CONSTRUCTION OF TIMBER BRIDGES BY PRESTRESSING PREFABRICATED SEGMENTS. Lars Gillingsrud Bergh and Hallvard Johnsen Aase

Transcription

1 PRESTRESSING PREFABRICATED SEGMENTS Lars Gillingsrud Bergh and Hallvard Johnsen Aase

2 BACKGROUND Idea for design consept by chief engineer Tormod Dyken at the Norwegian public roads administration Inspiration: Precast prestressed segmental concrete bridges Wooden toys prestressed by an elastic band 2

3 BACKGROUND The Norwegian public roads administration office by Alnafossen at Brynseng Footbridge from the parkinglot to the main entrance Significant elevation between the parking lot and the main entrance A curving bridge will reduce the rise 3

4 BACKGROUND Alternative to other known construction methods of timber bridges Many and new possibilities for design and architectonic expression Innovative timber construction Esthetical qualities 4

5 THE BASICS OF THE CONCEPT Glulam segments Milled paths Axial pressure keeping the segments fixed together Uniform upward reaction 5

6 FOCUS OF THE STUDY The study has mainly focused on alternative principles for prestressing, alternatives for connections between the segments and principles for manufacturing the segments 6

7 BRIDGES USED AS EXAMPLES 7

8 RELEVANT PRESTRESSING PROFILES a) F n.a F b) n.a F F c) F F n.a 8

9 RELEVANT PRESTRESSING PROFILES 9

10 RELEVANT PRESTRESSING PROFILES F F n.a 10

11 CONNECTIONS BETWEEN THE SEGMENTS The joints has to transfer shear and compressive stress between the elements Two alternative connections were proposed and studied Jointing based on casting a grout of acrylic based mortar between the segments Jointing based on carvings in the segments 11

12 CASTING OF ACRYLIC BASED MORTAR Common method for transferring axial pressure between glulam segments 12

13 CASTING OF ACRYLIC BASED MORTAR Studied by shear testing of prestressed samples 13

14 CASTING OF ACRYLIC BASED MORTAR- RESULTS The shear capacity of the joint is greater than of the woods capacity. This requires a certain compression for the whole section. 14

15 CARVED CONNECTION The carvings are milled as upscaled fingerjoints 15

16 CARVED CONNECTION Studied using FEM analysis and simplified calculations for reinforcement. 16

17 CARVED CONNECTION RESULTS The tensile strength perpendicular to the grain will limit the carved connections capacity Using screws to reinforcing will increase the connections capacity some It is assumed that reinforcing by fiberglass or wood fibers placed in an appropriate way will have great effect on the connections capacity 17

18 ALTERNATIVES FOR CONNECTIONS - CONCLUSIONS Casting a grout of acrylic based mortar between the segments gives a high shear capacity, given a relatively high compression. Carved connections gives a relatively low shear capacity, but does not require that a compression is provided. Experimenting with reinforcing the carved connections with wood-fibers or fiber-glass might give connections with high shear capacity, without high compression is required. 18

19 MANUFACTURING OF THE SEGMENTS Principles for manufacturing of the segments are proposed and discussed with Mr. Holmestad, director of Moelven Limtre AS The principles are considered to be feasible 19

20 MANUFACTURING OF THE SEGMENTS Beams build up to segments 20

21 MANUFACTURING OF THE SEGMENTS Beams build up to segments Recesses for the prestressing tendons ducts are milled out with a CNC machine 21

22 MANUFACTURING OF THE SEGMENTS The parts are glued together The outside shape of the segments is planed 22

23 Conclusion The research imply that the concept is feasible Two alternative bridges has been proposed and further studied The results from the thesis is compiled in two examples 23

24 ALTERNATIVE 1 3 spans Supports at A, B, D and E At the end supports, the bridge is fixed for torsion, but not for bending moment Centric tendons Carved connections 22 tendons in 2 ducts 24

25 ALTERNATIVE 2 One span At the supports the bridge is fixed for both torsional and bending moment Parabolic tendon Joint with grout of acryl based mortar 65 tendons in 4 ducts 25

26 REMARK The design of the profile for the prestressing steel is simpliefied, and some secondary effects has not been taken into account The upward reaction is assumed to reduce also the torsional stress on the crossection and connections. This opens the possibility for building bridges with very bold curvature. 26

27 FURTHER RESEARCH Detailed analysis of topics discussed in the paper Scale or full-scale tests 27

28 OTHER APPLICATIONS Alternative to external prestressing Long spans for load carrying wood structures Greater loads on load carrying wood structures 28

29 THANKS FOR YOUR ATTENTION 29

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