An assessment of Pre-stressing in post-tensioned Concrete Box Girder Railway Elevated Structures: a case study on Comparative Analysis of Continuous Vs Simply Supported Elevated Structure of Addis Ababa’s Light Rail Transit
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Date
2015-04
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Addis Ababa University
Abstract
Elevated structures are the key element in metropolitan city railways like Addis Ababa’s Light railway Transit. This is because it is much more expensive, controls the system’s capacity and so many more. This Paper which mainly focuses on the comparative analysis of the values of pre-stressing is done to predict percentages of the relative prestressing values if the simply supported segments need to be continuous in the future.
The objective of this research is to estimate the quantitative percentage values of pre-stress for continuously supported elevated structures in a reference to the simply supported ones and the resulting span increments, all are being straight line girders. Consideration of the varying cross-sectional area along the span makes the study more realistic and the results dependable.
In this study the main governing manual is AREMA 2010 - Volume II, manual for Railway Engineering. Again, almost all input values are exhaustively taken from Addis Ababa Light Rail Transit Construction Project Design Drawing (AA-LRT’s CDD, 2013). The four basic loadings considered are Dead load, Secondary dead load, Live load and pre-stress load. Others loadings like wind load, earth quake load, etc could be considered. Yet, they do not have major influence on the relative percentage values of pre-stressing. The modeling and analysis are done for a total of four structures. On the one hand 20m (one-span simply supported) & 23.002m (two-span continuous) single cell each. And on the other hand 30m (one-span simply supported) & 97.758m (three-span continuous) double cell each. But, full pre-stress design is done for the double cell ones. The reason is that the single cell one’s areal cross-section geometry does not meet the requirements of AREMA 2010’s specification independently.
The maximum span increment is calculated by making the 20m simply supported segment two span continuous and the 30m simply supported segment three span continuous. The parameter used to determine span increment is deflection, caused by the structure weight, prior to the application of pre-stress and is calculated to be 4.7mm for the single cells and 15.6mm for the double cells. The deflection caused by Live load after the application of pre-stress also meets the requirement of not greater than l/640 of their all respective span lengths. The relative percentage value of pre-stressing for each continuous structure is compared with the respective simply supported segment. Single cells are stressed from both ends only. But, the double cells are stressed using two prestressing schemes: tensioning from the start end and tensioning from both ends. From SAP 2000 modeling results the 20m spans are increased to 23.002m. Again, by keeping both end span 30m, the middle span of three span continuous box girders is increased to 37.758m.The Prestress analysis involves inputting values and comparing the results and reiterating the process until convergence.
Studying the comparative values of pre-stressing, making the simply supported one’s continuous, help us to know and figure out the relative advantages of it. For the same size of cross-section, the span in a continuous ones made increased than in a simply supported one. Generally, increasing the span has a key role in a significant reduction of the number of piers that are likely to be a key source of obstruction to the traffic flow.
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Keywords
simply supported elevated structures, continuously supported elevated structures, pre-stressing span increment, deflection and the relative percentage values of prestressing