ABSTRACT
Reliability and economy are inherently competing demands; the study of which is becoming an increasing concern for the designers of electric transmission line structures and more generally to the profession of Structural Engineering. The work covered in this study deals with the assessment of the reliability (safety) levels of the prestressed electric transmission concrete poles; as they are used for support of transmission line. Safety levels at various heights of tappered I - section poles have been investigated for the serviceability limit states in various categories Viz:- Compressive and Tensile working stress conditions, deflection requirements and stability criteria. Several combinations of load and resistance effects are considered and implied safety levels for this class of poles are estimated. A general purpose computer program package based on FORM5 (First Order Reliability Method; Version 5) was adopted for the estimation of the safety levels associated with the different limit states examined. As far as the serviceability limit states are concerned, it is found that the requirements for the working stresses in compressive and tensile and the criteria for deflection and stability are very conservative. In most cases, the critical points along the poles height fall within the solid rectangular section. It is also noted that the failure of the poles within the first 1.0 to 1.5m from the apex is quite alarming. The conditions governing the deflection requirements call for an immediate reappraisal since the associated safety levels are extremely very low. The results have been plotted so that the safety level (0) can be read off for any given limit state condition for a pole at various heights for any load and resistance effects desired. In addition, the cost-effectiveness of the design requirements for tensile criteria is also estimated.