Proceedings of the

International Conference Timber Bridges ICTB2010

Lillehammer, Norway September 12 -15, 2010

Editors: Professor Kjell A. Malo Chief Engineer Otto Kleppe Chief Engineer Tormod Dyken

Organizers: Norwegian Public Road Administration NTNU, Norwegian University of Science and Technology NTI, Norsk Treteknisk Institutt Innovation Norway

Secretariat: Norwegian Public Road Administration P.O Box 8142 Dep NO-0033 Oslo, Norway www.vegvesen.no

© ICTB 2010 & Tapir Academic Press, Trondheim 2010

ISBN 978-82-519-2680-5

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Preface Over the past twenty years, timber bridge construction has gathered headway in many countries. Research in the area has produced significant results; new materials and connections have been developed and various structural systems have been explored. New developments have been presented in journals and at general timber construction conferences. The time was now ripe for a specialized international conference on timber bridges to present the state of the art. In Norway, the ready availability of timber and the tradition of utilizing timber in houses and other structures make it natural to consider timber an adequate construction material for bridges with spans of up to 100 meters or even more. Today, there are many timber bridges in Norway – both road and pedestrian bridges. Since 1995, the Norwegian Public Roads Administration has built more than 100 timber bridges with spans up to 70 meters. The main objective of the conference ICTB2010 at Lillehammer was to showcase and discuss the state of the art in timber bridge technology. The conference topics were: • Design aspects • Environmental aspects • Historical bridges • Protection and durability • Monitoring • Timber bridge aesthetics • Components, connections and detailing • Pedestrian bridge projects • Bridge decks • Composite bridges The primary emphasis of the conference was on the design of durable, environmentally friendly and cost-efficient timber bridges. Our hope is that ICTB 2010 can serve as a source of inspiration for designers, researchers, architects and others, working within the field of timber bridges.

Lillehammer, September 2010.

On behalf of the facilitating organizations, Børre Stensvold Bridge Director, Norwegian Public Roads Administration Conference Chair.

International Scientific Committee Professor em. Heinrich Kreuzinger, Technische Universität München, Germany Professor Kurt Schwaner, Biberach University of Applied Sciences, Germany Professor Gerhard Schickhofer, Graz University of Technology, Austria Professor em. Aarne Jutila, Helsinki University of Technology, Finland Professor Robert Kliger, Chalmers University of Technology, Sweden Research Engineer James Wacker, USDA Forest Products Laboratory, USA Professor Kjell Arne Malo, Norwegian University of Science and Technology, NTNU, Norway Amanuensis Nils Ivar Bovim, The Norwegian University of Life Sciences, Norway

Steering Committee Bridge director Børre Stensvold - Conference Chair, Norwegian Public Roads Administration Mr Erik Aasheim, Conference Co-Chair, Norsk Treteknisk Institutt (NTI) Professor Kjell Arne Malo, Programme Co-Chair, Norwegian University of Science and Technology, NTNU Mr Otto Kleppe, Programme Chair, Norwegian Public Roads Administration

Organising Committee Mr Otto Kleppe, Chair Norwegian Public Roads Administration Mr Nils Ivar Bovim, The Norwegian University of Life Sciences Mr Rune B. Abrahamsen, Sweco Norway Mr Åge Holmestad, Moelven Limtre AS Professor Kjell Arne Malo, Norwegian University of Science and Technology, NTNU Mr Erik Aasheim, Norsk Treteknisk Institutt (NTI) Mr Trond Arne Stensby Norwegian Public Roads Administration Mr Tormod Dyken Norwegian Public Roads Administration

Contents Key note lecture Kurt Schwaner, Germany: Timber Bridges - different countries, different approaches .....................

1-20

Design aspects Part I Michael Flach, Austria: How to design timber bridges ...................................................................... Per Kr. Ekeberg, Norway: Technical concepts for long span timber bridges .................................... Hauke Kepp, Norway: Thermal actions on timber bridges ................................................................ Kolbein Bell, Norway: Structural system for glulam arch bridges ....................................................

21-28 29-36 37-48 49-66

Design aspects Part II João Nuno Amado Rodrigues, Portugal: Use of composite timber-concrete bridges solutions in Portugal ........................................................ 67-78 Jarle Svanæs, Norway: Environmental timber bridges – verification of material properties of Kebony modified wood ......................................................... 79-88 Hilde Rannem Isaksen, Norway: Construction cost of Timber Bridges in Norway –A comparison with Steel and Concrete ............................................................................................. 89-98 Ove Solheim, Norway: New 4-lane Mjoesbridge in timber? ............................................................. 99-106

Environmental aspects Johanne Hammervold, Norway: Environmental analysis of bridges in a life cycle perspective ........ 107-118 Jarle Svanæs, Norway: Environmental friendly timber bridges – Environmental improvement through product development .............................................................. 119-122

Historical Bridges Tsuneo Igarashi, Japan: The 62nd reconstruction of a traditional wood bridge ................................ 123-130 Guillermo Iñiguez-Gonzáles, Spain: Remarkable ancient timber bridges up to the 1850´s. Part I: general review........................................................................................................................... 131-138 Miguel C. Fernández-Cabo, Spain: Remarkable ancient timber bridges up to the 1850´s. Part II: case studies and breakthroughs................................................................................................ 139-156

Protection and Durability Otto Kleppe, Norway: Durability of Norwegian timber bridges ....................................................... Anna Pousette, Sweden: Outdoor tests of timber beams and columns .............................................. Masahiko Karube, Japan: Report of the collapsed wooden bridges in Japan .................................... Elisabet Michelson, Norway: Polyurea based bridge membrane on wooden bridges .......................

157-168 169-178 179-194 195-204

Monitoring Thomas Tannert: Structural health monitoring of timber bridges ...................................................... 205-212 Anders Gustavsson, Sweden: Health Monitoring of timber bridges .................................................. 213-222 Tormod Dyken, Norway: Monitoring the moisture content of timber bridges .................................. 223-236

Antti Karjalainen, Finland: Bridge Information Modelling (BIM) and Laser Scanning In Renovation Design, Case Pyhäjoki Bridge ..................................................................................... 237-242 Jim Wacker, USA: Development of a Smart Timber Bridge Girder with Fiber Optic Sensors ......... 243-252

Timber Bridge Aesthetics Richard J. Dietrich, Germany: Six timber bridges of special interest ................................................ 253-258 Yngve Aartun, Norway: Timber Bridge Aesthetics –Design and function (+ tradition) .................... 259-266 Bernt Jakobsen, Norway: Spectacular Wooden Truss Bridges as Traffic Safety Enhancing Measures ....................................... 267-276

Components, Connections and Detailing Lars Bergh, Norway: Construction of timber bridges by prestressing prefabricated segments ......... 277-280 Bjørn A. Lund and Matteo Pezzucchi, Norway: Development of a new barrier system for stress laminated timber road bridge decks ........................ 281-296 Kjell Arne Malo, Norway: On Connections for Timber Bridges ........................................................ 297-312 Abdy Kermani, United Kingdom: Developments in stress-laminated arch construction for footbridges ................................................. 313-320

Pedestrian Bridge Projects Rolf Broennimann, Switzerland: Design, construction and monitoring of a bowstring arch bridge made exclusively of timber, CFRP and GFRP ................................................................. 321-328 José L. Ferández-Cabo, Spain: Construction aspects of a 19.2 m Timber Truss cantilevered view walkway in Vitoria, Spain ...................................................................................... 329-334 Anssi Laaksonen, Finland: Malminmaki Pedestrian Overpass .......................................................... 335-340 Julio Vivas, Spain: Design and installation of a covered timber footbridge over the A8 motorway in Bilbao, Spain .................................................................................................................. 341-350

Bridge decks Mats Ekevad, Sweden: Prestressed Timber Bridges - Simulations and experiments of slip .................................................... 351-358 Roberto Crocetti, Sweden: Anchorage systems to reduce the loss of pre-stress in stress-laminated timber bridges ..................... 359-370 Rune B. Abrahamsen, Norway: Bridge deck rehabilitation using cross-laminated timber .................................................................... 371-382

Composite bridges Aarne Jutila, Finland: Wood Concrete Composite Bridges – Finnish Speciality in the Nordic Countries ............................. 383-392 Jeno Balogh, USA: Testing of Wood-Concrete Composite Beams with Shear Key Detail ................................................ 393-398 Leander A. Bathon, Germany: Performance of single span wood concrete - composite bridges under dynamic loading .................. 399-402

International Conference on Timber Bridges (ITCB 2010)

Development of a Smart Timber Bridge Girder with Fiber Optic Sensors Jim Wacker Research Engineer Forest Products Laboratory USDA Forest Service Madison, Wisconsin, USA [email protected]

Jim Wacker is currently chair of the American Society of Civil Engineers (ASCE) Timber Bridge Committee. The main focus of his research is the field performance of timber structures in the transportation sector.

Ursula Deza Graduate Student Bridge Engineering Center at Iowa State University Ames, Iowa, USA [email protected]

Ursula Deza is a PhD candidate at Iowa State University working on diverse bridge research for health monitoring systems at the Bridge Engineering Center.

Brent M. Phares Association Director Bridge Engineering Center at Iowa State University Ames, Iowa, USA [email protected]

Dr. Brent M. Phares is the Associate Director of the Iowa State University Bridge Engineering Center. Dr. Phares is an international expert in the field of advanced bridge evaluation technologies.

Terry J. Wipf Professor and Director of Bridge Engineering Center at Iowa State University Ames, Iowa, USA [email protected]

Dr. Terry J. Wipf is the Pitt Des Moines Professor at Iowa State University and Director of the Bridge Engineering Center. His expertise is bridge field testing, evaluations, and structural health monitoring.

Summary Past timber bridge evaluation and maintenance efforts in the USA have principally focused on the internal integrity of timber components using various non-destructive evaluation tools to supplement visual inspection data. This project is part of a comprehensive effort to develop smart structure concepts for improving the long-term performance, maintenance, and management of timber bridges. This comprehensive effort focuses on developing an integrated turnkey system to analyze, monitor, and report on the performance and condition of the most commonly constructed timber bridge type in the USA, the longitudinal glued-laminated girder with transverse gluedlaminated bridge deck. This paper describes an initial project to develop techniques for integrating fiber optic sensors (FOS) within timber bridge glued-laminated beam components.

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Keywords: Smart timber bridge, fiber optic sensor, FBG packaging, glued-laminated girder, health monitoring techniques.

1.

Introduction

The always present deterioration of bridges has alerted bridge owners and managers to develop new techniques for constructing, repairing, rehabilitating, and monitoring bridges. In the case of timber bridges, service conditions have historically been determined with visual inspection techniques and maintenance decisions being based upon the information gathered. To improve this situation, the development of an innovative timber bridge structure with capabilities to monitor long-term performance parameters through the implementation of fiber optic gages was undertaken as part of the previously developed Five-Year Research Plan [1]. The initial work of this research plan consisted of development of techniques for embedding FBG sensors with and without physical attachment to glulam members for detecting structural and non-structural attributes. In addition to this, small scale glulam members were constructed in the laboratory and instrumented with commercially available FBG strain sensors and specially designed sensor packages. These specimens were tested at common bending levels varying loading rates and temperature conditions to assess the performance of the different FBG sensor packages.

2.

Smart Timber Bridge Research Plan

The goal of this research plan is to develop smart timber bridge structures by using both existing and new forms of instrumentation types to measure the structural adequacy and the degree of deterioration of the bridge through the integration of health monitoring technologies, and bridge management approaches. A smart structure would typically incorporate the use of structural materials, sensors, data reduction techniques and remote systems that allow for the monitoring of the structure. With these elements, the smart structure is able to monitor the in-situ behavior of the structure, to assess its performance under service loads, determine the current condition and detect damage/deterioration [2]. In this context, a conceptual smart timber bridge was developed with the purpose of improving the longterm performance, maintenance, and management of timber bridges. Four concepts were established to develop the smart timber bridge comprising of: • Selection of the bridge structural materials. • Identification of the measured performance metrics (attributes). • Selection/development of the sensor types. • Communication/processing and reporting. Stress rated glued-laminated timber (glulam) members were selected as the material for the smart timber bridge due to the growth in usage. In contrast to the variable range of solid wood, glulam is an engineered, stress-rated product that provides distinct advantages over solid-sawn timber. To date, bridges constructed with glulam have received minimal attention from the bridge health monitoring community and there lacks of a body of data on the in situ behavior. In brief, the superstructure of the conceptual bridge composed of a series of transverse glulam deck panels supported on longitudinal glulam beams is the main focus of the smart timber bridge development. By identifying the bridge-specific behaviors and deterioration modes, the assessment of the smart timber bridge condition would be conducted through the evaluation of the structural adequacy and decay. Structural adequacy of the bridge would be determined by measuring the flexural strains to evaluate the lateral load distribution, dynamic load allowance and cumulative fatigue for comparison to acceptable design levels. In addition, the decay/deterioration of the timber structure, specifically due to moisture, metal corrosion and ultraviolet light would be evaluated through the application of novel sensors [3]. The overall health condition of the smart timber bridge might be monitored using commercially available as well as new sensors incorporating Fiber Bragg Grating (FBG) technology. Besides being linear and absolute in response, electrical interrupt immune and readily multiplexed, these FBG sensors have the ability to be both embedded and/or surface mounted. In recent laboratory as

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well as field tests conducted by the Bridge Engineering Center at Iowa State University, FBG sensors demonstrated 99% accuracy when compared to foil strain sensors [4]. In addition, sensors to detect moisture content, ferric ions and degradation in wood lignin would be developed and implemented to detect the decay/deterioration of the glulam members. As a part of the health monitoring technologies and bridge management approach, a communication/reporting system would be developed. This system would consist of a data acquisition system with developed data processing techniques and software applications to interpret and report the results of the data obtained during monitoring activities. The behavior of the superstructure would be summarized integrating all the responses related to the attributes of the smart timber bridge and be addressed to the bridge owner in a clear report. With this information, the owner could program routine maintenance and/or rehabilitation of the bridge. Also, this system would serve as an immediate alert to early damage or a catastrophic event (e.g., wood decay, collision, etc).

3.

Development of the Sensor Packages

The first of part of this research focused on the development of new packages that would protect the FBG sensors embedded within the glulam. 3.1 Structural Packages Structural packages were developed to protect the fragile bare FBG strain sensor during handling and installation while also providing mechanical connectivity between the FBG sensor and the glued-laminated specimen. The structural FBG sensor package conceptually consists of a backing material and a bare FBG strain sensor bonded together. The resulting system could be either attached to an exposed wood surface or embedded between the laminates of glulam members to measure the response of the member to external forces. In this work, five new backing material configurations were developed using either stainless steel shims or aluminum mesh sheets shaped as shown in Figure 1 (a). The dimensions of the structural packages were developed to resist the horizontal shear stresses and to allow for the redistribution of localized strain irregularities between the package and the wood laminates. The embedding technique consisted of surface preparation, followed by the application of a structural adhesive to bond the backing material to the wood laminate. After curing for a minimum of 24 hours, the bare FBG sensor was applied to the backing material using a similar structural adhesive and cured for an additional 24 hours.

(a) Backing materials (b) FBG strain sensor

(c) Instrumented internal wood laminates

Figure 1. Structural Packages: Materials and Installation In addition to the bare FBG strain sensors, one commercially available surface mounted FBG strain sensor bonded to a C-FRP package was evaluated (Figure 1 (b)). The FBG structural packages as bonded to the internal laminates are shown in Figure 1 (c). Nine small scale three-ply gluedlaminated specimens were instrumented with eighteen FBG sensor packages consisting of combination of six package designs and three structural adhesives. 3.2 Non-Structural Packages The non-structural FBG sensor package conceptually consists of a backing material and an adhesive or adhesive tape that protects and isolates the FBG sensor from load induced behaviors. In that sense, no physical attachment between the FBG sensor and wood laminate was allowed. Ten non-

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structural packages were prepared with a combination of stainless steel shims and aluminum foil as backing materials which were bonded with two different types of adhesives and two adhesive tapes. The process of installing the FBG non-structural packages consisted of the wood preparation by routing a recess area, to house the FBG sensor, isolation materials, and leads, and the application of the non-structural FBG package on an external wood surface (see Figure 2). In all cases, the packages were cured for a minimum of 24 hours. Five small scale three-ply glued-laminated specimens were instrumented with ten FBG strain sensors isolated with non-structural packages.

(a) FBG sensor in a recess area and applied adhesive tape.

(b) Installed backing material

Figure 2. Non Structural Package: Materials and Installation 3.3 FBG Sensors during Beam Manufacturing The three-ply glued-laminated specimens were assembled with a constant pressure of 0.689 MPa (100 psi) between laminates sustained throughout the curing period at room temperature. After completing the assembly process, twelve of the eighteen internal FBG sensors with structural packages were operative. The fragility of the gages and the robustness of the specimens made retrievals of the embedded gages impossible, so the cause of the failure could not be determined. Possible causes of the failure were attributed to the transition between the FBG sensor package and leads and/or differential displacements between glued laminates during assembling and stressing process. As for the non-structural packages, all ten FBG sensors were functioning.

4.

Testing Program

All specimens were tested in bending under third-point loading with a total load of 11 KN (2500 lbs) (Figure 3(a)). The tests were adapted from the ASTM 198 05a provisions [5]. Various loading rates and temperature variations were applied to investigate the behavior of the sensor packages. To test the compressive and tensile response of each sensor, the load was applied to each bending surface. Additional external sensors beyond those developed in this work were installed to provide comparative sensor performance data. In the structural package specimens, external structural FBG sensor packages, strain transducers and electrical resistance strain gages (foil strain gages) were installed for strain comparison (Figure 3(b)). Thermocouples were installed to record temperature variations during long duration tests. In the non-structural package specimens, external strain transducers were installed.

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(a) Test fixture

(b) Instrumented external glulam specimens

Figure 3. Test Setup: Small Scale Glulam Specimen with Structural Package 4.1 Structural Package Specimens’ Tests 4.1.1 Bending Test The basic bending test was performed to establish the response of the FBG sensor packages to flexural loading in the elastic range. The FBG strains were compared to theoretical values and to the data gathered from the foil strain gages and strain transducers. Upper and lower bound theoretical flexural strains were estimated from conventional beam theory by applying the modulus of elasticity values (E) contained in AASHTO Specifications [6]. Side 1 All FBG sensors immediately responded to loading and unloading with a 2 Side1 Loading 50.8 Linear Fit 1, nearly linear elastic strain R =0.997 response. In Figure 4, a typical plot of the external 25.4 1 and internal FBG strains for an applied load of 11 KN Side 2 Loading (2500 lbs) for both Side 1 0 0.0 Linear Fit 2 and 2 loadings compared to a R =0.996 linear regression model are shown. In all cases, the -25.4 -1 compressive and tensile flexural strains were dissimilar. The differences -50.8 -2 were attributed to anatomical wood factors in the vicinity -400 -300 -200 -100 0 100 200 300 400 of the FBG sensor packages. Side 2 In Specimen 1, material Strain properties varied [PH] significantly due to the Figure 4. Bending Test – Structural FBG Sensor Packages’ included knots and slope of Results: Strain levels vs. Linear Regression Model grain near the sensors; this is evident by the consistent difference of 50 PH between the Side 1 and 2 loadings. When comparing to the theoretical upper bound strains (with E = 10 GPa (1500 ksi)), all external and internal FBG strains were lower. With respect to the theoretical lower bound strains (with E = 14 GPa (2000 ksi)), the FBG strains differed by up to 40%. In Figure 5, the external FBG sensors, foil strain gages and strain transducers readings were plotted to evaluate their performance. Comparing the external FBG strain values with the average strain determined as the arithmetic mean of all external sensor readings, both strain values differed by up to 11%. Specimen 1 differed by up to17%. [mm]

Location of FBG Structural Package [in.]

2

2

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-600

Side 1 Loading

Side 2 Loading 522 PH 392 PH

-400

FSG1 (2) S1 (2): Foil Strain Gage 1 (2) Side 1 (2)

FBG 1 FSG1 S1 ST1 S1 FSG2 S1 ST2 S1

ST1 (2) S1 (2): Strain Transducer 1 (2) Side 1 (2)

FBG 2 FSG1 S2 ST1 S2 FSG2 S2 ST2 S2

200

FBG 1 FSG1 S1 ST1 S1 FSG2 S1 ST2 S1

Strain [PH]

0

FBG 2 FSG1 S2 ST1 S2 FSG2 S2 ST2 S2

-200

FBG1 (2): FBG Sensor Package Type 1 (2) at Side 1 (2)

-392 PH

400

---Theoretical External Upper and Lower Bound

-522 PH 600

Side 1

Side 1

Side 2

Side 2

Specimen 4

Figure 5. Strain Comparisons for the Bending Test Results

[N]

Load [lbs]

[PH]

Strain [PH]

o

[ C]

Laboratory Temperature [oF]

4.1.2 Sustained Loading Test The objective of this test was to evaluate the elastic and viscoelastic behavior of the structural FBG packages under a 24-hour sustained loading and uncontrolled ambient laboratory temperature. The effectiveness of the FBG sensors was evaluated by comparing both basic bending and sustained loading results in the short term. Both tests’ results differed by up to 8%. In Specimen 1, Side 2 Loading, a noticeable reduction in strains was observed attributed to the wood surface irregularities at the sensor packages’ regions (i.e., knot, spiral grain). Throughout the 24 hour test, the external 85 29.4 and internal FBG strains Temperature 80 26.6 visibly varied with the Ext. Laminates: 75 23.8 In compression temperature fluctuations (up In tension 70 21.1 to +/-2 oC (+/-4 oF)) (e.g., 65 18.3 Figure 6). When assessing -500 -500 the wood and package Structural FBG -375 -375 Sensor Package materials’ thermal -250 -250 Ext. FBG 1 properties, only the FBG Int. FBG 1 -125 -125 sensors are significantly Int. FBG 2 0 0 Ext. FBG 2 affected by temperature 125 125 1: Side 1 variations. A linear 250 250 2: Side 2 375 375 regression model was fit to 500 500 the strain and temperature data to investigate the 13345 3000 Sustained Loading 8896 2000 strain-temperature Load 4448 1000 relationship. The resulting 0 0 R2 coefficients varied from 0 20000 40000 60000 80000 100000 minimal to 0.96. The higher Time [sec] values may indicate the higher influence of the Figure 6. Sustained Loading Test – FBG Strains, Load and Temperature vs. Time temperature fluctuations in the strain variations. The minimal values may indicate the predominant effect of the time-dependant load over the temperature variations. The presence of residual strains at the end of the loading revealed that creep deformation, due to the combined effect of load and temperature effect, had occurred in all packages. For each sensor, the strain recovery was evaluated by calculating the rate of recovery as the strain difference per unit time. The rate of recovery was determined for the strain data collected for a minimum of 15 min. The positive rate of recovery values indicated that the FBG packages’ response would decrease to zero strain. In the case of Specimen 1, Side 2 Loading, the FBG sensors had higher residual strains which may indicate changes in the internal make up of this specimen.

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In the short term, all structural FBG sensor packages had comparable strain performance to the basic bending test results. After 24 hours, the strain levels were influenced by both time-dependant loading and ambient temperature fluctuations. After unloading, the residual strains of two internal and four external structural FBG sensor packages were found to decrease to zero strain over a period of an hour.

[N]

Load [lbs]

[PH]

Strains [PH]

o

[ C]

Laboratory Temperature [oF]

4.1.3 Fast Loading Test The main objective of these tests was to evaluate the viscoelastic behavior of the FBG structural packages when applying a load 11 KN (2500 lbs) with fast loading rate followed by stabilized loading and unloading. During testing, constant laboratory temperatures were observed for which temperature effects were neglected. All specimens were tested under loading rates of 11 KN/min (2500 lbs/min), 22 KN/min (5000 lbs/min) and 667 KN/min (150,000 lbs/min). The latter test was performed twice per specimen to verify the reproducibility of the strain data. No major differences between 11- and 22-KN/min fast test results were observed. At the fastest rate, both peak strains 85 29.4 were up to 2% higher than Temperature 80 26.6 External Laminates: the average strain In compression 75 23.8 determined from the In tension 70 21.1 stabilized strain data. In 65 18.3 the 667 KN-lbs/min fast 500 500 Structural FBG test, the increments in load 375 375 Sensor Package and strains were at least Ext. FBG 1 250 250 Int. FBG 1 30% larger than the 125 125 Int. FBG 2 average values (see Figure 0 0 Ext. FBG 2 7). After 5 seconds, the -125 -125 1: Side 1 2: Side 2 -250 -250 strain levels and load were -375 -375 stable. With the exception -500 -500 of Specimen 1, the average Fast Loading 17793 4000 strains were on the same Rate of loading: 13345 3000 order as the basic bending 8896 2000 667 KN/min 4448 1000 test results, indicating that (150,000 lbs/min) 0 0 Load the FBG sensor packages 0 50 100 1500 1600 had consistent flexural Time [sec] Figure 7. Fast Loading Test – 667 KN/min fast loading results stiffness after fast loadings. The residual strains were minimal in most cases showing a tendency to decrease over a period of approximately 15 minutes. 4.1.4 Pseudo Cyclic Loading Test This test was conducted with the purpose of examining the behavior of the structural packages for phase lag during loading and after removing the applied load. Two pseudo cyclic tests consisting of 10 cycles with rates of loading and unloading of +/-22 KN/min (+/-5000 lbs/min) and +/-6 KN/min (+/-1250 lbs/min) were applied on each specimen’s side. In all tests, the dispersion of the peak strains was minimal (i.e., below 3 PH), demonstrating that the strain phase lag was negligible. When comparing both tests, the peak strains differed by up to 10 PH; relatively high peak strains were obtained in the test with fast loading (i.e., +/-22 KN/min). Higher strains were associated to the higher rate of loading. In addition, the residual strains were also assessed and in all cases, the strain recovery was impending demonstrating that the FBG packages have a viscoelastic response. 4.1.5 Heat and Sustained Loading Test The specimens were subjected to a combined heat and sustained loading for 24 hours to evaluate the viscoelastic behavior of the FBG packages due to both effects during and after loading. The specimens were confined in a heat box in which the temperatures were increased from ambient laboratory current conditions to approximately 49oC (120 oF). As observed in Figure 8, the external FBG strains varied in phase with the temperature fluctuations. Internally, the FBG packages lagged behind the external temperature changes. In the absence of the internal thermocouples, the internal FBG strains were not evaluated for temperature correlation. A linear regression analysis was completed to assess the relationship between external strain and temperature. The quality of the

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Heat Box Temperature [oF] Strains [PH]

[PH] [N]

Load [lbs]

o

[ C]

linear model was determined through the associated R2 coefficients. These coefficients varied from 150 65.6 0.25 to 0.97, indicating that Temperature at External Laminates 125 51.7 strain data were impacted In tension 100 37.8 by the temperature In compression 75 23.9 fluctuations and creep. 50 10.0 After loading, the residual strains were above 80 PH. 500 500 Structural FBG After testing, each package Sensor Package 400 400 Ext. FBG 1 was visually examined to Int. FBG 1 300 300 detect any physical Int. FBG 2 Int. FBG 2 deterioration. In Specimen 200 200 1: Side 1 1, elevated temperatures 2: Side 2 100 100 above 66oC (150oF) were 0 0 accidentally applied. In -100 -100 this specimen, one package delaminated. In general, -200 -200 the FBG sensor packages had a viscoelastic behavior; 3000 13345 Heat and Sustained Loading 8896 2000 reduction of higher strain Load 4448 1000 levels after unloading and 0 0 returning the specimen to 0 20000 40000 60000 80000 100000 ambient temperatures Time [sec] confirmed the creep Figure 8. Temperature and Sustained Loading Tests recovery.

Cold Box Temperature [oF] Strains [PH]

[PH] [N]

Load [lbs]

o

[ C]

4.1.6 Cold and Sustained Loading Test The effect of cold temperatures and sustained loading were assessed to determine the viscoelastic behavior of the structural packages. The specimens were placed in a cold box for reducing the temperature to around -18oC (0oF). However, after enclosing the specimen, the temperatures were uncontrollably lower during the first three hours and steadily stabilized later. The tensile bending 37.7 100 surface was cooled to Temperature at external laminates: 50 10.0 temperatures near -18 C 1: In compression -17.7 0 (0oF), while the compressive 2: In tension -45.4 -50 bending surface was -73.1 -100 subjected to temperatures 500 500 that initially were lower than Structural FBG Sensor Package -46oC (-50oF) and increased 250 250 Ext. FBG 1 to approximately 10oC Int. FBG 1 0 0 Int. FBG 2 (50oF). Difficulties of Ext. FBG 2 generating a constant cold -250 -250 1: Side 1 2: Side 2 flow were due to the dry ice -500 -500 instable conditions and its distribution inside the cold -750 -750 box. As a result, the initial -1000 -1000 flexural stiffness and -1250 -1250 physical properties of the specimens and/or structural 13345 3000 Cold and Sustained Loading packages may have varied. 8896 2000 Load As observed in Figure 9, 1000 4448 0 0 strain lags with respect to the 0 20000 40000 60000 80000 100000 cold temperature fluctuations Time [sec] were observed in all external and internal FBG packages. Figure 9. Temperature and Sustained Loading Tests The lag was associated to the inherent insulation and thermal properties of dry wood members.

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4.2 Non-Structural Packages’ Test One bending test was performed in five specimens instrumented with non-structural FBG sensors to investigate the embedding techniques by obtaining no strain in the FBG sensors. During loading and maintaining a constant load of 11 KN (2500 lbs), the FBG sensors registered strains that were consistently less than 10 PH. Only one sensor had strain levels equivalent to the structural internal FBG sensor packages denoting an error in the non-structural package application. The source of error was attributed to the package adhesive that may have bled in the recess area and partially attached the sensor to the recess area. Among five non-structural package types, four had negligible strain levels demonstrating the effectiveness of installation techniques.

5.

Discussion and Conclusions

In general, the installation techniques for FBG sensors with and without physical attachment to the wood laminates using structural and non-structural packages were satisfactory. The developed embedding techniques were proven to be adequate at the laboratory level; however, the application of these techniques must be developed at the manufacturing level before being implemented at widespread scale. Regarding the laboratory testing over the short term loading, the structural FBG sensor packages demonstrated to perform within the tolerances of theoretical values (beam theory) and the foils strain gages and strain transducers’ response. Note that all small specimens were subjected to the same bending tests, while varying the duration of the load, rates of loading, cyclic loadings and ambient temperatures. In the short term bending tests, strain levels in all packages were consistent and in the order of the basic bending test results. During and after loading, strain levels were influenced by both creep and temperature fluctuations in a lower or higher degree depending on the duration of the applied load and the imposed temperatures. In all tests, after unloading, the presences of residual strains and the imminent strain recovery over time demonstrated the viscoelastic behavior of the structural FBG sensor packages inherent of the constituent packages materials and specimens. With the exception of one package, the developed non-structural FBG sensor packages and embedding techniques were proven to isolate the FBG sensor from strain response. Only one bending test was conducted to verify the no strain in the FBG sensors; however, these techniques are required to be tested under different loading conditions to demonstrate that the non-structural response would be registered by the FBG sensors.

6.

Acknowledgements

The authors would like to acknowledge the support obtained from USDA, Forest Product Lab, especially to Mr. Michael Ritter as well as ALAMCO for its involvement and contribution to this investigation.

7.

References

[1]

Phares, B., Wipf, T., Deza, U. (2007) “A 5-year Research Plan for the Development of a Smart Glue-laminated Timber Bridge” Bridge Engineering Center – CTRE – Iowa State University. Intelligent Sensing for Innovative Structures. (2001) “Guidelines for Structural Health Monitoring – Design Manual No. 2”. University of Manitoba, Winnipeg, Manitoba 2001. Ritter, M. A. (1992) “Timber Bridges: Design, Construction, Inspection and Maintenance”. United States Department of Agriculture, Forest Service, Washington, DC, 970 pp. Doornink, J. (2006) “Monitoring the Structural Condition of Fracture-critical Bridges using Fiber Optic Technology”. Doctor of Philosophy’s Dissertation, Iowa State University, Ames, Iowa 2006. American Standards for Testing Materials ASTM 198-05a. (2005) “Standard Tests Methods of Static Tests of Lumber in Structural Sizes” ASTM International, West Conshohocken, PA 19428-2959. American Association of State Highway and Transportation Officials. (2005) “AASHTO

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