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4 INTERNATIONAL CONFERENCE

THEORETICAL AND EXPERIMENTAL ANALYSIS OF TIMBER FOOTBRIDGE

Jozef GOCÁL Assistant lecturer University of Žilina Žilina, Slovakia [email protected]

Josef VIČAN Dean of faculty University of Žilina Žilina, Slovakia [email protected]

Marián SÝKORA Assistant lecturer University of Žilina Žilina, Slovakia [email protected]

Summary The paper presents results of experimental and numerical analysis of superstructure of an existing timber footbridge with steel bridge deck members. The main attention is paid to a problematic structural detail – joint of the steel crossbeam to the main timber girder. In order to investigate this detail more exactly, other laboratory tests on specially prepared specimens were performed and, consequently, numerical analyses of the specimens were realised. Experimentally and numerically obtained values are compared and discussed. Aim of the research is to describe real behaviour of the investigated joint with regard to its rigidity. Keywords: timber footbridge, steel crossbeam, static proof test, crossbeam to main girder connection, joint stiffness, experimental analysis, numerical analysis. 1.

Introduction

Within the solution of trans-border cooperation project between the Civil engineering faculties of University of Žilina in Slovakia and VŠB-Technical University Ostrava in Czech, we have been concerning ourselves with development of optimal types of the timber based bridge systems, which could be applied for pedestrian and short road bridges. Considering quite a large range of possible types of superstructures, we have focused only on plate-girder timber structures with bottom bridge deck consisting of seemly combined steel and wooden members. The connection of steel crossbeam to the main timber girder resulted as a problematic structural detail from the preliminary parametric studies [1], [2]. At these studies, the joint was considered as fully rigid in order to ensure the main timber girder stability as well as the global stiffness and dynamic properties of the superstructure. On the other hand, the connections of wooden structural members are usually considered as nominally hinged, which it is motivated by slip of the joint created with dowel-type fasteners. It may be expected that the true stiffness of the joint is somewhere between these two extremes. Real behaviour of this detail was observed during a static and dynamic proof test of recently built timber footbridge with possible vehicle access over the Bata’s channel in Hustenovice in Czech Republic. Our department participated at design of the bridge superstructure by working out structural analysis and proposal of structural details. The deflections and stresses in the main timber girders, longitudinal timber stringers as well as in the steel cross beam were measured during the proof test and, consequently, they were compared to the computed values. The global review of the proof test results was published in [3]. This paper deals with more detailed analysis of stiffness of the structural detail mentioned above. Since the results of experimental and numerical analysis of the real bridge superstructure, as they are presented hereinafter, have not provided satisfactory answers, additional laboratory tests of the investigated detail were performed on specially designed laboratory specimens. The laboratory test process as well as the results of numerical analysis of the tested specimens is also presented in this paper. 2.

Theoretical and experimental analysis of the existing footbridge superstructure

2.1

Description of the footbridge

The superstructure consists of two simply supported glued laminated girders 280/1800 mm with theoretical span 11.94 m and axial distance 5.28 m. The main girders are interconnected at their bottom edges by seven steel crossbeams made of hot rolled profiles IPE 360 and located at distances 1.99 m. The crossbeams support five longitudinal glued laminated

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stringers 280/330 mm, placed at distances 0.88 m and carrying the transverse plank pavement 160/100 mm. The crossbeams are connected to the main timber girders by means of front plates of thickness 15 mm and twelve bolts of diameter 20 mm, located in two lines. The joint stiffness is increased by vertical stiffener made of “T” profile of height 165 mm (1/2 of IPE 360) welded to the overlapping part of front plate and the upper flange of cross beam. After this manner, a steel semi-frame was created ensuring the main timber girders against lateral and torsional buckling. View at the bridge object is presented in Fig. 1. Cross section of the bridge superstructure is shown in Fig. 2. More detailed description of the bridge object may be found in [3].

Fig. 1 View at the bridge object

Fig. 2 Cross section of the tested timber footbridge

2.2

Numerical models of superstructure for theoretical analysis

Two numerical models were processed at the FEM program SCIA Engineer [4]. The first model was created for purpose of the preliminary static and dynamic analysis of the bridge superstructure. This model was modified after realisation of the experimental measurements by applying the real material characteristics of the main timber girders and stringers, which were determined using non-destructive techniques. The spatial computational model was processed using combination of shell and beam finite elements. The shell elements were used for modelling the main timber girders. The other members of superstructure, i.e. the crossbeams, longitudinal stringers and transversal plank pavement as well, were modelled by means of beam elements. The model respects all eccentricities occurring at the joints of particular

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members. The connections of steel semi-frames, consisting of crossbeams and vertical stiffeners, to the main timber girders were considered as fully rigid, which was achieved by putting the vertical stiffeners’ beam elements to the timber girders’ shell elements. The others joints, i.e. the connections of stringers to crossbeams as well as the connections of transverse planks to stringers, were considered as fully hinged in the both main flexural planes. Geometric scheme and visualization of the preliminary computation model loaded by testing vehicle is shown in Fig. 3A and Fig. 3B, respectively. A

B

C

Fig. 3 Numerical computation model A) geometric scheme of the preliminary model, B) visualization of the superstructure, C) geometric scheme of the improved model

The second numerical model was processed also by means of the program SCIA Engineer [4] in order to better describe the real behaviour of steel crossbeams, especially the connection of steel semi-frame to the main timber girders. The steel crossbeams and vertical stiffeners were modelled by means of shell members. Another improvement of the computational model was achieved by more precise simulation of the front plate connection to the main timber girder. The bolts were modelled by means of non-linear beam elements working only in tension. The contact between steel front plate and timber girder was simulated also by means of non-linear beam elements, but they were working only in compression. Geometric scheme of the improved computation model is shown in Fig. 3C. 2.3

Experimental analysis of the bridge structure

Based on the preliminary numerical analysis, several characteristic locations were chosen to record the stress-strain response of the superstructure to the testing load. The main girders were observed in the middle of the span and also at the place of the second cross beam connection because of bad access in the mid-span. The outer glued laminated stringer and the second steel crossbeam were also observed during experiment. Global review of the proof test results was published in [3]. With regards to the observed structural detail, the stress response of the steel crossbeam is interesting. The normal stresses were observed in the middle of the crossbeam, at the outer stringer place and at the connection to the main girder. Arrangement of the tensometric sensors installed on the chosen crossbeam is presented in Fig. 4.

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Fig. 4 Arrangement of tensometric sensors installed on the crossbeam

2.4

Comparison of measured and computed results

The measured and computed values of stresses and deflections on the observed crossbeam are presented in Table 1. The simpler model (Model 1 in Table 1) gives a good accordance in obtained stresses and deflections in the middle of the crossbeam, but, on the other hand, the stresses under the outer stringer as well as the stresses at the connection to main girder are very different. The improved model (Model 2 in Table 1) gives better accordance in obtained stresses under the outer stringer, however, the stresses at the connection to main girder are very close to those computed by the simpler model and, in addition, the stresses and the deflection in the middle of the crossbeam are considerably lower in comparison to the measured values. As it was expected, the real connection is not as rigid as the modelled joints. It may be stated out that neither of the theoretical models corresponds to the real behaviour of the bridge superstructure. In the first, simpler model the results are influenced by the fully rigid connection of the vertical steel stiffeners to the main timber girders. In the second, improved model the stress response in the upper flange is significantly affected by the local stress peaks at places of the discrete stringer connections. Also the slip of joints, which was not taken into account in numerical models, is another factor that has a considerable effect on the computation results. On the other side, the measured values may show smaller or bigger deviation, influence of which increases with decreasing absolute values. Table 1 Measured and computed values of stresses [MPa] and deflections [mm] on the steel crossbeam

Way of obtaining values

Under the outer stringer

At joint to the main girder

In the middle

T9

T10

T11

T12

T13

T14

T15

T16

T17

T18

Deflection

Measurement

-11.0

-17.5

18.5

14.4

6.9

-0.4

-1.0

-0.4

-42.1

42.3

2.8

Model 1

-27.5

-26.2

27.1

28.4

7.7

-6.8

1.6

-4.7

-39.6

40.7

2.4

Model 2

-14.8

-13.5

18.2

19.1

5.1

-8.7

5.5

-5.7

-27.1

32.1

1.9

3.

Theoretical and experimental analysis of the steel crossbeam connection to the timber girder

With regard to unsatisfactory results of the experimental and numerical analysis of the real bridge superstructure presented above, we have decided to farther investigate the problematic structural detail by means of additional laboratory tests on specially designed test specimens, which were subjected to detail experimental and numerical analysis. The tests were performed in laboratories of VŠB-Technical University Ostrava in Czech Republic.

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3.1

Description of the laboratory test specimens

Nine laboratory specimens were proposed and prepared in all in order to verify the real behaviour of the steel-to-timber member connection. The specimens consisted of two timber blocks 200×500×500 mm made of glued laminated timber of the strength class GL24h according to STN EN 1194 [5]. The blocks representing main girders of a bridge superstructure were interconnected at their bottom edges by steel hot rolled beam IPE 140 made of steel S235 according to STN EN 10025 [6], which represented a crossbeam of the bridge superstructure. The blocks were simply supported in the transversal direction (i.e. in direction parallel to the steel crossbeam). In order to eliminate rotation of the timber blocks they were stabilised at all four corners by steel bars made of hot rolled angles L 60×60×6 mm, joined to the blocks by means of steel screws of diameter 8 mm. The steel crossbeams were joined to the timber blocks by means of steel front plates of thickness 10 mm and steel bolts of diameter 12 mm and strength class 5.6 according to STN EN 1993-1-8 [7]. The joint itself was proposed in three different variants of the bolts arrangement with regard to the joint stiffness, designated as variant “A”, “B” and “C”, respectively. The individual variants differed one from each other by the number of bolts over the upper flange of crossbeam and by corresponding height of the front plate stiffener (see Fig. 5). Three specimens were manufactured for each of the variants.

Fig. 5 Arrangement of laboratory specimens

3.2

Numerical models of specimens for theoretical analysis

Each of three proposed specimen variants was numerically analysed by means of two different computational models. The first model was processed in program SCIA Engineer [4] using combination of shell and beam elements. The shell elements with associated corresponding thicknesses were applied for modelling the timber blocks as well as the steel crossbeam including front plates and corner stiffeners. The stiffening angles and the bolts connecting the front plates to the timber blocks were modelled by means of beam elements. The joint itself was modelled by the same way as in the second model of the bridge superstructure described in section 2.2. The slip between steel front plate and timber block

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was taken into account by means of elastic line supports of the beam elements simulating bolts. The support stiffness was derived from the modulus of elasticity perpendicular to the grains. The second, more sophisticated numerical model was processed by means of software ANSYS [8] using 3D finite elements. This system can take better into account plasticity and orthotropy of material as well as the contacts between connected members. Using the special contact finite elements it is possible to much better describe deformation of bolts as well as tenseness at the contact area between front plates and timber blocks. Visualisation of both kinds of numerical models of the specimen variant “C” is presented in Fig. 6. All the models were firstly used for preliminary numerical analyses of the specimens in order to optimise the course of laboratory tests. After realisation of experimental measurements, the models were modified applying the real material characteristics and, consequently, they were used for detailed numerical analyses of the tested specimens.

Fig. 6 Visualization of numerical models of the specimen variant “C” processed in SCIA Engineer and ANSYS

3.3

Experimental analysis of the specimens

Stress-strain response of the steel crossbeam to the load caused by transverse acting discrete forces was observed during laboratory tests. Based on the preliminary analyses, the specimens should have been loaded by one force in the middle of the span. However, during testing the first specimen of series “A”, the plastic behaviour of the crossbeam was noticed at much lower loading level in comparison to the preliminary analysis. It was caused by lower strength class of the applied steel than had been assumed. Therefore, the remaining eight specimens were loaded by two forces situated symmetrically at distance 150 mm from the midpoint. Arrangement of the specimen of series “C” subjected to the fourpoint bending proof as well as a location of strain and displacement sensors is presented in Fig. 7. During the tests, there were recorded: stresses in the middle of the span (tensometric sensors T0, T1 and T2), stresses at the crossbeam connection to timber blocks (tensometric sensors T3, T4, T5 and T6), stresses in the bolts (tensometric sensors T7, T8, T9 and T10), vertical deflection of the crossbeam in the midpoint (displacement sensors S6 and S10), pushing bolts into the timber (displacement sensors S1 and S2) and the relative rotations of the front plate against the timber blocks (displacement sensors S8, S9 and S11). The loading force was caused by hydraulic press. At first, every specimen was loaded by the force 5 kN (on the press), eventually distributed to two half forces. This force represented a basic loading level of all specimens. Then, the specimens were loaded by gradually increasing force with step 5 kN in case of specimens of series “A”, or 10 kN in case of the specimens of series “B” and “C”, respectively. After each loading step the specimens were unloaded to the basic loading level. The specimens were loaded until failure, which occurred either due to a bearing failure of the bolts or due to cutting grains caused by transversal bending of timber blocks.

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Fig. 7 Arrangement of the specimen “C” during test, T0 – T10 – localization of strain sensors, S1 – S11 – localization of displacement sensors

3.4

Comparison of measured and computed results

Development of the stress-strain response of specimens to the test load in the observed places of steel crossbeams was graphically processed in dependence on the acting force caused by the press. Figures 8 and 9 shows comparison of chosen characteristic experimental dependences with corresponding theoretical dependences obtained by both numerical models described in section 3.2. The theoretical values correspond to the static scheme at the four-point bending (the specimen A1 was subjected to the three-point bending test). The vertical displacements in the middle of crossbeam are purged from the pushing bolts into the timber blocks. The rotations of front plate were determined from the measured and computed values of horizontal displacements, respectively. Force at the press [kN] 0

40

20

40

60

80

100

120

140

0 20 0 -20 -40 -60 0

20

A2, A3 A - SCIA A - ANSYS

40 60 80 100 Force at the press [kN] B1, B2, B3 B - SCIA B - ANSYS

120

140

Normal stress [MPa]

Normal stress [MPa]

60

-50 -100 -150 -200 -250 -300

C1, C2, C3 C - SCIA C - ANSYS


B1, B2, B3 B - SCIA B - ANSYS


Fig. 7 Comparison of measured and computed normal stresses in upper flange of the crossbeam a) at the connection place, b) in the middle of span

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3,5

6,0

3,0

5,0

Rotation [mrad]

Deflection [mm]


2,5 2,0 1,5 1,0

4,0 3,0 2,0 1,0

0,5 0,0

0,0 0

20

40

60

80

100

120

140

0

20


60

80

100

120

140

Force at the press [kN]

Force at the press [kN] A2, A3 A - SCIA A - ANSYS

40





Fig. 8 Comparison of measured and computed deformations of the crossbeam a) deflection in the middle, b) rotation of the front plate towards the timber block

Basically, it may be stated out that the experimental as well as the computed values of the crossbeam stress-strain response confirm the assumed effect of the joint structural arrangement on its stiffness. The connection without corner stiffener (variant “A”) behaves as nominally hinged, which is confirmed by compressive stresses in the upper flange of crossbeam (Fig. 7a) even at the distance 50 mm from the timber block, corresponding to a small positive bending moment. Addition of stiffener with one or two rows of bolts above the upper flange of crossbeam results in an increase of the joint stiffness, which is demonstrated by tensile stresses in the upper flange of crossbeam (Fig. 7a). Decreasing absolute values of the stresses in the middle of the crossbeam with increasing stiffness of the joint (Fig. 7b) correspond to moving bending moment from the mid-span to the supports. Also the lower deflection values of the crossbeam in the mid-span (Fig. 8a) and the rotations of the front plate towards the timber block (Fig. 8b), respectively, document a higher joint stiffness of specimens “B” and “C”. In the case of specimens of series “C” with the most rigid connections, the normal stresses in the middle of the crossbeam decreased by about 33 % compared to the stresses noticed at the specimens of series “A” with quasi-hinged joints. However, providing the fully rigid joints the stresses should decrease even by 66 %, which corresponds to reduction of bending moment in the mid-span from the value M = P·l / 3 in the case of double hinged beam to the value M = P·l / 9 for the case of double-fixed beam. Similarly, the deflections in the middle of the crossbeam decreased in the case of specimens of series “C” by about 55% compared to the specimens of series “A”, while on the assumption of ideal fixation they should decrease even by 79%. The experimental values show naturally smaller or bigger dispersion connected by a large variance of physical and mechanical properties of wood. Especially the rotations of front plate, determined from measured horizontal deformations, show a great dispersion and, in addition, the positive effect of the joint stiffening by the corner stiffener may be observed only at the higher loading levels. This can be explained by a big sensitivity of determining the rotation value to the measurement accuracy of the horizontal displacements of the front plate towards the timber block. The way of the connection assembly represents another important factor influencing the joint behaviour, namely the prestress rate of the bolts. The prestressing force in bolts increases friction on the surface between the steel front plate and the timber girder, wherewith it enhances a shear resistance of the connection and mainly increases the initial rotation stiffness of the joint. However, this initial prestress was not taken into account at the numerical analyses. 4.

Conclusions

All the presented results of experimental and numerical analyses confirm the assumption of semi-rigid behaviour of the observed structural detail. The stiffness rate depends mainly on the joint arrangement, but also on the way of the connection assembly. Modelling this joint as ideal hinged may be accepted at preliminary global structural analysis or at local analysis of the crossbeam with regards to the maximum traffic load effects in the middle. In case of detailed structural analysis, mainly on investigation of the main girder stability as well as the dynamic behaviour of the superstructure, the partial stiffness of the joint should be taken into account, for example, by means of elastic fixation. Determination of the stiffness parameters of such structural details is discussed, for example, in work [9], where the

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component method, originally developed for joints of steel or steel and concrete combined structures, is applied. 5.

Acknowledgements

The paper was processed within the solution of trans-border cooperation project SR-ČR 2007/2013 supported by European Regional Development Fund. 6.

References

[1]

VIČAN, J. – GOCÁL, J – ODROBIŇÁK, J.: “Combination of Wood and Steel on Designing Pedestrian and Short Roadway Bridges”, Proceedings of Conference “DREVOSTAVBY”, Oščadnica, Slovakia, 2009, pp. 41-48. (In Slovak)

[2]

GOCÁL, J. – ODROBIŇÁK, J.: “Study of Plate-girder Timber Footbridges”, Proceedings of Conference “DREVOSTAVBY”, Oščadnica, Slovakia, 2009, pp. 75-81. (In Slovak)

[3]

Vičan, J. – Hlinka, R. – Bahleda, F. – Lokaj, A.: “Experimental Verifying of the Timber Bridge Superstructure Behaviour”, Proceedings of Conference “DŘEVOSTAVBY A KONSTRUKCE NA BÁZI DŘEVA”, Štramberk, Czech, 2009, pp. 114-119. (In Slovak)

[4]

SCIA Engineer 2010. Nemetschek Scia Group, 2010. WEB: http://www.scia-online.com./

[5]

STN EN 1194 Timber structures. Glued laminated timber. Strength classes and determination of characteristic values. SUTN, Bratislava 2001.

[6]

STN EN 10025-2 Hot-rolled products of structural steels – Part 2: Technical delivery conditions for non-alloy structural steels. SUTN, Bratislava 2004.

[7]

STN EN 1993-1-8 Eurocode 3: Design of steel structures. Part 1.8: Design of joints. SUTN, Bratislava 2005.

[8]

ANSYS Academic Research. ANSYS, Inc. WEB: http://www.ansys.com/Industries/Academic.

[9]

ODROBIŇÁK, J. – VIČAN, J.: “Establishment of the Component Method for a Semi-rigid Steel-Timber Joint”. Proceedings of Conference “DREVOSTAVBY”, Habovka, Slovakia, 2010, pp. 87-92. (In Slovak)