Experimental investigation of cable-stayed timber bridge

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1 Experimental investigation of cable-stayed timber bridge Just, Alar 1, Just, Elmar 2, Pousette, Anna 3, Õiger, Karl 4 ABSTRACT In this article static and dynamic behavior of a cable-supported timber road bridge by results of experimental investigations carried out on a 1/15 scale model is described. The aim of the work was to show the possibility of building long span timber road bridges. The prototype structure is assumed to be a cable-stayed timber road bridge with span of 1 m. Bridge width is 7.2 m. Pylon height is 19.5 m. Bridge is loaded according to Eurocode 1.3. A base of modeling was physical scale model. Model of the cable-stayed timber road bridge consisted of 4 sections with total span 6.67 m. There were hinges between the bridge sections. Height of pylons 1.3 m. Bridge deck represented stresslaminated timber plate with thickness of 17 mm. Stays were from round steel. Natural frequencies were measured in horizontal and vertical direction. The main mode of the vibrations determined was fundamental mode of natural frequency. The natural frequency reduced for prototype bridge was found 1.49 Hz in vertical direction. Damping factor determined on the model was ζ =.39. In horizontal direction natural frequency was.62 Hz. It has been confirmed that there are no big problems to achieve static carrying capacity in vertical direction, but the special problem for lightweight cable-supported bridge represents lateral vibration and the possibilities to achieve necessary lateral stiffness. Lateral stiffness increases noticeable connecting stress-laminated deck structure to the main beams. The long span timber cable-stayed bridges with the span of 1 m are possible to construct by reasonable consumption of materials. INTRODUCTION Until these days big skepticism dominates in Estonia concerning timber bridges. Today only some small-span glulam bridges have been designed and constructed. Concrete and steel have been the main and favorite building materials. Wood industry in Estonia has to expand because the raw material is easy-to-get and relatively cheap - Estonia has forest on 45% of area. As opposite, the production and use of timber and glulam structures is well developed in the Nordic countries. The Nordic timber industry has also in a research and development program increased the use of timber as a bridge material during the last years. Esthetics, environment and price can make a timber bridge competitive. There are many advantages when using wood in constructions: wood is a light, strong, durable and easy to shape material. Wood is a natural material and relatively little energy is needed to produce the material. The code based design requirements for timber bridges are often relatively conservative because of limited knowledge on 1 PhD student, Tallinn Technical University, Estonia 2 Senior Researcher, Tallinn Technical University, Estonia 3 Civil Engineer, Swedish Institute for Wood Technology Research, Skellefteå, Sweden 4 Professor, Dr.Techn, Tallinn Technical University, Estonia

2 the behavior of large timber bridges, especially for the dynamic behavior. Timber bridge decks has a relatively low stiffness, and long span timber bridges are light and slender structures that are sensitive to vibrations. The natural frequency and the damping are important factors for the dynamic behavior of a bridge. The damping of a construction can be difficult to determine, but generally timber constructions with mechanical connections has quite a large damping. For timber bridges damping of vibrations is a little bigger than for example by steel bridges. It is because of by vibrations part of energy will be absorbed and dissipates in environment, especially by timber structures and joints. PROTOTYPE STRUCTURE The prototype structure is assumed to be a cable-stayed timber road bridge with span of 1 m. Bridge width is 7.2 m. Pylon height is 19.5 m. Bridge is loaded according to Eurocode 1.3. One of two 3 m wide traffic lines is loaded with characteristic load 9 kpa. second line and residual zone with 2.5 kpa. Also a tandem with axle loads 3 kn on first line and tandem with axle loads 2 kn on second line is used. Distance between the two axles is 2 m. Bridge model is prepared in scale 1:15. MODEL Bases of modeling is physical scale model. Stresses in real structure and model should achieve the same level of values. Additional self weight 4.8 kpa is used for that reason. This permanent load represents the missing dead load per unit area because of the self weight of model is 3375 times smaller. Figure 1. Bridge model scheme Model of the cable-stayed timber road bridge consists of 4 sections with total span 6.67 m. Each section consists of 4 longitudinal timber beams with cross-section 33 x 1 mm and transversal beams. There are hinges between the bridge sections. Height of pylons 1.3 m. Bridge deck represents stress-laminated timber plate with thickness of 17 mm. Stays from round steel 6.5 mm and 12 mm. stress-laminated deck head beam 33x1 cross-beam 17x67 L15x22x2,5 l=4 Figure 2. Cross-section of the bridge model

Timber material used for model is spruce (picea abies). Strength class C22 (by Eurocode 5). Modulus of elasticity E = 82 MPa. Average density ρ = 463 kg/m 3.

Bridge model was loaded also without connections between stress laminated deck and beams.

Model joints between bridge sections do not correspond exactly to natural joints of prototype structure that will be constructed using dowels and steel plates.

Figure 3. General view of model Figure 4. Deck structure. Figure 5. Deflectometers. Figure 6. Middle joint.

3 Timber material used for model is spruce (picea abies). Strength class C22 (by Eurocode 5). Modulus of elasticity E = 82 MPa. Average density ρ = 463 kg/m 3. Stress laminated deck was connected to the beams to increase bridge stiffness in vertical and in lateral direction. Bridge model was loaded also without connections between stress laminated deck and beams. Moisture content and temperature have been the same during the test periods: RH 3-5 %, temperature ca 15-2 C, average moisture content of wood w = 8.3 %. Model joints between bridge sections do not correspond exactly to natural joints of prototype structure that will be constructed using dowels and steel plates. Different structures have been used for model joints, but with sufficient strength of the joints no big differences were found in deflections and stay forces. Figure 3. General view of model Figure 4. Deck structure. Figure 5. Deflectometers. Figure 6. Middle joint. TESTS Static tests Tests with model were carried out between April 1998 and January 2. Distributed load has the same value as prototype structure, point loads are 15 2 times smaller. The model was loaded with distributed load over whole bridge. Loading over half length of bridge was used as well as loading of only one traffic lane was tested.

4 The bridge was loaded with and without stress-laminated deck to see the influence of deck stiffness to bridge deflections and forces in stays. Vertical load used for testing was distributed load 14.1 kpa (includes additional load from self weight 4.1 kpa). Bridge was also loaded with two concentrated loads of 1.44 kn on one lane and 1.19 kn on two points on second traffic line. Equipment used for static loading was hydraulic system "Enerpack", forces were measured with manometer. Deflections were measured by deflectometers with exactness,1 mm, deformations of stays by strain gages. Maximum deflection under maximum distributed load was 23.1 mm which is L/288 for the span between the pylons. Static tests with and without stress-laminated deck did not show any significant difference in vertical displacement. Ratio of displacement in vertical direction in the middle of bridge (point C) was u deck /u o =.97. Dynamic tests Natural frequencies were measured in model with and without deck in horizontal and vertical direction. The main mode of the vibrations determined was fundamental mode of natural frequency. This circumstance allows us with sufficient exactness by comparison of model and prototype structure main dynamic parameters to use formulas of Eurocode 5 part 2. Vibrations were measured with Geiger s vibrograph. Results of testing are shown on table 1. The results are given as measured values from model and reduced values for prototype structure, using transition coefficients. It has been confirmed that there are no big problems to achieve static carrying capacity in vertical direction. but the special problem by lightweight cable-supported bridge represent lateral vibrations and the possibilities to achieve necessary lateral stiffness. Lateral stiffness increases noticeable connecting stress-laminated deck structure to the main beams. TABLE 1. Dynamic properties Natural model frequency Amplitude Speed Acceleration Damping time prototype Hz mm 1-2 m/s 1-2 m/s 2 s With deck Vertical point E point C Horizontal point E Without deck Vertical point E point C Horizontal point E Damping factor was found in model as ζ =.39. Logarithmic decrement.25.

Figure 7. Vibration mode at the lowest natural frequency for the model. THEORETIC ANALYSIS Theoretic analysis was carried out using different programs as IDEAS. Staad-Pro. Robot. Frame Analysis (2D).

CASE STUDIES Järna bridge In Sweden several cable stayed timber bridges for foot- and bicycle have been built. Some of them have been examined for vibrations.

5 Figure 7. Vibration mode at the lowest natural frequency for the model. THEORETIC ANALYSIS Theoretic analysis was carried out using different programs as IDEAS. Staad-Pro. Robot. Frame Analysis (2D). Main results are given in table 2. Different FEM-programs give quite similar results. Test results are compared with results from "Frame Analysis" on figures CASE STUDIES Järna bridge In Sweden several cable stayed timber bridges for foot- and bicycle have been built. Some of them have been examined for vibrations. Eigen mode analysis and vibration measurements have been performed on a bridge in Järna, crossing the motorway south of Stockholm, Sweden. The bridge was built in The cable-stayed part of the bridge is symmetric with two spans of 25 m each and a pylon in the middle. The bridge deck is prestressed and consists of glulam T-beams. The measured natural frequencies are higher than the natural frequencies from calculations. The natural frequencies for the bridge were calculated with a finite element model. The lowest frequency obtained for the inner spans was 3.1 Hz. Measurements of traffic and wind accelerations of the deck. force excited accelerations of the deck and accelerations from traffic and wind on one supporting cable were measured. From the measured data the lowest natural frequency evaluated was 3.48 Hz and the damping was 1.72 %. Figure 8. The Järna bridge Figure 9. Vibration mode at the lowest natural frequency for the Järna bridge Vaxholm bridge Another bridge was built in 1996 in Vaxholm near Stockholm, Sweden. It is a foot- and bicycle bridge with a span of 9 m. It is symmetric with pylons and back stays at both bridge ends. The bridge deck consists of longitudinal main beams of glulam, with transverse and diagonal glulam beams forming a horizontal truss. On top of this truss there are longitudinal beams and a plank deck. The natural frequency was calculated with a finite element model. The lowest frequency for the deck in the vertical direction was 1.9 Hz. Full scale measurements were performed, where accelerations caused by wind excited vibrations

6 were recorded. From the measured data some frequency peaks and damping values could be evaluated, but there were too few points of measurement to assign a frequency peak to a particular mode of vibration. The frequency peaks were 1.4 Hz in the vertical direction. and 2.2 and 2.8 Hz in the horizontal direction. The damping was in the vertical direction 3.2 %, and in the horizontal direction %. [5]. Figure 1. The Vaxholm bridge Figure 11. Vibration mode at the lowest vertical frequency for the Vaxholm bridge According to [4] the lowest natural frequency for Hiroshima airport bridge was calculated as 1.43 Hz. Hiroshima Airport Bridge is the longest cable-stayed timber road bridge with 77 m span between pylons. DISCUSSION Comparison of analysis, tests and existing bridges is made in this section. Table 2 and 3 give some comparable results of calculations and tests. The values are reduced to the model values. Deflections of model are 2 times bigger than theoretic values. This is because the impact of straightening of stays and displacements in joints is not taken into account in some FEM-program. It could be taken into account using reduced modulus of elasticity of stays. Table 2. Deflections. Comparison of theoretic and test results FRAME FRAME Staad Robot Model Deflections, mm ANALYSIS ANALYSIS test Point No (1 m span) model 1/15 (1 m) model 1/15 C E G N, VA,VB 1, VK,VL 1,

7 Table 3. Forces in stays. Comparison of theoretic and test results FRAME FRAME Staad Robot Model Forces, kn ANALYSIS ANALYSIS test Bar No (1 m span) model 1/15 (1 m) model 1/15 1, , , , , , , , , , Long span timber bridges, especially foot- and bicycle bridges, are light and slender structures that are sensitive to vibrations. The requirements for vibrations may be the decisive load case for long span timber bridges. The frequency of the bridge can be made higher by increasing the stiffness of the deck, the stiffness of the pylons or the sectional area of the stays. The stiffness of the pylons have little influence on the frequency. The stiffness of the deck has some influence on the frequency, but most important is the sectional area of the stays [2]. Damping found in model is ζ =.39. Usually damping coefficient for steel structures ζ =.2...3, for reinforced concrete ζ =.3...5, for timber ζ = CONCLUSIONS The long span timber cable-stayed bridges with the span of 1 m are possible to construct by reasonable consumption of materials. Adjustment of structure and joints of prototype structure according to model tests must be done. At the same time, the influence of long time loading has to be researched in the future. Finally, it has been confirmed that there are no big problems to achieve static carrying capacity in vertical direction. but the special problem by lightweight cable-supported bridge represent lateral vibrations and the possibilities to achieve necessary lateral stiffness. Lateral stiffness increases noticeable connecting stress-laminated deck structure to the main beams. REFERENCES 1. Eurocode 5. Part 2 Bridges. 2. Pousette, A Cable-Stayed Timber Bridges. Nordic Timber Bridge Project, phase 2. Nordic Timber Council. Stockholm. 3. Nordic Wood. Eigen mode analysis and vibration measurements. B189. Gång- och cykelbro över E4 vid Järna. Saltå Kvarn Stockholms Kommun. Rambøll Gilham, P, Iimura,Y Design, testing and erection of the Hiroshima Airport Bridge. International Wood Engineering Conference. New Orleans. 5. Handa, K Calculation of natural frequencies and damping for Waxholm Timber Bridge.

9 1 2 3 A C E G K Figure 15. Deflections. Point TESTED Deflection, mm theor 5,6 7 8theor 7, THEORETIC Figure 12. Forces in stays Load, kpa E F theor E,F C D G H theor C,D theor G,H Figure 13. Forces in stays Figure 16. Displacements of points C - H. 1,8,6,4,2 Deflection, mm Deflection, mm theor 13, theor 15, Load, kpa theor 17, theor 19, Load, kpa 5 1 Load, kpa N theor N Tension force, N Tension force, N Tension force, N Figure 14. Forces in stays Figure 17. Horizontal movement of support N.

Field Load Testing of the First Vehicular Timber Bridge in Korea

Field Load Testing of the First Vehicular Timber Bridge in Korea Ji-Woon Yi Ph.D. Student Department of Civil & Environmental Engineering Seoul National University Seoul, Korea jwyi@sel.snu.ac.kr Wonsuk