23 Jan 2023

Hardwood columns – Compressive strength and Young’s modulus parallel to grain direction and influence of wood moisture content

 

WHFF Project:2020.11  

Projectmanagement: Prof. Dr. Andrea Frangi

 

The most important in brief 

  • The relevant properties for glulam (GL) made of beech (Fagus sylvatica L.), oak (Quercus robur L), ash (Fraxinus excelsior L.) and chestnut (Castanea sativa Mill.) as well as for laminated veneer lumber (LVL) made of beech were investigated for use in columns. 
  • The influence of wood moisture content on compressive strength and compressive modulus of elasticity parallel to grain direction was quantified, models were developed to account for it, and design values for compressive strength (fc,0,d) and compressive modulus of elasticity parallel to grain direction (Ec,0,mean and Ec,0,05) in moisture content class 1 and moisture content class 2 were derived for all hardwood products investigated. 
  • By the results a participation in the standardization on European level can be guaranteed (especially prEN 14080-2 & EN 1995-1-1).  
  • On a national level, the results are to find their way into the SIA 265 standard. 

 

Project description 

The use of hardwood in structural timber engineering offers the answer to two major challenges of the coming decades. On the one hand, the generally higher performance potential of hardwood compared to softwood allows the expansion of the application range of structural timber construction as a whole. This will make it possible to substitute mineral and metallic building materials and thus reduce greenhouse gas emissions in the construction sector. The greatest possible savings are achieved when local resources are used and processing and ultimately use take place without long transport routes.  

On the other hand, the Swiss forest is currently undergoing major restructuring as a result of climate change. One consequence of this will most likely be that the current “bread tree” of the timber industry, spruce, will be available to a lesser extent in the future. It is the order of the day to investigate those wood species that benefit from climate change – or at least suffer less from it – with regard to their suitability for use in structural timber engineering and thus to set the course for a future ecologically and economically sensible use of the Swiss wood supply.  

In this project, the properties of glulam (GL) made of beech (Fagus sylvatica L.), oak (Quercus robur L), ash (Fraxinus excelsior L.) and chestnut (Castanea sativa Mill.) as well as those of laminated veneer lumber (LVL) made of beech that are relevant for use in columns were systematically investigated. These are the compressive strength parallel to grain direction (fc,0), the compressive modulus of elasticity parallel to grain direction (Ec,0), and the influence of wood moisture content (u) on the above properties. Spruce glulam (strength class GL 28h) was tested as the “reference product” in this study.  

A total of 300 compression tests were performed to determine the compressive strength and compressive modulus of elasticity parallel to the grain. The wood moisture content range investigated was from 6 to 18%. The wood moisture content of all test specimens was determined by the Dry- oven method. The density of the test specimens as well as the swelling and shrinkage mass were also determined within the scope of the project. 

Conclusions 

Based on the results of the experimental investigations, the influence of wood moisture on the compressive strength and the compressive modulus of elasticity parallel to the grain direction was quantified, models for their consideration were developed, and design values for the compressive strength (fc,0,d) and the compressive modulus of elasticity parallel to the grain direction (Ec,0,mean and Ec,0,05) in moisture class 1 and moisture class 2 were derived for all hardwood products investigated.  

The extensive and systematic investigations of this study allow a participation in the standardization on the European level and thus the representation of Swiss interests secured by data. In particular, reference should be made in this regard to the development of a product standard for glulam made of hardwood (prEN 14080-2), which is currently underway in CEN/TC 124, as well as to the revision of Eurocode 5 (EN 1995-1-1). At the national level, the results are to be included in the design manual “Lignatec Hardwood”, which is currently being prepared, and in the long term in the SIA 265 standard.  

The following is an overview of the adaptation proposals from the project:  

  • Definition of service classes or moisture classes – Adjustments in the standards EN 1995-1-1 and SIA 265  
  • Review of the factors ηw defined in the SIA 265 standard to take into account the influence of wood moisture on the load-bearing resistance and stiffness  
  • Conversion of test results to the reference wood moisture contents – adaptations in the standard EN 384  
  • Determination of wood moisture content – adaptations in standard EN 13183  
  • Climatization and testing – adaptations in standard EN 408 

The project was supported by the Forest and Wood Research Promotion Switzerland WHFF-CH of the Federal Office for the Environment FOEN and the Conference for Forests, Wildlife and Landscape KWL of the cantons.

Click here to read the report about the project on ARAMIS

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