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Strength grading of structural timber and EWP laminations of Norway spruce - Development potentials and industrial applications
SP Wood Technology, SP Technical Research Institute of Sweden, Videum Science Park, SE-351 96 Växjö.ORCID iD: 0000-0002-8513-0394
2014 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Machine strength grading of structural timber is a sawmill process by which considerable value is added to sawn products. The principle of such grading is that the strength of a timber member is predicted on the basis of a so called indicating property (IP) which, in general, represents an averaged value of the modulus of elasticity (MOE) measured over a board length of about one meter or more.

A limitation of today’s grading methods is that the accuracy of strength predictions is often rather poor, which results in a low degree of utilization as regards structural potential of sawn timber. However, it has for many years been well known to researchers that much better strength predictions can be made by using localized MOE values, determined over a very short length, as IP. Still, the determination of such values in a sawmill production environment has been technically very difficult to achieve.

In the research presented in this thesis, dot laser scanning with high resolution was utilized for detection of local fibre orientation on the surfaces of timber members. Since wood is an orthotropic material with superior structural performance in the longitudinal fibre direction, information about fibre orientation was, in combination with beam theory and measured wood material properties, used to determine the bending MOE variation along boards. By application of an IP defined as the lowest MOE found along a board, more accurate strength predictions than what is obtained by common commercial grading techniques was attained.

The thesis also involves flatwise wet gluing of Norway spruce side boards into laminated beams. As side boards, being cut from the outer parts of a log, have excellent structural properties it was not surprising to find that the beams had high strength and stiffness, even when laminations of sawfalling quality were used. The possibility of grading boards in a wet state by means of axial dynamic excitation was investigated with a positive result and application of simple grading rules resulted in considerable improvement of beam bending strength. Finally, bending MOE variation determined on the basis of laser scanned fibre directions was used for identification of weak sections in laminations. Elimination of such sections by means of finger jointing showed that average lamination strength of a board sample could be improved by more than 35 percent.

Abstract [sv]

Hållfasthetssortering av konstruktionsvirke är en sågverksprocess som innebär att värdet av det sågade virket ökar väsentligt. Principen för denna typ av sortering är att styrkan hos ett virkesstycke predikteras med utgångspunkt från en så kallad indikerande egenskap (IP) som oftast representeras av medelvärdet av elasticitetsmodulen (E-modulen) mätt över en sträcka av minst en meter utmed virkesstyckets längd.

De sorteringsmetoder som används idag ger ofta prediktioner med relativt låg noggrannhet, vilket innebär att endast en begränsad del av det sågade virkets konstruktiva potential kan utnyttjas. Det är dock väl känt att avsevärt bättre prediktioner kan erhållas genom att använda en lokal E-modul, uppmätt över en mycket kort sträcka, som IP. I dagsläget saknas dock teknik för att kunna bestämma ett sådant lokalt värde vid produktionshastighet i ett sågverk.

I den forskning som presenteras i denna avhandling har punktlaserskanning med hög upplösning använts för att bestämma fiberriktningens variation på ytorna av virkesstycken. Eftersom trä är ett ortotropt material med högst styvhet och styrka i longitudinell fiberriktning ger skanningsresultaten värdefull information om hur dessa egenskaper varierar längs en planka. Genom att kombinera informationen om fiberriktning med uppmätta virkesegenskaper och klassisk balkteori, kan böjstyvhetens variation utmed en planka beräknas med hög upplösning och därefter omräknas till en E-modul i böjning. Med en IP definierad som det lägsta värdet på nämnda E-modul utmed en planka kan en högre noggrannhet i prediktionen av hållfasthet uppnås, jämfört med vad som kan erhållas med dagens sorteringsmetoder.

Avhandlingen omfattar också limträbalkar tillverkade av sidobräder av gran limmade i rått tillstånd. Eftersom sidobräder sågas från de yttre delarna av en stock har de vanligtvis utmärkta konstruktiva egenskaper. Det var därför inte förvånande att balkarna uppvisade hög styrka och styvhet, även i de fall lamellerna var av sågfallande kvalitet. Möjligheten att med hjälp av axiell dynamisk excitering sortera sidobräder i rått tillstånd undersöktes med positivt resultat och genom att använda sorterade lameller kunde balkarnas styrka förbättras avsevärt. Den ovan beskrivna metoden att med utgångspunkt från bl.a. skannade fibervinklar bestämma styvhetens variation längs virkesstycken utnyttjades sedan för att identifiera svaga snitt i lameller av sidobräder. Genom att eliminera sådana snitt med hjälp av fingerskarvning kunde medelhållfastheten för ett stickprov sidobrädor höjas med mer än 35 %.

Place, publisher, year, edition, pages
Växjö: Linnaeus University Press, 2014. , 155 p.
Series
Linnaeus University Dissertations, 170/2014
Keyword [en]
fibre angle, grain angle, knots, laser scanning, machine strength grading, modulus of elasticity, side boards, strain, structural timber, wet gluing, wood
National Category
Building Technologies
Research subject
Technology (byts ev till Engineering), Civil engineering
Identifiers
URN: urn:nbn:se:lnu:diva-40319OAI: oai:DiVA.org:lnu-40319DiVA: diva2:790042
Public defence
Södrasalen (M1083), Hus M, Växjö, Lückligs plats 1, Växjö (English)
Opponent
Supervisors
Available from: 2015-02-23 Created: 2015-02-23 Last updated: 2016-12-15Bibliographically approved
List of papers
1. Strain fields around knots in Norway spruce specimens exposed to tensile forces
Open this publication in new window or tab >>Strain fields around knots in Norway spruce specimens exposed to tensile forces
2012 (English)In: Wood Science and Technology, ISSN 0043-7719, E-ISSN 1432-5225, Vol. 46, no 4, 593-610 p.Article in journal (Refereed) Published
Abstract [en]

Two-dimensional strain fields around knots in two Norway spruce specimens subjected to tension loading were detected using a contact-free measuring technique based on white-light digital image correlation. The first specimen included a traversing edge knot and the second one a centric knot. The development of strain fields as function of load level were measured by consecutive cyclic load tests in which one side of a specimen was studied during each test. The objectives were to examine to what extent the strain fields could be detected, to investigate the correlation between strain fields measured on different sides of a specimen, and to analyse the strain distributions around the knots. The results show that the applied technique is very useful for catching both overall and detailed information about the behaviour of knots in wood members exposed to loading. Clear wood defects that could not have been detected by neither visual inspection nor scanning were observed and conclusions could be drawn regarding release of internal stresses. The correlations between strain fields on different sides of the specimens were excellent and the correspondence between measurement results and comparative finite element calculations was surprisingly good, considering the fact that the employed FE models were fairly simple.

Place, publisher, year, edition, pages
München: Springer, 2012
Keyword
knots, contact-free, strain measurement, strain distribution, wood
National Category
Building Technologies
Research subject
Technology (byts ev till Engineering), Civil engineering
Identifiers
urn:nbn:se:lnu:diva-10185 (URN)10.1007/s00226-011-0429-8 (DOI)2-s2.0-84863727713 (Scopus ID)
Note

online first 27/5-2011

Available from: 2011-01-18 Created: 2011-01-18 Last updated: 2017-12-11Bibliographically approved
2. Prediction of timber bending strength and in-member cross-sectional stiffness vartiation on basis of local wood fibre orientation
Open this publication in new window or tab >>Prediction of timber bending strength and in-member cross-sectional stiffness vartiation on basis of local wood fibre orientation
Show others...
2013 (English)In: European Journal of Wood and Wood Products, ISSN 0018-3768, E-ISSN 1436-736X, Vol. 71, no 3, 319-333 p.Article in journal (Refereed) Published
Abstract [en]

Machine strength grading of structural timber is based upon relationships between so called indicating properties (IPs) and bending strength. However, such relationships applied on the market today are rather poor. In this paper, new IPs and a new grading method resulting in more precise strength predictions are presented. The local fibre orientation on face and edge surfaces of wooden boards was identified using high resolution laser scanning. In combination with knowledge regarding basic wood material properties for each investigated board, the grain angle information enabled a calculation of the variation of the local MOE in the longitudinal direction of the boards. By integration over cross-sections along the board, an edgewise bending stiffness profile and a longitudinal stiffness profile, respectively, were calculated. A new IP was defined as the lowest bending stiffness determined along the board. For a sample of 105 boards of Norway spruce of dimension 45 × 145 × 3600 mm, a coefficient of determination as high as 0.68-0.71 was achieved between this new IP and bending strength. For the same sample, the coefficient of determination between global MOE, based on the first longitudinal resonance frequency and the board density, and strength was only 0.59. Furthermore, it is shown that improved accuracy when determining the stiffness profiles of boards will lead to even better predictions of bending strength. The results thus motivate both an industrial implementation of the suggested method and further research aiming at more accurately determined board stiffness profiles.

Place, publisher, year, edition, pages
Springer, 2013
Keyword
machine strength grading, bending stiffness, laser scanning, fibre angle, grain angle, wood, structural timber, lumber, dynamic stiffness, MOE
National Category
Materials Engineering
Research subject
Technology (byts ev till Engineering), Civil engineering
Identifiers
urn:nbn:se:lnu:diva-23846 (URN)10.1007/s00107-013-0684-5 (DOI)000317977800004 ()2-s2.0-84882925541 (Scopus ID)
Available from: 2013-01-29 Created: 2013-01-29 Last updated: 2017-12-06Bibliographically approved
3. Localized modulus of elasticity in timber and its significance for the accuracy of machine strength grading
Open this publication in new window or tab >>Localized modulus of elasticity in timber and its significance for the accuracy of machine strength grading
2014 (English)In: Wood and Fiber Science, ISSN 0735-6161, Vol. 46, no 4, 489-501 p.Article in journal (Refereed) Published
Abstract [en]

From previous research, it is well known that a localized modulus of elasticity (MOE) is a better indicating property (IP) of strength than an MOE averaged across a longer span. In this study, it was investigated to what extent the relationship, in terms of coefficient of determination (R2), between strength and localized MOE was dependent on the length across which the MOE was determined. Localized MOE was calculated with MOE profiles based on dot laser scanning of fiber directions, axial dynamic excitation, and a scheme of integration across a board's cross-section. Two board samples were investigated. Maximum R2 values, which were as high as 0.68 and 0.77, respectively, were obtained for localized MOE determined across lengths corresponding to about half the depth of the investigated boards. Consequently, application of a highly localized bending MOE as an IP will result in very competitive grading.

Place, publisher, year, edition, pages
Society of Wood Science and Technology, 2014
Keyword
fibre angle, indicating property, laser scanning, machine strength grading, modulus of elasticity, strain fields
National Category
Building Technologies
Research subject
Technology (byts ev till Engineering), Civil engineering
Identifiers
urn:nbn:se:lnu:diva-40316 (URN)000343355600007 ()2-s2.0-84946014778 (Scopus ID)
Funder
VINNOVA
Available from: 2015-02-23 Created: 2015-02-23 Last updated: 2017-12-04Bibliographically approved
4. Strength grading of narrow dimension Norway spruce side boards in the wet state using first axial resonance frequency
Open this publication in new window or tab >>Strength grading of narrow dimension Norway spruce side boards in the wet state using first axial resonance frequency
Show others...
2011 (English)In: International Wood Products Journal, ISSN 2042-6445, Vol. 2, no 2, 108-114 p.Article in journal (Refereed) Published
Abstract [en]

Strength grading of Norway spruce (Picea abies (L.) Karst.) side boards in the wet state was investigated. For a sample of 58 boards, density and dynamic modulus of elasticity in the axial direction, MOEdyn, were determined in the wet state. The boards were then split into two parts and the procedure of determining MOEdyn was repeated both before and after the boards were dried to a target moisture content of 12 %. Finally, tensile strength of the split boards was measured and its relationship to MOEdyn for both wet and dried split boards determined. The investigation also included an evaluation of a so called reversed lamination effect on the stiffness caused by the splitting of boards into two parts. The results show that strength grading of split boards in the wet state can give just as good results as grading performed after drying. The reversed lamination effect on the stiffness of split boards was found to be of lower order.

Place, publisher, year, edition, pages
Leeds/London United Kingdom: Maney Publishing, 2011
Keyword
strength-grading, axial stiffness, side boards, wet state, green state
National Category
Building Technologies
Research subject
Technology (byts ev till Engineering), Civil engineering
Identifiers
urn:nbn:se:lnu:diva-17871 (URN)10.1179/2042645311Y.0000000015 (DOI)
Note

Published online first 1/11-2011

Available from: 2012-03-06 Created: 2012-03-02 Last updated: 2016-12-15Bibliographically approved
5. Green-Glued Products for Structural Applications
Open this publication in new window or tab >>Green-Glued Products for Structural Applications
2014 (English)In: Materials and Joints in Timber Structures: Recent Developments of Technology, Springer, 2014, 45-55 p.Conference paper, Published paper (Refereed)
Abstract [en]

The results from bending tests on 107 laminated, green-glued, beams manufactured from Norway spruce side boards are presented. The beams were made by face gluing 21-25 mm thick boards using a commercial one-component moisture curing polyurethane adhesive. In addition to the bending test results, results from shape stability measurements after climatic cycling and bond line strength and durability test results are also presented. The results from the bending tests show that, by applying very simple grading rules, it is possible to obtain beams with high bending strength (with a 5%-percentile characteristic value of 40,1 MPa) and substantial stiffness (mean value of 14360 MPa). Also the shape stability of the beams and the strength and the durability of the interlaminar bonds were found to be satisfactory.

Place, publisher, year, edition, pages
Springer, 2014
Series
RILEM Bookseries, ISSN 2211-0844 ; 9
Keyword
green gluing, glulam, bond line shear strength, durability, adhesive bonds
National Category
Building Technologies
Research subject
Technology (byts ev till Engineering), Civil engineering
Identifiers
urn:nbn:se:lnu:diva-32453 (URN)10.1007/978-94-007-7811-5_4 (DOI)000329970600004 ()2-s2.0-84885443561 (Scopus ID)978-94-007-7810-8 (ISBN)
Conference
RILEM International Symposium on Materials and Joints in Timber Structures, OCT 08-10, 2013, Stuttgart, GERMANY
Available from: 2014-02-24 Created: 2014-02-24 Last updated: 2016-12-15Bibliographically approved
6. Improving Strength of Glulam Laminations of Norway Spruce Side Boards by Removal of Weak Sections Using Optimized Finger Jointing
Open this publication in new window or tab >>Improving Strength of Glulam Laminations of Norway Spruce Side Boards by Removal of Weak Sections Using Optimized Finger Jointing
2014 (English)In: Materials and Joints in Timber Structures: Recent Developments of Technology / [ed] Simon Aicher; H.-W. Reinhardt ;Harald Garrecht, Springer, 2014, 801-811 p.Conference paper, Published paper (Refereed)
Abstract [en]

Recent research has shown that glulam laminations of Norway spruce side boards possess excellent structural properties. This investigation concerns the possibility of improving the performance of such laminations through elimination of weak board sections by means of finger jointing. Sections to be removed were identified using profiles of edgewise bending stiffness determined on the basis of scanned fibre angle fields on board surfaces. The difference in average tension strength and average tension stiffness, respectively, between a group of finger jointed boards and a reference group of non-jointed boards was evaluated. Joints were inserted in the first group with an average distance of 2.4 m. It was found that the finger jointing gave a considerable increase of strength (36 %), whereas the stiffness improvement was not as evident. Based upon the results, it can be assumed that application of finger jointed side board laminations will result in glulam beams with very high strength.

Place, publisher, year, edition, pages
Springer, 2014
Series
RILEM Bookseries, ISSN 2211-0844 ; 9
Keyword
Dot laser, fuinger jointing, glulam, scanning, side boards
National Category
Building Technologies
Research subject
Technology (byts ev till Engineering), Civil engineering
Identifiers
urn:nbn:se:lnu:diva-33062 (URN)10.1007/978-94-007-7811-5_71 (DOI)2-s2.0-84885462961 (Scopus ID)978-94-007-7810-8 (ISBN)
Conference
Rilem - Materials and Joints in Timber Structures
Available from: 2014-03-19 Created: 2014-03-19 Last updated: 2016-12-15Bibliographically approved

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