| 1. |
- Crocetti, Roberto, et al.
(författare)
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Stress-laminated-timber decks: state of the art and design based on Swedish practice
- 2016
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Ingår i: European Journal of Wood and Wood Products/Holz als Roh - und Werkstoff. - : Springer Science and Business Media LLC. - 1436-736X .- 0018-3768. ; 74:3, s. 453-461
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Tidskriftsartikel (refereegranskat)abstract
- Stress-laminated-timber (SLT) bridge decks area valid alternative to conventional short- and medium-spanbridges in terms of cost and performance. SLT decks aremade from a number of planks or glulam beams positionedside by side and stressed together using high-strength steelbars. A concentrated load can therefore be distributed fromthe loaded beams onto adjacent beams due to the resistingfriction caused by the pre-stressing of all beams in thedeck. This paper describes the state of the art of SLT bridgedecks, with special emphasis on Swedish practice. Theeffect of butt joints on deck deflection and solution appliedto accommodate the loss of pre-stress are shown. Simpledesign tools for the preliminary design of road bridges areillustrated. Best practice with regard to some detailing,water protection and durability is also discussed. Finally,possible developments of SLT bridges are discussed.
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| 2. |
- Ekholm, Kristoffer K J, 1982, et al.
(författare)
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Effect of vertical interlaminar shear slip and butt joints in narrow stress-laminated-timber bridge decks
- 2014
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Ingår i: Engineering Structures. - : Elsevier BV. - 1873-7323 .- 0141-0296. ; 72, s. 161-170
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Tidskriftsartikel (refereegranskat)abstract
- Interlaminar slip occurs in stress-laminated-timber (SLT) decks subjected to concentrated loads, such as vehicles. Butt joints in SLT decks can be regarded as local discontinuities with increased load transfer resulting in increased frictional forces. Multiple tests were performed with different combinations of pre-stress level, number of loads, span-to-depth ratio, beam width and butt-joint configurations. The results of the tests have been analysed and compared with non-linear finite element analyses using solid elements and contact properties. The experimental results and the analysis showed that, in the case of a high span-to-depth ratio in the deck and without butt joints, there is little or no interlaminar slip. Test elements without butt joints showed little slip at pre-stress levels of 300 kPa or higher. A significant reduction in post-slip stiffness was observed for the test elements with butt joints. (C) 2014 Elsevier Ltd. All rights reserved.
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| 3. |
- Ekholm, Kristoffer K J, 1982, et al.
(författare)
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Full-Scale Ultimate-Load Test of a Stress-Laminated-Timber Bridge Deck
- 2012
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Ingår i: Journal of Bridge Engineering. - 1084-0702 .- 1943-5592. ; 17:4, s. 691-699
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Tidskriftsartikel (refereegranskat)abstract
- A general assumption reported in the literature is that the structural response of a stress-laminated-timber (SLT) bridge deck islinear until failure. However, few studies of ultimate-load tests (ULTs) on timber bridges have been reported. A full-scale test of a SLT deck(span, 4.9 m; thickness, 270 mm) was performed to obtain the deformations at various prestress levels as well as at the ultimate-load capacityof such a structure. Prior to the ULT, nondestructive tests (NDT) were performed at three different prestress levels. The load was applied as anaxle load positioned both centrically and eccentrically. The deflections were about 10% larger at a prestress level of 300 kPa compared with aprestress level of 600 and 900 kPa. For applied loads larger than 150–250 kN, the deflection of the deck was nonlinear at certain positions.This was most likely owing to large concentrated shear forces that resulted in interlaminar slip between the laminates. The limit for linearityseems to be dependent on the applied prestress. A prestress of 600 kPa and an eccentrically positioned load was used for the ULT. Failureoccurred at a load level of 900 kN. The existing design codes and new procedures in development may be verified and calibrated against theresults in this paper.
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| 4. |
- Ekholm, Kristoffer K J, 1982, et al.
(författare)
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Mechanical Properties of Stress Laminated Timber Decks – Experimental study
- 2009
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Ingår i: CIB-W18 Proceedings meeting forty-two 2009, Dübendorf, Switzerland. - 1864-1784.
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Konferensbidrag (refereegranskat)abstract
- In several European countries, the design of stress laminated bridges has been conducted according to a simplified method developed by M. A. Ritter in the 1990’s, valid for American wood species such as Douglas fir. According to this method a stress laminated deck can be designed as a beam with cross section dimensions beff x t, where beff is a fictitious (or effective) width - determined according to orthotropic plate theory - and t is the thickness of the deck. However, there are several uncertainties concerning the validity of this model concerning its application to European stress laminated bridges. These uncertainties are related to: (1) wood species - in Europe boards or glulam of Norway spruce or Scots pine are generally used; (2) design philosophy - Ritter’s model was developed according to the “allowable stresses design method”, whilst the philosophy behind the Eurocodes is the design method according to the ultimate limit states.In this paper, an experimental investigation on specimens manufactured with pre-stressed glulam will be presented. The glulam is made from Norway spruce. The aim of the study is to estimate the mechanical properties of stress laminated timber decks with varying pre-stressingforces. The methods for measuring (a) transversal modulus of elasticity ET , and (b) the transverse shear modulus GLT will be presented. Comparison with results obtained by this research and results obtained by other researcher will be shown.
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| 5. |
- Ekholm, Kristoffer K J, 1982, et al.
(författare)
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Modeling Slip in Stress-Laminated Timber Bridges: Comparison of Two Finite-Element-Method Approaches and Test Values
- 2014
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Ingår i: Journal of Bridge Engineering. - : American Society of Civil Engineers (ASCE). - 1084-0702 .- 1943-5592. ; 19:9
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Tidskriftsartikel (refereegranskat)abstract
- Finite-element (FE) simulations of the deformation behavior of a 5.4-m-long, 8-m-wide, and 0.27-m-thick stress-laminated timber bridge deck were conducted. The simulation results were compared with full-scale test results when using a load resembling an axle load placed near the edge and when cycling the load between a high and low value. Two separate approaches to nonlinear FE modeling were used. The first FE model simulates a frictional slip between the glulam beams with an elastic-plastic material model. The second FE model simulates a frictional slip by modeling each discrete contact surface between each beam in the deck. The results show good agreement between simulation and test results and reveal that the simulation model that models contact surfaces produces slightly better results at the expense of a greater modeling effort and increased computational time. Hysteresis in the load versus deformation curves is clearly visible and was due to significant slip between the glulam beams, which was successfully simulated.
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| 6. |
- Ekholm, Kristoffer K J, 1982, et al.
(författare)
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Non-linear analysis of a stress-laminated-timber bridge loaded to failure
- 2012
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Ingår i: Proceedings of the Sixth International Conference on Bridge Maintenance, Safety and Management, IABMAS 2012, Stresa, Lake Maggiore, 8-12 July 2012. - : CRC Press. - 9780415621243 ; , s. 1879-1886
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Konferensbidrag (refereegranskat)abstract
- The general assumption in stress-laminated-timber (SLT) bridge design is that the structural response is linear both in the serviceability limit state (SLS) and in the ultimate limit state (ULS). However, this has been shown not to be the case according to a full-scale test performed in Sweden where the SLT deck was subjected to a failure load. When an SLT deck is loaded to failure, non-linear behaviour must be considered when a structure of this kind is analysed. Both horizontal and vertical slip occur in the interlaminar interface between the stressed glulam laminations. This behaviour does not occur in a solid timber plate. Once slip has occurred, the stresses are redistributed between the laminations. Interlaminar slip is of great importance and is affected by several factors such as the pre-stress level, surface roughness and surface moisture content. A rectangular SLT deck, 5.00 × 7.98 × 0.27 m 3 (length, width and thickness), with two patch loads positioned close to the edge, was tested. The deflection values for both ultimate and non-destructive loads were compared with finite element (FE) models of the SLT deck in order to evaluate the behaviour of the deck. The results from the experiments were compared with both linear and non-linear FE models.
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| 7. |
- Ekholm, Kristoffer K J, 1982
(författare)
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Performance of Stress-Laminated-Timber Bridge Decks
- 2013
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Stress-laminated-timber (SLT) bridge decks are a satisfactory alternative to conventional short-span bridges in terms of cost and performance. SLT decks are made from a number of timber or glulam beams positioned side by side and stressed together using high-strength steel bars. A concentrated load can therefore be distributed from the loaded beams onto adjacent beams due to the resisting friction caused by the stressing. A project has been conducted in three parts in order to fill some of the knowledge gaps relating to SLT decks constructed in Sweden. The first part of the project focused on determining the ultimate-load capacity of SLT decks, as well as studying their behaviour when subjected to non-destructive loads. A full-scale test of a 270 mm deep SLT deck showed that interlaminar slip already occurs at load levels equivalent to serviceability limit state (SLS) loads. The interlaminar slip resulted in non-linear load-deflection behaviour in the deck, making linear design models insufficient. The ultimate load capacity of the tested SLT deck was 4.5 times higher than the SLS load. The deck showed a great redundancy when reloaded after failure.The second part of the project aimed to study the cause and effects of interlaminar slip. Non-linear finite element (FE) models, which successfully simulated the interlaminar slip between the deck beams, were developed. Non-linear FE models can produce valuable knowledge about the stress redistribution that occurs in the deck and cannot be measured during tests. Even though the magnitude of the interlaminar slip is very small, it has a major influence on the load redistribution in SLT decks. Significant variations in the normal stress in the SLT deck were observed when the laboratory-tested full-scale test was simulated. The variations in normal stress have a significant influence on the amount of interlaminar slip in SLT decks, since the critical shear stress is dependent on the magnitude of normal stress.The final part of the project focused on studying timber bridges in service and summarising the existing literature on the durability of SLT decks. A reduction in prestressing has been a major durability concern for SLT decks constructed using wet timber beams which shrunk with time, causing a prestressing reduction. Field inspections have shown that the prestressing loss in Swedish SLT decks made from dry glulam is much smaller than that reported in the literature.
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| 8. |
- Ekholm, Kristoffer K J, 1982
(författare)
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Stress-laminated-timber bridge decks subjected to ultimate loads
- 2011
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Stress-laminated-timber (SLT) bridge decks are a satisfactory alternative to conventional short-span bridges in terms of cost and performance. SLT decks are made from several deck laminations positioned side by side. The laminations in the deck are normally timber or glulam beams that are stressed together using high-strength steel rods positioned in pre-drilled holes perpendicular to the length of the laminations. A concentrated load is distributed from the loaded lamination onto the adjacent laminations due to the resisting friction between the stressed laminations. The pre-stress between the laminations should be of such a magnitude that movements between the laminations are prevented.The general assumption regarding the design of stress-laminated-timber (SLT) bridge decks is that the structural behaviour is linear in both the serviceability-limit state (SLS) and the ultimate-limit state (ULS). In this thesis, the validity of this assumption is investigated. Several full-scale tests have been conducted in order to determine the structural response and performance of SLT decks subjected to high concentrated loads causing the failure of the decks. Several complementary tests have also been conducted in order to investigate the behaviour of specific details in SLT decks.Several approaches to the design of SLT decks are currently in use. Both “hand calculation” methods and numerical finite element methods are commonly used. The hand calculation methods are generally based on the “equivalent beam theory”. In this method, the three-dimensional deck is replaced by a two-dimensional beam which has a width corresponding to the load distribution in the transverse direction. Numerical FE analyses are normally conducted using three-dimensional shells. The shell is assigned orthotropic mechanical material parameters in order to include the load distribution in the transverse direction.Tests have shown that the load-deflection relationship of the decks was non-linear when subjected to eccentric loads. Interlaminar slip occurred between the laminations in the deck. These small movements caused significant stress redistribution in the deck. The interlaminar slip was dependent on the pre-stress level. The horizontal interlaminar slip occurred at low loads of approximately 10 20% of the ultimate load capacity. None of the linear design models proved sufficient to include these non-linear effects.
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| 9. |
- Ekholm, Kristoffer K J, 1982, et al.
(författare)
-
Stress-Laminated Timber Decks Subjected to Eccentric Loads in the Ultimate Limit State
- 2013
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Ingår i: Journal of Bridge Engineering. - 1084-0702 .- 1943-5592. ; 18:5, s. 409-416
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Tidskriftsartikel (refereegranskat)abstract
- Stress-laminated timber (SLT) bridge decks are generally designed using either linear hand calculation methods or linear finite-element models. Several studies have shown, however, that the behavior of SLT decks is nonlinear when loaded until failure. In this paper, several linear design methods are compared with one another and with an ultimate load test of a full-scale SLT deck subjected to an eccentric load. Some of the linear hand calculation methods show significant discrepancies in results, depending on the load position. There are also variations in the results from finite-element models, depending on the material properties assigned to the deck. All the design methods failed to predict the deflection of the tested deck when loaded to failure. A larger deflection was observed in the full-scale test than that predicted by the design methods. As a result, the linear design method could underestimate the bending stresses in the deck. Several hand calculation methods are also unable to calculate the transverse forces and moments necessary for design according to Eurocode 5.
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| 10. |
- Ekholm, Kristoffer K J, 1982, et al.
(författare)
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Ultimate limit state load test of stress-laminated-timber deck
- 2010
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Ingår i: World conference on timber engineering 2010, June 20-24 2010, Riva del Garda, Italy. - 9781622761753 ; 700:2, s. 343-352
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Konferensbidrag (refereegranskat)abstract
- Glulam beams made of Norway spruce are stressed together with high strength steel bars creating a Stress laminated timber deck (SLTD). Deflections and stresses can be estimated with both finite element models based on orthotropic plate theory as well as with simplified hand calculation methods based simple beam theory. If SLTDs areloaded until failure in the ultimate limit state (ULS), phenomena which generally do not occur in serviceability limit state (SLS) must be taken into account. Slip will occur in the interlaminar surface between the stressed glulam laminations. Once slip has occurred, stress will redistribute between the laminations. A rectangular SLTD was tested for several load levels and pre-stress levels. Deflections were compared with a finite element orthotropic model. These results were also compared with several established design methods. After failure occurred in the ULS, the load was first removed and then applied again, in order to emphasize the reliability of stress redistribution within SLTD.
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