ABSTRACT
Vertical Elements The structure for the 77-storey Plaza Rakyat office tower is formulated based on the desire for economy through simplicity and repetition. The structure is framed entirely in reinforced concrete due primarily to the predominant use of the material over structural steel in Malaysia. Typical lowrise and high-rise floor framing plans are shown in Fig. 2. Gravity loads are collected on the
Figure 2. Typical Low-rise and High-rise Floor Framing Plans
exterior perimeter by large rectangular columns spaced 9.0m on center. For the lower half of the building, these columns have a constant face dimension parallel to the plane of the exterior wall of 1200mm and vary in depth along the height of the building from 2700mm at the base to 1500mm at the Level 51 transfer. Reinforcement in the large columns is organized such that access points for concreting and vibration of the concrete are specifically accounted for in the design. (Fig. 3) Column size transitions are made on the inside face of the column only in order to simplify
formwork and detailing with respect to the exterior wall. The column sizes above Level 53 are reduced to 800mm square in order to provide less encumbrance to sightlines for the upper level office spaces. The concrete strengths utilized are quite low (C50 and C40 grades) for a building of this height in order to avoid the introduction of foreign concrete technologies. The design for the exterior columns was controlled primarily by considerations of strength under gravity loads alone. The remainder of the gravity load is supported by the rectangular core which is organized around the central elevator, stair and services areas. Vertical walls on the perimeter of the core vary in thickness from 850mm at the base to 450mm at the roof. Internal walls in each direction of constant 300mm thickness frame the various elevator banks in the core. Besides designing the vertical load-carrying elements for strength under gravity loads, the design was developed from a standpoint of trying to equalize the working stresses of the core walls and exterior columns in order to reduce the effects of differential vertical shortening which can result in some out of levelness in the floors of very tall concrete buildings. It is anticipated that the concrete core walls will be constructed some 5 to 6 stories above that of the floor and exterior column construction. Between Levels 51 and 53, the building exterior steps inward approximately 3.0 metres. All exterior columns are transferred through two-storey high reinforced concrete shear panels which avoid the necessity of deep transfer girders at Level 51. The transfer is accomplished through the strength and rigidity of the floor slabs in compression at Level 53 and tension at Level 51. There is no net horizontal load on the building as the loads from the two sides of the building oppose each other. A
two-dimensional finite element analysis model was created in order to determine the stresses in the columns above and below the transfer, the two-storey high shear panel (wall), and the floor slabs at Level 51 and 53 as well as those within two to three stories of the transfer. (Fig. 4)
Vertical deformation analysis (elastic, creep and shrinkage effects) The estimation of vertical deformation including the effects of inelastic concrete creep and shrinkage is one of the most important concerns in the design and construction of tall reinforced concrete and composite (mixed) buildings. For the Plaza Rakyat project, a state-of-the-art computer analysis program was developed to determine the magnitude of the vertical deformations of the tower columns and core walls. This program was used to develop initial compensation values for the construction based on an assumed construction schedule and material coefficients. While computer analysis and simulation of the column shortening process is critical, concrete material laboratory analysis and testing for creep and shrinkage characteristics is equally important. To this end, a concrete creep and shrinkage testing program was initiated at the local University of Malaysia to provide material coefficients based on the mix designs to be utilized on the project. Based on the results of the material testing and the assumed construction sequence, compensation values (cambers) for the construction of the core wall and for the differential deformation between the core and exterior were generated to be used during construction. Fig. 5 summarizes the vertical deformation values for the core wall and exterior columns. Total deformation is subdivided into elastic, creep, and shrinkage values.
