Effects of Differential Shortening on the Design and Construction of Highrise Concrete Structures

Abstract

The load transfer from the horizontal members (slab or beam) to the supporting vertical members can be considered by either taking the tributary area, or by analysing the whole structure using a refined finite element analysis method. With the tributary area method, the stiffness and movement of the vertical members are not taken into account and the load redistribution between the two adjacent members is often ignored in the design process. On the other hand, if the construction sequence is not properly modelled, the refined analysis method often leads to the extreme case of the load redistribution as the self weight would be considered acting instantaneously. Load redistribution is a result of the differential shortening in the adjacent members, which causes additional bending stresses in the connecting horizontal members. Differential shortening in highrise concrete structures can in some cases be very significant and lead to impaired serviceability to both structural and non-structural elements of the building. The effects of the differential shortenings on the design and construction of concrete buildings will be highlighted based on the reported case studies.Based on this study, it is found that the effect of differential shortening could still be overlooked in the design process unless more specific guidelines are provided in a form of a code of practice. Finally, a parametric study is performed to investigate a reasonable limit for differential shortening and the required moment capacity of the horizontal member.