To predict the residual strength of an engineered structure, it is first necessary to predict the effect of decay. A model was developed in which the effective area of structural decay in large section timbers was defined as that area that could be picked out with a pen-knife. Some limited studies indicated that for practical purposes the remaining wood could be assumed to have its full residual strength. Thus once the extent of the decay has been computed, the residual load capacity of a structure can be evaluated through the use of conventional engineering mechanics. A preliminary model was developed by analysing data obtained from 5000 in-ground stakes placed around Australia. These stakes were located on 5 sites and were studied over a period of 30 years. Some more widely dispersed short term tests were also undertaken. The tests comprised untreated timber of 77 species (both hardwoods and softwoods) and timber of 2 species treated with CCA and creosote. The preliminary model was then calibrated and modified by examination of full sized structures comprising round poles and rectangular house stumps from a limited number of sites. These included the effects of maintenance activities, such as the effects of diffusion paste bandages. Using the completed model, it is now possible to compute the deterioration over the years of the load capacity of structural elements constructed with timber in ground contact. The model is directly applicable to about 80 species of timber, CCA and creosote treatments to various levels and a variety of maintenance procedures. Through use of a climate index (based on rainfall and temperature parameters) the procedure is applicable to all locations in Australia, from temperate to hot tropical regions and from rainforest to desert areas. With slight modifications, it should be possible to apply this model to any structure, fabricated from any species and located anywhere in the world.

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