El desarrollo rural en la última década: el Desarrollo Rural

Department of Civil Engineering, Faculty of Engineering, University of Peradeniya

Design for steel compression members can be made by either allowing or restricting the possible occurrence of the local buckling of component plates. Generally it is common practice to restrict the local buckling by specifying the maximum width- thickness ratios for component plates to keep the design process simple. Most hot rolled sectioned used in design of structures fall in to this category. However when built–up sections are used the designer may permit the local buckling by specifying the higher width–thickness ratio (b/t). In British Standard Structural use of steel work in building (BS 5950-1., 2000) specifications, widely used for design of steel buildings in Sri Lanka, classifies the steel cross sections as Class 1 plastic, Class 2 compact, Class 3 semi- compact and Class 4 slender. Only in Class 4 slender sections the load carrying capacity of the section is reduced due to local buckling.

An efficient design may be obtained by allowing the possible local buckling of component plates and thus enlarging the freedom for the determination of cross-sectional shapes with arbitrary selections of width-thickness ratio. The objective of this paper is to see that permitting the local buckling of component plates gives an efficient design for columns.

The design method of BS 5950-1., 2000 was initially compared with the limited experimental data from Usami T. et al. (1982), which includes the experiments of the built- up box shaped columns of the HT 80 steel grade.

Optimizations for steel compression members were performed, focusing mainly on whether allowing the local buckling of component plates will bring benefits for efficient design or not. Ultimate strengths were found for cross sections by varying width-thickness ratio (b/t) while having the same gross sectional area and member length to ensure constant weight of material for that particular member and plotted. This was repeated for different L2_{/A ratios, where L is member length and A is gross sectional area. This study} was limited to welded box sections under pure compression.

Comparison of design method with the test data available indicates that the design method has sufficient accuracy for the ultimate strength of the column. The optimization study indicates that there exists a range where allowing local buckling produce economical benefits for ultimate strength.

Proceedings of the Peradeniya University Research Sessions, Sri Lanka, Vol. 16, 24th _{November 2011}

20

Retrospective Analysis of Victoria Dam

K.C. Chandrasiri, L.G.S.T. Jayathilaka, P.M.D.D.B. Pathiraja and A.P.N. Somaratna

Department of Civil Engineering, Faculty of Engineering, University of Peradeniya

Built across the river Mahaweli at Theldeniya during 1980-84, Victoria is a doubly-curved arch dam of 122 m height that impounds a reservoir of 722 million cubic meters capacity. It is the largest concrete dam in Sri Lanka. Equipped with three 70 MW generators for a total installed capacity of 210 MW, Victoria provides about 780 GWh of energy annually to the national grid. From the energy point of view alone, Victoria dam assumes great importance to the country’s economy.

During construction the dam was provided with extensive instrumentation to continuously monitor its behaviour. Among these instruments are, normal and inverted pendula, crest leveling studs, and survey targets on the downstream face. Over the past 27 years, they have yielded a great volume of data regarding the deformations of the dam, among other things. Such data can be used in two ways. First is that they can be compared with the expected behaviour to verify compliance with design assumptions, and in the case of any deviations, to understand the reasons for such deviations. Second is that they can be used to develop models to predict the future behaviour of the dam which will assist engineers in making operation, maintenance, and repair decisions. In fact these can be two phases of a single process. The present study attempts the first task: comparing observed deformations with those expected. A linear elastic, 3-d finite element analysis was performed to estimate the expected deformations. In view of the approximate symmetry, only one half of the dam was modeled. Even though there are some galleries inside the dam (the cross sectional area of which is about 2.35 % of that of the dam) and a set of openings for spillway gates near the top of the dam, they have been neglected in the model. Fixed boundary conditions are employed at the base and the abutments of the dam.

Comparison of deflections on the dam cross section at the centre line (Fig.1) shows reasonable agreement with respect to the deflection pattern between the finite element analysis and data obtained by means of the pendula soon after impounding. This indicates the validity of the adopted approach. However, it is noted that the finite elements consistently underestimate the observed values. This may be due to (a) possible variations in the Young’s modulus (b) non-inclusion of the dam galleries and the spillway gate openings in the finite element model, and (c) the assumption of fixed boundary conditions. It is believed that the rectification of these shortcomings could improve the co-relation between finite element estimates and actual observations.

Financial assistance from Tokyo Cement Company (Lanka) Ltd. to undergraduate projects in the Department of Civil Engineering, University of Peradeniya is gratefully acknowledged.

Proceedings of the Peradeniya University Research Sessions, Sri Lanka, Vol. 16, 24th _{November 2011}

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Forecasting Extreme Rainfall in Kelani River Basin under Changing Climate