Precast Concrete in Framed Tube High-Rise Structures

Abstract

In this thesis a particular structural system for high-rise structures is discussed: the prefabricated concrete framed tube and tube-in-tube structures. These system consist of a prefabricated concrete perforated elements in the perimeter of the structure that are the main lateral load resisting elements in the building. For tube-in-tube structures, the perimeter elements work together with a structural core that also resists the high lateral loads that are due to the wind loading on high rise structures. The thesis starts with a literature study on multiple aspects that are associated with the subject. First, an introduction into prefabricated high-rise and framed tube structures is given and some key behaviors and elements of this kind of structures are explained, such as the shear lag effect and aspects of connections between prefabricated elements. Hereafter, previous research associated with the subject is described. Most of these titles are Master's Theses from Delft University of Technology, which is at the forefront for research on prefabricated concrete high-rise structures. Other references found are articles published in Asian countries or the United States of America, but those tend to focus on cast-in-place framed tube structures. Finally, sources describing the connection between elements are described, which is an important parameter in prefabricated concrete shear wall structures. The literature study continues with an analysis of prefabricated high-rise structures in the Netherlands. It presents different structures already constructed and their relevance to the subject of the thesis. Also a description of the Dutch high-rise market is given as well as the justification for the construction of high-rise towers in this market. The literature study leads to the section on the missing links in research and the research questions associated with these missing parts of information available in literature. As a result, the research part of the thesis is divided into three parts: - Shear lag effect of non-rectangular framed tube structures - Structural behavior of prefabricated framed tube structures - Case Study Zalmhaventoren: Potential of fully prefabricated tube-in-tube structures These missing links lead to two separate parts of this thesis. Firstly a parameter research was done to answer the research questions associated with the first two missing links. This parameter research tries to answer research questions such as: “How do framed tube structures without a rectangular floor plan behave with respect to shear lag?” and describes the relation between parameters such as aspect ratio, slenderness and corner stiffness on the shear lag effect and other structural aspects. It answers these questions by changing the parameters of a basic structure in the Finite Element Method (FEM) program AxisVM. This structure has a floor plan of 14.4m x 14.4m and has 25 floors with a 3.5m floor height, adding up to a total height of 87.5m and a slenderness just over 6. All models are made in 4 different configurations: a cast-in-place configuration, a prefabricated configuration with a masonry configuration for the elements and two similar prefabricated configurations with small and large windows. Two parameters are also formulated in this section to keep track of the shear lag effect: These are the shear lag factor P, for flange walls and the shear lag factor P* for web walls. All these previously mentioned characteristics, except for the floor plan, are used for the answering of the research question on non-rectangular structures. The conclusions from this part of the parameter research are that the changes in shear lag effect are small if inside corners or corners with an angle not being 90 degrees are used. For structures with the floor plan of higher polygons, such as octagons and hexadecagons improved structural behavior was found. The reduction in peak stresses in the corners can be attributed to the shorter elements where shear lag can form and the more corners over which the peak stresses can be divided. For the parameters researched using the square floor plan it was found that both the aspect ratio (the ratio between width and depth of the structure) and the introduction of structural vertical joints between the elements in masonry configuration had no to a negligible influence on the structural behavior. The effect of the corner stiffness on the structural behavior was more profound: especially reducing the corner stiffness has a noticeable negative effect on co-operation between the flange and webs walls, which in turn increases the deflection at the top of the structure. For the relation between the shear lag and the slenderness of the structure it was found that structures with a lower slenderness, that depend more on their shear stiffness to resist lateral loading, experience more shear lag than structures with a higher slenderness. In the second part the following research question was answered: “Is it possible to build a structurally feasible prefabricated tube-in-tube high-rise structure in the confines of the Dutch building market of approximately 200m high and how does it compare to other prefabricated solutions?” In the literature report a building was described that could be used to answer both parts of this research question. The Zalmhaventoren is a proposed residential shear wall high-rise structure in Rotterdam which has been researched by ten Hagen for a Master's thesis at Delft University of Technology. In this thesis, the original cast in-situ concrete Zalmhaventoren is redesigned to a prefabricated shear wall structure that is 201.3m high. A redesign of this structure gives insight in the possibilities of a fully prefabricated concrete structure and has a model that as comparable to the model researched by ten Hagen. During the redesign the basic dimensions of the original structure were kept. The floor plan of the structure is 30m x 29.5m, has a core of 14.4m x 12m and the floor to floor height is 3.05m. Although originally designed as a residential structure, this floor height is also sufficient for other functions, such as an office function. To keep the structure comparable to the one designed by ten Hagen, 66 floors were used to reach the total height of 201.3m. Besides these basic dimensions however, significant changes were made to convert the structure into a prefabricated tube-in-tube structure. The removal of the shear walls and addition of the lateral load resisting façades changed the floor plan of the structure. To give the tube the stiffness that is required when loaded by wind, extra material was added in the corners of the structure, where windows were present in the shear wall design. Also an extra column was added in the center of the frame for the largest balcony on each side of the structure. In the lobby of the structure similar changes were made to fit the new structural system of the building. Another important part of the structural design was the determination of the stiffnesses of the different connections between the elements. Especially for the horizontal connections in the tube of the structure this proved difficult, because of the differences in normal stress between different parts in the same element. A 2-level stepped stiffness was given to these tube elements to approximate the actual stiffnesses in this type of connections. For the corner connections (of the Interlocking Halfway Connection type) the stiffness was determined for the connection in compression and in tension. An iterative calculation ensured that the correct stiffness was attributed to the correct corner connection. After the model was made, the results were analyzed. Probably the most important result, for it is often governing in high-rise structures, is the deflection at the top of the structure. For a structure of this height, the maximum allowable deflection is 403mm. For the four different wind directions, the deflection was between 282mm and 327mm. Also the shear stress between elements was checked and found to be within the limits set in the Eurocode. For two elements an estimation of the reinforcement was made. The reinforcement of the lintel was viable, but for the column near the lobby of the structure, a reinforcement percentage was found that was higher than the recommended maximum value in the Eurocode. However, if the integrity of the concrete can be assured or the less strict Dutch national annex is used, the found percentage does not interfere with the feasibility of the structure Along with the main model, some alternate models were made to research the effect of certain aspects on the most important result: the top deflection of the tower. The first one was the difference between a cast-in-place model and the prefabricated one. The increase in top deflection for the prefabricated structure was around 8%. Not negligible, but not high enough to stop considering prefabricated tube-in-tube structures from being viable. The second check was if removing the balconies, and with them the less stiff frames around the balconies, and replacing them with ordinary stiffer windows had a positive effect on the top deflection. It was found that implementing this change at the same element thickness reduced the top deflection by 34% to 42%. Even when the thickness of all the elements was reduced from 400mm to 300mm, the top deflection of a structure without the balconies was better than for the model with balconies and a element thickness of 400mm. In the comparison with the structure from the work of ten Hagen, it became clear that both lateral stability resisting systems could work for the Zalmhaventoren. Both had advantages and disadvantages that could be weighted either way by different developers if the structure was designed purely as a residential building. However, if the tower was designed as being a multipurpose building, the open floor plan of the tube-in-tube structure provides not only more flexibility at completion, but especially over the entire lifetime of the structure. Ultimately this research gives insight into the structural behavior of the prefabricated tube-in-tube structure and proved that the possibilities of the prefabricated tube-in-tube structure lie even beyond the 200m, especially if the technologies associated with this stability system keep improving over the coming years.