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\ . Journal of Structural Engineering Vo119 No. July 1992 pp. 65-75 No.19-8 Exhibit 3.1 Reliability-based analysis and design of transmission tower structural systems Sriram Kalaga A simple method for designing structures using probabilistic principles is proposed. The procedure involves in , deriving design strengths of system members for a specified level of reliability. An approach for evaluating the system s overall reliability using the concept of utilization ratios is suggested. Application of the proposed Reliabil· ity Based Design (RBD) method is illustrated through the analysis and design of a small planar transmission tower structure composed of axially-loaded angle sections. With the help of the results obtained, a regression analysi s is made to evolve a polynomial expression for empirical failure probabilities. Lastly, the failure ot:. . the selected tower is erified through a Monte Carlo simulation of its collapse modes. Suggestions for further extensi6n of the proposed RBD method are made. Reliability-Based Design (RBD) is a powerful, realistic de sign tool involving the application and use of probabilistic principles to predict the performance of structural systems. The appeal of the method lies in the fact that it explicitly allows for consideration of the inherent statistical variability of the loads arid resistances associated with the structure. Environmental loading phenomena such as wind, icc and snow, which are basically random in nature, can be appro priately accounted for in the design process. This realistic design basis is leading, RBD to be increasingly preferred to conventional design in many engineering environ ments.1·12·15 In the U.S., electrical utility industry, for ex ample, a formal transition to RBD is already underway for the analysis and design of transmission towers. Lattice steel, self-supporting tower structures are very common in transmission line systems. These towers are general ly composed of axia l members (steel angle sections, single and double), capable of carrying either tension or compression. Prescribed design loadings for such towers currently include wind, ice and snow loads, in addition to other forces relevant to groundwires and cables. This, the stochoistic loadings and the structural simplicity of the tower, makes a transmission tower appropriate for RBD. Theoretically, reliability procedures and probabilistic structural analyses r q u i r ~ quantitative evaluation of the member (or component) failure probabilities. This informa tion is used to examine the explicit chance that the structure shall fail during its life span. Any RBD procedure must account for individual members of the system and their role in the overall system failure event Howe ver, as oppose d to single woolk n poles often used to support elecfTical cables and w i r s ~ the tower is a complex, 3-dimensional structure. Considering the fact that a typical tower consists of over 25 primary and secondary members, the system failure event is a difficult numerical problem to evaluate. The presence of both tension and com pression members, with different performance criteria for the various limit states, adds to the complexity of the prob lem. Furthermore, there is always a possibility of member failures being correlated in several combinations; one may also have to consider member interdependencies while for- mulating the soluti on. · Structura l Engineer/Consultant, Structural Analysis SciVices , Inc ., 16-6-12 Official Colony, Visakha patnam 530 002. JOURNALOFSTRUCTIJRALENGINEERING VOL 19N0.2JULY 1992 6 --- ~ : . : · ~ · : · . · ~ ~

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