles, the concrete portion of specimens S4, S5 and S6, which were loaded to failure, was broken to verify the causes of failure. The primary failure mode occurred parallel to the grain of the timber by crushing around the connectors and flexural yielding of the connectors. While some local crushing of the concrete may also have occurred, its magnitude was small compared with the local crushing of the timber observed in the failed specimens, as indicated Fig 12. In summary, the ultimate shear strength of the X-shaped connectors appeared unaffected by fatigue cycling and the accumulated fatigue damage resulted from initial slipping, which subsided after the first cycles. These positive results warranted further investigation of composite girders. Description of the girder tests Six composite girders (G1-G6) were also manufactured for the flexural tests. The dimensions of the girders were defined such that the failure would occur first in the connectors, then in the concrete slab, and finally in the timber girders. In addition, the physical limitations of the DARTEC testing machine were also considered. The physical properties of the materials and the gluing conditions of the connectors in the girders were similar to those of the specimens subjected to the direct shear tests. Table 3 lists the mechanical properties of the materials of the girders. Girders G1, G2 and G3 were loaded quasi-statically up to failure at 0.10kN/mm, using the H-frame hydraulic press and the fatigue tests were carried out in the DARTEC M1000/RC universal testing machine. Girders G4, G5 and G6 were subjected to cycles of constant amplitude sinusoidal fatigue, at a frequency of 3Hz, to 6 The Structural Engineer 87 (1) 8 January 2009

6 Failure of the concrete slab 19 Failure at the neutral line 20 Stiffness (EI) of the girders recorded at intervals of cycles 21 Connector anchoring details (units: mm) 22 View of composite bridge ready for use 23 Placement of the logs. Source: Pigozzo (2004) simulate the effect of traffic, prior to being loaded quasi-statically to failure. The minimum (S min ) and maximum (S max ) midspan fatigue loads for each girder: G4 (6.60kN and 39.61kN), G5 (6.60kN and 54.81kN) and G6 (6.60kN and 66.82kN), were selected based on the work of Weaver et al 8. The shear in the girder connections was estimated based on the method of transformed sections proposed by Mohler, which is presented in the Eurocode 5 9 standard. Thus, the maximum loads (S max ) used for the cyclic loading of girders G4, G5 and G6 were 30, 40 and 50% of the connectors average capacity. The minimum load (S min ) applied was 5% of the connectors average capacity, maintaining the load relationships of R = 0.167, R = and R = applied on the specimens in the shear tests. At intervals of cycles, girders G4, G5 and G6 were loaded quasi-statically to 39.61kN 54.81kN and 66.82kN, respectively, and midspan deflections were recorded using displacement transducers. Based on the values of the vertical deflections, the stiffness (EI) values of the girders were determined from equation (1). ^EIh= Smax : L / 48 : u 3...(1) Girder test results A preliminary analysis based on the method of transformed sections indicated that the failure modes observed in girders G4, G5 and G6, which were loaded quasi-statically to failure after undergoing cycles, were crushing of the upper face of the concrete slab by compression, and shear at the neutral line of the girder. As a result of the increase in midspan deflections, the stiffness (EI) of girders G4, G5 and G6 declined as a function of the number of cycles. The loss of stiffness (EI) of the girders was 15.84% for girder G4, 18.44% for girder G5 and 20.82% for girder G6, as indicated in Fig 20. The mean ultimate strength of girders G1, G2 and G3 was kn, while that of girders B4, B5 and B6 was kN. Table 4 lists the capacity of all the girders subjected to flexural tests and loaded to failure. Practical application in a bridge structure To complement this study, the researchers of the Laboratory of Wood and Timber Structures LaMEM (São Carlos, Brazil) built and monitored a bridge with a log-concrete composite deck. Located in the state of São Paulo (GPS coordinates S and W, altitude 450m), the bridge is 4.0m wide and has a 7.0m span. This case illustrates the application of the connection system (X-shaped) analysed here, as indicated in Figs 21 and 22. Photographs, illustrations and construction details are given, followed by a discussion of the results of the first load tests. The bridge was designed with a Class 30 structure for vehicle traffic, in line with the NBR standard, which specifies vehicles with a total weight of 300kN. The Structural Engineer 87 (1) 8 January

7 Preparation of the holes. Source: Pigozzo (2004) 25 Timber deck before concreting 26 Concreting of the timber deck. Source: Pigozzo (2004) 27 Load test of the timber deck (midspan of the bridge). Source: Pigozzo (2004) 28 Comparison of vertical displacements measured at midspan. Source: Pigozzo (2004) The superstructure of the bridge was composed of 12 log beams of Eucalyptus citriodora timber treated with CCA, with an average diameter of 32cm. The log beams were laid side by side, alternating thick and thin ends to eliminate the effect of the conical shape of the logs. The X-shaped steel bar connectors were glued with Sikadur 32 adhesive epoxy. A reinforced concrete slab with an average compressive strength of 28MPa was cast directly on the timber deck. The concrete slab had a minimum thickness of 8cm at the top of the log beams and, because of the depressions caused by the log cross-sectional profile, an average thickness of approx. 12cm. The concrete slab was reinforced in the longitudinal direction with a steel mesh comprising 4.2mm bars at 20cm and 10 cm centres and in the transverse direction with a steel mesh of 4.2mm and 10mm bars at 10cm and 20cm spacings respectively giving a total of 5.17cm 2 /m. The design of the composite bridge deck and the connectors spacing were adopted considering the Mohler s model using the stiffness K value determined by experimental X connections tests, that is according to the Eurocode 5 9 standard. Figures 23 to 26 show details of the construction of the bridge. Two load tests were carried out to evaluate the performance of the structural system. The first load test was carried out on the deck containing only the log beams. The second load test was carried out on the composite deck after the concrete was cured. The load tests were conducted on October 2002, using a fully loaded truck with a gross vehicle weight of 349kN. The vertical displacements were measured with rulers mounted on the bridge soffit. The readings were taken with an optical level with 1mm precision. Figure 28 shows the measured vertical displacements. The vertical displacements recorded in the second load test were lower than those measured in the first test, as predicted by the research. Excellent lateral load distribution is achieved in the composite deck because of the integrity of the connections and because the two materials (eucalyptus and concrete) both have a similar modulus of elasticity. This prevents the potential problem of high differential deflections occurring from truck wheel loads. In 2008 a visual inspection of the bridge confirmed it to be in good condition. Summary and conclusions The purpose of this work is to provide alternatives for the construction of safe and low-cost bridges, especially on secondary roads. The composite deck consisting of log beams and a layer of reinforced concrete joined with X-shaped steel connectors is an excellent option, particularly from the standpoint of cost. The combination of these materials joined with steel bar connectors results in stiffer, stronger and much more durable decks than those built solely of timber, particularly in the case of bridges with small and medium-sized spans. Thus, based on the research described here, the following conclusions were drawn. X-shaped shear connectors show good performance. However, despite their good performance both as-built and after fatigue testing, the effect of gaps that develop around the connectors due to local crushing of the wood during fatigue can be significant. Nevertheless, although connection fatigue resulted in increased slipping, the ultimate strength of the connectors was not affected. The results after fatigue cycles, showed that the initial slip appears to stabilise and remains at a relatively constant value up to cycles. Consideration is being given to improving performance by using split-ring connectors in order to increase the local bearing and shear resistance the timbers (Weaver 8 ). The comparison of the experimental displacements for the 7m span bridge deck confirmed the excellent performance of the composite bridge deck system. Although the sensitivity of the device employed to measure the experimental displacements precluded a more accurate comparison, it sufficed for the load tests. However, the relevance of the test results indicated that the real movement of composite bridge deck systems can be determined with a significant level of accuracy. Acknowledgments The authors thank FAPESP (São Paulo State Research Support Foundation, Brazil) for its financial backing of this work. References 1 Pigozzo, J. C.: Estudos e aplicações de barras de aço coladas como conectores em lajes mistas de madeira e concreto para tabuleiros de pontes (Studies and applications of glued steel bars as connectors in timber-concrete composite slabs for bridge decks). Thesis (Doctorate in Structural Engineering) São Carlos School of Engineering, University of São Paulo, São Carlos, 2004, 358p 2 Hellmeister, J. C.: Pontes de eucalipto citriodora (Eucalyptus Citriodora Bridges). Thesis (Full Professorship in Structural Engineering) São Carlos School of Engineering, University of São Paulo, São Carlos, 1978, 85p 3 Molina, J. C, Calil Jr., C. Fatigue analysis on glued steel bar connectors for log-concrete composite bridge decks. 19th Inter. Cong. Mech.Eng., 2007, 8 The Structural Engineer 87 (1) 8 January 2009

8 6p 4 NBR 7190: Projeto de estruturas de madeira (Design of timber structures). Associação Brasileira de Normas Técnicas ABNT, NBR 5739: Concreto Ensaio de compressão de corpos-de-prova cilíndricos (Concrete Compression tests on cylindrical specimens). Associação Brasileira de Normas Técnicas ABNT, NBR 5738: Concreto Procedimento para moldagem e cura de corpos-deprova (Concrete Procedure for molding and specimens cure). Associação Brasileira de Normas Técnicas ABNT, NBR 8522: Concreto Determinação dos módulos estáticos de elasticidade e de deformação e da curva tensão deformação (Concrete - Determination of the static modules of elasticity and of strain and of the curve stress strain). Associação Brasileira de Normas Técnicas ABNT, Weaver, C. A., Davids, W. G., Dagher, H. J.: Testing and Analysis of Partially Composite Fiber-Reinforced Polyner-Glulam-Concreto Bridge Girders. Journal of Bridge Engineering ASCE, July/August, p , Eurocode 5: European Prestandard. ENV , Part 1-1, Design of timber structures: General rules and rules buildings, Brussels, Belgium, European Committee for Standardization, Dec, 110p, NBR 7188: Carga móvel em ponte rodoviária e passarela de pedestre (Live loads on bridges for vehicles and pedestrians). Associação Brasileira de Normas Técnicas ABNT, Matthiesen, J. A.: Contribuição ao estudo das estruturas mistas: Estudo experimental de estruturas mistas de madeira e concreto interligados por parafusos (Contribution to the study of the composite structures: Experimental study of composite structures of wood and concrete connected by screws), Thesis (Liberate Teaching), Ilha Solteira University of Engineering Department of Civil Engineering, São Paulo State University, p 12 Madsen, B.: Timber connections with strength and reliability of steel, Inter. Wood Conf., Toronto, Canada, p , 1996 The Structural Engineer 87 (1) 8 January