Differential Axial Shortening of Columns in Tall Buildings

Abstract

The axial shortening of columns becomes significant with increase in the height of the structure. Differential shortening between adjacent vertical members arises primarily from the different stress levels in the adjoining vertical members, emerging from different tributary areas associated with them. Difference in volume-to-surface area ratio of the adjoining members also contributes towards the differential shortening between the members. The differential movements results in tilted slabs and also affects non-structural elements like partitions, claddings, finishes, mechanical equipment etc. Additional forces are generated in the beams and slabs as result of support settlement. Therefore, it is important to study the phenomenon of differential shortening of vertical members in tall buildings. The total column shortening is caused by the elastic, creep and shrinkage deformations of the column. Various creep and shrinkage models are available for estimating the DAS. Three widely used creep and shrinkage models namely, ACI 209R-92, CEB-FIP 90 and IS: 1343-2012 were studied and its impact on DAS was investigated by carrying out column shortening analysis in a 20-storey building in ETABS. The IS: 1343-2012-model gave highest value of DAS followed by CEB-FIP 90. The ACI model gave least amount of DAS. DAS is affected by a large number of factors such as the material properties, design assumptions and loading assumptions. To find out the significant factors affecting DAS parametric studies were carried out on a 40-storey building in ETABS. The DAS was found to be 2-2.5 times higher when the effect of creep and shrinkage was considered. Volume-to-surface area ratio, floor load intensity and grade of concrete were found to be the critical parameters influencing DAS. Therefore, use of columns with high plan aspect ratio and use of higher grade of concrete in columns and shear walls are recommended. An analytical examination of the phenomenon of column shortening was carried out. The process of estimation of DAS involves tremendous calculation. A program was developed for the estimation of DAS. The column shortening values obtained from the program was comparable to the results obtained from the ETABS analysis. The program gave slightly higher values of DAS as compared to the results obtained from ETABS.