Otago Polytechnic

How brittle does timber become as it dries out? Our staff and students investigated.

In 2016 the balcony of a student flat collapsed when it was overloaded with people attending a backyard concert by the band 660. The incident sparked some interesting speculation amongst carpentry and engineering staff at Otago Polytechnic which resulted in a collaborative carpentry and engineering research project. Could very dry timber be more brittle than new timber with a higher moisture content? And could that be tested using our Dennison universal testing machine? 

For the first stage, to answer the second question, an Engineering Technologies student designed and tested a machine to fit onto the Dennison so as to apply pressure to test the shear strength of timber.  Then in 2018 another student, Cody Laing, took on the second stage of the project. Cody's work was supervised by Senior Lecturers Kevin Dunbar and Robert Cooke, with assistance from Mark Harmer for analysis and presentation of the results. Lengths of radiata pine and New Zealand oregon (Douglas fir) were purchased, in the common floor joist size of 125mm x 50mm. The timber was dried to varying degrees, outside, in our boiler room, and in a kiln drying machine. Samples were cut and labelled for testing. Each sample was weighed, its moisture content measured, and the saw cut (affecting grain) recorded. The samples were then tested to breaking point using the Dennison. 

The testing process confirmed that the NZ oregon is more elastic than the pine - it would give rather than fracture - which made it harder to determine when it failed. However for both types of timber, contrary to expectations, as the timber dried the shear strength did not decrease. This is a reassuring result for owners of older homes. 

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NATURAL & BUILT ENVIRONMENT

September 2019