Investigation into material resistance factors and properties of young, engineered Eucalyptus grandis timber

Pagel, Calvin (2019-04)

Thesis (MEng)--Stellenbosch University, 2019

Thesis

ENGLISH ABSTRACT: Only 1 % of South Africa’s total land area is used for forestry with 51 % of that being used for Pinus and 40 % for Eucalyptus species. A country wide shortage of adequate structural timber has been forecast for the near future where SA pine is the predominant structural timber species. Recent research has looked into Eucalyptus grandis to be used as a structural timber resource due to its fast growth rate and large plantation area. This hardwood has not traditionally been used structurally due to the physical defects commonly present in sawn, dried Eucalyptus grandis timber. The development of engineered timber products has been investigated as one way to mitigate some of these defects. Young Eucalyptus grandis which has been finger-jointed in the green state was recently introduced into the South African market. Tests completed by Crafford (2013) on the material showed promising mechanical strength results with low variability when compared to similar SA pine. This lead to the consideration of determining a specific material resistance reduction factor for use with the young Eucalyptus grandis as the current factor prescribed in the code is deemed to be overly conservative as it was calibrated for softwood timber. The finger-jointed timber had still experienced many physical defects, such as twist and checking. Face lamination of the material was thus proposed with the aim of reducing defects and to further decrease the mechanical strength variation. In this study two sets of samples were tested, a group of 100 standard green finger-jointed Eucalyptus grandis and a set of 100 finger-jointed and face laminated samples. This allowed for comparisons to be made to determine if the face lamination is a worthwhile inclusion and to produce material strength results to be used for partial factor determination. The face lamination was completed using a one-component polyurethane adhesive with timber in the wet, unseasoned state. Both sets were dried to equilibrium moisture content to assess physical defects, which are usually aggravated during drying. Four point bending tests were completed to determine the flexural characteristics, modulus of elasticity (MOE) and modulus of rupture (MOR), of the samples. Only flexural strength tests were completed due to the limited material available and as they are the indicator properties listed by the JCSS along with being the most important properties for roof truss design which is the predominant end use of structural timber in South Africa. Both sets performed well in terms of warp deformation with no cases above rejection limits for bow and cup. A total of 1 % of laminates had excessive twist compared to the 9 % of the standard boards. The standard set had significantly more check (20 %) and split (15 %) defect rejections compared to the 4 % for check a 0 % for split achieved by the laminated set. Structural grade S7 requirements were achieved for MOE and MOR of both sets. Only a small difference in coefficient of variation (COV) of approximately 1 % was recorded between the sets but a 12 % lower COV was achieved for the MOR of the sets compared to equivalent SA pine. The material resistance reduction factor calculated for MOR was found to be governing, with essentially equivalent factors for the laminates and standard boards of 0.776 and 0.769 respectively. It is thus proposed that a reduction factor of 0.77 be implemented in the code for the Eucalyptus grandis to be used instead of the current 0.68 which was originally devised for softwood timber. Although the lamination process did not reduce the variation in strength results to a significant extent, significantly lower defects were recorded for the laminated set. This would result in a higher yield of material with a better visual appeal which could allow for it to be a more valuable product. For both sets, the 0.09 gain in reduction factor may not seem to be a large advantage, but when coupled with the higher structural grade being achieved and the shorter rotation age than SA pine, the young, finger-jointed and face laminated Eucalyptus grandis is a promising option for structural timber use in a country as timber scarce as South Africa.

AFRIKAANSE OPSOMMING: Slegs 1 % van Suid-Afrika se totale grondoppervlakte word vir bosbou gebruik en daarvan word 51 % vir die Pinus spesie en 40 % vir die Eucalyptus spesie gebruik. ‘n Landswye tekort aan voldoende struktuurhout is voorspel vir die nabye toekoms waar Suid-Afrikaanse denne die oorheersende strukturele houtsoort is. Onlangse navorsing het Eucalyptus grandis as ‘n strukturele houtbron ondersoek as gevolg van dié spesie se vinnige groeikoers en die groot plantasie oppervlakte wat huidiglik beskikbaar is. Hierdie loofhoutspesie word nie tradisioneel vir strukturele doeleindes gebruik nie as gevolg van die fisiese afwykings wat algemeen voorkom in gesaagde en gedroogte Eucalyptus grandis hout. Die ontwikkeling van verwerkte houtprodukte is ondersoek as 'n manier om sommige van hierdie afwykings te verminder. Jong Eucalyptus grandis wat in die nat en onbehandelde toestand gevingerlas is, is onlangs aan die Suid-Afrkaanse mark bekendgestel. Toetse wat Crafford (2013) op die materiaal voltooi het, het belowende meganiese sterkte-eienskappe getoon met lae veranderlikheid in vergelyking met soortgelyke SA dennehout. Dit het gelei tot die oorweging om 'n spesifieke materiaal weerstand gedeeltelike-faktor te bepaal wat gebruik kan word vir jong Eucalyptus grandis aangesien die huidige faktor wat in die kode voorgeskryf word gekalibreer was vir naaldhout en dus te konserwatief is. Die gevingerlasde Eucalyptus grandis het steeds verskeie fisiese afwykings getoon, soos vervorming en oppervlakbarse. Platkant-laminering van die materiaal was dus voorgestel om fisiese afwykings te verminder en om meganiese sterkte variasie verder te verminder. In hierdie studie is twee stelle monsters getoets: 'n groep van 100 standaard nat en onbehandelde Eucalyptus grandis monsters en 'n groep van 100 gevingerlasde en platkantgelamineerde monsters. Dit het toegelaat dat vergelykings gemaak kon word om te bepaal of platkant-laminering 'n waardevolle insluiting is en om materiaalsterkte resultate te lewer wat gebruik kon word om die gedeeltelike-faktor te bepaal. 'n Eenkomponent poliuretaan gom is gebruik om hout in die nat en onbehandelde toestand te lamineer. Beide stelle is gedroog tot veselversadigingspunt om fisiese afwykings te evalueer, wat gewoonlik tydens die droog proses vererger word. Vierpunt buig toetse is voltooi om die buigkarakteristieke, modulus van elastisiteit (MOE) en breukmodulus (MOR) van die monsters te bepaal. Slegs buigsaamheidstoetse is voltooi as gevolg van die beperkte materiaal wat beskikbaar was en aangesien dit die aanwyser-eienskappe is wat deur die JCSS gelys word, asook die belangrikste eienskappe vir dakkappontwerp is - wat die oorheersende eindgebruik van strukturele hout in Suid-Afrika is. Beide stelle het goed presteer in terme van buigvervorming, met geen gevalle bo verwerpingsgrense vir boog en koppie vervorming nie. Slegs 1 % van die gelamineerde borde het oormatige vlakke van draai ervaar in vergelyking met die 9 % van die standaardborde. Die standaard stel het aansienlik meer oppervlakbarse (20 %) en spleting (15 %) afwykings gehad wat bo verwerpingsgrense geval het in vergelyking met die 4 % vir oppervlakbarse en 0 % vir spleting wat deur die gelamineerde stel bereik is. Strukturele graad S7 vereistes is behaal vir die MOE en MOR van beide stelle. Die koëffisiënt van veranderlikheid (KVV) van die twee stelle het met slegs 1 % verskil, maar die MOR van die stelle het 'n KVV van 12 % laer as ekwivalente SA dennehout verwerf. Daar is gevind dat die materiaal weerstand gedeeltelikefaktor vir die MOR beherend is, met byna ekwivalente waardes vir die gelamineerde- en standaardborde van onderskeidelik 0.776 en 0.769. Daar word dus voorgestel dat 'n gedeeltelike-faktor van 0.77 geïmplementeer word in die kode wat vir die Eucalyptus grandis gebruik word, in plaas van die huidige 0.68 wat oorspronklik vir naaldhout bepaal is. Alhoewel die lamineringsproses nie die variasie in sterkte tot 'n aansienlike mate verminder het nie, is aansienlik minder afwykings waargeneem by die gelamineerde stel. Dit lei tot 'n hoër opbrengs van materiaal, asook 'n materiaal wat visueel meer aantreklik is. Die gevolg is 'n produk wat pontensieel meer waardevol is. Dit mag dalk voorkom dat die 0.09 toename in die gedeeltelikefaktor van beide stelle nie 'n groot voordeel is nie, maar gepaard met die hoër strukturele graad wat bereik is en die korter rotasie ouderdom van die Eucalyptus grandis in vergelyking met SA dennehout, is die jong, gevingerlasde en platkant-gelamineerde Eucalyptus grandis 'n belowende opsie vir strukturele houtgebruik in 'n land wat so houtskaars is soos Suid-Afrika.

Please refer to this item in SUNScholar by using the following persistent URL: http://hdl.handle.net/10019.1/106071
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