CREEP OF SMALL DIAMETER TIMBER POLES

Abstract

It has become necessary to encourage the use of small diameter timber poles (plantationthinnings) as building timber particularly in roof construction, as the supply of betterknown indigenous species can no longer meet the increasing demand for constructiontimber. Eucalyptus and Pinus species were introduced to Sri Lanka some years ago and atpresent there exists a large area under their plantation. While the larger diameter trees areused in pole form for transmission poles or are processed for furniture, the thinnings havehad little use. Preliminary studies were carried out to assess the feasibility of using thethinnings as roof timber. The mechanical properties were found to be adequate. Boththese species however need to be preservative treated to increase their durability. Borondiffusion was considered a cost effective method of treatment provided the poles are usedwhile still green. Previous studies have shown that the poles removed from a Boronpreservative tank took 3 to 4 months to reach equilibrium moisture content. Thus theymust be allowed to dry out under load.The limiting of deflection often controls the design of seasoned timber beams. It is thusexpected that limiting deflection, will be more the controlling design requirement for thegreen poles drying out under load. The magnitude of long term or creep deflection thatoccurs as the poles dry out under load is thus significant and needs to be determined.This paper describes an investigation into the creep behaviour of initially green smalldiameter timber poles drying out under load. The magnitude of a factor that can be appliedto instantaneous elastic deformation to evaluate long term deformation is determinedexperimentally for timber poles of Eucalyptus and Pinus species of different diametersdrying out under a range of bending stresses.