EFFECT OF STAGES OF CONSTRUCTION ON THE BEHAVIOUR OF POST-TENSION SLABS IN HIGH-RISE BUILDINGS

Abstract

Abstract Adopting post-tensioned (PT) slabs for high-rise buildings (HRB) became an efficient solution to achieve thinner slabs, longer spans, and better durability compared to non-PT slabs. Building codes require designing of PT slabs under both service and ultimate loads. During service stage, accurate analysis of PT slabs is critical because redistribution of moments in continuous flexural members is not allowed which is different from ultimate stage. One-step analysis (OSA) is commonly adopted during design of HRB, where gravity and lateral loads are applied on a complete structure at once. However, inadequacy of this approach was proven due to neglecting the sequential nature of loading, time-dependent behaviour of concrete, and leveling of floors during construction. The current thesis investigates the impact of using staged-construction analysis (SCA) of HRB on the behaviour of PT slabs as a more realistic approach for HRB analysis. Comprehensive finite element models (FEM) are developed to conduct SCA of different HRBs with PT slabs having various heights and layouts. The FEM accounts for both geometric and material nonlinearities while including prestressing loads. Hence, PT slab analysis is conducted using 3D modelling of a complete building rather than 2D modelling of one slab only. A series of analysis comparisons are made between OSA, SCA, and SCA with time-dependent effects (SCAT). Then, comparisons are made to evaluate the design of the PT slabs during service and ultimate stages due to the different analysis approaches. Afterwards, this thesis investigates the impact of using non-PT slabs within a PT HRB. Using different floor systems within the same high-rise building (HRB) is common practice, based on functionality and loading of each floor. Interaction between PT and non-PT slabs is usually neglected in traditional design practice where a separate analytical model for each PT floor is developed. To account for this interaction in various HRBs, a 3D nonlinear staged-construction analysis is conducted including time-dependent effects (SCAT). Structural behaviour of two representative office buildings, including both PT and non PT floors, is investigated. Three cases for each building are considered: Case 1 includes PT system in all floors, Case 2 includes PT system in all floors except for five non-PT floors equally distributed along the building height, Case 3 includes PT floors in all floors except for the lower five non-PT floors.