ABSTRACT
Predicting the behavior of reinforced concrete structures beyond the construction phase is a critical aspect of structural design. In particular, the differential shortening of columns due to varying stress conditions over time can significantly impact the long-term health of a structure. This effect is not always considered at low building heights as the comparative effect might be minimal – e.g. 1% shortening in a single story building is on the order of millimeters. However, as the number of stories increases, this effect compounds, such that column shortening for a 60 story structure might be on the order of 30 mm. At this scale the shortening can cause problems not only within the structural members but in the nonstructural components as well, such as partitions and pipe lines. It is therefore necessary to design a structure such that it is able to mitigate the shortening which occurs in the years and decades following construction.
Such is the case for the twisted columns of the New Marina Casablanca Tower in Morocco. This 160 m tall tower (currently in conception) was designed as part of a new convention center along the coast in Casablanca. The primary architectural feature is exhibited in the form of a spiraling tower where each successive floor is rotated as the building ascends, amounting to a total of 135° twist. The structural system consists of an inner core and inclined columns which follow the angle of rotation. Although a system of core and columns is common, the introduction of the inclined columns leads to unique behavior of the structure, and the time-dependent shortening effects cannot be assumed based on previous work intended for straight columns.
In this study the authors analyze the time-dependent behavior of these columns at two scales. At the building scale, the construction procedure is simulated to determine the load history of the columns and initial elastic deformations. Multiple construction analysis methods are considered. Then, based on the results from this global simulation, a single-story column is modeled to the material level, providing more nuanced analysis. First the hygro-thermal-chemical (HTC) theory is used to determine the temperature and relative humidity in the column under actual environmental conditions. This is then coupled with the mechanical loads determined from the global analysis to model the mechanical behavior using the solidification-microprestress-microplane (SMM) model. This analysis is then repeated at multiple angles to determine the effects of inclination on the shortening behavior.
