AN INTEGRAL PRODUCTION CHAIN TO RELIABLY PRODUCE GLUED LAMINATED TIMBER

AN INTEGRAL PRODUCTION CHAIN TO RELIABLY PRODUCE GLUED LAMINATED TIMBER Christophe Sigrist 1, Martin Lehmann ABSTRACT: Glue laminated timber (GL) is a commodity product today. The quality and strength of the finished product depend on many parameters: strength class of individual boards composing GL, strength of finger joints, face gluing of the laminates and on the production process itself comprising planning, pressing curing and more. The product ranges from industrial, homogeneous low strength gluelam from visually graded timber to high strength gluelam using inhomogeneous layups formed by machine graded timber. Tests on high strength gluelam (GL3 and GL36) in various countries showed however that the strength and stiffness properties stated in the today s design codes are overestimated. In a new standard regulating the properties and minimal production requirements (pren 14080) for gluelam, the strength properties for GL are lowered on one side and the proprieties regarding the strength of the boards and the finger joints are increased on the other side. The program presented here mainly deals with the strength properties of GL produced from visually graded timber (maximum GL8) as SME may not be in the position of acquiring the expensive installations to machine grade the timber. In order to optimise the flow of material, to carry out the adequate grading process at the right moment, the saw millers and the producers of GL work out a common process along the production line in order to become more competitive on the market. Extensive tests on ungraded and graded boards, finger joints and gluelam beams are carried out and verified by computer models in order to support the actual requirements regarding production and GL properties. KEYWORDS: Strength and stiffness of gluelam, board properties, production process, visual grading, modelling 1 INTRODUCTION 13 The project develops an integral production method for gluelam and is based on a close collaboration within the timber processing industry. The goal is to reduce grading / production costs, to optimise the utilisation of the resource and to guarantee the strength and stiffness properties of both the individual boards used for the production of gluelam and the final product. Individual grading steps specific to the players involved allow placing high and low quality timber at the correct position in the gluelam beam. The focus lies on visual grading methods. However all investigations are backed up using several non destructive, machine grading systems in order to find out if the visual grading could eventually be improved by the usage of simple and efficient grading tools. The results obtained from testing 1 Christophe Sigrist, Bern University of Applied Sciences, Architecture, Wood and Civil Engineering BFH-AHB, Solothurnstrasse 10, P.O.Box, CH-500 Biel-Bienne 6, Email: christophe.sigrist@bfh.ch Martin Lehmann, Bern University of Applied Sciences, Architecture, Wood and Civil Engineering BFH-AHB, Solothurnstrasse 10, P.O.Box, CH-500 Biel-Bienne 6. Email: martin.lehmann @bfh.ch are verified by computer modelling. All mayor sawmills, gluelam producers from Switzerland and three producers of grading chains from Europe are involved in the project. Ten gluelam producers in Switzerland currently produce about 45 to 50 000 m 3 of gluelam per year. The largest output by one company is about 15 000 m 3. About 80% of the gluelam corresponds to the standard grade GL 4 according to the design code SIA 65: 01. Some of the innovative companies also occasionally produce high strength GL 3 and GL 36. All these SME carry out the required factory controls and are audited. Some of the companies exclusively use Swiss timber; only about 0% of the required boards are imported from surrounding countries. All but one producer participate in the project. The association of the Swiss timber industry encloses about 350 sawmills which process about.5 million m 3 of saw logs per year. Most of the extremely small mills produce construction timber for the local building market. The 9 largest mills involved in the project are responsible for about 40% of the processed volume and provide the boards for the gluelam production. Compared to international standards the mills are small and process about 00 000 m 3 yearly at most. For this reason and as next to boards many other products are

sawn the mechanical grading of timber represents a costly step in the production chain. Therefore to date most of the timber is graded visually. In special cases simple grading devices (hand tools) are used to replace or backup visual grading. As the (international) market increasingly asks for stress graded timber the potential and suitability of various grading machines / grading chains for SME is also investigated and supported by additional partners. STATE OF THE ART KNOW-HOW Investigations by Gloss [1] have indicated that the separation of lighter heart-in material and denser side boards leads to a better utilisation of the timber properties. Based on these results grading chains using a balance or other methods to determine the density of the timber were developed. The strength properties of gluelam primarily depend on the strength properties of the boards to be used, the coefficient of variation of these properties and the position of such boards within the gluelam beam. The combined results and conclusions by Schickhofer [] from investigations on gluelam over the last decades in Europe are presented in the simplified Figure below. Visually graded timber presents a higher coefficient of variation (35% +/- 5%) than mechanically graded timber (5% +/- 5%). Also visually graded timber is generally sorted into a smaller number of grades than if mechanical grading is applied. As the visual grading is less precise both low and high quality bards will be found within an attributed stress grade. Due to this effect and according to the simplified representation of the problem in the graph below, boards with lower characteristic strength properties in tension (f t,k 16 to 17 N/mm ) compared to mechanically stress graded timber (f t,k 0 N/mm ) could be used to obtain a desired strength for a gluelam beam. The graph shows the example of GL8 with a characteristic target bending strength of f m,k = 8 N/mm. The higher system effect on material showing higher variation further increases this effect. Biegefestigkeit Biegeträger [N/mm ] 34 3 30 8 6 4 0 18 10 1 14 16 18 0 Zugfestigkeit Lamelle [N/mm ] visuell maschinell Figure 1: Simplified representation of strength models for GL from visually (blue) and mechanically (red) graded timber (tensile strength of boards on abscise, bending strength of gluelam on ordinate) The above result is of particular interest to SME producing GL who generally process smaller amounts of timber than the large, industrial counterparts. A focused and simple grading for the gluelam factory depending on the timber quality delivered by the saw mill must still be determined. The efficiency of the grading depends on the interaction between the different processing industries. 3 MOTIVATION 3.1 STAGE BY STAGE GRADING OF BOARDS The experience shows that timber may be visually graded quite easily if a pre-assessment has taken place at an earlier stage. An ideal situation in view of further processing would be to divide the timber in two to or maximum three quality classes at an early stage of the production chain (saw mill). Such a separation may occur when processing the logs in considering the position of the board within the log (heart-in material, free of heart material), the density of the boards or other easy to assess characteristics. In order to simplify the process or to increase the efficiency of such an operation simple and reliable tools or machinery which may be installed at low cost in the mill are investigated. Focusing on single measurements the costs for such an installation may be kept at a minimum. Such a procedure does not focus on the attribution of a (precise) stress grade to individual boards, but focus on an initial distinction of the quality of boards and should lead to a more efficient process within the production chain at a later stage. Machine grading of boards may be installed at any time and the benefits (and costs) are known to the saw milling industry. The better a pre-assessment in the primary processing step in separating weak boards form boards with potentially high strength properties the simpler and faster the grading process in the gluelam factory can be carried out. The grading process at this stage then concentrates on specific parameters which are relevant for the strength of the final product only and will speed up the production process in the gluelam factory. Less docking and loss of timber is also expected. The grading process in the gluelam factory as well as the sawing strategy and primary break down has to be linked. 3. UTILISATION OF THE RESOURCE The aim of an efficient grading is not only to separate high quality boards from bards with less strength potential. Even if high strength boards can be sold for a better price the saw mill has a strong interest to fully utilise the potential of the resource. Earlier investigations [3] on tension properties of boards from the local Swiss resource have clearly indicated that a large percentage of boards have a high potential regarding strength and stiffness to be assigned to a C30 strength class or better. Boards of lower quality or strength may efficiently be used for certain products (cross laminated timber) or in the inner zone of a combined gluelam beam. Reduced strength and stiffness properties of boards and finger joints are required and larger defects are tolerated. This leads to less docking and less waste as often full length boards can be processed.

3.3 GUARANTEED GLUELAM QUALITY The gluelam producer must guarantee the strength and stiffness of his product. Earlier investigations on strength properties of C4 boards visually graded during the production process of gluelam fabrication [3] have indicated that the required strength properties are at times difficult to be reached which lead to various actions regarding factory control and auditing. The changes of the requirements regarding production and properties of GL according to pren 14080 led to a re-assessment of the state of the art of production of gluelam in Switzerland. The increased requirements regarding the properties of the outer laminates in combined GL8 products may now be critical if the boards are visually graded. As a matter of fact the required strength class C35 (f t,k = 1 N/mm ) can so far not be obtained by visual grading when applying DIN 4074-1 used in Switzerland. The investigations address the following issues: to demonstrate sufficient strength and stiffness properties of GL4 and GL8 produced from visually graded timber to demonstrate the applicability of the currently used standard to visually grade timber to reach the required strength properties of the boards and the gluelam alternatively to revise certain grading criteria or to use grading tools suitable to the company size in Switzerland to strengthen confidence in glued laminated products in general to assure compliance with the latest international requirements regarding properties, quality and conformity. 3.4 COLLABORATION WITHIN THE PRODUCTION CHAIN The close collaboration between saw millers (primary industry) and gluelam producers (secondary industry) will be enhanced by this project. As the requirements regarding quality will be well defined unsuitable material will not be delivered. Less time and money will be lost and the quality of work and product will generally increase. 4 WORK ITEMS The investigations are still running. The goal is to involve all partners to an equal part. The focus lies on one side on the board properties and grading options in the saw mill, on the other hand on the properties of gluelam beams produced from these boards and the duties attached to this. In any case all material will be randomly selected and samples of sufficient size in view of statistical evaluation will be provided. The saw mills involved allow covering most of the Swiss resource in terms of provenance. Working through the various work items revealed after an initial phase that in particular the estimated number of boards to be provided by the mills was set too high to be handled. Therefore the number of boards to be provided was adapted to the capacity / size of the mills and the number of test specimen reduced. The number, strength classes and size of gluelam beams to be provided by the gluelam producers were also slightly adapted respecting the capacity of the production and the main products in terms of dimension and quality the producer in question usually provides. The initial procedure and idea presented below is however still respected. 4.1 PRIMARY RESOURCE Every saw mill provides standard board material. Typical log diameters and sawing patterns are considered, no special procedures are to be adopted. Before sawing the centre part of the logs was marked in colour in view of distinguishing properties form heart-in material and side boards and to study the effect of sawing patterns and log diameters / provenance. Figure : Marking of board position In a first step about 300 boards are graded using hand tools (frequency, frequency and density, ultrasonic velocity, penetration) and an accredited grading chain. The following apparatus for the non destructive evaluation NDE have been considered: Viscan+ (MiCROTEC Innovation wood): the frequency measurement is based on an optical signal, no contact with the timber surface is required; for grading the frequency measurement is combined with a density measurement Timbergrader MTG ( Broohuis Micro-Electronics): the frequency measurement is based on stress waves and contact to the timber surface is required; for grading the frequency measurement is combined with the density measurement Silvatest (CBS-CBT): elastic waveforms at an ultrasonic frequency (50-100 KHz) are detected by a receiver and the runtime is measured; for grading the calculated ultrasonic velocity is combined with an estimation of the density Pilodyn 6J (Proceq): Penetration measurement at a given load to estimate the timber density Goldeneye 70 (MiCROTEC Innovation wood): laser scanner to detect timber defects and x-ray scanner for density estimation (industrial grading chain). The measurements have been extended to fresh sawn timber in some saw mills in the mean time. All these NDE measurements allow the characteristics of the resource to be controlled and the samples for further investigations to be drawn. 10% of the boards totally provided are reserved for tension tests, and another 10% of the boards find their way into the gluelam production.

In terms of visual grading the main focus is laid on single knots and knot clusters. The knot area ratio (KAR) of the first 3 predominant defects is recorded and mapped. Additionally a visual appearance grade (not structural grade) has been attributed by the saw mill following established criteria. Tension tests according EN 408 are then carried out using an accredited 850 KN tension machine. Finally a huge data base for more than 1300 tested boards will be available for further processing, checking existing grading rules or developing new grading routines considering visual grading only or combining visual and machine grading. Firstly a purely visual grading routine will be confirmed / adapted. Secondly suitable machine measurements in order to initially assess the quality potential of a board combined with a coarse visual override applying a minimum number of criteria will be addressed. Based on the data it will be possible to assess the suitability of grading chains as well. 4. GLUELAM As the production of the beams is closely followed by the research team the precise position of most boards within the beam, and in particular in the tension zone, is known. Due to the sampling procedure the mechanical and physical properties of the input material is known and the strength and stiffness potential may be retraced. Marking the boards by RFID to trace back the original properties of the boards has also been tried out. The timber is graded before or during production into T11, T14 and T18 or better material. The focus lies on GL4h and GL8c. Every gluelam producer fabricates about 8 test beams (160 mm / 600 mm / L=1 m). As many beams of the highest quality as possible are fabricated according to the available resource. With the remaining boards the standard quality GL4 is provided. In special cases beams from machine graded boards (hand tool) have also been produced. Finally the results from more than 50 full size beams tested according to EN 408 in four point bending will be ready for analysis. The test results will be backed up by modelling in order to demonstrate that the procedure fulfils the requirements of pren 14080. 4.3 PRODUCTION CHAIN The implementation of the findings in order to establish a well functioning collaboration within the production chain represents an important part of the current investigations. Guidelines and other measures will be developed in order to guarantee fabrication and factory control at various relevant production stages. The tasks within the different companies have to be described, assigned and implemented. The procedure and the findings of the research program that are communicated to the industry undergo continuous adaptations and improvements as a step by step procedure has been retained. Training of staff and production personnel takes place on regular bases. In a first step the overall process, the feasibility of the investigation and the suitability of the results has been confirmed with two of the leading companies (one saw mill and one gluelam producer). In a second step the project has been extended to three additional pairs. In the near future the five remaining partnerships will bring in their share to the program. 5 RESULTS FROM BOARD TESTING 5.1 TENSION PROPERITES OF VISUALLY GRADED BOARDS The analysis targets S10 (C4) and S13 (C30) boards. The goal is to derive a method to pre-sort the boards and to then achieve the required properties for the material to be used to produce GL according pren 14080 when grading the timber during production. The results from visual grading according to DIN 4074-1, an empirical separation of sawn bards into non structural appearance grades and grading using simple mechanical grading tools and a sophisticated grading chain (back up only) are presented here. The following intermediate results have been obtained: Large differences regarding strength and stiffness between various log diameter and log provenance were observed. Both small and large diameter logs are used to saw boards for the production of gluelam according to availability, season and order. It has been observed that some of the log sorts yield substantial better strength and stiffness properties. Due to the natural fluctuation of the resource it seems not to be reasonable / possible to distinguish between diameter and provenance to pre-sort the boards. No useable difference of strength properties between outer (free of heart) and inner (heart-in) material was observed. In particular in small diameter logs (red marking) where usually 4 to 6 boards (splitting of the heart) are obtained the difference of density from the center to the outer part of the log is too small to be used to pre-sort the boards. In some occasions and surprisingly for some of the larger log diameters the density decreased from the center to the side. Figure 3: Board density in function of the proportion of heart-in HI material (0%=outer board: <5% of colour marking) Slightly higher MOE for sideboards compared to heartin material was obtained in most cases. The sorting of boards by the saw mill into non structural grades (appearance) seems to be quite efficient and promising indicating that the required strength and stiffness properties may be achieved. The boards are classified being suitable for the production of beams with high requirements regarding appearance as they will remain exposed in a structure. The grading

criteria for such a pre-sorting focus on splits, deformation (spring, bow, twist), resin pockets, coloration and more. The timber industry however wishes not to mix appearance grades and strength classes. The strength class of C30 can be achieved following the grading rules of DIN 4074-1. If additionally all lateral knots / spike knots are removed the tension strength of this strength class may be further increased and the requirements for a T1 class are achieved. However recovery would be quite small. Previous to processing the boards have been assessed by at least one NDE method. Therefore the dynamic MOE, the density and the estimated strength class of every board is known. The production of the beam has been closely followed and the position of each board in every laminate has been recorded manually. The application of RFID in order to locate certain boards has also been tried out successfully. This valuable information is used to explain / understand failures of tested beams and to refine grading requirements on boards for laminates. Figure 4: Efficiency of grading timber pre-sorted according to appearance grades A1:low and A: high It is generally difficult to achieve the requirements for strength for C4 as low density / low stiffness boards can not be identified easily. 5. TENSION PROPERITES OF BOARDS GRADED BY SIMPLE HAND TOOLS Preliminary indications regarding machine grading by hand tools and considering one grading parameter only have been obtained so far analysing the first samples. The elimination of boards with very low densities (ρ 380 kg/m3) combined with a visual pre-sorting upon purely non structural criteria would allow to guarantee a C4 grade at least. 13% of the boards would have to be downgraded or rejected. Grading by MOE dyn allows distinguishing between stress grades C4 and C30. Setting the threshold at 11 500 to 11700 N/mm about 35% of the samples would lead to grade C35. About 10% of the C35 boards are obtained from a pre-sort not meeting the requirements regarding appearance grade. Only 7% of the boards would have to be rejected or assigned to a C16 stress grade. 6 PROPPERTIES OF GLUELAM BEAMS 6.1 PRODUCTION OF BEAMS To date 4 producers have produced 30 gluelam beams (160 mm / 600 mm / L=1 m). GL4 and GL8 were produced and tested using boards with comparable properties as tested on tension before. The board material for the production of the beams has been selected from the parent sample showing an identical stiffness distribution (Figure 5). Figure 5: Selected C30 boards in the tension zone of GL beams compared to overall board stiffness Various strategies have been applied to consider both the specific production of each factory and the potential of the resource which from mill to mill turned out to be quite distinct. Table 1: GL strength classes produced and applied grading strategy company GL n remarks 1 GL8k GL4h 8 boards visually graded by the institute GL8k GL4h GL4k 3 3 boards visually graded during production by company grader 3 GL36k GL3k GL8k GL4k boards graded before production using a hand tool (NDE) 4 GL4h 4 visual pre-sorting into two non structural grades 6. BENDING PROPERTIES OF GL4 AND GL8 All beams have been tested according to EN 408. The test setup is presented in Figure 6. Good results have been obtained from these tests. The so far limited number of tests, various individual approaches selected in order to satisfy the company s established way of producing gluelam and the large variability of the obtained bending stresses makes the statistical analysis difficult. Only one beam composed by visually graded boards (n=11) of the strength class GL8k failed at 5,3 N/mm below the characteristic value. Reasons for this failure

may be found when analysing the physical and mechanical properties of the boards as presented below. Bending strengths as high as 48,1 N/mm have been recorded for this strength class. The requirements of GL4 have been achieved (n=11) in all but two cases where failure has been obtained at stresses below the characteristic value. The detailed analysis of the boards composing the tension zone at the location of failure will in combination with the results from tension tests on boards give valuable information regarding visual grading criteria for C4 boards to be adopted in the future. The high bending values for GL4 which were achieved in most cases are remarkable. GL36. The stiffness of the two beams GL8 is slightly lower than the required mean MOE of 1 000 N/mm. In all critical cases regarding insufficient strength of the gluelam beam it can be demonstrated that the strength requirements of single boards have not been met by visual grading. The figure below shows the example of a GL8k beam. The density and MOE of the boards composing the three outer layers have been analysed in detail. The failure was triggered by a reasonably small edge knot with local fibre deviation at mid span. The bottom lamella had few and small defects in general. Considering KAR and local fibre deviation would allow attributing a C30 grade according to DIN 4074-1 to all visually graded individual boards composing the first lamella. This failure also demonstrates that any edge or spike knot must be avoided in the outer layers of GL8k built up by visually graded boards. Previously carried out NDE measurements on these boards indicate that either MOE, density or the combination of both would lead to a downgrade of these boards if mechanically graded and that they are not suitable to be placed in an outer lamella. Figure 6: Bending test on full scale gluelam beams according to EN 408 Figure 8: Analysis of failure of beams considering visual and mechanical board properties Figure 7: Strength and stiffness from bending test on full scale gluelam beams (GL) Tests on 8 beams composed by boards graded by a hand tool also fulfilled the requirements. Little difference regarding strength and stiffness is obtained for GL3 and 7 CONCLUSIONS First results from this research program are very promising. The investigations will be extended to the remaining industry partners. This will lead to statistically significant data. The findings on strength and stiffness properties of gluelam will be verified by modelling. The grading routines for visual grading only, visual grading supported by hand tools and machine grading in consideration of the yield in the stress classes attributed are being determined now. The benefit of machine grading the boards in the saw mill is obvious but only suitable for few mills (SME). Good ways of pre-sorting the timber and the implementation in small mills are still under discussion. The results from bending tests demonstrate that workmanship is high in Switzerland. All gluelam is produced according to the newest standards and the required properties are mostly met. The need of thorough and specific grading is well understood. Interestingly it appears to be easier to grade high strength boards than to achieve a C4 stress grade. According to the tests on boards and beams the knots on the narrow side of the boards are not tolerated in the outer laminates of GL8. The consequence is that such beams present a clear band

in the outer zones where as the inner zones may show many knots. ACKNOWLEDGEMENT The research is carried out thanks to funding from KTI / CTI, Switzerland, Project Nr. 9843.1 PFES-ES. REFERENCES [1] Glos P., Pahler A. (005). Chances of lumber quality grading. Proceedings of 5th COST Action E40 Conference on large diameter timber - problem or chance? HSB Biel, Switzerland, pages 9-33. [] Brandner, R.; Schickhofer, G. (007). Bearing model for glued laminated timber in bending new aspects concerning modeling. Proceedings of the COST E55 Workshop - Graz. (007), pages 1-31. [3] Sigrist C., Engels I. (004). Zugfestigkeit von BSH- Lamellen. Kontrolle der Wirksamkeit der visuellen Sortierung zur Erzeugung von BSH gemäss Entwurf SIA 65: Holzbau, Forschungsbericht Buwal 00.06.