STOCHASTIC ASSESSMENT OF NIGERIAN TIMBERS FOR BRIDGE DECKS IN ACCORDANCE TO AASHTO LFRD

This work provides results for stochastic safetyevaluation of Nigerian timbers for bridge decks in accordance to American Association of State Transportation and Highway Officials (AASHTO) Load Resistance Factored Design (LRFD) design specifications.A timber bridge is modeled in accordance to AASHTO LRFD, 2010, to represent real life experiment in order to depict the structural behavior of planks when used as a bridge deck. This model was then subjected to some degree of entropy using Advanced Second Moment Reliability Assessment (ASMRA) method, which was subsequently analysed using JAVA library with the help of Flanagan polynomial.Experimental data collected from literature was comparedwith the current Nigerian Code of Practice, NCP 2, 1973 for timber, and were used for the assessment. It was observed that, strength classes, timber thicknesses and stringer spacing are the major factors among others influencing the structural behavior of Nigerian timber proposed as bridge decks. Therefore, the major classes of Nigerian timber recommended for bridge decks are timber within the strength classes N1 to N4 with dimensions ranging from 100 x 250mm to 150 x 300mm on stringers spaced not greater than 300mm, depending on the strength class adopted; with timber belonging to the higher strength classes taking the lower dimension. An exception to this, are timber which belong to the strength class N1, where the stringers can be spaced at 450mm using timber with dimension not lower then 100mm thick and width not less than 250mm.The recommended strength classes with associated material properties can be a source of sustainable bridge deck material over a reasonable period of time as indicated by the probability of failure as a result of damage due to load accumulation. In view of this, timber which is a locally available material can be used as substitute for the expensive concrete and steel which are the most commonly used materials.

The need for local content in construction of engineering infrastructure is now a serious engineering challenge in Nigeria. This is because vast quantities of local raw materials, which must be processed and used for cost effective construction abound. Construction activities based on these locally available raw materials are major steps towards industrialisation and economic independence for developing countries (Aguwa and Sadiku, 2011).

Timber is one of the natural occurring raw materials which abound in Nigeria and it had been put to use as a building material for construction since prehistoric times. It is available inlarge quantities in the forested parts of the country. The extent of its usage by professionals in the buildingindustry is determined not only by their understanding of the material, but also by their perception of thematerial (Adedeji and Ogunsote, 2004).

The major use of timber in Nigeria and most part of the third world has been limited to domestic use as an alternative source of heat energy or household kitchen appliance. In this respect, timber which is supposed to be used to sustain the development of the economy is been burnt as coal. With critical analysis of our environment and careful exploration of the structural properties of timber, one can adequately establish and design an environmentally friendly structure which is cost effective.