Today’s tall buildings usually exhibit some extraordinary features such as extreme heights, elevation set-backs, overhangs, or free-form exterior surfaces, all of which make the construction process difficult, complex, and possibly unsafe for certain stages of construction. In addition to the elaborately planned construction sequence, prediction and monitoring of the building’s movement during construction and after completion are required for precise and safe construction.
Building movement in this context means the vertical and horizontal displacements that result from the sum of axial and lateral deformations of the vertical members of each level. The major factors affecting building movement include loads, geometry, time-dependent material properties of concrete, and sequence of construction. The building movement has an impact on the load distribution on the structural elements and can cause substantial serviceability problems. The shortened vertical structural elements inevitably transfer some forces to neighboring non-structural elements such as partitions, cladding, piping, and elevator rails, which are not designed to carry the additional loads. The effects of differential shortening between adjacent vertical members are evident particularly in tall buildings with a central core and perimeter columns. Because the central core generally has less stress than the perimeter columns and may be constructed in advance using a climbing form, the amount of shortening is much less than the perimeter columns.
For buildings with mass eccentricity or irregularity, the differential shortening combined with an applied moment could cause the building to move laterally because accumulated differential shortenings cause curvature, which is integrated along the height of the building and results in vertical and lateral movements depending on the symmetry and placement of the vertical elements. This lateral movement gradually develops during construction and constantly increases after completion because of the time-dependent properties of concrete. These properties can have adverse effects on the serviceability of non-structural elements such as partitions, elevators and curtain walls. Therefore, the lateral movement induced by differential shortening as well as vertical shortening should be accurately predicted for tall buildings.
Most of the previous studies focused on the prediction of the vertical shortenings only. In an earlier study, individual vertical members were analyzed without the restraining actions of horizontal members, and deviation from vertical was not considered. Although two dimensional frame analysis including the time-dependent properties of concrete was utilized in recent studies, prediction and monitoring of the lateral movement induced by differential shortenings have not been adequately researched.
This research describes a theoretical study on the behavior of the lateral movement induced by differential shortenings and proposes a method of construction stage analysis that includes the time-dependent effects of concrete. The proposed construction stage analysis is a series of static analyses where new construction stages are applied to a stressed and deformed structure of the previous stage. The proposed analysis method consists of a deformation analysis of individual structural members to evaluate the creep and shrinkage deformation, and a structural analysis of the overall structure to consider the restraining effect of adjoining members.
The algorithm of the construction stage analysis performs a series of structural analyses for every main construction stage, such as concrete casting of a floor and installation of curtain walls, and combines the result with time-dependent deformation to determine the overall vertical and horizontal movements. For every construction stage, a two-step analysis is performed before proceeding to the next stage. For the first step, shrinkage and creep deformations of individual structural members are calculated. The calculations are conducted for the time interval between construction stages. Movements of the structural members due to shrinkage and creep are changed to a strain load for the second step analysis. For the second step, a structural analysis of the deformed structure is performed to account for the newly applied loads and strain loads that are calculated in the first step. Structural restraints of the shrinkage and creep movements by adjoining members are considered in this step.
The developed analysis method was used to predict the lateral movement of a 58 story reinforced concrete building located in the city centre of Kuala Lumpur. The time-dependent properties that caused the lateral movement were discussed. Finally, the analysis results were verified against field survey data. These results show that the lateral movement induced by differential shortening can be calculated with a reasonable degree of accuracy by the proposed construction stage analysis method.