On the time dependent axial shortening of tall concrete buildings with framing action

Long term axial shortening in a tall concrete structure is a second-order effect that is of some impediment to the serviceability, and in some cases, structural integrity of the building. The research here involves analysis of axial shortening elements that interact through framing action. In many tall concrete buildings framing is inherent as columns, beams and floors are quite often connected with rigid connections. The mechanics of differential axial shortening and framing action due to rigid connections causes load sharing between vertical elements that fundamentally affects the degree of both absolute and differential axial shortening. Evaluation of axial shortening by analysis of a structural system consisting of discrete elements over-predicts the level of axial shortening when framing is present. A methodology to calculate axial shortening, accounting for load sharing of axial loads for tall concrete buildings, is developed. The application of this methodology to the construction of a tall concrete building allowing for the building cycle is presented. A program is written that incorporates both the methodology and its application to a tall building, using the ACT concrete creep and shrinkage models. Correlation of the program is made against an existing previously tested program that does not allow for framing action. A comparison is made between the two sets of data for an actual 85 storey building. From this it is concluded that; i) Framing action reduces the differential axial shortening between two elements ii) The core generally experiences less change to axial shortening than columns iii) The effect of framing cannot be estimated by a relative percentage adjustment applied uniformly to each storey.